The increasing concentration of atmospheric carbon dioxide (CO2) 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 15N tracing study conducted with a sheep-grazed pasture soil that had been under 10 years of CO2 enrichment. Our study aimed to investigate changes in process-specific gross N transformations in a soil previously exposed to an elevated atmospheric CO2 (eCO2) 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 eCO2, which was driven by the mineralization of recalcitrant organic N. The retention of N in the grassland was enhanced by increased dissimilatory NO3− reduction to NH4+ (DNRA) and decreased NH4+ oxidation. Our results indicate that heterotrophic processes become more important under eCO2. We conclude that higher MIT of recalcitrant organic N and enhanced N retention are mechanisms that may alleviate PNL in grazed temperate grassland.
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