Grazing intensity and micro-topographical effects on some nitrogen and carbon pools and fluxes in sheep-grazed hill country in New Zealand

A significant proportion of grazed land in New Zealand is in hill country and this area supports a large proportion of the national sheep and beef herd. Little is known about the effect of increased grazing intensity on nitrogen (N) and carbon (C) cycling and losses from these hill country grazing systems, nor of the effect of micro-topography on N and C pools and fluxes. A three year grazing trial was conducted to gather information on the effect of an increase in grazing intensity on soil chemistry and N and C leaching in three distinct micro-topographical classes. In this temperate environment, micro-topography was the dominant influence on annual net herbage accumulation, soil chemistry and N and C leaching. Net herbage accumulation was two- and five-fold greater on low (<13°) compared to medium (13–25°) and high (>25°) slopes (14.6, 7.8, and 2.9 t DM ha−1 yr−1, respectively). Low slope soils had generally higher concentrations of phosphorus and N and C species compared to soils on medium and high slopes. Nitrogen leaching was greatest in low compared to medium slope areas: three-year mean estimates of annual leaching were 80 vs. 7 kg inorganic N ha−1, and 40 vs. 8 kg dissolved organic N ha−1, respectively. Per ha estimates of inorganic N leaching in this landscape (where low, medium and high slopes occupy 10, 60 and 30% of the land area) are 15 kg N ha−1 if we make the assumption that high slopes leach as much inorganic N as medium slopes, or 12 kg N ha−1 if it is assumed that high slopes do not leach at all. The significant effect of slope on parameters measured is a reflection of micro-topographical influences on livestock grazing, resting and excreting patterns. Estimates of annual dissolved organic C leaching were not significantly different between low and medium slopes (185 vs. 132 kg C ha−1, respectively). Grazing intensity, at the two contrasting grazing pressures employed (annual mean standing biomass of 1900 vs. 1475 kg DM ha−1), had no significant impact on the variables measured in this three year study, demonstrating the robust nature of these grazing systems in the short-term. Our study provides new and quantitative information on net herbage accumulation, soil characteristics and nutrient leaching in grazed hill country. This new information will enable the development and refinement of N and C models to inform sustainable farming practices for this nationally important land use.