UNIT 6 CYCLES

Structure Introduction Objcctivcs Biogeochemical Cycling Cxrbon Cycle Nitrogen (~'yclc Sulphur Cycle Nutrient Budgcts.and ('ycling in Forests Nutrient Budgets Nutrient Cycling in Tropical and Temperate Forests Summarj, Terminal Questions Answers

- - 6.1 INTRODUCTION

You have already learnt in the earlier units (Units 1 arid 5)that all have certain common basic features of structure and function. They all have living and non-living components through which there is a flow of energy and exchange of materials. Birth. growth, death and decay are the four pillars that keep life going on the planet. The does not offer an endless supply of minerals to land living organisms nor do fresh water and sea to their inhabitants. Yet in the millions of years during which life evolved and flourished on land, the soil has not been exhausted of the required by plants. Similarly, the atmosphere has not run out of its oxygen or . Shortages of thesc substances are prevented because they are circulated and recycled in a in a delicately balanced cycle of events. In this unit we will consider the dynamics of major nutrient elements in the biosphere. We will study the various chemical forms in which they occur in nature and the way they cycle. You will learn specifically about the cycling of carbon, nitrogen, sulphur and phosphorus through the abiotic and biotic components of the . The role of in liberating the nutrients back into the environment for their is crucial in the cycling of these nutrients. The nutrient cycles are delicately balanced and each step is critical for their normal functioning. However, man is seriously influencing the rate and quantum of these nutrient cycles through his activities. Suitable examples are provided to show the impact of human activities on the nutrient cycles. A study of this unit will help you to understand that a community of plants and animals in an ecosystem survives primarily by a combination of material cycling and . Objectives After reading this unit you will be able to: define and use in proper context the term and explain the Importance of the concept. distinguish betweell gaseous and sedimentary cycles, outline the course of carbon, nitrogen. sulphur and phosphorus cycles, describc the importance of micro-organisms in nutrient cycling, differentiate between nutrient cycling in tropical and temperate forest, identify the consequences of human intervention in nature in terms of nutrient cycles.

You have studied in Unit 5 that energy flows through ecosystems'enabling the organisms to perform various kinds of work, and is uItimately lost as heat. It is gone for ever in terms of usefulness to the system. On thc other hand, nutrient materials Nutrient Cycling

I never get 'used up'. They can be recycled again and again indefinitely. For example. when we breathe, wk inhale several million atoms that may have been inhaled by say, Akbar or any other person you may care to choose from history. First let us explain .what we mean by nutrients. As you have learnt in Unit 5, of morc than I00 1 chemical elements, about 40 are present in living organispls. Some are needed in relatively li~rgcamounts and n, arc c;illcd macronutrients while some ;ire needed in only trace amounts ancl so nanictl niicronutrients (scc Tablc 6. I).

Table 6.1 Relative amounts of some chemical elements that make up living things

Element Main Reservoir

Major Macronutrients Carbon Apospher~ (> 1 X, dry organic weighf) Hydrogen Hydrosphere Oxygen Atmosphere Nitrogen Atmosphere and Soil Phosphorus Lithosphere

Relatively Minor Macronutrients Calcium Lithosphere (0.21% dry organic weight) Chlorine Lithosphere Lithosphere Iron Lithosphere Magnesium Lithosphere Sulphur Lithosphere and At rno\phere Sodium Lithosphere Lithosphere

Some Micronutrients Aluminirlm Lit hospb :e (<0.2% dry organic weight) Boron Lithosp!,i-re Bromine Lithos- .,ere Zinc Lith~..~,here Cobalt 1.ithospherr Iodine Lithosphere Chromium Lithosphere

Individual nutrients can exist in combination with other elements forming different compounds. But living organisms may not he able to obtain the essential nutrients from all, those compounds. For example, plants can use carbon only in the form of carbon dioxide (CO,). Similarly all organisms necd nitrogen but most of them are incapable of utilising the gaseous Nz present in the atmosphere unless it is available in form of soluble nitrates (NOT) or ammonia (NH3). The mineral nutrients move from the non-living to the living and then back to the non-living components of the ecosystem in a more or less circular manner. This is known as biogeochemical cycling (bio for the living, geo for atmosphere. water, rocks and soil and chemical for the elements and processes involved). We generally call them nutrient or mineral cycles. You should, however, remember the important role of a) green plants which organise the nutrients into biologically useful compounds, b) decomposers which ultimately return them to their simple elemental state. c) air and w.ater which transport the nutrients to long,distances between the abiotic and biotic components of the ecosystem.

You'should also get familiar with the two important terms associated with biogcochcinical cycles : a) the different reservoirs or pools of nutrients likc thc atmosphere and rocks. These are large and the relative size of thesc pools is important when one assesses the effect of human activities on nutrient cycles. h) tlic compartments of the cyclcs t hl.o~~gIiwhich the nu~l-icnthmove. l'hcy are I relatively short-term stores of nutrients in comparisoli with ~eservoirs;for cxaniplc, the plants and ;~nimalsthrough which rhc nutt-icnrs 11iovcand in which they arc srorcd for short periods in n cyclc. 1 I Fig. 6.1 shows a nlodcl. The reservoirs are the ;~ttnosphcrc.and rocks; the major I Fcmystem : Functioning and Types compartments are sea and sediment, freshwater and soil. The5e include the primary producers and consumers and dead in the system.

ATMOSPHERE r-----1 A,I I 4, I* I SEA It

ROCK

Fig. 6.1 : A model of the biogeochemical cycle. The arrows indicate the outgoing and incoming of minerals.

Now let us study two types of biogeochemical cycles.

