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

Abstract

It is still unclear whether elevated CO 2 increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO 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 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 2 and using soil from the New Zealand FACE site that had >4 years exposure to CO 2 enrichment. Using 14CO 2 pulse labelling, the effects of elevated CO 2 on C allocation within the plant-soil system were studied. Under elevated CO 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 2, N availability to plants stayed unchanged. Soil previously exposed to elevated CO 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 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.