Inferring Carbon Sequestration in Sugarcane Soils From Carbon Dioxide Exchange and Biomass Production.

The advent of carbon trading and biofuel production highlight the need to document the carbon balances of cropping systems.  This is particularly true for sugarcane, a C4 plant with high carbon gains of around 30 t C/ha/y and high biofuel potential. Modelling and soil sampling suggest that whole crop removal, as in burnt-cane farming systems or biofuel production, will deplete soil C while trash retention systems may increase it.  In both cases, likely changes in soil C are less than 1 t C/ha/y.  Published accounts indicate that confirming such small changes by traditional soil sampling is error-prone and requires investigations of more than 10 years.  Can we determine gains or losses of soil C over an annual growing season by subtracting the carbon stored in the crop biomass from the carbon gained by the crop through the uptake of atmospheric carbon dioxide?  We report results from one–year measurements in each of two sugarcane crops for which carbon turnover differed by a factor of 2.  We conclude that errors and uncertainties in the measurements and calculations are presently too large to detect the small differences in the gain or loss of soil C claimed for different management practices or predicted by modelling, at least in the short term.