Pihlblad et al_df

Elevated atmospheric CO₂ (eCO₂) affects soil carbon (C) dynamics by altering the balance between soil organic matter decomposition (SOM) and plant-derived C accumulation. Even minor changes in this balance can significantly impact future climate. Soil nutrient availability to decomposers, particularly nitrogen (N) and phosphorus (P), is crucial for both rates of SOM decomposition and plant-derived C accumulation, yet these processes are rarely assessed simultaneously. To investigate the interaction between eCO₂ and nutrient availability on these soil C cycling processes we cultivated a tree (<i>Eucalyptus</i> <i>grandis</i>) and a grass species (<i>Microlaena</i> <i>stipoides)</i> in a controlled ¹³C-depleted atmosphere for 22 weeks. The altered δ¹³C signature of CO₂ allowed separation of plant-C from SOM-C. N and P availability was manipulated by nutrient additions, while assessing multiple C pools, fluxes, and gross N processes. We found that plant-derived and SOM-derived C pools were decoupled in poor soils (a larger plant-C pool in soil does not affect the magnitude of the priming effect). The rhizosphere priming effect was suppressed when P limitation was alleviated, with a simultaneously increase of the N mineralization rate, leading us to conclude that microbes are mining for P in SOM and N in rhizodeposition products. The impact of CO₂ was highly dependent on nutrient availability. Findings highlight that in this low P soil the processing of SOM-C and plant-C were not coupled and were more dependent on P availability than N availability. Further exploring these controls will be crucial for understanding ecosystem C, N and P cycling in future climates.