Grazing exclusion reduces soil N2O emissions by regulating nirK- and nosZ-type denitrifiers in alpine meadows

Purpose Knowledge of soil N cycling and the associated functional microbial groups of N2O production under different management measures could provide clues for the restoration of degraded meadows in alpine ecosystems. Materials and methods We investigated soil N2O emissions, the genes related to N2O production and reduction (AOA-amoA, AOB-amoA, nirK, nirS, and nosZ), and associated microbial communities in four meadows (continuous grazing, grazing exclusion by fencing, grazing exclusion by combined fencing and reseeding, and undisturbed meadow) in the Tibetan Plateau to reveal the mechanism underlying potential N2O emissions in alpine meadows. Results and discussion Compared to the grazing meadow, fencing and fencing + reseeding meadows had lower N2O emissions and lower abundances of AOA-amoA, AOB-amoA, nirK, nirS, and nosZ genes, suggesting that grazing exclusion could decrease the soil N-turnover potential. However, the higher N2O emissions compared to those of undisturbed meadows indicated that longer restoration periods were necessary. Seeding the fenced meadow did not alter soil N2O emission or the abundance of AOA-amoA, AOB-amoA, nirK, nirS, and nosZ genes, possibly owing to the similar soil nutrient status compared to that of the fencing meadow. Grazing exclusion also resulted in significant changes in the community diversity and composition of microbes harboring these functional genes, especially nirK and nosZ communities. N2O emissions were significantly associated with microbial communities involved in N2O production and reduction but not with the gene abundance of AOA-amoA, AOB-amoA, nirK, nirS, and nosZ. Soil dissolved organic nutrients, including C and N, and soil moisture were the controlling factors for N2O production by altering the community composition of nirK- and nosZ-type denitrifiers, such as Bradyrhizobiaceae, Rhizobiaceae, Brucellaceae, Ochrobactrum, and Proteobacteria. Conclusions Our results indicated that grazing-induced elevation of potential N2O emissions from meadow soil could be alleviated by grazing exclusion, including sole fencing and a combination of fencing and reseeding, by changing soil dissolved organic nutrients and moisture thus regulating the microbial communities.

Automated summary

Our study investigated the effects of different management measures on soil N2O emissions and associated microbial communities in alpine meadows of the Tibetan Plateau. Grazing exclusion by fencing and reseeding decreased N2O emissions and gene abundances related to N2O production, but longer restoration periods were needed. Soil dissolved organic nutrients and moisture were key factors controlling N2O production by altering the community composition of denitrifiers.