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16HOW MUCH ORGANIC CARBONREMAINS IN SOIL?Microbes digest up to 90 per cent of organic carbonthat enters a soil in organic residues. In doing so,they respire the carbon back into the atmosphereas carbon dioxide (CO 2 ). Microbes continually breakdown organic residues eventually converting a smallproportion of them to humus, which gives the topsoilits dark colour. While up to 30 per cent of organicinputs can eventually be converted to humusdepending on soil type and climate, in Australianagricultural soils this value is often significantly less.Examples of the influence of soil type, climateand management on the amount of soil organicmatter (and carbon) accumulated in and lost fromagricultural soils is presented in Table 1.4. To maintainor increase a soil’s stock of organic carbon longterm,increased amounts of organic matter mustbe continually added. Any decline in the amount oforganic material being returned to soils will result ina decrease in the soil’s organic carbon content.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDESoils naturally higher in clay content generallyretain more organic matter than sandy soils.Soil type, rainfall and temperature limit the amountof soil organic matter generated via plant biomassand subsequently stored as humus for the longterm.As a result, soils rarely reach their theoreticalpotential for organic matter storage (see Figure1.7). Management practices also have a significantinfluence on whether actual soil organic matter (andcarbon) reaches its attainable level as determinedby climate (see Figure 1.7). Beyond this threshold,continuous inputs of external organic carbon sourcesare required, which can be logistically difficult andexpensive, and risk the depletion of organic carbonin another location.Plate 1.1 The organic horizon is oftenrelated to a darkening of soil colour and isparticularly evident on the soil surface.Source: Department of Primary Industries, Victoria
Table 1.4 Rate-limiting influences on the accumulation of soil organic matter.InfluenceCause17Soil typeNaturally occurring clay in soil binds to organic matter, which helps to protect it from being broken down or limitsaccess to it by microbes and other organisms.ClimateLand and soilmanagementIn contrast, organic matter in coarse textured sandy soils is not protected from microbial attack and is rapidlydecomposed.In comparable farming systems with similar soil type and management, soil organic matter increases withrainfall. This is because increasing rainfall supports greater plant growth, which results in more organic matteraccumulating in the soil.Organic matter decomposes more slowly as temperatures decline. Under moist conditions each 10°C increasein temperature doubles the rate of organic matter decomposition (Hoyle et al. 2006). This means moist, warmconditions will often result in the most rapid decomposition of organic inputs.Maximising crop and pasture biomass via improved water-use efficiency and agronomic management willincrease organic matter inputs.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEAs a large proportion of organic matter is present in the top 0-10 cm of soils, protecting the soil surface fromerosion is central to retaining soil organic matter.Tillage of structured soils decreases soil organic matter stocks by exposing previously protected organic matter tomicrobial decomposition.Adding off-farm organic residues such as manures, straw and char can increase soil organic matter dependingon the quality of the added residues.Landscape can influence water availability. Transfer of soil and organic matter down slope via erosion canincrease the amount of soil organic matter in lower parts of the landscape.Soil constraints can influence humus formation by constraining plant growth and decomposition rates. This couldslow both the amount and transformation rate of organic matter moving into more stable fractions.Microbes and particularly bacteria grow poorly in strongly acidic or alkaline soils and consequently organic matterbreaks down slowly. Soil acidity also influences the availability of plant nutrients and in turn the amount of organicmatter available for soil biota growth.
Page 1 and 2: MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 8 and 9: 6INTRODUCTIONMANAGING SOIL ORGANIC
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 16 and 17: Figure 1.7 The influence of soil ty
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 22 and 23: 20MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM

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