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Quality – Soil Technical Note No. 4

Soil Biology and

Table of contents

The goal of management...... 2

Why should land managers understand soil biology?...... 2 The underground ...... 4 What controls soil biology? ...... 5 General management strategies ...... 7 Considerations for specific land uses...... 10 Cropland...... 10 – Soil Biology Forestland...... 13 Technical Note No. 4 Rangeland...... 15 Assessment and monitoring...... 17 January 2004 Summary...... 19

NRCS file code: 190-22-15 References and further resources...... 19

Other titles in this series: #1 Soil Biology Primer Introduction #2 Introduction to Microbiotic Crusts The purpose of this technical note is to provide information about the #3 Soil Biology Slide Set effects of land management decisions on the belowground component of the . It points out changes in soil biological function that land managers should look for when changing management practices. This information is not a set of prescriptive guidelines, but is designed to increase awareness and prompt field trials.

NRCS Soil Quality http://soils.usda.gov/sqi The goal of soil biology management

The goal of managing the soil biological knowledge to support the development of community is to improve biological functions, detailed management strategies. A particular including forming and stabilizing , practice may have the desired result in one cycling , controlling pests and disease, situation but have little effect in another because and degrading or detoxifying contaminants. biological communities respond to the interaction of multiple factors including food Research shows that management practices and sources, physical , moisture, and impacts disturbances impact soil biological functions of historical . Therefore, before a new because they can 1) enhance or degrade the product or practice is applied to a large parcel of microbial habitat, 2) add to or remove food land, it should be tested on a limited area and resources, or 3) directly add or kill soil results should be monitored in comparison to an . Although management practices are untreated plot. known to impact soil biology, there is limited

Why should land managers understand soil biology?

associations for exchange. Even applied Energy and the food web may pass through soil organisms Through , the sun’s energy is before being utilized by crops. converted into food, feed, and fiber. However, Soil stability and most of the solar energy captured by is not directly harvested when crops are gathered; Soil organisms play an important role in forming instead, it feeds the belowground food web. and stabilizing soil structure. In a healthy soil Feeding the “underground livestock” is essential , fungal filaments and from to productive , rangeland, and farmland. microbes and help bind soil Figure 1 shows how energy is recycled particles together into stable aggregates that repeatedly through belowground soil organisms. improve , and protect soil from The is part of energy, nutrient, and erosion, crusting, and compaction. Macropores water cycles. The energy cycle begins when the formed by earthworms and other burrowing sun’s energy is captured by the -based creatures facilitate the movement of water into (aboveground) food web. Nutrient availability is and through soil. Good soil structure enhances governed by the -based (belowground) development, which further improves the food web. The is also influenced by soil. the interaction of plants, , and quality and quantity organisms. By improving or stabilizing soil structure, soil dynamics help reduce runoff and Functions of the soil food web improve the infiltration and filtering capacity of Nutrient cycling soil. In a healthy soil ecosystem, soil organisms In a healthy soil ecosystem, soil biota regulates reduce the impacts of pollution by buffering, the flow and storage of nutrients in many ways. detoxifying, and decomposing potential For example, they decompose plant and pollutants. and other microbes are residue, fix atmospheric , transform increasingly used for remediation of nitrogen and other nutrients among various contaminated water and soil. organic and inorganic forms, release plant available forms of nutrients, mobilize , and form mycorrhizal (-root)

Page 2 of 20 Soil Biology and Land Management Plant health Complexity and function A relatively small number of soil organisms Many soil biological functions emerge from the cause plant disease. A healthy soil ecosystem complex interactions of soil organisms and are has a diverse soil food web that keeps pest not predictable by adding up the activity of organisms in check through and individual soil organisms. How well the soil . Some soil organisms release community performs each of these functions compounds that enhance plant growth or reduce depends in part on the complexity of the disease susceptibility. Plants may exude specific biological community. Complexity is a factor of both the number of and the different substances that attract beneficial organisms or kinds or functions of species. Greater repel harmful ones, especially when they are complexity may imply more diversity of under stress, such as . functions and more redundancy of functions, and For more information about what in the therefore more stability. For example, when soil and how they function, see the “Soil multiple populations of microbes convert to nitrate, even if one population Biology Primer” (Tugel and Lewandowski, dies out, the function () will continue 1999), and the soil biology glossary on the to be performed. Functional redundancy is the NRCS Soil Quality web site. underlying idea behind the "insurance hypothesis," which states that insures against declines in function.

Figure 1. The plant-based (aboveground) and detritus-based (belowground) food webs. Arrows represent (commonly measured in units). Of the aboveground organic

entering the pool of soil , 60%-80% of the carbon is eventually lost as CO2. (Based on Chapin et al., 2002, Fig. 11.12.)

Herbivores CO2 Plants

Plant-based food web

Microbivores (, (bacteria, fungi) )

Soil organic matter

Detritus-based food web

Predators (, some , , etc.) Stabilized organic matter (Heterogeneous pool of slowly decomposable, physically protected, compounds)

Soil Biology and Land Management Page 3 of 20 The underground community

Soil organisms can be grouped by size as shown , scarab beetles, and earthworms, are in figure 2, or by functions as described below “ecosystem engineers” that physically change (Wardle, 2002; Coleman & Crossley, 1996). the soil habitat for other organisms by chewing and burrowing through the soil. Microbes Decomposers (decomposers) living within their guts break Bacteria, actinomycetes (filamentous bacteria), down the plant residue, dung, and fecal pellets and saprophytic fungi degrade plant and animal consumed along with the soil. residue, organic compounds, and some . Bacteria generally, but not Mutualists exclusively, degrade the more readily Mycorrhizal fungi, nitrogen-fixing bacteria, and decomposed (lower C:N ratio) materials, some free-living microbes have co-evolved compared to fungi, which can use more together with plants to form mutually beneficial chemically complex materials. (See boxes on associations with plants. Mycorrhizae are pages 7 and 8.) Bacteria often degrade what they associations between fungi and plant in can of a particular material; then fungi which the fungus supplies nutrients and perhaps decompose the remainder. water to the plant, and the plant supplies food to the fungi. These fungi can exist inside Grazers and predators (endomycorrhizae) or outside (ectomycorrhizae) Protozoa, mites, nematodes, and other organisms the plant root . The common arbuscular “graze” on bacteria or fungi; prey on other mycorrhizae (AM or VAM fungi) are species of protozoa and nematodes; or both endomycorrhizae. graze and prey. Grazers and predators release plant-available nutrients as they consume , parasites, and root feeders microbes. Often organisms specialize in one Organisms that cause disease make up a tiny type of prey, such as either bacteria or fungi. fraction of the organisms in the soil, but have Certain collembolans () even been most studied by researchers. Disease- specialize on specific species of fungi. Other causing organisms include certain species of organisms are generalists and will feed on any bacteria, fungi, protozoa, nematodes, insects, microbial species they encounter. and mites. Litter transformers

