Yak and Tibetan sheep dung increase the proportional biomass of grasses and alleviate soil nitrogen limitation in degraded Tibetan alpine grassland

The dung of Tibetan livestock serves as a natural fertilizer that plays a crucial role in maintaining plant productivity and restoration of degraded alpine grasslands. However, research on how livestock dung affects vegetation composition and soil quality in Tibetan alpine grassland on the time scale remains uncertain. We established three types of nutrients (yak dung, Tibetan sheep dung and urea) and control on a Tibetan alpine grassland in 2017, and measured the vegetation (species composition, aboveground biomass, composition of functional group biomass, and species diversity) and soil characteristics (soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP)), and calculated the SOC/TN, SOC/TP and TN/TP ratios) at the end of August 2017, 2018, and 2019. The study aimed to compare the effect organic fertilizers (two Tibetan livestock dungs) and inorganic nitrogen fertilizer (urea) on vegetation characteristics and soil chemical properties, and to assess whether the livestock dung can improve soil nutrient availability level in degraded Tibetan alpine grassland. Our results showed that: (1) Tibetan sheep dung delayed the increase of aboveground biomass, and urea increased the aboveground biomass in 2017 and 2018; (2) yak and Tibetan sheep dungs and urea increased proportional biomass of aboveground grasses and plant species richness index in the three years; and (3) livestock dungs increased SOC, TN, the ratios of SOC/TP and TN/TP in 2018. Our study implies that yak and Tibetan sheep dung stimulated plant growth and altered soil nutrients, while alleviating soil nitrogen limitation in degraded alpine grasslands. The return of livestock dung to soil may be a sustainable alternative to chemical fertilizers in near-natural recovery of degraded Tibetan alpine grasslands.