Variability of δ15N in soil and plants at a New Zealand hill country site: correlations with soil chemistry and nutrient inputs

Abstract

This study investigated ¹⁵N enrichment and nutrient cycling in hill country used for semi-extensive pastoral agriculture, at a site where pre-European seabird breeding occurred. Soil (0–15 cm) and plant samples were taken from 18 ridgeline and sideslope transects. Three stock camps (locations which grazing animals frequent) were identified within the study area, two on the ridgeline and one on the sideslope. Soil ¹⁵N enrichment was greatest at stock camps, and lowest where stock input was minimal. Soil natural abundance ¹⁵N (δ¹⁵N) was therefore an index of stock nutrient inputs. Soil δ¹⁵N increased with decreasing C: N ratio, consistent with N loss through volatilisation and/or nitrate leaching from net mineralisation. Plant δ¹⁵N from stock camps was lower than its associated soil, implying that ¹⁵N enrichment of plant-available N was lower than that of total soil N. However, the correlation between plant δ¹⁵N and soil δ¹⁵N varied between stock camps, indicating differences in N cycling. Olsen P was higher at stock camps, although again differences were found between stock camps. Total P and N were correlated neither with stock camps nor topography, but were higher than expected from parent material concentrations and literature results, respectively. It is postulated that significant contributions of both elements from former seabird breeding remain in the soil.