Forage taste agents modifying yak grazing decrease soil microbial diversity in alpine meadow

Taste is a crucial sense of livestock for their selective feeding. As feed additives, taste agents such as salty and sweet agents are often added to livestock supplementary feed to increase feed intake, and bitter agents are commonly used as repellents to protect vegetation from browsing animals. Selective feeding of grazing livestock affects community structure and ecosystem function in natural grasslands. Regulating livestock feed intake by applying taste agents to pasture is a novel grazing management tool. However, the effects of these taste agents on the soil microbial community following selective feeding have so far not been fully evaluated. In this study, three different taste agents, Denatonium benzoate (bitter agent), NaCl (salty agent), and Sodium cyclamate (sweet agent), were sprayed onto alpine meadow pasture on the Qinghai-Tibet Plateau. Resulting evidence indicates that spray-application of taste agents to alpine pasture over three consecutive growing seasons, indirectly modifies soil bacterial diversity by stimulating appetite, and encouraging more selective feeding of pasture species. Compared with control, treatment with the salty agent increased both yak dry matter intake, and soil bulk density, but decreased plant diversity. Yet, increased forage uptake was accompanied by alterations to soil nutrient availability and bacterial community structure, including lower soil available nitrogen and bacterial a-diversity. Soil bulk density, NO3--N, soil moisture, aboveground biomass, and quantity of yak dung, were the main forces driving changes to soil bacterial diversity. Moreover, the structural equation model showed that applying taste agents to pasture affects soil bacterial diversity directly, and indirectly. This study further highlights that pastoral application of taste agents has impacts on the soil bacterial community, supporting the necessity to include bacterial components in evaluation of the consequences of using taste agents as a grazing management tool.