Arthropods are invertebrates Figure 2. Body width of soil organisms (Swift et al., 1979). with jointed legs, including Bacteria insects, spiders, mites, Fungi springtails, centipedes, and Nematodes . Many soil Protozoa shred and consume Mites and other organic matter, Collembola Enchytraeid increasing the surface area Symphylan accessible to decomposers. The Termites organic matter in their fecal Centipedes pellets is frequently more Millipedes Spiders physically and chemically Earthworms accessible to microbes than was Beetles the original litter. Some litter Isopods transformers, especially ,

1 um 10um 100um 1 mm 10 mm 100 mm

Page 4 of 20 Soil Biology and Land Management What controls soil biology?

People can adapt management strategies to that use organic compounds from other affect the factors that control soil biological organisms as their source of both carbon and communities. Soil biological activity is energy. A small group of bacteria get their determined by factors at three different levels. 1) energy from inorganic nitrogen, , or At the scale of individual organisms, biological compounds, rather than from sunlight or organic activity is determined by conditions such as compounds. These bacteria are important in temperature and moisture in the microbial cycling some nutrients required by plants. Soil . 2) At the scale of populations, organisms also require varying amounts of biological activity is determined by the amount macronutrients (N, S, and P) and of habitat diversity, the types of habitat (e.g. Fe, Cu, Zn). The amount of all these disturbances, and the diversity and interactions nutrients and the quality of nutrient sources will among various soil populations. favor some organisms over others, depending on 3) At the scale of biological processes, functions each species’ requirements and preferences. such as nutrient cycling or pest control are affected by the interaction of biological populations with physical and chemical soil and most soil organisms are obligate properties. aerobes, meaning they require oxygen. Some bacteria are obligate anaerobes, meaning they For example, consider the effect of on require oxygen-free conditions to function. earthworms at each of these scales. At the scale Many organisms are facultative anaerobes, of individual organisms, a single tillage event meaning they can switch metabolic pathways may kill as many as 25 percent of individual and function as either aerobes or anaerobes earthworms. At the scale of populations, a single depending on environmental conditions. tillage pass may have little effect after a few Anaerobes use nitrate, sulfate, or iron instead of months as the earthworms reproduce and rebuild oxygen as an electron acceptor. Aerobic their population. At the scale of soil processes, respiration is the most common form of tillage will weaken soil structure over time and metabolism and typically produces ten times reduce the amount of surface residue available to more energy per unit of organic matter than that fungi and earthworms. As fungal and generated through anaerobic metabolism. activity declines, soil stability declines and alters Anaerobic conditions and anaerobic microbes the microhabitats for other organisms. dominate in marshes and other saturated soils. However, even well-drained soils can have Microscale factors anaerobic and aerobic microsites within The following environmental factors affect the millimeters of each other. See box (next page) types and activity levels of soil organisms. for a list of processes performed by aerobic and These factors may vary over short distances in anaerobic organisms. the soil. Consider how each factor is impacted by , , time of day or season, and management practices including tillage, , and . Food (nutrients and energy) All organisms require a source of food that supplies nutrients and energy. “Primary producers” are organisms that use to make their own food from sunlight and CO2. “Consumers” are organisms

Soil Biology and Land Management Page 5 of 20 Heterogeneity of resources Anaerobic soil biological processes Heterogeneity can refer to variation in food Fermentation – Conversion of to sources or any of the other microscale conditions alcohol. listed above. Heterogeneity of soil habitats – Reduction of nitrate to gaseous nitrogen. creates diversity and complexity in the structure of the soil food web. Plant diversity is an production – Reduction of CO2 to important means for creating heterogeneity methane (CH4) in marshes and rumens. Sulfur reduction – Reduction of sulfate to because plants affect the food sources, the sulfide or sulfur. physical habitat (e.g. root structures and soil Iron reduction structure), and the chemical attractants and Aerobic soil biological processes deterrents for soil organisms. Respiration – The conversion of oxygen eventually to and water. All human land uses, especially agriculture, are Ammonification – The creation of ammonia subject to natural and human disturbances from organic compounds. May also including , harvest, tillage, compaction, occur anaerobically slowly. , disease, or applications. Nitrification – The oxidation of ammonium to The frequency, severity, and timing of nitrite and then nitrate. disturbances determine their effect on soil biological activity. According to the Other physical factors intermediate disturbance hypothesis, the greatest Moisture, temperature, light, pH, and electrical level of biological diversity and stability occurs conductivity (salinity) are other critical factors with a “Goldilocks” amount of disturbance – not determining the level of biological activity so much that processes are continually disrupted, within an ecosystem. Each species has different and not so little that just a few species gain optimal conditions, but overall bacterial activity . Conventional cropping systems are is highest at temperatures between 20°C and highly disturbed systems. Low-input, 40°C, at pH levels between 6 and 8, and when conservation tillage systems with crop rotations pore spaces are about 60% water-filled. Soil may be an example of an intermediately texture and determine the amount of disturbed ecosystem. space available for soil organisms and for the Interactions with other organisms movement of air and water through the soil. Soil populations are affected by interactions Thus, porosity, aeration, and moisture levels are with other soil organisms. One type of biotic linked. Relatively larger organisms, such as interaction is competition for limited food and nematodes and small mites (figure 2), require habitat. A second type is predation by larger large pore spaces to move. Many organisms, organisms, such as nematodes and mites. A third including protozoa and nematodes, are type is – interactions beneficial to essentially aquatic and require water films. both parties, such as those involving mycorrhizae, symbiotic nitrogen fixers, many Community-scale factors microbes, and microbes living in The microscale factors listed above directly earthworm guts. When land management affect soil organisms. However, to understand practices disproportionately affect one group of soil biological function we also have to consider organisms over another, they impact the large-scale factors such as heterogeneity of interactions among soil organisms. habitat, disturbances, and biological interactions.

