Soil recalcitrant but not labile organic nitrogen mineralization contributes to microbial nitrogen immobilization and plant nitrogen uptake

Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral-associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (M-Nrec) and labile (M-Nlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting M-Nrec (299 observations) and M-Nlab (299 observations) from 57 N-15 tracing studies, we found that soil pH and total N were the master factors controlling M-Nrec and M-Nlab, respectively. This was consistent with the significantly higher rates of M-Nrec in alkaline soils and of M-Nlab in natural ecosystems. Interestingly, our analysis revealed that M-Nrec directly stimulated microbial N immobilization and plant N uptake, while M-Nlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that M-Nrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, M-Nlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing M-Nrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing M-Nlab may stimulate the potential risk of soil N loss.