Spatial and temporal variations of grassland soil organic carbon and total nitrogen following grazing exclusion in semiarid Loess Plateau, Northwest China

Grazing exclusion is an efficient management practice for preventing grassland degradation in semiarid Loess Plateau. However, little information was known about the spatial and temporal variations of soil organic carbon (SOC) and total nitrogen (TN) following grazing exclusion. In this study, grasslands with grazing exclusion for 11 years at 29 different locations were investigated. The results showed that SOC and TN concentrations decreased with increasing of soil depth. SOC and TN concentrations in 0-20 cm layer were 1.4-2.0 times of that in the other layers. The coefficients of variation of SOC and TN concentrations were about 20%, indicating that there was a moderate variation among different locations. The mean SOC and TN densities were 63.36 and 6.62 t ha(-1) respectively which were significantly affected by altitude and the mean annual precipitation (MAP) gradients (P < 0.001). Grasslands with altitude of 500-2000 m or MAP of 450-570 mm were optimal for SOC and TN sequestrations. Otherwise, the effects of slope position and mean annual temperature on SOC and TN densities were insignificant (P > 0.05). To explore temporal variations of SOC and TN, grasslands with different grazing exclusion ages (5, 10, 15, 20, 25, and 30 years) were investigated. SOC and TN concentrations increased by 89-385% and 86-330% respectively along the chronosequence within 30 years. Based on the values of SOC and TN concentrations, the grasslands were divided into two groups, grazing grassland and grazing exclusion grasslands. Within the group of grazing exclusion grasslands, grazing exclusion for 25 and 30 years facilitated the grasslands to accumulate higher SOC and TN than that for 5, 15, and 20 years. To conclude, SOC and TN accumulations of grazing exclusion grassland were depended on attitude, MAP, and grazing exclusion years.