Simulating root responses to grazing of a Mongolian grassland ecosystem

A new Sim-CYCLE grazing model has been obtained by combining a grazing model (Seligman et al. 1992, Ecol. Model. 60:45-61) with the Sim-CYCLE model (Ito and Oikawa 2002, Ecol. Model. 151:143-176). The new model has been validated against a set of field data obtained at Kherlen Bayaan-Ulaan (KBU) grassland. On the basis of the model, the root responses to grazing of KBU grassland have been studied under different conditions of stocking rates and precipitation. Model results indicate that both belowground biomass (BB) and below-ground net primary production (BNPP) generally decrease with increasing stocking rate. However, if stocking rate is not higher than 0.7 sheep ha(-1), a sustainable state of the grassland ecosystem can be achieved after about 100 years, which suggests that the maximum sustainable stocking rate at KBU should be 0.7 sheep ha-1. At the sustainable state, the maximum BB in a year is about 11 Mg DM ha(-1) under non-grazing condition, 5 Mg DM ha(-1) under 0.4 sheep ha(-1) stocking rate, and 4 Mg DM ha(-1) under 0.7 sheep ha(-1) stocking rate; the BNPP is 1.3 Mg DM ha(-1) year(-1) under non-grazing condition, and 0.6 Mg DM ha(-1) year(-1) under 0.4 sheep ha(-1) stocking rate, and 0.4 Mg DM ha(-1) year' under 0.7 sheep hat stocking rate. Ratio of non-assimilation organ to assimilation organ (C/F) increases with increasing stocking rate. The C/F ratio is 10.99 under non-grazing conditions, and 12.11 under 0.7 sheep hat stocking rate. Root turnover rate decreases with increasing stocking rate. The rate is 12% each year under non-grazing conditions, and 11% each year under 0.7 sheep ha(-1) stocking rate. In addition, the effect of grazing on the grassland ecosystem under different scenarios of precipitation is also analyzed. Both BB and BNPP increase with increased precipitation, and vice versa. When precipitation is set to be 10% higher than the averaged from 1993 to 2002, the maximum sustainable stocking rate is 0.8 sheep ha(-1), and when the precipitation is set to be 15% lower than the averaged, the maximum sustainable stocking rate is 0.6 sheep ha(-1).