Types of Biogeochemical Cycles : There are two basic types of biogeochemical cycles, gaseous and sedimentary. In the gaseous type of biogeochemical cycle there is a prominent gaseous phase. Cycling of carbon and nitrogen represents gaseous biogeochemical cycles. In sedimentary cycles the main reservoir is the lithosphere from which the nutrients are released largely by weathering of rocks. The sedimentary cycle is exemplified by phosphorus and sulphur. When we describe biogeochemical cyclc;we often say that a cycle is perfect or imperfect. A perfect is one in which the nutrients are replaced as fast as they,are used up. Most gaseous cycles arc generally considered perfect. In contrast, sedimentary cycles are considered relatively imperfect, as some nutrients are lost' from the cycle into the soil and wdiments and becomc unavailabIe for immediate cycling i.e., there are morc stages in which short-term or long-term stagnation occurs. Most significant of the stagnation stages is sedimentation in oceans and deep continental lakes. So if portions of nutricnts. such as phosphorus or sulphur are lost. they are unavailable to organisms for comparativelj, longer periods. Human beings have so speeded up the movement of many nutrients that the cycles tend to becomc imperfect or rather acyclic resulting in too much of nutrients at onc stayc or too little at another. We will discuss this in detail when wc comc to thc phos~)Iio~-~~scycle. Factors or processes which promote nutrient loss from the compartments of biogeochemical cycles to the reservoir can impoverish ecosystems over long run. For example, continuous cultivation and cropping without the use of fertilisers is bad for the soil. Small particles and nutrients wash with runoff waters or leach down to groundwater and rivers through subsoil to the sea, where they may get buried with sediments which may eventually be incorporated into rocks. , forestry operation (e.g. deforestation). and other activities can profoundly affect the rates of nutrient cycling. For instance, burning of fossil fuels contributes towards the build up of carbon dioxide in the atmosphere. We will learn more of this as we discuss each cycle inciividually 6.3 Nutrient Cycling

Carbon is the basic constituent of all organic compounds. Next to water, carbon is the most significant element constituting 49 per cent of the dry weight of organisms. The carbon cycle is essentially a perfect one, that is, carbon is returned to the environment about as fast as it is removed. The source of all carbon in living organisms, dead.organic material and fossil deposits is carbon dioxide found in the atmosphere.

Table 6.2 : Carbon in major biospheric compartments

Major Compartments in the Biosphere Carbon in lo9 tons

Atmosphere Terrestrial Oceans (mostly as carbonates) Fossil fuels

Source : Data from 1981 report of the Council on Environment Quality. The atmosphere has an average concentration of about 0.032 per cent or 320 ppm of C02. Apart from the atmospheric pool, a considerably large amount of CO, is found dissolved in the ocean$. It is estimated that the oceans contain more than 50 times as much carbon as there.is in the atmosphere. The oceanic reservoir.tends to regulate the amount in the qtmosphere. Tdble 6.2 shows the major biosphere compartments involving carbon. The cycling of carbon involves the atmospheric reservoir, from where it is taken.up by the producer to and from both these groups to the decomposer and then back to the reservoir (Fig. 6.2). Let us now consider each stage of the cycle. . . CO, in atmosphere

Fig. 6.2 : Simplified carbon cycle

Through , green plants pick up carbon from carbon dioxide they take m from the atmosphere. AS much as 4 to 9 Y 1013kg of carbon is fixed In the biosphere through photosynthesis annually. Respiratory activity in the producers and consumers accounts for-tbreturn of a considerable amount of carbon as C02 to the atmosphere. The most substantial -Ystem : and Types return, however, is through the activity of decomposers in their processing of the materials and dt ad remains of other trophic levels. Burning of wood, forest fires and combustion of organic matter also are additional man-made sources for releasing C02into the atmosphere. The rate of release of carbon depends on environmentabco~lditionssuch as soil, Turnover Rate : Rate of replace- moisture, temperature and precipitation. In tropical forests most of the carbon in merit Of a substance Or a 'pecies plant remains is quickly recycled, for there is little accumulation in the soil. The when losses to a system are replaced by addition. turnover rate of atmospheric carbon over a tropical forest is about 0.8 year. In drier regions such is grasslands, carbon is stored as . In swamps and marshes where dead material falls in water and is not completely decomposed, carbon is stored as humus or peat and circulated very slowly. The turnover rate here is of the order of 3-5 years. More than 99 per cent of the is present in the earth's crust'as deposits of coal, petroleum, peat and limestone. These as you know are deposits of plant and animal remains. On weathering of carbonate rocks, burning of fossil fuels and volcanic activity, the bound carbon is returned to the atmospheric-aquatic reservoir. A. number of aquatic plants In aquatic environments, the ..~hvto~lankton . utilises the CO? that is dissolved in the in PlkPline water release water, or is present as bicarbonates and carbonates and convert this CO, into calcium carbonate (CaCOs) as a byprodud of pho(osmtbesis. For phytoplankton . The phytoplankton is used as food by the aquatic . enmple 180 kg of Elodea The CO, produced in respiration is reutilised by the 'phytoplankton to produce more ~e~ precipitate 2 kg biomass. The carbon bound in the shells of snails and fcyraminifera as carbonates is hr of , deposited in the sediments when thesemimals die. In this manner a significant

