The maintenance of soil organic matter levels and the optimization of nutrient cycling are essential to the sustained productivity of agricultural systems. Both are related closely to the bioturbating activities of macrofauna and the microbially-driven mobilization and immobilization processes, which the activities of large invertebrates also encourage. Maintaining soil organic matter content requires a balance between addition and decomposition rates. As changes in agricultural practices can engender marked changes in both the pool size and turnover rate of soil organic matter, it is important to analyse their nature and impacts.
Crop production worldwide has generally resulted in a decline in soil organic matter levels and, consequently, in a decline of soil fertility. Converting grasslands and forestlands to arable agriculture results in the loss of about 30 percent of the organic C originally present in the soil profile. On reasonably fertile soils with reliable water supply, yields in long-term arable agricultural systems have been maintained at very high levels by applying substantial amounts of fertilizer and other soil amendments. In low-input agricultural systems, yields generally decline rapidly as nutrient and soils organic matter levels decline. However, restoration is possible through the use of fallow lands, mixed crop-livestock and agroforestry systems, and crop rotations.
Traditional mould-board plough and disc-tillage cropping systems tend to cause rapid decomposition of soil organic matter, leave the soil susceptible to wind and water erosion, and create plough pans below the cultivation depth. By contrast, reduced- or zero-tillage systems leave more biological surface residues, provide environments for enhanced soil activity, and maintain more intact and interconnected large pores and more soil aggregates, which are better able to withstand raindrop impact. Water can infiltrate more readily and rapidly into the soil with reduced tillage and this helps protect the soil from erosion. In addition, organic matter decomposes less rapidly under reduced-tillage systems. No-tillage systems have proved especially useful for maintaining and increasing soil organic matter.
Crop rotation is the basis for the sustainability of direct sowing systems. A production system that includes cover crops, legumes for N fixation, crop rotation and no tillage can be adapted regionally and, therefore, contribute to the sustainability of soil management in the region. Where rainfall intensities are very high, or biomass management options are limited by a water shortage, maintenance of soil surface cover by crop canopies or crop residues during periods of high erosion risk is essential. With improved land management, at least part of the organic matter lost can be restored. The increase in soil organic matter in the absence of tillage can transform agricultural soils into carbon sinks.
The relatively low levels of active organic matter fractions in zero-tillage systems have highlighted the extreme dependence of such systems on the maintenance of high levels of surface protection by crop residues. Residue accumulation, including cover crops and crop residues, increases the levels of some soil nutrients and soil organic C. The active fraction of organic matter plays a very important role in aggregate stability and rainfall infiltration. Building up active C levels in the soil in rainfed cropping systems may have a greater impact in reducing surface crusting and improving rainfall infiltration capacity than would simply changing to zero-tillage systems. Management practices designed to maximize C inputs and to maintain a high proportion of active C should be seen as essential steps towards more sustainable cropping systems.
Cover crops, intercropping and crop rotations can also help to promote biodiversity both below the soil surface and aboveground. This diversity is important to maintaining a well-functioning and stable ecological system. Where many different types of organisms coexist, there are: fewer problems with diseases, insects and nematodes; more competition among species; and more possibility for many types of predators to thrive. In such a situation, no single pest organism is able to reach a population of sufficient size to affect crop yield seriously.