Hobbs et al. hypothesized that grazing by large herbivores results in conservation of nitrogen that would otherwise be lost from burned grasslands. They tested this hypothesis in a series of experiments on burned and unburned tallgrass prairie grazed by cattle. Nitrogen loss was directly proportional to the aboveground standing crop prior to burning. Loss in the ungrazed treatment (1.8 g/m2/year) equaled about 1/2 of the nitrogen in pre-burn aboveground standing crop and was double that of the grazed treatment (0.9 g/m2/year). The authors estimate nitrogen loss through grazing and excretion and conclude that grazing conserves 3 to 5x more nitrogen than is lost. Fire temperatures and energy release were reduced by grazing. Mowing did not influence patch utilization or residual biomass when pastures were burned. Thus, the effects of fire on grassland N budgets were modified by grazing, and the effects of grazing on the patch structure of grasslands were modified by fire. The influence of fire on ecosystem N budgets was altered by grazing to the extent that grazing may control whether burned tallgrass prairie gains or losses N.
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