APPLICATION OF THE ROTHAMSTED CARBON TURNOVER MODEL TO SOILS IN DEGRADED SEMIARID LAND IN NEW-ZEALAND

The Rothamsted soil-carbon turnover model was used to determine rates of change of organic carbon in soils of a degraded semi-arid land syst em of New Zealand. Estimates of annual inputs, net primary production (NPP), and recovery time to raise carbon from a degraded state to a su stainable level of production were obtained. The annual inputs and hen ce the NPP were estimated by running the Rothamsted soil-carbon turnov er model in reverse. Inputs included the organic C content of the soil , clay content, monthly temperature and rainfall and the rate of decay of annual inputs. An estimate of the biologically inert organic matte r fraction was obtained from the radiocarbon content of soil organic m atter. The soil recovery time was estimated by determining NPP for a r elatively undegraded soil and and applying carbon at this rate to a de graded soil. The predicted total annual plant residue inputs were 1.81 tha(-1) y(-1) and the predicted inert organic matter content from rad iocarbon data is 5.12 tha(-1). The estimated time needed to raise carb on from a degraded state to the level of an undegraded site is 48 year s. This is most likely an underestimate, because in initial years the vegetation will not be capable of adding the same annual carbon input as the undegraded site. The refinement of this estimate of recovery ti me will require data on the rate of increase in net primary productivi ty with revegetation.