The impact of a kānuka silvopastoral system on surface runoff and sediment and nutrient losses in New Zealand hill country

Abstract

New Zealand hill country is a pastoral region that has high levels of surface runoff and sediment and nutrient losses. Silvopastoralism has potential to mitigate these losses via the interaction of trees with the soil, pasture and livestock. This study compares surface runoff and sediment and nutrient losses (nitrogen and phosphorus) in pasture plots with and without Kunzea robusta (kānuka) trees and investigates the reasons for the variation in these losses between kānuka pasture and open pasture. Annual surface runoff was 54.0 mm in kānuka pasture and 7.6 mm in open pasture despite kānuka pasture having improved soil conditions (in terms of porosity, organic matter and soil fertility). Kānuka pasture had species indicative of good pasture condition even though the plots had significantly less standing grass biomass. This was most likely due to livestock preferentially grazing the pasture under kānuka which led to less attenuation of surface runoff. Sediment and nutrient losses were 10–100 times greater in kānuka pasture, mainly due to the increase in surface runoff, but also because of livestock concentrating nutrients under the trees through excreta deposition. These results are contrary to past silvopastoral research and highlights that an extreme choice between shaded areas of good pasture condition and unshaded areas of poor pasture condition within a pastoral hillside can lead to negative consequences in terms of surface runoff and contaminant losses because of livestock interactions. More research is required to understand these processes in other hill country farms and grazing conditions, and explore mitigation options.