CaCakxes with ClaV to portion of the carbon gets buried in the sediment and is removed from circulation. baa bdeover a nulndcr This mav later surface as limestone rock or coral rccf. of years. The atmospheric gaseous C02remains in dynamic equilibrium with the C02 dissolved in oceans. The interchange between the two phases occurs due to diffusion, the direction of which depends on the relative concentrations of carbon dioxide. Carbon dioxide dissolves in water easily and some of it enters the aquatic phase through precipitation. A litre of rain water contains about 0.3 ml of gaseous C02. The C02 dissolved in the water, in soil or in oceans forms carbonic acid (H2C03). The carbonic acid dissociates into hydrogen and bicarbonate ions (H+ and HCO;). The bicarbonate ions can furthen dissociate into hydrogen and carbonate ions. All these steps are fully reversible as shown in the following equation. Dissolved CO, + H20 = H2C03 H+ + HCO; -*Hf + COT, 1c. Atmospheric CO, The direction of the reaction depends on the concentration of the critical component. For example, a local depletion of C02 would result in the movement of C02from the dissolved phase into the atmosphere. Similarly the assimilation of bicarbonate ibns (HCO;) through photosynthesis by aquatic plants would tend to shift the equilibrium in the oiher direction. The equilibrium system actually is not as simple as it seems. It depends on several factors, pH of the water being one. At higher pH values i.e., alkaline conditions more carbon is present as carbonates; in acidic ' conditions mo:e carbon is in the dissolved phase. It may now be apparent to you that what seemed like a simple cycle is actually quite complicated. However, it is important to recognise that there are limited avenues by which carbon is utilised and a much larger number by which it is restored to the atmosphere. Human Impact on Carbon Cycle Human activities have greatlvinfluenced the carbon cycle. The discharge of CO, into the atmosphere is steadily increasing owing to burning of fossil fuels and destruction of forests. At the beginning of the Industrial Revolution about 1800, it is believed that C02concentration in the atmosphere was 290 ppm (parts per'million) which is equal to 0.29 per cent. In 1958 when accurate measurements were first taken, the concentration of CO, was already 315 ppm, while in 1988 it had risen,to 350 ppm. A major concern over the increasing concentration of C02in the atmosphere is its possible effect on the average ambient global temperature. Carbon dioxide is one of the gases that helps to produce the 'greenhouse effect' (recall FST-1 Block 4, Unit 16). Rise in the ambient global temperature would have pronounced ecological effects. The warming would cause icecapsdo mel~and ocean levels to rise, as a result Nutrient Cycling the continental coastal regions would be flooded. The rise in temperature would also change the rainfall and vegetation patterns which would disrupt agricultural 'production. This has been verified by comparing with predictions of climatic patterns of the past through computer modelling studies.

You have already learnt that nitrogen is an essential constituent of protein - the building block of all living cells. It is also a major constituent of the atmosphere (79 per cent). Although organisms live in an atmosphere rich in gaseous nitrogen yet the organisms cannot use this nitrogen. It can be utilised only after gaseous nitrogen has been 'fixed' into some ~micallyusable form. The transformation whereby molecular nitrogen is converted into a variety of nitrogenous compound and its release again into the atmosphere, is what constitutes the (Fig. 6.3).

Fig. 6.3 : Nitrogen cycle. A sikplified diagram representing major steps in the circulation of nitrogen involving various organisms and different forms of inorganic and organic nitrogen. . F- Trps The largest reservoir of nitrogen is the atmosphere but the critical pools are represented by itsprganic and inorganic forms that can be used by plants and animals. As we have said before, atmospheric nitrogen cannot be used by plants or animals. It has to be first fixed. The term nitrogen fixation refers to the oxidation or reduction of atmospheric nitrogen to nitrates (NO;) and ammonia (NH,) which can be used by living organisms. In nature nitrogen fixation into these compounds occurs primarily in two ways : i) High energy fixation : Through cosmic radiations, lightning, volcanic activity and meteorite trails which provide the high energy needed to combine atmospheric N, with oxygen and hydrogen of water. The resulting ammonia and nitrates are brought to the earth by rainwater. ii) Biological fixation : Approximately 63% of all nitrogen fixed is through biological fixation. Nitrogen fixing organisms are primarily prokaryotes; and blue green algae. Nitrogen fixation requires activation of molecular nitrogen by splitting nitrogen into two atoms of free nitrogen N2 + 2N. This is an energy requiring step, which in biological fixation requires 160 kcallmole. The actual fixation step, in which two atoms of nitrogen combine with three molecules of H2 to form two molecules of ammonia (NH,) releases 13 kcallmole. Therefore, the net energy requirement for nitrogen fixation is 147 kcallmole. Except for the photosynthetic ones, all nitrogen fixing organisms need an external source of carbon compounds to provide the energy for this endothermic reaction. It is an interesting fact that nitrogen fixation regulated by two nitrogenase and hydrogenase in nature requires low energy. In contrast, industrial nitrogen fixation requires very high temperature (400' C) and pressure (2 x 10' Pascal). Table 6.3 illustrates the kind of organisms known to fix nitrogen. Symbiotic nitrogen fixatiun occurs largely in terrestrial situations whereas, fixation by free living organisms occurs in both terrestrial and aquatic situations.

