Ten years of elevated atmospheric carbon dioxide alters soil nitrogen transformations in a sheep‐grazed pasture

Abstract

Abstract The increasing concentration of atmospheric carbon dioxide (CO 2 ) is expected to lead to enhanced competition between plants and microorganisms for the available nitrogen (N) in soil. Here, we present novel results from a 15 N tracing study conducted with a sheep‐grazed pasture soil that had been under 10 years of CO 2 enrichment. Our study aimed to investigate changes in process‐specific gross N transformations in a soil previously exposed to an elevated atmospheric CO 2 (eCO 2 ) concentration and to examine indicators for the occurrence of progressive nitrogen limitation (PNL). Our results show that the mineralization–immobilization turnover (MIT) was enhanced under eCO 2 , which was driven by the mineralization of recalcitrant organic N. The retention of N in the grassland was enhanced by increased dissimilatory NO 3 − reduction to NH 4 + (DNRA) and decreased NH 4 + oxidation. Our results indicate that heterotrophic processes become more important under eCO 2 . We conclude that higher MIT of recalcitrant organic N and enhanced N retention are mechanisms that may alleviate PNL in grazed temperate grassland.