Page 6 of 20 Soil Biology and Land Management General management strategies

Four broad management strategies are presented production should be combined with below. The diversity and functioning of a soil other organic matter management practices biological community are likely to improve including minimizing residue removal and when these strategies are used. Management tillage, growing cover crops, and adding , plans should consider both the timing of mulch, or other amendments. management practices and disturbances, and the duration and degree of their effects on soil 2) Manage for diversity. biology. The effects of management and The diversity of plant assemblages across the disturbances vary by season, and the capacity of landscape and over time promotes a variety of the soil community to recover from a particular microbial habitats and soil organisms. Up to a practice or disturbance ranges widely. point, soil biological function generally improves when the complexity or diversity of 1) Manage organic matter. the soil biological community increases. Regular inputs of organic matter are essential for Many types of diversity should be considered, supplying the energy that drives the soil food such as diversity of land uses (buffers, , web. Each source of organic matter favors a row crops, grazing land), plant types (perennial, different mix of organisms. (See boxes, this page annual, woody, grassy, broadleaf, , etc.), and next.) Thus, a variety of sources may root structures (tap, fibrous, etc.), and soil pore support a variety of organisms. The location of sizes. Diversity is desirable over time as well as the organic matter—whether at the surface, across the landscape. Land managers can mixed into the soil, or as roots—also affects the increase diversity with appropriate grazing type of organisms that dominate in the food web. management, patchy or selective harvest (in Under any land use, organic matter inputs to the contrast to broad clear-cutting), vegetated soil can be increased by improving plant fencerows, buffer strips, strip cropping, and and increasing annual biomass small fields. These landscape features provide production. In particular, good root growth is refuges for beneficial arthropods. Diversity over important for building . High time can be achieved with crop rotations. Rotated crops put a different food source into the soil each year, encouraging a wide variety of Components of organic matter organisms and preventing the build-up of a Organic matter is composed of a single pest species. heterogeneous mix of compounds with various chemical bonding and branching 3) Keep the ground covered. characteristics. Each organism has the Ground cover at or near the surface moderates necessary for decomposing some compounds but not others. For example, soil temperature and moisture; provides food is a recalcitrant that and habitat for fungi, bacteria, and arthropods; can only be broken down by white and and prevents the destruction of microbial habitat brown rot fungi. by erosion. Minimize the length of time each year that soil is bare by maintaining a cover of The composition of organic matter from plants varies considerably, but generally living plants, biological crusts, or plant residue comprises 60-70% at the surface. (), Living plants are especially important as cover 15-20% lignin, and 15% other compounds because they create the rhizosphere—that area including , nucleic acids, , within one or two millimeters of living roots waxes, and pigments. where soil biological activity is concentrated.

Soil Biology and Land Management Page 7 of 20 Microbes around roots take advantage of plant and other land uses. However, some exudates and sloughed-off root cells. disturbances significantly impact soil biology Maintaining a rhizosphere environment is one of and can be minimized to reduce their negative the important benefits of using cover crops. In effects. These disturbances include compaction, addition to preserving microbial habitat, cover erosion, soil displacement, tillage, catastrophic crops help build and maintain populations and , certain pesticide applications, and diversity of arthropods by preserving their excessive pesticide usage. habitat for an extended portion of the growing Compaction season. Ideally, soils are approximately 50 to 60% pore 4) Manage disturbances. space comprising a variety of pore sizes and lengths. The size and continuity of pores Some soil perturbations are a normal part of soil controls whether larger microbes, such as processes, or are a necessary part of agriculture protozoa, can prey upon bacteria and fungi. Compaction reduces the diversity of pore sizes Carbon-to-nitrogen ratio and the amount of space and pathways available for larger organisms (figure 2) to move through The carbon-to-nitrogen ratio (C:N) of organic the soil. This favors bacteria and small predators matter can vary from about 4:1 (low carbon, high nitrogen) for bacteria to more than over fungi and the larger predators. Arthropods 200:1 to 600:1 for woody materials. Ratios are severely affected by compaction. Among for wheat straw are about 80:1, and young nematodes, the predatory species are most may be 15:1. The C:N ratio of soil sensitive to compaction, followed by fungal- organic matter in agricultural soils averages feeders, then bacterial-feeders. Root-feeding 10:1. Fungi have a fairly constant C content nematodes are least sensitive to compaction— of 45%, but N levels vary, resulting in C:N perhaps because they do not need to move ratios of 15:1 to 4.5:1. through soil in search of food. Compaction Low N materials have a low nutritional changes the movement of air and water through quality for microbes. When organic materials soil, can cause a shift from aerobic to more are added to soil, the carbon triggers anaerobic organisms, and may increase losses of microbial growth. If the amount of N in the nitrogen to the (denitrification). added material is inadequate to support the Rooting depth may be limited in highly increased growth, the microbes will absorb compacted soils. This restricts the depth of the nitrogen from the soil and immobilize it in rhizosphere environment that supports microbes. their tissues, thus depriving plants of nitrogen, at least temporarily. As a rule-of- Erosion and thumb, materials with C:N ratios less than 25 Most soil organisms – especially larger ones – or 30:1 will not trigger this N deficiency in live in the top few inches of soil. Erosion plants. Materials with lower C:N ratios tend to decompose quickly. Materials with higher disrupts and removes that habitat. Sedimentation C:N ratios are slow to decompose and can buries the surface habitat and deprives lead to carbon storage or sequestration in organisms of space and air. the soil when accompanied by additional Soil displacement and tillage nitrogen inputs in reduced tillage systems. Displacement and mixing of the soil occur C:N ratios of soil organic matter provide during many activites including tillage, land clues about the microbial community. For leveling, grading, intense grazing, and site example, higher ratios tend to support more fungi compared to bacteria. A labile pool of preparation and harvesting on forestlands. Some soil organic matter with a low C:N ratio soil displacement can be useful such as tillage implies that the SOM consists of a high for preparation in cropland, limited proportion of microbes. disturbances in highly productive