Table 6.3 : Examples of symbiotic and free living nitrogeri fixing organisms

Symbiotic HOST PLANT N, FIXING ORGANISMS Legumes (pea, alfalfa, pulses like arhar, beans, clover, etc.) Rhizobium Non-legumes (~lnus,hfyrico, Ctasuorino, Hippophe, Eloeognus, Coriqri4 c&.) Actinomycetes Tropical grasses (Pispalurn, Digitaria, maize, sorghum) Azotobacter, Spirillum Klebsiella Cycads Blue green algae Ferns., (Azolla) Blue green algae (Anabaena) Lichens Blue green algae

Free-living

Aerobic bacteria *- (Azotobacter) Anaerobic bacteria - (Clostridium) Anaerobic - photosynthetic bacteria - (Chromatium, ~hodos~irillum,Chlorobium) Blue green algae - (Nostoc) i) Symbiotic Nitrogen Fixers Of the symbiotic nitrogen fixing bacteyia, species of Rhizobium form root nodules in legumes and are the most studied nitrogen fixers and the best understood. Species of Rhizobium are host specific to particular species of legumes. The rhizobia penetrate the root hair and once inside the root, the bacteria rapidly multiply and form swollen, irregular - shaped bodies in roots of legumes. Some non-legume w6ody plants also have root nodules and fix nitrogen symbiotically. The organisms that cause the formation of nodule and fixation of nitrogen are actinomycetes (a kind of primitive ). Some examples of non-legumes are species of Alnw, Elaeagnus Myrica, Araucaria, Ginkgo, Casuarina. Unlike legumes, which are largely tropical in origin, these nitrogen fixers orginate in the temperate zone. Nitrogen fixation by blue green algae or cynobacteria may take place In free living Nutrient Cycling forms or in with fungi as in certain lichens, mosses. ferns and at least one seed plant. The frohds of the small free floating aquatic fern Azolla contain small pores filled with symbiotic blue green algae Anubaena that actively fix nitrogen. For centuries this fern has played an important role in the rice fields of China. Before the rice fields are planted. the water filled paddy fields are covered with the aquatic fern which fixes enough nitrogen for the crop as it matures. This practice permits rice to be grown without further addition of nitrogen fertilisers. Symbiotic nitrogen fixers are more efficient than free living ones. ii) Non Symbiotic Nitrogen Fixers There are certain groups of free living bacteria both aerobic and anaerobic and blue green algae that fix nitrogen. Aerobic nitrogen fixing bacteria such as Azotobacter and anaerobic form Clostridium are widely distributed in as well as in fresh and marine waters. In fact accumulating evidence indicates that many soil a?d water bacteria are capable of nitrogen fixation and because they occur in the total amount of nitrogen fixed is considerable. The N, fixed in the soil and root nodules is used by the plants to form numerous nitrogenous compounds mainly proteins which then enter the food chain. Nitrogen is returned to the soil in the form of organic compounds through manure, dead plants, and animals and micro-organism. But most of this nitrogen is insoluble and not immediately available for plant use. The organic nitrogenous compounds have to be changed to inorganic compounds to be used by plants. This is done by two processes - ammonification and .

Ammonification Many heterotrophic bacteria. actinomycetes and fungi in soil and w iier, metabolise the organic nitrogen and release it in an inorganic form as ammonia. This process is known as ammonification or mineralisation. This is an energy relea. lng reaction. For example, glycine-based protein releases 176 kcallmole. This energy is used to maintain the life 'process of the organisms that accomplish the transformation. Because the ammonium ion has a Nitrification positive charge it tends to be ~mmbniaor ammonium salts, are converted into nitrate in a process termed retained on the clay particles which are negatively charged as nitrification. to be useful to most autotrophic and heterotrophic organisms. This soon it is formed, till it is oxidised. process occurs in warm moist soil with near neutral pH and takes place In two steps : The nitrate ion being negatively ammonia salt or ammonia is oxidised and converted into nitrite by Nitrosomonas charged moves freely through the i) soil and readily travels down to 2NH, + 30, 2N0; + 2H+ + 2~,065 kcallmole the root zone. ii) Nitrite is further oxidised and converted into nitrate by Nirrobactor 2NO; + 0, - 2NO; 17 kcallmole These nitrifying bacteria obtain their energy from this oxidation process. Now let us see how nitrogen is converted back into its gaseous form.

Denitrification Nitrates are readily leached from the soil and also lost through the process by which molecular or gaseous nitrogen (N,) as well as nitrous oxide (NO) 'and nitric oxide (N20) and nitrogen dioxide (NO,) are formed from NO; by bacteria (such as ~seudomonus)and fungi. They use the nitrate as a source of oxygen in the presence of glucose and phosphate. Denitrifing bacteria prefer anaerobic or partially aerobic such as estuaries, bogs, lake bottoms and water-logged soils. The bacteria reduce the nitrates to nitrites which are finally converted to free nitrogen. Figure 6.4 shows the processes involved in N, cycle namely fixation, assimilation, denitrification, , leaching, runoff in ra!nwater, etc., along with some estimates of annual global movements. ?'he magnitude of the two flows is dirk~tly related to human activities - emmissions into the atmosphere and industrial fixation that is largely added to farms in the form of nitrogen fertilisers are also shown. The total annual nitrogen fixation is estimated to be 92 x lo6 metric tonnes, whercas total amount denitrified and returned to the atmosphere is only 83 x lo6 metric tonnes. The extra nitrogen added each year in the biosphere causes disbalance of nearly 9 x lo6 metric tonnes and is being largely built up in groundwater, reservoirs, rivers, lakes and the ocean. kZ0syY~: Rtnctioning and Typs

ATMOSPHERE

Aubndnle and Induslnal ernlsstons Jz

-----'& -----'& Bicommunity Leaching, runoff etc to sediment

Industrial fixation (30)

Fig. 6.4 : Estimates of the magnitude of key flows in the nitrogen cycle. Numbers in parentheses are in lo6 metric tomes per year (Data from Delwich 1970 Scientific American)

