The accumulation of carbon (C) in soil as organic matter (SOM) is considered to be a win-win situation for agriculture and the environment. Retention of crop and pasture residues should contribute to this accumulation of organic C in soil. However the retention of crop stubble has not necessarily resulted in increases in soil organic matter in long term field trials in southern New South Wales (NSW), Australia. Even under permanent pasture, annual or perennial, the rates of accumulation of soil organic matter are only about 500kg C/ha/year at best in the environment of southern NSW. Hence the C accumulation rates in 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 mineralisation of nitrogen (N) (and hence C) during the cropping phase. A change from this system requires increased reliance on the use of N fertiliser. Either way, an additional problem is that the accumulation of organic matter during a pasture phase, or the use of N fertiliser, is commonly associated with soil acidification, requiring the application of limestone and the release of CO2 in order to maintain agricultural productivity. In addition to these soil based processes, the climatic variability that is commonly experienced in the semi-arid environment of southern NSW means that year to year variability in dry matter return to soil and in decomposition rates of residues will be relatively large. The net result is that the signal to noise ratio of any trend in SOM accumulation is low.
Conyers, M., Liu, D., Kirkegaard, J., Orgill, S., Oates, A., Li, G., Poile, G., & Kirkby, C. (2015). A review of organic carbon accumulation in soils within the agricultural context of southern New South Wales, Australia. Field Crops Research, 184, 177-182. https://doi.org/10.1016/j.fcr.2014.07.013