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.