The self-regulating feedback shownin Fig. 6.4 makes the nitrogen cycle a relatibely perfect one when a large area or the biosphere as a whole is considered. Some nitrogen is lost to the ocean sediments and gets out of circulation but this is compensated by nitrogen entering the air by volcanic gases. Let us now assess lhe impact of human activities on this cycle. Human Impact on Nitrogen Cycle Human activities are profoundly affecting the cycling of nitrogen in nature. Over 30 x lo6 metric tonsjyr. of N2 is fixed in the commercial production of fertilisers, an amount almost equal to that fixed biologically. The use of N2 fertilisers affect the distribution of N2 on earth. Much of the nitrogen in the harvested crops becomes animal and human waste in sewage waters and eventually enters the through runoff and leaching. Nitrogenous compounds leached into the groundwater may be abundant in irrigation and drinking water where they can cause serious health hazards. Nitrogenous compounds entering the lakes have fertilising effect resulting in algal blooms and promote cultural eutrophication. You already have an idea what eutrophication means (from'ufiit 26 Block 4 of'FST-1). Excessive growth of phytoplankton in eutrophic lakes produces huge quantities of biomass and finally collapse due to nutrient exhaustion. The dead organisms are consumed by , detrivores which use up the oxygen supply. This problem of cultural eutrophication is, however, more severe in the case of phosphorus additions rather than nitrogen. When fossil fuels are burned we add nitrogenons compounds to the air. Large quantities oi nitrogen oxide (NO) are released from vehicles and most of the NU is converted to NO2 by combining with ozone (03) in the atmosphere. NO2 is a toxic gas for 6umans and a cause of smog. It combines with water to form nitric acid, HN03, which forms 30% of the strong acids in the . You will read mc about acid rain jn Section 6.5. Now that you have learnt about the biogeochemical cycles where the main reservo are in gaseous phase, we will discuss two sedimentary cycles namely phosphorus a1 sulphur. These are different from the earlier two gaseous cycles because the main reservoirs and major reactions involving their transformation are largely confined to the sediment.

6.5 SULPHUR CYCLE

The sulphur cyclc is mostly sedimentary except for a short gaseous phase (Fig. 6.5). The large reservoir of sulphur is in the soil and sediment where it is tied up in organic (coal, oil and peat) and inorganic deposits (pyrite rock and sulphur). It is released by weathering of rocks. erosional runoff and decomposition of organic matter, and is carried to terrestrial and aquatic ecosystems in salt solution. The smaller reservoir is in the atmosphere. Sulphur can circulate on a global scale along with carbon. oxygen and nitrogen because of its gaseous phase.

Fig. 6:s : he Sulphur cycle linking air, water and soil. The centre circular arrows show oxidation (0)and reduction (R) reactions that bring about key transformations between available su!phate (SO,) pool, organic sulphur and iron sulphide deep in the sediment and soil. . mystem : fund ion in^ and Types Sulphur enters the atmosphere as hydrogen sulphide (H2S) and sulphur dioxide (SO;?) from several sources like combustion of fossil fuels, volcanic eruptions, and the surface of oceans and gases released by decomposition. Hydrogen sulphide also oxidises into sulphur dioxide (SO,). Atmospheric SO2 is carried back to the earth dissolved in rainwater as weak sulphur~cacid (H2S04).Sulphur in the form of sulphates (~04~)is taken up by plants and incorporated through a series of metabolic processes into sulphur bearing aminbacids. From the producers the amino ac~dsare' taken up by the consumers. 4 Sulphur bound in living organisms is carried back to the soil, to bottoms of ponds and lakes and seas through excretions and decomposition of dead organic material by bacteria and fungi. The oxidation-reduction transformations have been summarised in Table 6.4. These are carried out by specialised bacteria that obtain their energy from these transformations.

Table 6.4 Role of some microbes in the sulphur cycle (%represents oxidation while R-represents reduction reactions)

Microbes Reactions, transformations 0 0 Colourless, green and purple sulphur bacteria H2S :- S -4 SO, R Desulphuvihriu (anaerobic) bacteria SO, --r H2S 0 Thiobacillus (aerobic) tizS -+ SO, 0 Aerobic Organic S -+ SO, R Anaerobic Heterotrophs Organic S -+ H2S .

Look at Fig. 6.5 again. From the reactions shown in the figure and the table you can see some parallel with the nitrogen cycle, since the sulphate is used as a hydrogen acceptor by the heterotrophic sulphate retiucing bacteria just asthe denitrifying bacteria use nitrite and nitrate. Species of colourless sulphur bacteria such as Reggia~oaoxidises hydrogen sulphide to elemental sulphur and species of Thiobacillus oxidise it to sulphate. For some species oxidation processes can occur only in the presence of oxygen; for others oxygen is not necessary. These bacteria are chemosynthetic . They obtain their carbon from the reduction of CO,. 6C02 + H2S ChH120h + 6Hz0 + 12s These bacteria are again comparable to the chemosynthetic autotrophic nitrifying bacteria that oxidise ammonia to nitrite and nitrite to nitrate. The green bacteria apparently are able to oxidise HIS to elemental sulphur S, whereas purple bacteria can carry the oxidation to sulphate stage.

