The effects of reduced tillage practices on soil carbon content

Defra has published a new review of the potential for reduced tillage practices and organic matter returns to increase the carbon content of English and Welsh arable soils as a means of mitigating greenhouse gas emissions.

This review, published by Defra in 2008, finds that there is limited scope for additional soil carbon storage/accumulation from zero/reduced tillage practices and organic material applications, over and above present day normal farm practice. It says there are questions over the implications of such practices for nitrous oxide (N₂O) emissions and the overall balance of greenhouse gas emissions (expressed on a CO₂-C equivalent basis). SOC accumulation is finite and reversible, and SOC levels will only remain elevated if the practice is continued indefinitely.

Only the application of biosolids (treated sewage sludge), compost and paper crumble appear to offer the same level of CO₂-C ‘savings’ that have been predicted for land-use change options (e.g. reversion of arable land to permanent grassland, woodland or willow/poplar biomass production). With the probable exceptions of compost and paper crumble applications (which are largely a result of recent diversions away from landfill), almost all of the other organic materials considered were already applied to land, so it is questionable whether this can be regarded as genuine additional carbon storage (against a recent/present day baseline). Probably of more importance, is the maintenance of existing SOC levels and the avoidance of ploughing out permanent grasslands.

It concludes that the predominant justification for returning all organic materials to soil should therefore be for maintaining existing SOC levels, and completing natural nutrient and carbon cycles, not additional carbon storage for climate change mitigation per se. Similarly, should reduced tillage be encouraged, it should be for its protection of existing SOC levels and benefits to soil water retention and prevention of erosion, as well as reduced production costs and energy use, rather than for additional carbon storage per se.