Plant carbon allocation and nitrogen cycling in a perennial grassland: The role of herbivory

I investigated patterns of nitrogen cycling in an herbivore affected landscape at Wind Cave National Park, South Dakota, focusing on plant and microbial responses to herbivory. Following prairie dog (Cynomys ludovicianus) colonization, plant communities change in both species composition and structure. Long-term colonization creates a greatly altered core area which grades into a less altered state towards the edge of the colony. Net nitrogen mineralization was highest in the more greatly altered area of colonies and lowest in adjacent uncolonized grasslands. Areas grazed repeatedly over a period of years had substantially reduced root biomass. Decreased root carbon in grazed areas apparently limited carbon availability to decomposers causing decreased nitrogen immobilization and increased net nitrogen mineralization. Such increases in plant available nitrogen may partially explain frequently reported grazing-induced increases in plant tissue nitrogen concentrations.; Simulation analysis was used to further evaluate feedbacks between plant carbon allocation and production patterns and nitrogen cycling with herbivory. In addition, physiological attributes of different grass ecotypes were linked to nutrient cycling and evaluated. One ecotype is found on intensively grazed prairie dog colonies and the other in uncolonized grasslands. Data analyses showed that plant production responses to grazing differed for the two ecotypes. The on-colony ecotype showed compensatory growth for aboveground biomass at low defoliation intensities. Aboveground production was lower for defoliated grasses than undefoliated grasses at all defoliation intensities. Carbon allocation to root production declined with grazing intensity for the on-colony ecotype. Equations describing ecotypic responses to defoliation were incorporated into CENTURY, a nutrient cycling simulation model. Model simulations showed increases in plant production that paralleled increases in net nitrogen mineralization for both ecotypes. Positive plant production was maintained at higher grazing intensities for the on-colony than the off-colony ecotype. Decreased biomass allocation belowground influenced nitrogen cycling for the on-colony ecotype. Feedbacks between plant production and nitrogen cycling were more important determinants of plant production than fertilization effects of urine and feces deposition.; These studies demonstrate that carbon allocation to roots and plant production responses to herbivory are key links determining nutrient cycling responses to herbivory.