Light-Intensity Grazing Increases Soil Bacterial-Based Energy Channels and Phosphorus Availability Under Climate Warming in an Alpine Meadow of the Tibetan Plateau

Grazing exclusion has been used extensively in alpine meadows of the Tibetan Plateau. However, in recent years, studies have shown a decreasing trend in soil nutrients because of grazing exclusion and climate change. The effects of grazing exclusion on the soil biogeochemical process of phosphorus cycling in alpine meadows are unclear, especially under climate warming. Here we conducted a 5-year grazing exclusion and warming manipulation experiment to examine the effects of grazing exclusion on soil phosphorus fractions, microbial and nematode communities, and enzyme activities in light-intensity grazing, grazing exclusion, and combined grazing exclusion and warming treatments. Our results showed that grazing exclusion significantly decreased bacterivore and omnivore-predator densities, phoD gene abundance and alkaline phosphomonoesterase activity but had few effects on acid phosphomonoesterase activity. Structural equation modeling indicated that bacterivores had a positive effect on phoD gene abundance, alkaline phosphomonoesterase activity and inorganic phosphorus fractions. Combined grazing exclusion and warming significantly decreased bacterivore and omnivore-predator densities but significantly increased fungivore density. Structural equation modeling indicated that the combined grazing exclusion and warming treatment increased the soil fungal-based energy channel, but the higher abundance of fungi and fungivores did not have significant effects on acid phosphomonoesterase activities and inorganic phosphorus fractions. Omnivore-predators play an important role in maintaining acid phosphomonoesterase activities. Alternatively, the combined grazing exclusion and warming treatment significantly increased the concentrations of amorphous Al and free Al, which had a positive correlation with the maximum phosphorus adsorption. Thus, the combined grazing exclusion and warming treatment significantly decreased soil phosphorus availability. Our results suggest that light-intensity grazing can increase soil nematode food web stability and phosphorus availability, which might allow grasslands to better withstand climate warming through increases in soil phosphorus storage and phosphorus cycling in alpine meadows.