Nitrogen enrichment amplifies the effects of litter treatments on soil microorganisms and ecosystem functions in grazed and non-grazed grasslands

Abstract

Litter removal or addition and nitrogen (N) enrichment are two important methods for restoring grasslands. Despite the litter alteration and N enrichment effect on soil microorganisms in different ecological systems of independent have got the good evaluation, it is still unknown how N enrichment modifies the effects of litter removal and addition on soil microorganisms and ecosystem function in different grasslands. Here, we conducted two independent experiments to assess the effects of litter addition/removal and N enrichment on soil microorganisms and ecosystem functions in a typical grassland. In a litter-removal experiment, we evaluated how N addition regulated the effect of litter removal in a long-term non-grazed grassland. In a litter-addition experiment, we evaluated how N addition regulated the effect of litter addition in a long-term over-grazed grassland. We found that the response of plant communities to removal/addition of litter and N enrichment was relatively minor in both non-grazed and grazed grasslands; the high precipitation during the sampling year may mask the effects of the litter and N treatments on plant community. Litter addition in the grazed grassland increased fungal and AMF biomass and decreased bacteria: fungi ratio, but litter removal in the non-grazed grassland had only minor effects on soil microbial variables. Litter removal in the non-grazed grassland and litter addition in the grazed grassland had different effects on soil carbon (Cmin) and N mineralization (Nmin), but N enrichment consistently reduced soil Cmin and Nmin in non-grazed and grazed grasslands. In addition, N enrichment amplified the effects of litter treatments on soil microorganisms and mineralization. These findings indicate that removal of litter from non-grazed grasslands and addition of this litter to grazed grasslands may help to restore grasslands, and the amount of N enrichment should be adjusted to optimize the positive effect of nitrogen enrichment and weaken the negative effect of nitrogen enrichment. Overall, our experimental results revealed that the effects of litter treatment on soil microorganisms and mineralization in grazed and non-grazed semi-arid grasslands are amplified by N enrichment.