Stocking Rate and Grazing Season Modify Soil Respiration on the Loess Plateau, China

The influence of sheep grazing on carbon cycling in the midarid steppe of the Loess Plateau, Gansu, China, was investigated by measuring the CO2 exchange rate in pastures with a 7-year history of zero, light, moderate, and heavy grazing. Farming systems in the area are characterized by heavy grazing pressure and low vegetation productivity. The effect of stocking rate on soil respiration (Rs) was determined using field trials to investigate factors influencing spatial and temporal variation in Rs in August 2008 on two sites: summer grazed and winter grazed. Soil respiration is an important component of the carbon cycle in rangeland. Measurements included daily Rs, soil temperature, soil moisture, and root biomass. Daily Rs was also measured in late April and middle December 2008. The response of Rs to increasing stocking rate in August 2008 differed with site; on the summer-grazed site increased stocking rates reduced Rs (P < 0.02), whereas on the winter-grazed site, stocking rate had little effect on Rs. Path analysis revealed that on the summer-grazed site, soil moisture had the greatest influence on Rs, whereas on the winter site soil temperature was the most important factor; stocking rate had the least influence on Rs at both sites. The results highlight the importance of environmental variability in determining the effects of grazing on Rs.