Slope class and grazing intensity effects on microorganisms and nitrogen transformation processes responsible for nitrous oxide emissions from hill pastures

Abstract

Nitrous oxide emissions are a product of soil microbial transformations such as nitrification and denitrification that are regulated by a range of biophysical factors. As the majority of farmed land in New Zealand is grazed hill country, this represents a potentially large source area for nitrous oxide (N2O) emissions nationally. In this study we investigated the impact of slope class and grazing intensity on hill soil biogeochemistry with a focus on processes leading to N2O emissions. Soils under permanent grass/legume sheep-grazed pasture on hill country in the North Island (temperate climate) of New Zealand were studied. Soil samples were collected from experimental pastures representing 3 slope gradient classes: low (0–12°), medium (13–25°), and high (>25°) slope and 2 grazing intensities which we define as extensive and intensive (mean annual standing biomass of 1900 vs.1475kgDMha−1). Soil properties, nitrifying and denitrifying enzyme activity, and abundance of soil microbial functional genes were determined.Soil physicochemical properties, nitrifying and denitrifying enzyme activity, and abundances of soil microbial functional groups were higher in the low compared to medium and high slope classes. In contrast, heterotrophic nitrification enzyme activity was marginally higher in soils from medium and high compared to low slopes. In general there were no significant effects of grazing intensity on the soil properties studied, or on autotrophic/heterotrophic nitrification enzyme activity. However, in low slopes only soil nitrate content, nirK (nitrite reductase) gene abundance and total denitrification enzyme activity were significantly higher under more intensive grazing pressure.Our results suggest that the potential for N2O emissions is higher in low compared to medium and high slope areas. At our experimental site, three years of comparatively more intensive grazing pressure did not show conclusive evidence for grazing-induced impacts on N2O emission potential. Of the microbial functional units assessed, the abundances of nirK were most closely linked with function. Our work indicates the importance of taking into consideration the effect of slope class when accounting for N2O emissions in hill country pastures in New Zealand.