Page 8 of 20 Soil Biology and Land Management systems, and soil scarification to ensure success Pesticides and herbicides of some types of reforestation. However, soil All pesticides impact some non-target disturbances significantly change the biological organisms. Heavy pesticide use tends to reduce habitat of the soil. If the extent of the soil biological complexity. Total microbial disturbance is limited to small areas, the overall activity often increases temporarily as bacteria impact will also be limited. Broadly applied and fungi degrade a pesticide. However, effects practices such as tillage, grazing, or clear-cutting vary with the type of pesticide and species of can impact large areas. Even a single tillage or soil organism. Labels generally do not list the compaction event can significantly affect the non-target organisms affected by a product. In location and quantity of the food supply and the fact, few pesticides have been studied for their physical habitat of soil organisms. If enough effect on a wide range of soil organisms. nitrogen is present, tillage and other practices Pesticides that kill aboveground insects can also that mix the soil usually lead to a flush of kill beneficial soil insects. Foliar insecticides microbial activity and nutrient release, and loss applied at recommended rates have a smaller of soil organic matter via CO2 respiration. impact on soil organisms than fumigants or Where there is a loss of soil organic matter, fungicides. Herbicides probably affect few microbial activity will eventually drop to a rate organisms directly, but they affect the food and that is lower than the initial rate. Over time, habitat of soil organisms by killing . A tillage shifts the food web from being dominated pulse of dead vegetation may trigger a flush of by fungi to being dominated by bacteria. biological activity and . Crop rotations are useful for breaking pest cycles, reducing pesticide application rates, and for varying the families of pesticides used.

Soil Biology and Land Management Page 9 of 20 Considerations for specific land uses

The effects of management practices, operations, and natural disturbances often are specific to particular land uses, such as those discussed in this section. Each of the considerations below relates to the general management strategies described in the previous section.

Cropland The highly disturbed soils of cropland may have application of N or P (whether from organic as many bacteria and protozoa as other amendments or synthetic ) can suppress ecosystems, but tend to have far fewer fungi, certain soil organisms, especially mycorrhizal nematodes, and arthropods. Reduced tillage and fungi, as well as lead to degradation of air and perennial cropping systems will support more of water quality. these larger soil organisms. Crop biomass additions Compost can be used to inoculate the soil with a Roots and surface residue from crops are wide variety of organisms and to provide a high convenient and valuable sources of soil organic quality food source for them. have matter and food for soil organisms. Corn also been credited with reducing the incidence of harvested for grain will generate 3 to 4 tons of plant disease (Ceuster and Hoitink, 1999). Some surface residue per acre and 1 to 2 tons of root species thrive in both compost and soil, but biomass. Dense, sod-type crops produce many prefer one or the other. For example, the generous amounts of root biomass. Recent redworms (Eisenia fetida) commonly used to research suggests that root contributions are make vermicompost do not survive well in soil. more significant for building soil organic matter The quality of compost varies substantially than are contributions from aboveground plant depending on the material used, peak residue. temperature during the composting process, and the level of aeration. Organic materials Surface residue encourages the decomposers— decomposed with little oxygen (e.g. liquid especially fungi—and generally increases food manure) will contain a very different set of web complexity. Residue provides food and organisms and compounds than well-aerated habitat for surface-feeding organisms, such as compost. (For information about how to make some earthworms, and for surface-dwellers, such compost, see NRCS conservation practice as some arthropods. It also changes the moisture standard #317, Composting Facilities.) and temperature of the soil surface, and protects soil organic matter from erosion. The residue sludge will increase some pathogens and reduce others. Like manure, sludge can be an excellent food , peanuts, and many vegetables leave source for organisms. However, high levels of little surface residue and should be rotated with metals or salts in some sludge kill or reduce the high residue crops or cover crops. activity of some organisms. Animal manure Cover crops Dung pats provide food and habitat for a variety Cover crops have several positive effects on soil of larger soil organisms. Manure in any form is a communities. Most soil organisms live in the significant source of nutrients. Manure rhizosphere – the area directly around living application substantially changes the mix of roots. By planting cover crops (also called green organisms in the soil compared to plant sources manure) the rhizosphere environment is of organic matter. The implications of these available to soil organisms for a longer portion differences are not clear, but they probably of the year. Cover crops typically increase the affect disease levels and nutrient cycling. Over- amount and diversity of roots and aboveground

