Short and long-term effects of elevated CO sub(2) on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO sub(2) increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO sub(2) on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO sub(2) on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO sub(2) and using soil from the New Zealand FACE site that had >4 years exposure to CO sub(2) enrichment. Using super(14)CO sub(2) pulse labelling, the effects of elevated CO sub(2) on C allocation within the plant-soil system were studied. Under elevated CO sub(2) more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO sub(2), N availability to plants stayed unchanged. Soil previously exposed to elevated CO sub(2) exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO sub(2) was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.