Relative importance of soil microbes and litter quality on decomposition and nitrogen cycling in grasslands

Microbial decomposers absorb nitrogen (N) to sustain growth during litter decomposition releasing surplus N into the soil. The balance between N release (mineralization) and absorption (immobilization) can alter nutrient cycling based mostly on the functional diversity in microbial communities. We examined the effects of soil microbes and litter quality on N cycling in an artificial grassland and two seminatural grasslands through a microcosm study. The artificial grassland soil had a higher respiration rate and a lower N mineralization rate, irrespective of the litter identity during incubation, when compared with the seminatural grassland soil. The release of mineralized nitrogen from litter was small for the artificial grassland because the soil microbes absorbed most of the mineralized N during decomposition. The artificial grassland soil had a low fungi:bacteria ratio (i.e., a high abundance of bacteria) and showed a clear separation of bacterial and fungal communities from the seminatural grassland soil. Our results suggested that stoichiometric constraint, not the biomass turnover rate, of microbial decomposers plays a key role in N cycling between aboveground and belowground communities in grasslands.