The effects of grazing on grassland soils

The effects of grazing on physical properties (bulk density, water infiltration), chemical properties (soil organic matter, nitrogen) and soil microbes of grassland soils were reviewed based on published literature. The effects of grazing on soil properties were inconsistent, because of the complexity of the soil system, time lag after disturbance and resilience of soil system to perturbation. In general, because of the impact of animal trampling, there are changes in soil pore size distribution as well as total porosity with increasing stocking rate. The decrease in macropores (50μm) and larger mesopores (9～50μm) could lead to higher bulk density, greater penetration resistance and a decreased soil water holding capacity. However, bulk density may decrease in sandy soil with lower organic matter content, because overgrazing causes soil organic matter to decrease, influences the stability of soil aggregates and results in collapsed soil structure. There exists a complex interaction between grazing and soil organic matter. Many factors determine the response of soil organic matter to livestock grazing. These factors include the initial conditions of vegetation and soil, environmental factors, especially moisture and temperature, and grazing history (intensity, frequency, duration, and type of animal). Soils with inherently low soil organic matter are more prone to change in response to grazing than soils high in organic matter. Microbial biomass is the most labile C and N pool in soil and can be used as a rapid and sensitive indicator of change in the soil management system. Grazing at high stocking rates may have opposing effects on soil fertility depending on the time-scale considered. In the short term, benefits may occur because of increased nutrient cycling efficiency. But, in the long term, overgrazing without management may cause a decline in soil fertility due to imbalanced nutrient input and output, which finally leads to the degradation of grassland, especially in the more fragile arid and semi-arid regions.