The influence of grazing exclosure on soil C stocks and dynamics, and ecological indicators in upland arid and semi-arid rangelands

Soil carbon (C) storage and quality in rangeland ecosystems may change as a result of long-term heavy grazing, especially in upland arid and semi-arid rangelands characterized by low soil organic C contents. The impact of grazing exclosure on soil C stock and mineralization, microbial biomass C and the activities of invertase and alkaline phosphatase involved in C and P cycling as soil ecological indicators were studied in woody and non-woody rangeland ecosystems of two sites (viz., Sabzkouh with woody cover and Boroujen with non-woody cover) located in Zagros Mountains, Central Iran. At each site, soil samples were taken from 0 to 15 cm depth of the two rangelands including heavy grazing (HG) and ungrazed (UG) regimes, and analyzed for their chemical and microbial properties. Results demonstrate that soil organic C and total N contents, and C/N ratios were not affected by grazing regime at Boroujen, while ungrazed rangelands had higher C (17%) and N (19%) contents compared with heavily grazed rangelands at Sabzkouh. Grazing exclosure increased soil C mineralization (44.7-73.6%) and turnover (24-64%) at both study sites, and increased the amount of soil microbial biomass C (5.4-fold) and decreased the metabolic quotient (90%) only at Sabzkouh. Soil invertase activity was significantly higher (1.3-2.1 times) in ungrazed than heavily grazed rangelands at both sites, whereas ungrazed rangelands had significantly greater soil alkaline phosphatase activity (19%) compared with heavily grazed rangelands only at Sabzkouh. The variations in the effect of grazing exclosure on soil organic matter (SOM) dynamics and C sequestration in these upland arid and semi-arid rangelands were due to differences between rangelands in initial SOM contents, and probably could be due to differences in plant productivity and composition, and the amount of rainfall between the study sites. Non-woody rangelands contained less soil C and N contents, and C mineralization but greater microbial biomass C than woody rangelands, reflecting a clear difference in substrate quality and soil C availability. Soil biochemical and microbial indices as ecological indicators can reflect grazing exclosure effects on soil restoration only in woody rangelands.