The role of rhizosphere in enhancing N availability in a mature temperate forest under elevated CO₂

Enhanced growth of trees under elevated atmospheric CO₂ concentration (‘CO₂ fertilisation’) can potentially reduce a fraction of anthropogenic CO₂ emissions but is anticipated to become progressively constrained by nitrogen (N) limitation in temperate ecosystems. However, it is believed that this constraint may be mitigated if trees under elevated CO₂ (eCO₂) prime microbial activity in their rhizosphere to release available N. We assessed whether mature trees under eCO₂ regulate N availability in their rhizosphere to meet increased N demand. We hypothesized that eCO₂ primes N mineralization in the rhizosphere while reducing N losses through nitrification and denitrification. This study was conducted in a mature English-Oak-dominated temperate forest in central England, in the sixth year of Free Air CO₂ Enrichment (FACE). In the summer of 2022, we measured N transformations, enzyme activities, and nutrient pools in the rhizosphere and bulk soil of the organic layer (0–7 cm) under laboratory conditions. While the rhizosphere was found to be inherently more active (i.e. positive N priming) than the bulk soil, the effect of eCO₂ were not consistently stronger in the rhizosphere. Available soil N, dissolved organic carbon and microbial biomass were enhanced under eCO₂ in bulk and rhizosphere soils. Net N mineralization was enhanced under eCO₂ in the bulk and rhizosphere soils while leucine aminopeptidase activity, associated with organic N depolymerization, was enhanced solely in the rhizosphere. Despite higher C and N availability creating potential hot spots, nitrification was reduced under eCO₂ and denitrification remained unaffected in the rhizosphere, demonstrating a more efficient conservation of N under eCO₂. Our findings demonstrate that eCO₂ stimulates N-mining and reduce N losses in the rhizosphere. Furthermore, the tenfold difference in N turnover rates between rhizosphere and bulk soils suggests that expanding rhizosphere mass from increased root biomass may help trees under eCO₂ to meet higher N demand.