The Impact of Yak Dung Deposition on Litter Decomposition and Multi-Nutrient Cycling in Grassland Ecosystems

Yak dung is an input to the carbon (C) and nutrient cycles that maintain ecosystem functions on the Tibetan Plateau. Yak dung is C and nutrient-rich excreta that is conducive to the growth and metabolic activities of bacterial communities, thus predicting that more bacterial than fungal processes are responsible for the degradation of yak dung. A three-year yak dung degradation experiment in a yak-grazing alpine rangeland was designed to investigate the changes in dung moisture content, chemical and enzymatic properties, and bacterial and fungal communities during degradation, as well as to explore how these parameters may regulate the degradation of yak dung. After three years of decomposition, yak dung had a 79 % reduction in mass, and most of the mass loss occurred within the first 2 years. Cellulosic polymers, especially cellulose and hemicellulose, determined the rate of yak dung degradation. The main changes in dung bacterial communities occurred during the first 2 years of degradation, largely related to changes in moisture and available substrates (e.g., dissolved organic C, dissolved organic nitrogen (N), ammonium, nitrate, and available phosphorus). In contrast, dung fungal communities did not change until 1.5–3 years of degradation, in response to the total substrates (e.g., total C and N). The relative abundances of Proteobacteria, Bacteroidota, Firmicutes, Basidiomycota, and Ascomycota, and the activities of endo-cellulases, exo-cellulases, β-1,4-glucosidase, and β-1,4-xylosidase, which were associated with cellulose and hemicellulose degradation, decreased during decomposition. The relative abundances of Actinobacteria, and activities of peroxidases and polyphenol oxidase were positively correlated with dung lignin content. Structural equation modeling suggested that degradation of lignocellulose in dung was mainly the consequence of bacterial community activities. Additionally, moisture was the most important abiotic factor influencing lignocellulose degradation, as it can directly affect dung substrate availability, and ultimately bacterial communities and associated enzyme activities. As the microbial degradation of lignocellulose in yak dung is strongly related to moisture, any change to the rainfall pattern in the future is expected to influence yak dung degradation in this alpine region.