Page 10 of 20 Soil Biology and Land Management growth that become part of soil. Because of each Irrigation and salt build-up crop’s unique physiology, populations of Where irrigation increases plant yield, it specific soil organisms will increase or decline increases biomass production and soil organic depending on the crop. For example, some cover matter, and therefore tends to increase biological crops exude compounds that inhibit disease- activity and to alter the biological community causing organisms. structure. However, irrigation water can contain Inorganic fertilizers salts. To prevent salt accumulation that can reduce biological activity and crop yield, Fertilizer provides some of the nutrients needed additional water must be applied to leach these by soil organisms and will favor those species salts from the root zone. Some irrigation that can best utilize the forms of nutrients found techniques, such as furrow irrigation, require in fertilizers. The effect of acidity, alkalinity, or extreme soil disturbance that is detrimental to salt of some fertilizers (e.g. ammonium nitrate, biological habitat. When the disturbance is a ammonium sulfate, and formaldehyde) one-time event (i.e., intense but infrequent), as tends to reduce populations of fungi, nematodes, with installation of subterranean irrigation pipes, and probably protozoa. It is not clear how the disturbance is less likely to do lasting persistent these population reductions are in damage. various situations. Soil inoculants and compost tea Judicious fertilizer use can be positive for overall biological activity because it increases Some commercially available inocula are plant growth and organic matter inputs to the intended to increase populations of specific soil soil. organisms. Some products have a long track record of effectiveness, including nitrogen- Genetically modified (GMO) crops fixing bacteria and some pest predators, such as Each type of GMO is likely to have a different bacteria, nematodes, or insects. Some products effect on soil biology. For example, Bt crops are unproven or unpredictable. seem to have little direct effect on the Inoculants will have little effect or only a composition of the soil biological community, temporary effect if the organisms cannot yet decomposition rates of the crop residue compete in their new environment. Because they differ from that of other corn varieties – perhaps must have supplies of organic matter as a food because of changes in plant lignin composition, source, soils low in organic matter will not see which may indirectly affect soil organisms via long-lasting results unless a recurring supply of changes in food resources. (Lignin is a plant organic matter is added to the system. compound that is highly resistant to microbial Furthermore, many functions performed by soil attack.) However, and crop variety organisms result from the interactions of seem to be more important than the presence of organisms, not from a few individual species. the Bt gene in determining decomposition rates (Saxena and Stotzky, 2001a, b). When considering using inoculants, ask the following questions: • Do you have assurances that the Improved water drainage tends to improve organisms claimed to be in the product microbial activity by increasing oxygen are viable? availability. Poorly drained soils have a high • Will the organisms survive in your soil level of anaerobic microsites and therefore a environment long enough to have the higher rate of denitrification (conversion of desired effect? nitrate to gaseous nitrogen) compared to well- • If you achieve positive results, was the drained soils. change caused by the inoculated organisms or by associated management

Soil Biology and Land Management Page 11 of 20 practices, such as changes in tillage or The environment for soil organisms can differ added organic matter? significantly in no-till compared to conventionally tilled soils. For example, because Before committing a whole farm to a new the surface soil structure is not regularly product, test it on a small area and compare disrupted, no-till soils are more likely to have: results to a control strip managed identically but without application of the product. Monitor both • Anaerobic micro-environments, short- and long-term effects. • Cooler spring soil temperatures because of greater surface cover, Land leveling • More macropores to facilitate Land leveling may have effects similar to infiltration, erosion and sedimentation because biologically • Greater and carbon near active surface soil is removed from some areas the surface, and and deposited in other parts of the field. It may • Uneven distribution of organic matter also with a less desirable texture throughout the . or lower organic matter levels. This effect can be • In addition, surface compaction may be reduced by intense soil building practices after a problem if compaction was present land leveling or by removing and stockpiling the before the conversion to no-till, and if topsoil just prior to land leveling and then biomass inputs are low or traffic spreading it over the newly leveled surface. patterns are not controlled. Terraces and grassed waterways Organic matter decomposition rates are lower in Permanent vegetative structures add diversity to no-till vs. conventionally tilled soils because of a landscape and thus can enhance the the lower level of soil disturbance. The lack of biodiversity of the area. They serve as a refuge disturbance and the presence of surface residue for larger soil organisms such as arthropods and encourage fungi and large organisms such as pest predators. However, like land leveling, the arthropods and earthworms. No-till soils soil movement involved during construction of generally have a higher ratio of fungi to bacteria. terraces can significantly disrupt soil biological Fallow periods habitat. Because microbes concentrate around living Tillage and no-till roots, fallow periods of even a few weeks at the Tillage enhances bacterial growth in the short- beginning or end of a growing season reduce an term by aerating the soil and by breaking apart important microbial habitat. During long fallow soil aggregates to expose organic matter that had periods, most arthropods will emigrate or die of been protected from microbial decay. The starvation. Some organisms can form cysts, bacterial activity increases the loss of carbon allowing them to lie dormant until conditions respired as CO2, and triggers population become more favorable. Mycorrhizal fungi also explosions of bacterial predators such as “starve” during a fallow period and take time to protozoa. Ultimately, recurrent tillage reduces recover after the fallow period ends. Growing the amount of soil organic matter that fuels the non-mycorrhizal plants is equivalent to a fallow soil food web. year from the perspective of mycorrhizal fungi. The mechanical action of tillage tends to Plants that do not support mycorrhizae include temporarily reduce populations of fungi, brassicas (mustard, broccoli, canola) and earthworms, nematodes, and arthropods. Over chenopods (beets, lamb’s-quarters, chard, the long term with repeated tillage, these spinach). (See section on mutualists, page 4, for populations are likely to decline because of the more information about mycorrhizae.) lack of surface residue rather than because of the mechanical action of tillage.