The sulphate may be recirculated and taken up by the producers or used by sulphate reducing bacteria. The sedimentary aspect of the cycle involves the precipitation of sulphur in the presence of iron (Fe) and calcium (Ca) as highly insoluble ferrous sulphide (FeS) and ferric sulphide (Fe2S,) also known as pyrite; or relatively insoluble calcium sulphate (CaS04) thus contributing to the reservoir of sulphur. From this it enters the cycle through weathering and erosion. Not as much sulphur is required by the ecosystem as nitrogen and phosphorus. Nonetheless sulphur cycle is important in the general pattern of production and decomposition. For example when iron sulphides are formed in the sediments, phosphorus is converted from insoluble into soluble form as shown in the Fig. 6.7, and phosphorus enters the pool available to the living oyganisms. This is an excellent epample of how one cycle regulates another. Human Impact on Sulphur Cycle On account of combustion of large amounts of fossil fuels sulphur dioxide is emitted. Globally some 147 million tonnes of SO, are poured into the atmosphere each year. Normfilly oxides of nitrogen (No2'and N20) and sulphur (Soz)Are.only. transitionary steps in their respective cycles and are present in most environments in low concentrations. Combustion of , however. has greatly increased the concentration of these oxides in air especially in urban and industrial areas to a point where the biotic components are adversely, affected. SO2 is damaging to Nutrient Cycllng photosynthesis, it is one of the most potent phytotoxic pollutant. Furthermore it interacts with water vapour to produce H2S04and ultimately returns to earth as acid rain. (You have already read about the effects of acid rain in FST-1 Block 4 Unit Theoretically normal rain ba- 81 pH of 6.5, slightly more aridi: than 16). Acid-rain affects the land, vegetation and aquatic systems in a variety of ways. saliva and milk. Acid rail; is Importapt plant nutrients like, calcium, magnesium and potassium are progressively precipitation...... rain. %now leached out of the soil, aluminium and zinc accumulate. Useful micro-organism in sleet, fog that contains dilute the soil are replaced by harmful disease causing fungi. solutions of sulphusic and nitric acids. Acid rain is no longer a local problem, of urban areas. Its impact is greatest on lakes or streams and already acidic soils that lack pH buffers such as carbonates, calcium i salts and other bases.

SAQ 4 Complete the diagrams (i) and (ii) which explain the subcycles of the sulphur cycle. Faystem : Zunrt~onu~gand Types 6.6 PHOSPHORUS CYCLE

Phosphorus is a very important nutrient because of its role in the form of phosphate, in reactions that store and release energy. The availability of phosphates often becomes a limiting factor in ecosystem . The reservoir pool of phosphorus is in crystalline phosphate rock and the compartments in phosphorus cycling involve organisms, soil and shallow marine sediments.

. Tie natural form in which phosphords is available is inorganic phosphate. Through erosion and weathering of rocks, inorganic phosphate is made available to plants that absorb it from soil or in the case of aquatic plants from the water. Once taken up by the plant the phosphate may become part of ATP (adenosine triphosphate), nucleic acid or some other organic compound. The phosphate may be returned to the soil or sediment when the plant dies and decomposes. Phosphorus may also be passed to the consumer or get incorporated into the cell body of the decomposers. In consumers the phosphorus may be incorporated into the bones and teeth and thus it remains bound for a long period of time. Some of it is excreted as waste and is immediately available to the decomposer. It may by a short loop be converted back to inorganic phosphate and be assimilated by the plants (see Fig. 6.6). On the other hand, it may be tightly bound to iron, calcium and aluminium as insoluble compounds and be washed off or lost in sediments.

Organic Phosphates

Consumers (animalsetc) \ I Producers (Plan&)

Fig. 6.6 : A short loop in the cycling of phosphorus in the terrestrial ecosystem

In marine and fresh water ecosystems the phosphorus cycle moves through three compartments.

Inorganic Phosphate

Dissolved Organic phosphate <- Particulate Organic Phosphate Inorganic phosphat~sare taken up rapidly by the phytoplankton which in turn may be ingested by zooplankton or feeding organisms. Zooplankton in turn may excrete as much phosphorus daily as stored in their biomass. More than half the phosphorus excreted is inorganic which is again taken up by the phytoplankton thus keeping the cycle running. The rest of the phosphorus in aquatic systems is in the form of organic phosphates that may be utilised by bacteria, that in turn may be consumed by microbial grazers which then excrete the phosphorus they ingest. Part of the phosphorus is deposited in shallow sediments and part in the deep water because phosphorus is precipitated largely as calcium compounds much of which become immobilis,ed for long periods in the bottom sediments from where it is later recirculated by upwelling. Figure 6:7 shows the phosphorus cycle in terrestrial and aquatic ecosystems. Phosphorus is the key limiting factoj in aquatic systems. The turnover rate may -- actually determine the productivity in many aquatic systems. For example, excess phosphates can stimulate explosive growth of algae and photosynthetic bacteria populations, resulting in disruption of aquatic ecosystems. Fig. 6.7 : The phospnorus cycle In terrestrial and aquatic ecosystems. The rate of cycling of phosphoi 1.1 is extremely important for growth and activity in living things.

Human Impact on the Phosphorus Cycle Like other biogeochemical cycles, human activities have altered the phosphorus cycle. Human beings mine phosphate rocks and guano deposits to make phosphorus available for production of fertilisers, detergents, animal feed, medicines, pesticide: and numerous other products. This mining exposes phosphate deposits made over millions of years. Phosphates are removed from soil through cropping of vegetation,and to replace it phosphate fertilisers have to be added. Because of the abundance of calcium, iron and aluminium in the soil much of the phosphates get immobilised as insoluble salts. Thus more fertilisers have to be added. This results in high concentration of phosphates in agricultural runoffs. Similarly concentration of phosphorus in detergents, of food processing plants, animal feed lot, sewage, etc., add to a considerable quantity of phosphorus poured in natural waters. This problem becomes acute in urban areas. As said earlier, in aquatic ecosystems the phosphorus is taken up rapidly by the vegetation resulting in a sudden explosive growth of algae. Like nitrogen, this leads to cultural eutrophication of the water body. The producers cloud the water and forms a scum on the surface, blocking sunlight for the submerged plants. This is one example of the result of accumulation of nutrients at one stage of the nutrient cycle. It is important to note that the means of returning phosphorus to the cycle are inadequate to compensate for the loss. Sea birds have traditionally played-an important part in returning phosphorus to the cycle via their droppings (for example guano ueposlts off the coast of Peru) but apparently not at the rate at whlch it has occurred in the past. Unfortunately human activities appear to hasten the rate at which phosphorus is lost and thus make the cycle 'less perfect'. You could think our present use of phosphorus which is washed out into the rivers and finally into the oceans as an accelerated 'pouring' of phosphorus from the source to the sink. Ecosystem : Functioning and Types

