The Role of Soil Biota in Soil Fertility and Quality, and Approaches to Influencing Soil Communities to Enhance Delivery of These Functions

Department for Environment, Food and Rural Affairs Research project final report Project title The role of soil biota in soil fertility and quality, and approaches to influencing soil communities to enhance delivery of these functions. Sub-Project A of Defra Project SP1601: Soil Functions, Quality and Degradation – Studies in Support of the Implementation of Soil Policy Defra project code SP1601 Contractor SKM Enviros organisations Cranfield University Rothamsted Research Report authors Karl Ritz ([email protected]), Jim Harris, Phil Murray Project start date October 2009 Project end date March 2010 The role of soil biota in soil fertility and quality, and approaches to influencing soil communities to enhance delivery of these functions. Sub-project A of Defra Project SP1601: Soil Functions, Quality and Degradation – Studies in Support of the Implementation of Soil Policy 1. INTRODUCTION The importance of soil and the functions it performs are unquestionable. Soil is a living entity that needs to be maintained and managed in a sustainable way. Soils are highly complex systems, both literally in that they are constituted of vast range of constituents that show great spatial heterogeneity across some ten orders of magnitude, and in the more formal construct of complexity science (Ritz 2008). Factors that contribute to effective soil fertility, i.e. the production function, are diverse and concomitantly complex (Gregorich & Carter 1997; Mader et al. 2002). However, it is apparent that the soil biota contribute substantially to effective soil functioning from many perspectives (Bardgett 2005), including the basis and maintenance of sustainable agricultural fertility (Kibblewhite et al. 2008). The soil biota can be conceived of as the 'biological engine of the earth' (Ritz et al. 2004) driving and modulating many of the key process that occur within soils. The biomass typically only constitutes a small proportion of the total mass of soils, but has a hugely disproportionate effect upon soil functions. For example, Jenkinson (1977) appositely describes the biomass, which is predominantly microbial in constitution, as the "eye of the needle through which all organic materials must pass". However, the soil biota consists not just of the microbes but of a myriad of larger multi-cellular organisms, and the entirety interacts via series of complex food-webs (Van der Putten et al. 2004). Microbes function as primary decomposers and biochemical transformers at the core of such webs, and larger organisms provide higher-order ecosystem services such as organic matter comminution, decomposition, and ecosystem engineering. It is important to take an holistic systems viewpoint when attempting to understand the complex interactions in the soil which affect the soil biota. For example, the addition of fertilizers can have direct impact on the soil biota, but also can have an indirect influence via the plant and the two are inextricably linked (Murray et al. 2006). Whilst the mineralogy, physics and chemistry of the soil system provides the context, and sets the boundaries in which the soil biota operates, the unique feature of the biota is that it is adaptive to changes in environmental circumstances, which occur by processes of natural selection, in ways that the abiotic systems of the soil are not (Kibblewhite et al. 2008). This has important implications for the way in which soil systems function, and the ways they can be manipulated and managed. Whilst the emphasis on the production function is to maximise yield1, and this was historically perceived as the primary goal for agriculture, it is becoming increasingly recognised that the production function has to be reconciled with provision of other ecosystem goods and services to avoid degradation of the wider environment and detriment to society. Given the imperative to produce sufficient food to support a global population currently projected to exceed 8 billion by 2030 (FAO 2006), this is an extremely challenging task. Agricultural systems can be classified within a conceptual space that varies in many factors that include the origin of energy sources, nature and intensity of fertiliser use, complexity, biodiversity, cultural tenets, etc. These can be broadly categorised, for example, as a 1 Yield is here defined to mean the mass of agriculturally prescribed product. For example this can be grain cereal or legume or herbage forage for stock or biomass for energy crops. Page 1 spectrum of industrial – integrated – organic – biodynamic1, accepting this is not an entirely comprehensive list. However, it is important to move away from some of the more extreme caricatures of these different approaches to production, to recognise the