Page 12 of 20 Soil Biology and Land Management Forestland Forest soils have high ratios of fungi relative to bacteria and actinomycetes, such as bacteria, especially under coniferous forests. The streptomyces, are more prominent in clear-cut fungi are predominately ectomycorrhizae that areas, between in thinned areas, and infect tree roots and then extend their hyphae between intact forest patches. When reclaiming into the soil. This greatly increases the tree’s clear-cut forests it is beneficial to inoculate new effective root zone, allowing access to a greater tree seedlings with mycorrhizal fungi, forest area of soil from which to extract water and soils, or litter containing these fungi. Retaining nutrients. The mantle created by mycorrhizae patches of intact forests among clear-cut areas around the root also prevents pathogenic fungi provides a source of soil organisms to re- and bacteria from attacking the root system. inoculate the harvested areas. Arthropods can be quite numerous in forests Thinning and fuel management because the soil is rarely disturbed. Earthworms Thinning of forests has a positive effect on soil are common in deciduous forests, but rare biological diversity, especially in single-age among . Where non-native earthworms stands. Thinning increases the diversity of (e.g., fishing bait) have been introduced into understory vegetation and thus creates more deciduous forests, significant changes in diversity of habitats and food sources for soil understory vegetation have been observed organisms. (This is an example of the (Minnesota DNR, 2004). intermediate disturbance hypothesis described Tree harvesting on page 6.) However, excessive compaction and soil displacement can have negative effects. Tree harvesting removes nutrients from the area, reduces the total uptake of nutrients by plants, Roads, trails, and landings and can accelerate biological activity and Compaction created under roads, skid trails, and decomposition. Tree harvest can change the landings compresses soil particles together and activity, amount, and diversity of the microbial reduces pore sizes. This restricts the habitat for community. The degree of site disturbance soil organisms, especially the nematodes and during harvesting and the amount of slash larger arthropods. The use of designated skid remaining after harvest determines the effects on trails and restoration of landings after harvest soil organisms. Techniques that minimize soil minimizes the amount of forestland affected. disturbance and compaction will have the least detrimental effect on soil organisms. Tractors, Slash piling and woody debris wheeled skidders, and mechanized harvesting Machine or hand piling of slash concentrates equipment disturb the soil surface and can cause nutrient-rich branches, foliage, and sometimes compaction that restricts biological activity. topsoil. This increases soil organism populations Cable, helicopter, and horse logging produce the and activity locally. Excessive nutrient least disturbance. Soil surface displacement and can occur in areas where microbial activity mixing of soil, duff, and slash temporarily increases. Windrowing of slash has severe increase microbial activity and may interfere detrimental effects on areas between windrows with activity. because topsoil rich in soil organisms is scarified and placed in the windrows. Runoff may also Clear-cutting generally has a negative effect on erode nutrient-rich surface soil and organic soil biological activity. The large number of matter from slash areas, potentially degrading roads, skid trails, and landings compact soil and water quality of nearby streams and lakes. are particularly susceptible to erosion. When erosion and loss of mycorrhizal host plants is Wind damage increases the amount of dead severe mycorrhizal fungi may be lost from the available for soil organisms. Woody ecosystem. Compared to soil under trees, debris can enhance biological function because

Soil Biology and Land Management Page 13 of 20 dead wood mitigates environmental extremes, A short, cool fire rarely eliminates any group of such as heat and cold, in the microclimate of a organisms. If the fire is cool, nitrogen will not be disturbed area. In forests, downed logs serve as volatilized, and the nitrogen in ash may centers for biological activity including stimulate plant growth and diversity of species. mycorrhizal hyphae, nitrogen-fixing bacteria, Arthropods will repopulate readily after a patchy other microbes, arthropods, and even small fire. . In some systems, fungi extract water Catastrophic – Hot, long-duration fires from large rotting logs and supply the water to will kill most organisms, including microbes at growing trees during times of moisture stress. or near the soil surface. The ignition of litter and Downed logs also serve as natural dams to help duff as well as erosion after the fire reduces food reduce erosion and increase infiltration and thus available to soil organisms. The mineralization improve recolonization by desirable species in and subsequent leaching of nitrogen can highly disturbed areas. significantly decrease . Hydrophobic Fire layers formed during hot fires can restrict the Stand replacement fires – Stand replacement penetration of water into the soil for several fires are common in single age stands of fire- years. This restricts plant and root growth thus adapted tree species such as lodgepole , jack reducing the food supply for soil organisms. pine, longleaf pine and black spruce. Fire is an Insects and disease integral part of the of these forests. The The reduction of fire in some forests, such as microbial community tends to drop back to its Ponderosa pine forests, has led to an increase in previous level of activity after an initial flush of insect infestations and disease. Diseased or dead activity after a fire. trees increase the amount of woody debris that Cool or patchy fires – Frequent ground fires are serves as fuel for hotter-burning, potentially an integral part of some forest ecosystems, such catastrophic fires. as ponderosa pine stands. These fires are not hot An invasion or increase of less disease-resistant enough to burn the overstory trees, but the tree species will make a forest stand more understory trees and brush are killed. susceptible to insect infestation and disease.

Page 14 of 20 Soil Biology and Land Management Rangeland Soil biological activity in rangelands may vary Fire greatly over short distances. Activity may be Prescribed burning in grassland communities high under shrubs and grasses and almost none generally produces cool temperature fires that in the bare spaces between plants. have little or no direct effect on soil organisms. Some seemingly bare spots are actually There can be short-term losses of habitat or food encrusted with soil organisms, such as sources, but patchy fires leave refuges for larger cyanobacteria. Biological soil crusts are soil organisms in adjacent unburned litter and important for nutrient and water cycling, grass. particularly in arid and semi-arid environments. Absence of fire or an increased length of time When plant assemblages change dramatically between fires commonly promotes vegetation over time, for example, from grass- to shrub- shifts from grass-dominated to shrub- or tree- dominated, the character of the soil biology may dominated plant communities. Such vegetation change to the extent that it may be difficult to shifts affect the soil organisms through the convert the system back to the original plant change in residue composition and the depth of assemblage with its associated soil community. root zones that contribute food sources. Woody residue and roots will increase fungal Grazing and vegetation management populations. Bare soil between shrubs provides Grazing and vegetation management are the little food and will result in a decline in soil most important tools for maintaining the benefits organism populations. of the soil food web. Timely grazing, the proper Catastrophic fire is likely to occur in dense frequency of grazing, and control of the amount shrub- or tree-dominated plant communities. of vegetation removed will maintain or enhance The resinous wood and longer burn times plant production and the supply of organic promote hot fires that can effectively scorch the matter. Both overgrazing and non-grazing upper few inches of the soil. This will kill some reduces root growth and thus the amount of soil organisms and reduce their food supply organic matter inputs to the soil. Grazing while also increasing the availability of some stimulates root growth and production of root nutrients. exudates, but overgrazing reduces the amount of surface for photosynthesizing. (This is Invasive weeds another example of the intermediate disturbance Invasive weeds can cause a shift in the types of hypothesis. See page 6.) With reduced food soil organisms present because the quantity and supplies, biological activity decreases along with quality of plant residue and root exudates will important , including nutrient change. Weeds that cause increased litter cycling, water infiltration, and water storage. As buildup tend to promote more fungal dominance these functions decline, the ability of the plant in soil. The encroachment of annuals into and soil biological communities to replenish soil systems will cause changes in organic matter also declines. Heavy grazing can organism community composition because soil reduce the of nitrogen-fixing plants, biological activity corresponds with plant causing a decrease in the nitrogen supply for the growth stages and periods of litter fall and root entire plant community. Where biological crusts die-off. provide important functions such as protection from erosion, the timing and intensity of grazing Shrub removal should be managed to minimize damage to Removing shrubs by chaining, thinning, or crusts. Ensuring even manure distribution is applying herbicides promotes a shift to a another mechanism by which good grazing different plant community and affects food management enhances soil biological activity. sources for soil organisms. Less woody material

