Effects of grazing pattern and nitrogen availability on primary productivity

Abstract

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

Historical and recent land use affects ecosystem functions in subtropical grasslands in Brazil

“…Aboveground primary productivity, measured by peak standing biomass, was highest in low‐intensity permanent grasslands but did not differ between the other grassland types. This finding, at first sight, contrasts with numerous previous findings that have shown significant positive effects of grazing and burning on grassland primary productivity (Semmartin and Oesterheld , , Oesterheld et al. ), especially compared to abandoned grasslands where accumulated biomass limits light availability for emerging plants (Knapp ).…”

Grazing history effects on above- and below-ground litter decomposition and nutrient cycling in two co-occurring grasses

“…Because the potential influence of canopy microclimate on soil surface processes was removed, we discard differences in incident radiation, soil temperature and moisture as driving factors (Shariff et al 1994;Bardgett et al 1998). If, however, grazed vs. ungrazed sites with intact vegetation had been compared, we suspect that observed differences in litter decomposition might have been even stronger, as decreased light penetration and soil temperatures below the ungrazed canopy (Semmartin and Oesterheld 2001) would have further reduced decomposer activity inside the exclosure. Alternatively, it is plausible that long-term cattle grazing had modified the soil environment for decomposition.…”

Grassland responses to grazing: effects of grazing intensity and management system in an Inner Mongolian steppe ecosystem

“…De Mazancourt et al 1998;Patton et al 2007;Turner et al 1993), negative (e.g. Leriche et al 2003;Semmartin andOesterheld 2001), or no (e.g. Biondini et al 1998;Turner et al 1993) effects on net primary production.…”