6.7 NUTRIENT BUDGETS'AND CYCLING IN FORESTS

In this unit we have so tar considered the movement of individual nutrients with major emphasis on their global, biological and .chemical aspects. We will now study the nutrient dynamics in terms of input, output and flows of nutrients technically called nutrient budget with particular reference to forest ecosystem'. 6.7.1 Nutrient Budgets Nutrients are constantly being added'and removed by natural and artifical processes (see Fig 6.8). The measure of the input and outflow of nutrients through the various components of an ecosystem form its nutrient budget. The nutrient budget of an ecosystem can be considered under two sections.

a) Internal budget : This is concerned with the circulation of nutrients through various biotic and abiotic compartments of a given ecosystem. In other words the input and output that occurs along the producer -----+ consumer - decomposer food chain and the exchanges between the reservoirs and sediments within the gcosystem. b) External budgets :In contrast the external budgets pertain to the input and output of the entire ecosystem in relation to other'ecosystems. For instance volcanic eruptions throw materials into the atmosphere or spread lava over a terrain, thus distributing nutrients over large areas. Wind and water transport nutrients to long distances and serve as carriers for their action. such as weathering of rocks or wind that carries nutrients between different ecosystems. Animals feed in one ecosystem defecate or die in another, or trees grow in one ecosystem and are burnt elsewhere. Humans are without question the most powerful agents that affect the internal and external nutrient budgets.

6.7.2 Nutrient Cycling in Tropical and Temperate Forests From this study of the nutrient cycles you must have realised the importance of the role of green plants that take up nutrients from the substratum and air, representing Fig. 6.8 : Nutrient budgel in forest ecosystem. Input of nutrients is through precipitation, dust, littedall and, through weathering and root decomposition. Outflow is through wood harvest, hunting, runoff, erosion and leaching. ,.

thi abiotic components and decomposers that release the nutrients back into the i abiotic pools for reuse by. the plants. In tropical forests a large percentage of the total nutrients are held in the biomass and not in the soil, but in the temperate regions a large portion of organic matter and available nutrients is at all times in the soil and sediments. Figure 6.9 shows ihe contrast in the distribution of organic carbon matter in a northern coniferous and a tropical rain forest. Interestingly both ecosystems contain the same'amount of organic carbon but more than three fourths is in vegetation in the tropical forest. of nutrients in the organic structure of the tropic4 forests is aided by a Recycling of nutrients withid the number of nutrient conserving, biological adaptation. These adaptations depend on organic structure means that nutrients move within the plant the geology and basic fertility of the region and some of the mechanisms that are in the leaves and woody tissue., especially well developed in tropical rain forests are: Watever nutrients are washed - away tkom the leaves by air, water or lost from the plant in i) Root mats consisting of many fine feeder roots which penetrate the surface of litter is quickly taken up by the the litter and quickly recover nutrients from leaf fall and rain before they are plants again so that very little leached away. Root mats also inhibit the activity of denitrifying bacteria, thus nutrients remain in the soil. preventing loss of nitrogen. ii) Mycorrhizal fungi associated with root systems act as nutrient traps and help in the recovery and retention-of nutrients. This symbiosis is also present in temperate forests of areas that are basically poor in nutrients. Ecosystem : Functioning and Types iii) Evergreen leaves have thick waxy cuticles that retard loss of water and nutrients, also leaves have pointed tips or 'drip tips' that drain off water fast, thereby reducing leaching of leaf nutrients. iv) Algae and lichens that cover surfaces of many leaves pick up nutrients from .'rainfall some of which becomes available to the leaves immediately. Lichens also %x nitrogen. v) Thick bark inhibits diffusion of nutrients out from the phloem and subsequent loss by stem flow i.e., rain running down the trunks of trees.

Leaf I Leaf

Litter

L~ner

NORTHERNCONIFEROUS FOREST TROPICAL RAIN FOHESl

Fig. 6.9 : Distribution of organic carbon accumulated in abiotic (soil and litter) and biotic (wood and leaf) coylpartments ofa tropical and a temperate forest. Note that the tropical forest has a huch larger percentage of organic carbon in plant btomass.

Although soils of tropical forests are generally poor in nutrients they are able to maintain high productivity under natural conditions due to these nutrient-conserving mechanisms that almost bypass the soil by having a plant to plant cycling. When such forests are cut or cleared for agriculture these mechanisms are destroved and productivity declines very rapidly. Forest removal takes away the land's ability to. hold nutrients as well as to combat pests in the face of year round high temperatures. Crop production declines and in a few years the land is abandoned. Soils in temperate forest have relatively large nutrient pools and when these forests are cleared, the soil retains nutrients and may be cultivated for many years by ploughing one or more times a year, planting short season annual plants and applying inorganic fertilisers. During winter, freezing temperatures help hold in nutrients and combat disease and pest. It is for these reasons that agricultural practices suitable for temperate areas may be inappropriate for tropical areas and should not be applied unmodified in the tropics.