Soil Biology and Land Management Page 15 of 20 and more herbaceous material will promote can reduce or eliminate habitats for larger bacterial increases compared to fungi. Arthropod organisms in the food web, such as insects and species will also change to those supported by arthropods. the new plant community. As with tillage, the Compaction from traffic soil displacement and mixing that occurs during chaining will enhance bacterial decomposition Grazing animals and vehicles may cause of soil organic matter and residues. Compaction compaction, especially when traffic is and herbicide effects, as described in “General concentrated in small areas or soil is too wet. Management Principles” above, may also be an See “General management strategies” for a issue during shrub removal. description of the implications of compaction on soil biology. Plowing and seeding Erosion Plowing and seeding disturbances are related to the degree of soil mixing. Tillage that Off-road vehicle traffic or heavily used trails can completely mixes or inverts the topsoil will create ruts that compact soil and channel water. result in a sudden, drastic change in habitat, The resulting accelerated erosion, rills, and increased organic matter decomposition rates, gullies can strip or bury topsoil and have a and thus reduced food sources for soil negative effect on soil organisms. Erosion organisms. It also destroys larger pores and associated with vegetation shifts often results in some macrohabitats. Use of a seed drill the redistribution of topsoil, organic matter, minimizes soil mixing and is much less of a resources, and habitat across short distances. At disturbance to soil biology. Changes in the the shrub-intershrub scale, bare areas between amount of residue either from plowing or the shrubs provide the least habitat and resources for establishment of the seeded species will alter the soil organisms. Areas of grass, shrubs, or trees food sources for soil organisms. Loss of residue have more diversity of soil organisms.

Page 16 of 20 Soil Biology and Land Management Assessment and monitoring

In contrast to soil physical and chemical the soil functions of interest to a land manager. parameters, there are few specific guidelines for However, they are likely to change more slowly managing soil biological properties. Thus, it is than biological measures and thus are more especially important that land managers track delayed indicators. When deciding what to changes in soil biological functions over time to assess or monitor, keep in mind the objectives monitor the effects of management choices. and the time and resources available. Monitoring is the identification of trends by For more about planning appropriate soil quality systematically collecting quantitative data over assessments see “Guidelines for Soil Quality time from permanently marked locations. Assessment in Conservation Planning” (NRCS, Objectives for monitoring soil biological 2001c). function include: • Evaluation and documentation of the Types of tests progress toward management goals, As discussed in the Soil Biology Primer, a • Detection of changes that may be an variety of approaches can be used to describe the early warning of future degradation, and soil community, including 1) counting soil • Determination of the trend for areas in organisms or measuring biomass, 2) measuring desired condition, at risk, or with their activity, or 3) measuring diversity, such as potential for recovery. diversity of functions (e.g., biolog plates) or diversity of chemical structure (e.g. cell Assessment is the estimation of the current components or DNA). Each approach provides functional status of soil biological processes. It different information. requires a standard for comparison. Objectives of assessments can be: Methods for measuring biomass identify either the total number of organisms or only those that • Selection of sites for monitoring, are active. A pitfall trap or Berlese funnel • Gathering of inventory data, (NRCS, 2001b) can be used to collect larger • Identification of areas at risk of organisms living in litter from a , degradation, and pasture, or cropland. • Targeting management inputs. Activity measurements provide a better Techniques for measuring soil biological understanding of soil biological function than do properties range from informal, qualitative biomass measurements. One measure of observations to quantitative laboratory biological activity is testing for various techniques. These techniques can be useful for microbial enzymes (Dick, 1997). By choosing learning about organisms’ requirements the appropriate , an enzyme assay can be and functions. However, soil biological tests can used to assess the rate of carbon, nitrogen, or be difficult to interpret, and thus provide limited phosphorus cycling, or overall microbial support for making specific management activity. Enzyme assays have potential as a decisions. More information may be gleaned by useful indicator and can be done with the assessing and monitoring properties affected by equipment found in typical soil analysis labs, but soil biological activity including soil surface most labs do not yet offer these tests. stability (aggregate stability and slake tests), Respiration, or the amount of CO produced water infiltration rates (ring infiltrometer and 2 from the soil, is another measure of biological rainfall simulation tests), decomposition rates, activity. The test can be done in the field pest activity, and soil nitrate and carbon levels (NRCS, 1998), but results are difficult to (microbial biomass and interpret. Respiration rates are extremely tests). These measures of soil properties assess variable hourly, seasonally, and by region and

Soil Biology and Land Management Page 17 of 20 soil type, so baseline or reference data are nearly Collecting samples for laboratory meaningless. Furthermore, high respiration may analyses indicate a healthy and active biological A small number of commercial labs will test soil community, or it may indicate recent biological properties. Typical measures are disturbance, such as tillage, that has triggered a microbial biomass and direct counts of soil flush of activity. High respiration represents a microbes. When choosing a lab and soil biology loss of to the atmosphere, which is tests, consider the following. counter to the goal of . Yet there are no guidelines for determining how • What quality control measures does the much is too much. For these reasons, soil lab use to ensure reliable results? respiration tests can be useful in side-by-side • What is the significance of each test in demonstrations, but are of limited value as a soil terms of soil function? biological indicator. • How will the test assist in your management decisions? Cotton strip tests (NRCS, 1998 and 2001b) and a few other techniques measure decomposition • Do interpretations of results consider rates over days or weeks, and therefore are not your specific soil type and cropping confounded by short term variation as much as systems? are respiration tests. However, results are still The biomass (total amount of organisms) difficult to interpret and require a standard or changes seasonally, but does not change control for comparison. drastically from day to day. However, activity With any of these measures of the soil biological levels (e.g., respiration) can change quickly, so community, refer to a specialist for help note the time of year and the temperature and interpreting test results. moisture conditions when sampling, and sample under similar conditions for future observations.

