Fire and grazing in the tallgrass prairie: Contingent effects on nitrogen budgets
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Authors: N. T. Hobbs, D. S. Schmiel, C. E. Owensby, and D. S. Ojima
Date: 1991
Journal: Ecology
Volume: 72
Number: 4
Pages: 1374-1382
Summary of Methods: 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.
Article Summary / Main Points: None
Vegetation Types:
MLRA Ecoregions:
Agrovoc Control Words: Riparian zones Rangelands Wildlife
Article Review Type: Refereed
Article Type: Experimental Research
Keywords: burning, cattle, fire, grazing, herbivory, nitrogen budgets, nutrient cycling, patch dynamics, spatial variation
Annotation: Two pastures were moderately grazed between May and October at a rate of 1.4. One of the two grazed pastures was burned in May 1985, April 1986, and April 1987. The other pasture was not burned. To study the effects of presence or absence of grazing, they constructed 2 grazing exclosures within each pasture to provide 2 replications of ungrazed treatments within the burned and unburned pastures. To study the effects of heavy defoliation within patches, they established pairs of mowed and unmowed patches within each pasture. One member of each pair was chosen and mowed to a height of 5-cm to simulate effects of locally heavy grazing.
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