Grazing-induced changes in plant composition affect litter quality and nutrient cycling in flooding Pampa grasslands.

Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G-) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G- grasses. While no consistent differences between G+ and G- grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species' "responses" to grazing may only partially overlap with functional groups based on species "effects" on C and nutrient cycling.

Effects of grazing pattern and nitrogen availability on primary productivity.

A major part of the impact of grazing on primary productivity results from the joint action of tissue removal and nutrient return to the soil via dung and urine. Grazing, however, is not uniformly distributed in space: grazed grasslands show a matrix of grazed and ungrazed patches, which in turn, may or may not be affected by faecal or urine deposition. This paper investigates the effects of grazing spatial pattern and nitrogen availability on primary productivity. We propose that grazed plants located at the edge of a grazed patch are more shaded by their taller ungrazed neighbours than plants at the center. Since the border effect is less important as patch size increases, the effects of grazing will be more positive, or less negative, when grazing pattern is coarse-grained than when it is fine-grained. We also propose that nitrogen availability will affect this response to grazing through its effects on the intensity of competition for light and on the amount of compensatory growth. We performed a field experiment in a grassland community of the Flooding Pampa, Argentina, in which we compared the productivity of undefoliated controls and defoliated patches of different size, with and without nitrogen application. Defoliation reduced primary productivity and this effect was greater in the smallest, fertilized patches. Productivity was highest at patches of intermediate and large sizes. Nitrogen addition increased productivity by two-fold. The integrated photon flux density reaching the base of the canopy was affected by defoliation and by patch size: it was lower in controls than in defoliated patches and increased with patch size. Our results showed that (a) the size of the defoliated patch modified the response of this grassland to defoliation, (b) this response was correlated with light availability, and