Carbon and nitrogen pools in a tallgrass prairie soil under elevated carbon dioxide

Soil is a potential C sink and could offset rising atmospheric CO2. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Our objective was to measure pools of soil C and N to assess the potential for C accrual and changes to N stocks as influenced by elevated atmospheric CO2. Treatments (three replications, randomized complete block design) were ambient CO2-no chamber (NC), ambient CO2-chamber (AC), and two times ambient CO2-chamber (EC). Long-term (290 d) incubations (35degreesC) were conducted to assess changes in the slow soil fractions of potentially mineralizable C (PMC) and potentially mineralizable N (PMN). Potentially mineralizable C was enhanced (P < 0.1) by 19 and 24% in EC relative to AC and NC soil at the 0- to 5- and 5- to 15-cm depths, respectively. Potentially mineralizable N was significantly greater by 14% at the 0- to 5-cm depth in EC relative to AC, but decreased by 12% in EC relative to NC (P < 0.1). Measurements of PMC indicate that increases in total soil C under elevated CO2 in a previous study were a consequence of accrual into the slow pool. Relatively large amounts of new C deposited as a result of elevated CO2 (C-new) remained in the soil after the 290-d incubation. In contrast to accumulation of C into the slow fraction, C-new was integrated into a passive fraction of soil organic matter (SOM). Accumulation of N was also detected in the whole soil, which cannot be explained by current estimates of ecosystem N flux.