Effect of stocking rate on Stipa breviflora desert steppe soil

Grassland grazing ecosystem is one of the most efficient consumption systems of the terrestrial ecosystem. There are few repeated studies on long-term effects of stocking rate on stability of grazing ecosystems. With Inner Mongolia Wulanchabu Stipa Breviflora desert steppe as a case study, nine years stationary experiment was conducted to study the effect of grazing on soil physical and chemical properties. The study tested the processes and mechanisms of soil degradation at different stocking rates with the aim of beneficial adoption of reasonable grazing management measures and promoting sustainable development of grassland animal husbandry. A randomized complete block design was set up with four stocking rate treatments and three replicates. The stocking rate treatments included the control non-grazed enclosure (CK), light stocking rate (LG), moderate stocking rate (MG) and heavy stocking rate (HG). Adult Mongolia sheep were used in continuous grazing system from June to November of each year in 2004–2012. The stocking rates were 0 sheeps·hm2 (CK), 0.91 sheeps·hm2 (LG), 1.82 sheeps·hm2 (MG) and 2.71 sheeps·hm2 (HG). Soil physical and chemical properties were measured in 2004–2012. The results showed that soil water content in the topsoil decreased significantly with increasing stocking rate after 9 years of continuous grazing. Soil bulk density decreased by 17.25% with extended grazing time (P < 0.05), but not significantly changed with stocking rate. The stocking rate had no significant influence on soil mechanical composition, pH, and contents of available phosphorus and potassium. However, soil organic matter significantly decreased (P < 0.05) in the 010 cm soil depth under HG treatment compared with other treatments. With increasing stoking rate, the content of sand in the soil increased and that of clay decreased. Also HG treatment significantly decreased soil respiration rate in 2012. Based on statistical results of stocking rate, stocking year and their interactions, soil physical and chemical conditions of S. breviflora desert steppe did not significantly change over the nine years of grazing. However, organic matter apparently showed a certain degree of degradation. Because of resilience, hysteresis and complexity of natural soil systems, it was recommended that studies on the effects of stocking rates on soil bulk density should be done for longer time periods.