Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2

摘要

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, C-a). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be-constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C-3/C-4 grassland was exposed to a gradient in C-a from 200 to 560 mu mol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with C-a for litter of the C-3 forb Solanum dimidiatum, whereas decomposition of litter from the C-4 grass Bothriochloa ischaemum was unresponsive to C-a. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to C-a, reaching a maximum at similar to 440 mu mol/mol C-a. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased C-a. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising C-a could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing C-a, there was no net soil C sequestration at elevated C-a, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.