managing soil organic matter - Grains Research & Development ...

Carbon cycling between the soil, plants andatmosphere involves the continuous transformationof organic and inorganic carbon compoundsby plants and organisms (see Figure 3.1). Soilrepresents a reservoir able to both store and releasecarbon within the global carbon cycle and as such isconsidered both a sink and source for carbon.Soils contain carbon in both organic and inorganicforms, with the exception of calcareous soils, whichis largely held as soil organic carbon. This organiccarbon continually enters and leaves the soilresulting in both carbon accumulation and loss. Atany one time the amount of organic carbon in soilrepresents the balance between inputs and losses.Since carbon turnover can beconstrained by available nutrientsas suggested by the carbon tonutrient ratio, it is likely that morefertile soils will lose organic matterat a faster rate than lower nutrientcontent soils.A significant amount of the organic carbonaccumulated in soils has resulted fromphotosynthesis where plants convert atmosphericcarbon dioxide into above-ground shoot growth andbelow-ground root growth. As primary productivityincreases, organic inputs resulting from shoots,roots and micro-organisms grow and contribute toa build-up in soil organic carbon.Carbon emissions from soil back to theatmosphere occur in the form of carbon dioxide,largely as a result of agricultural practices drivingchanges in microbial processes. These emissionshave resulted primarily from the decomposition oforganic matter, reflecting the historical declines thathave been measured in soil organic matter for manyagricultural soils and contributed to the measuredincreases in atmospheric carbon dioxide resultingfrom human activities.CARBON BALANCE IN SOILSSoil organic carbon is in a constant state of flux,slowly responding to environmental or managementchanges and moving to reach a new equilibriumlevel after changes occur (see Figure 3.2). Forexample, in systems where plant production isconstrained, organic matter inputs decline and soilbiota increasingly deplete stored soil organic carbonfor energy. This results in declining soil organiccarbon levels until a lower limit, determined by soiltexture and a decline in biological activity, whichis essentially starved of decomposable carbon, isreached. In contrast, systems with increasing organicinputs to soil can attain a higher level of soil organic25MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE Figure 3.1 Organic carbon cycling in soils.