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

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.