Trampling and Dung and Urine Addition of Livestock Increase the Soil Organic Carbon in Mountain Meadows by Augmenting the Organic Carbon in Different Aggregates

Grassland soil carbon stocks contain substantial amounts of organic carbon and play a crucial role in the global carbon cycle. Grazing is one of the most primary land use types in grasslands. However, few studies have focused on the impact of three grazing behaviors (mowing (M), trampling (T), and dung and urine addition (D)) on the soil organic carbon (SOC) of mountain meadows. In this experiment, we simulated three grazing behaviors to explore the impacts of grazing behaviors on plant characteristics with plant growth, soil physicochemical properties, soil aggregate, and analyzed the main factors influencing the changes in SOC. After six years of treatment, the experimental results showed that M significantly decreased plant height, density, and aboveground biomass and significantly decreased soil organic carbon (SOC) (no M vs. M, −3.64%). T significantly increased soil bulk density, the proportion of macroaggregates, the organic carbon of microaggregates, and silt and clay aggregates and significantly increasing SOC (no T vs. T, +3.17%). D significantly increased plant density, soil total nitrogen and the organic carbon of macroaggregates, significantly increasing SOC (no D vs. D, +9.74%). Correlation and principal component analyses indicated that SOC was significantly negatively correlated with soil bulk density and plant coverage and significantly positively correlated with soil total nitrogen, soil C/N, microaggregate proportion, and the organic carbon of macroaggregates. Redundancy analysis indicated that the proportion of microaggregates and the organic carbon of macroaggregates were the main factors influencing SOC. The following conclusions were drawn: SOC responds differently to three types of grazing behaviors, D primarily increases the organic carbon in macroaggregates, while T mainly enhances the organic carbon in microaggregates and silt and clay aggregates, thereby affecting the SOC in mountain meadows.