Why does overgrazing promote nitrifiers and denitrifiers abundance and the resultant soil N2O emissions?

Abstract

AbstractOvergrazing by livestock is a global environmental problem, influencing global warming via soil N2O emissions. However, it remains unknown why overgrazing leads to increased N2O emissions. Here, we used a paired design of subalpine meadows in undisturbed and overgrazed sites at four different elevations (from 3000 to 3600 m) located over an area of ~200 km2 in the Qinghai‐Tibetan Plateau (QTP), China, to evaluate relationships among plant diversity, soil ammonium and nitrate N, the abundance of soil nitrifiers (AOA and AOB) and denitrifiers (nirK, nirS and the N2O reductase gene [nosZ]) and soil N2O emissions. Using a generalized linear mixed effects modeling framework with Poisson error, we found that the influence of overgrazing on increased abundance of soil nitrifiers and denitrifiers and associated increased soil N2O emission was a general phenomenon in QTP. More importantly, by using forward selection analysis and structural equation models, we showed overgrazing decreased plant richness, and this resulted in decreased ammonium N, but increased nitrate N at all elevations. Accordingly, decreased ammonium N, but increased nitrate N led to increased abundance of soil nitrifiers (AOA and AOB) and denitrifiers (nirK and nirS), but decreased nosZ abundance, which finally gave rise to increased soil N2O emission at all the elevations. Our results highlight the key role of plant diversity in regulating soil N2O emissions from soils. Thus, performing active ecological restoration to recover native plant species in overgrazed sites may help mitigate the influence of overgrazing on global warming.