Long-term effects of cattle grazing on nitrogen dynamics in a montane riparian ecosystem

Montane riparian ecosystems have high primary productivity, serve as buffers between terrestrial and aquatic systems, and can decrease nitrogen inputs to surface and groundwaters. As the ecological and economic values of riparian systems have been recognized, the understanding of multiple-use impacts (recreation, wildlife habitat, livestock grazing) has become increasingly important. Studies in riparian ecosystems have investigated the effects of large ungulates on aboveground plant dynamics. But, little consideration has been given to belowground soil dynamics. Our goal was to investigate the effects of moderate cattle grazing on soil microbial communities and functions in comparison with sites exclosed from grazing for 50 years in a montane riparian zone of north-central Colorado. To evaluate the long-term effects of cattle grazing, we characterized spatial and temporal variation in soil microbial biomass, soil respiration, potential net nitrogen mineralization, nitrification, and denitrification in both grazed and control treatments. Preliminary results suggest that nitrogen cycling in this montane riparian ecosystem has not been affected by low to moderate cattle grazing. Even though nitrogen processes did not exhibit significant differences, soil microbial biomass was lower in control streambank sites compared with grazed sites in 2005. Further analyses of the 2006 dataset should clarify these patterns. We sampled at streambank, transition, and upland-edge locations in 2006, thus, capturing more spatial heterogeneity of the riparian zone. If these analyses suggest no significant cattle grazing effects on belowground soil dynamics, then we can demonstrate that montane riparian zones on public lands can be managed beneficially for multiple uses.