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

Abstract

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 ¹⁵N 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.