spectrum of practices adopted, and to avoid presenting “conventional” and “sustainable” farming as opposites, incapable of being mixed (Shennan 2008). When environmental problems occur with agricultural production they usually hinge around poor management, and not the mode of agriculture per se (Trewavas 2004). In essence, the soil biota underpins five key ecosystem services that are fundamental to agricultural productivity, viz. carbon cycling, nutrient cycling, soil structural integrity and dynamics, biotic regulation and mutualism. Agricultural systems utilise or circumvent soil biota to differing degrees depending where they fall in the management spectrum above. Industrial agriculture, for example, typically substitutes services provided by the soil biota in other systems by industrially-derived substitutes such as inorganic fertilisers, synthetic biocides and ploughing. This distorts the natural balance of the ecosystem and may compromise the output of other environmental services (Kibblewhite et al. 2008). If production is taken as the sole aim of the system, then it can be seen as ‘efficient’, but there is likely a trade-off with other ecosystem services being compromised, such as water storage and biodiversity. The aims of this review are to briefly explain how soil biology operates with respect to production function, and to explore potential strategies for the management of the soil biota to maximise outputs whilst minimising inputs and impacts on delivery of other ecosystem goods and services. 2. THE ROLES THE BIOTA PLAY IN SUPPORTING THE PRODUCTION FUNCTION OF SOILS The soil biota underpin five key ecosystem services that are critical to the ability of soils to produce crops, viz. carbon cycling, nutrient cycling, soil structural integrity, biotic regulation and mutualism. These will be reviewed in turn, followed by a systems-level consideration. Carbon cycling Soil organic matter (SOM) originates from primary production, and in large part from terrestrial vegetation. There are strong links between SOM, soil functions (Tate 1992) and agricultural sustainability (Magdoff & Weil 2004). Carbon fixed by photoautotrophs organisms that carry out photosynthesis enters the soil via deposition in roots, rapidly in the form of soluble exudates that emanate from growing roots, and more slowly by the deposition of cells and tissues. Above ground, plant parts are deposited on the soil surface as they senesce, and en masse in the case of annual crops, unless such residues are harvested and removed from the field. In cropped grassland systems, such as for hay or silage, litter return to the soil surface is restricted by such harvesting, whilst in grazed pastures, part of the herbage consumed by livestock is deposited on the ground surface as dung. A managed return of organic matter to the soil can also be made, using such materials as slurry, manure, compost, industrial waste such as sewage sludge, food processing by-products, biochar, etc. A standing crop of vegetation can also be deliberately utilised as source of organic matter, for example in the case of green manures. All of these practices have short term outcomes, and long term effects (McLauchlan 2006). Organic material in soils is more or less continuously transformed by a very wide variety of chemical and biochemical mechanisms into a diverse range of compounds, with the majority of such transformations being carried out by the soil biomass (Marschner & Rengel 2007). The rates of such transformations range from seconds to centuries, as do the residence times of the associated compounds. Soil organic matter (SOM) contains energy- rich bonds which are the primary energy source for the soil biota. Soil organisms have appropriate biochemistries and life strategies to assimilate such energy for their growth and 1 The ‘industrial’ end of this spectrum tends toward a substitution of biotically-mediated processes with substantial use of fossil-fuel based energy consumption to create fertilisers, biocides and extensive soil disturbance via tillage. At the other end , ‘biodynamic’ type systems seek to avoid the use of any such sources and are founded on what are asserted to be holistic tenets. There are then a range of intermediate systems that are not straightforwardly discriminated. Page 2 reproduction; in doing so, the compounds are further transformed and cycled between compartments (Paul 2007). Importantly, this process yields stable SOM which contributes to structural development and buffering. Obviously, for such processes to occur, OM must be brought into physical contact with organisms. Many soil organisms are motile, and move though the soil matrix foraging for such substrate. However, motile bacteria, protozoa and nematodes are confined to water films to realise such passage. Filamentous fungi,

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