Samples should be placed in sealed bags and chilled (but not frozen) immediately. Get to know your community

To gain a general awareness of soil organisms and their effects, try these simple methods. Choose a few places to take a close look at what lives in your soil. Look under a shrub, in the , along a fence line, in a , in a field, etc. Take time to examine the litter on the surface and look for organisms that move. Look for biotic crusts, burrows, fungal hyphae, and other evidence of soil organisms. Over the seasons, look for picking out earthworms behind a tillage implement. Observe the rate that dung pats decompose. Notice the amount of runoff or ponding after a rain.

Page 18 of 20 Soil Biology and Land Management Summary

Soil organisms are integral to soil processes, Soil biological health generally improves when including nutrient cycling, energy cycling, water the following management practices are applied: cycling, processing of potential pollutants, and • Regularly adding adequate organic plant pest dynamics. These processes are matter, essential to agriculture and , and for • Diversifying the type of plants across protecting the quality of water, air, and habitat. the landscape (in all land uses) and Therefore, land managers should consider the though time (in cropping systems), effects of their actions on the health and function • Keeping the ground covered with living of the soil biological community. plants and residue, Despite the well-known importance of soil • Avoiding excessive levels of biological processes, the development of disturbances including soil mixing or monitoring and management guidelines is in its tillage, compaction, pesticides, heavy infancy. However, land managers can learn the grazing, and catastrophic . general principles of how their choices affect biological processes and can monitor changes in soil function.

References and further resources

Atlas, Ronald M. and Richard Bartha. 1981. Liebig, Mark and Tom Moorman. 1999. Soil , Fundamentals and Biology for the Northern Great Plains. Applications. Addison-Wesley Publishing USDA ARS and NRCS. Mandan, ND and Company, Inc. Ames, IA. Chapin, F.S.III; P.A. Matson, H.A. Mooney. Madigan, Michael T., John M. Martinko, Jack 2002. Principles of Terrestrial Ecosystem Parker. 1997. Brock Biology of Ecology. Springer-Verlag, New York. , Eighth edition. Prentice Coleman, D.C., D.A. Crossley. 1996. Hall, Upper Saddle River, NJ. Fundamentals of . Academic Minnesota DNR. 2004. Invasive Earthworms Press, San Diego. [Online]. Ceuster, T.J.J. de. and H.A.J. Hoitink. 1999. http://www.dnr.state.mn.us/exotics/ Using compost to control plant diseases. terrestrialanimals/earthworms/index.html Biocycle. 40(6):61-64. (verified 20Jan2004). Dick, R.P. 1997. Soil enzyme activities as NRCS. 1997. Introduction to Microbiotic Crusts. integrative indicators of . In: http://soils.usda.gov/sqi/ Biological Indicators Of Soil Health. New NRCS. 1998. Soil Quality Test Kit Guide. York : CAB International. P. 121-156. http://soils.usda.gov/sqi/ Gilkeson, L., P. Peirce, M. Smith. 1996. The NRCS Soil Quality Test Kit Guide Rodale’s Pest and Disease Problem Solver. includes instructions and interpretations for Rodale Press, Emmaus. tests of biological activity, aggregate stability, water infiltration and more. Gliessman, Stephen R. 1998. : Ecological Processes in Sustainable NRCS. 2001a. Agricultural management effects Agriculture. Sleeping Bear Press. on earthworm populations. Soil Quality─Agronomy Technical Note No. 11. http://soils.usda.gov/sqi

Soil Biology and Land Management Page 19 of 20 NRCS. 2001b. Soil Biology Classroom Saxena, D., G. Stotzky. 2001a. BT corn has a Activities. http://soils.usda.gov/sqi/ higher lignin content than non-BT corn. Describes methods for the pitfall trap, American Journal of Botany. 88(9): 1704- Berlese funnel, and cotton strip 1706. decomposition tests. Saxena, D., G. Stotzky. 2001b. Bacillus NRCS. 2001c. Guidelines for Soil Quality thuringiensis (Bt) toxin released from root Assessment in Conservation Planning. exudates and biomass of Bt corn has no http://soils.usda.gov/sqi/ apparent effect on earthworms, nematodes, NRCS. 2003. Glossary of Soil Biology Terms protozoa, bacteria, and fungi in soil. [Online]. Soil Biology and . 33(9):1225- http://soils.usda.gov/sqi/soil_quality/soil_biolo 1230. gy/sbglossary.html (verified 19Jan2004). Swift, M.J., O.W. Heal, and J.M. Anderson. NRCS. 2003. Conservation Practice Standards 1979. Decomposition in terrestrial [Online]. ecosystems. University of California Press, http://www.ftw.nrcs.usda.gov/practice_stds.ht Berkeley. ml (verified 19Jan2004). Tugel, A.J. and A.M. Lewandowski, eds. 1999. Paul, E.A. and F.E. Clark. 1989. Soil Soil Biology Primer. NRCS Soil Quality and Biochemistry. Academic Institute, Ames, IA. Press, Inc., San Diego California. The Soil Biology Primer is a colorful and easy-to-read introduction to the organisms Perry, David A. 1994. Forest Ecosystems. The that live in the soil and why they are Johns Hopkins University Press, Baltimore. important to agriculture and environmental Reeder, J.D., C.D. Franks, and D.G. Milchunas. quality. Copies are available through the Root biomass and microbial processes. In: Soil and Society at Follett, R.F., J.M. Kimble and R. Lal eds. www.swcs.org or 1-800-THE-SOIL. The Potential of U.S. Grazing Lands to Wardle, David A. 2002. Communities and Sequester Carbon and Mitigate the Ecosystems: Linking the Aboveground and Greenhouse Effect. Lewis Publishers. Belowground Components. Princeton University Press. Princeton, New Jersey.

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