Farm Table says:
What is the problem?
Accumulation of carbon in the soil as organic matter is considered desirable from an environmental perspective as it mitigates increased atmospheric CO2. This trial investigated the contribution to an increase in the concentration of organic C in soils as organic matter as a result of the retention of crop and pasture residues.
What did the research involve?
In southern NSW, several long-term experiments (15–34 years) measured soil organic C. These trials included crop and pasture sequences either continuously or in rotations, and various forms of tillage and crop residue management. This article is a review of the previous data.
What were the key findings?
- Even under permanent pasture, annual or perennial, the rates of accumulation of soil organic matter are only about 500 kg C/ha/year at best in the environment of southern NSW. As a result, the C accumulation rates in the soil are not likely to generate a significant income stream from C trading.
- Further, in the mixed farming systems of the inland areas, the traditional rotation has relied upon the build-up of organic matter during the pasture phase and the mineralization of nitrogen (N) (and hence C) during the cropping phase. A change from this system requires increased reliance on the use of N fertilizer.
- Either way, an additional problem is that the accumulation of organic matter during a pasture phase, or the use of N fertilizer, is commonly associated with soil acidification, requiring the application of limestone and the release of CO2 in order to maintain agricultural productivity.
Net organic C accumulation rates are slow in soils of southern NSW Australia, with maximum rates under a crop of the order of 0 kg C/ha × 30 cm/year where the soil is already at >1% organic C by mass.
Crop residue retention does not provide increases in soil organic C concentration in such soils.
There are productivity and environmental benefits from C accumulation that are held as sacred, however, the benefits to soil chemical fertility stem from the organic matter being dynamic in the soil, not from continuously accumulating.