SUMMARY

In this unit you have studied that : Nutrients circulate from the environment to organisms and back to the environment in perpetual cycles referred to as biogeochemical cycles or nutrient cycles. There are two types of cycles: gaseous, where the major reservoir is the atmosphere, these are represented by carbon and nitrogen and sedimentary cycles represented by sulphur and phosphorus with major reservoirs in the earth's crust. The carbon cycle involves the assimilation and respiration of carb,on dioxide by plants, its consumption as carbohydrates in plants and animal tissue and its release through respiration and decomposition and combustion. Carbon is also withdrawn froni the cycle into long-term reserves. The equilibrium of carbon dioxide between the sea, atmosphere and land is being disturbed by rapid release of carbon dioxide into the atmosphere by burning wood and fossil fuels. Increased carbon dioxide in the atmosphere has the potential to raise the ambient global temperature of the earth with serious ecological implications. Nitrogen cycle is characterised by fixation of atmospheric nitrogen by nitrogen ~ulrientCycling fixing organism and industrial processes, its assimilation by planks in the form of nitrate and ammonium ion. Involved in the nitrogen cycle are the processes of ammonification, nitrification and denitrification. Human intrusion into the nitrogen cycle involves release of oxides of nitrogen into the atmosphere which cause air pollution and smog. Excessive nitrates released into the aquatic ecosystems cause cultural eutrophication. Sulphur cycle is a combination ot gaseous and sedimentary cycles. It involves a long-term sedimentary phase in which sulphur is tied up in organic and inorganic deposits Irom wnere,it 1s released by weathering and decomposition and taken up by the plants as inorganic sulphates. Sulphur enters the atmosphere as SO2 .released during fossil fuel combustion and as H2S released during decomp~sitio.~ of organic matter. Sulphur aioxide, soluble in water is carried to earth as sulphuric acid in acid rain. The phosphorus cycle is wholly sedimentary with major reservoirs in phosphate rocks. It is released by weathering and taken up by plants as inorganic phosphates. Major part of the phosphates added as fertilisers are immobilised in the soil but gr'eat quantities used in detergents and in wastes are carried in the sewage effluents. These ultimately become part of shallow sediments of the sea and a large portion is lost into the deep sediments. Phosphorus is carried back to the terrestrial ecosystems tnrougn fish meal and bird droppings but the amount returned is not enough to compensate the loss. .Green plants, by taking up nutrients and decomposers by releasing the nutrients for reuse, play an important role in nutrient cycling. Nutrients are constantly bcing added or removed from the ecosystems. A measure of the inflow and outflow forms the nutrient budget of an ecosystem. In tropical forests a large portion of the nutrients are held in the biomass and not in the soil. This is due to the plant to plant recycling of nutrients aided by various nutrient-conserving biological adaptations. Therefore, if tropical fprests are cleared the soil's ability to hold nutrients is lost, making it unsuitable for long-term agriculture. In temverate forests a large portion of the nutrients are in the soil rather than in plant biomass. Therefore, if.these &rests are cleared, the soil still retains nutrients and may be farmed for many years.

6.9 TERMINAL QUESTIONS

1) What are the two types of biogeochemical cycles and what are their distinguishing features.?

, 2) Describe three pathways whereby atmospheric nitrogen is converted into fixed I forms that are usable by plants, and two pathways whereby fixed nitrogen is returned to the atmosphere. I

...... L '. 3) Describe briefly how carbon and sulphur cycles are affected by human activities. Ecosystem : Functioning and Types

4) a) Why is phosphorus considered an example of an imperfect cycle?

b) Phosphorus is often one of the limiting factors in aquatic ecosystems yet heavy discharge of phosphates into a clear lake 'kill' it. Comment.

5) Explain why removal of tropical forests often reveal poor quality agricultural land.

,- 6.10 ANSWERS

. Self-assessment. Questions 1) ii) compartment; iii) reservoir iv) biogeochemical cycles 2) a) iv) b) ambient global temperature; greenhouse gases; icecaps; continental coastlines 3) a) i) b) ammonium (ion); ammonium (ion); nitrate;. amino acid; dinitrogen (N2) 4) i) b) H2S; c) S. ii) a) SO,; b) SO?; c) H2S04 5) i) b) weathering; d) organicqhosghate e) consumel, -g) Inorganic phosphate Terminal Questions 1) a) Gaseous cycles where the primary reservoir is the atmosphere ag far as living organisms are concerned, examples carbon and nitrogen. b) Sedimentary Cycles where the principal reservoir lies in the earth's crust and Nutrient Cycling is released into the ecosystem by. weathering, mining and erosion. Examples are phosphorus and sulphur. 2) Atmospheric nitrogen is f~xed(i) into ammonium by biological fixation through nitrogen fixing bacteria and blue green algae, (ii) by lightning as photochemical fixation into nitrates, (iii) by industrial fixation in the form of nitrate and 1 ammonium fertilisers. Nitrogen is returned to the atmosphere through the process of denitrification of nitrates and as oxides of nitrogen in automobile exhaust and industrial combustion. 3) Hints : i) burning of wood and fossil fuels (coal, oil, gas) adds more C02 to the atmosphere which may lead to rise in global temperatures. Higher , temperatures can cause increased melting of polar ice, milder winters and changed rainfall patterns. I ii) clearing of forests would remove carbon-sinks. iii) increased release of SO, in atmosphere due to burning of fossil fuels and heavy industry. SO, is a toxic component of smog and forms H2S04 with water vapour and falls to earth as acid precipitation.

) a) because more phosphorus is lost from the cycle in deep sediment than is returned to the cycle. b) addition of phosphates to lakes leads to cultural eutrophication i.e. algal blooms and most of the oxygen requiring organisms die, as the oxygen in the water is used up by detrivores for the process of decomposition. 5) Hints : i) nutrients at any time are circulating in the biomass rather than residing in soil. ii) various biological adaptations in tropical plants conserve the nutrients.