SYNTHETIC CONSTRAINT OF SOIL C DYNAMICS USING 50 YEARS OF 14C AND NET PRIMARY PRODUCTION (NPP) IN A NEW ZEALAND GRASSLAND SITE

Time-series radiocarbon measurements have substantial ability to constrain the size and residence time of the soil C pools commonly represented in ecosystem models. C-14 remains unique in its ability to constrain the size and turnover rate of the large stabilized soil C pool with roughly decadal residence times. The Judgeford soil, near Wellington, New Zealand, provides a detailed 11-point C-14 time series enabling observation of the incorporation and loss of bomb C-14 in surface soil from 1959-2002. Calculations of the flow of C through the plant-soil system can be improved further by combining the known constraints of net primary productivity (NPP) and C-14-derived C turnover. We show the Biome-BGC model provides good estimates of NPP for the Judgeford site and estimates NPP from 1956-2010. Synthesis of NPP and C-14 data allows parameters associated with the rapid turnover "active" soil C pool to be estimated. This step is important because it demonstrates that NPP and C-14 can provide full data-based constraint of pool sizes and turnover rates for the 3 pools of soil C used in nearly all ecosystem and global C-cycle models.