Dissolved Organic Carbon Concentration and Denitrification Capacity of a New Zealand Hill Country Soil After Forage Crop Establishment

Conversion from perennial pasture to forage cropping to increase animal feed production is widely adopted on steep pastoral hill country landscapes in New Zealand. However, the effect of this conversion on the leaching and availability of dissolved organic carbon (DOC) and soil denitrification is unknown. This study examined the effect of a forage crop (swede— Brassica napobrassica Mill.) establishment on the DOC concentration and denitrification capacity of a hill country soil (Typic Eutrudept) in the lower North Island of New Zealand. Pasture was cleared using selected agrochemicals and sown to swede via a unique surface sowing technique (no cultivation). Soil was sampled once before and multiple times after crop establishment. The sampled soils were compared to soils from adjacent pasture plots. The results suggest that DOC concentration in the subsoil (> 30 cm) and denitrification capacity of the soil profile (100 cm) were generally not affected by the establishment of the forage crop within a 1-year period, without animal grazing. However, forage crop establishment resulted in an initial increase (by > 55%) in the nitrate concentration of the surface 20 cm soil depth on the sixth month after crop establishment. However, the subsoil (> 20 cm) nitrate concentrations in the cropped system were similar to the pasture system. This suggests that the establishment of the brassica forage crop, using the surface sowing technique, did not pose a risk of increased nitrate leaching from this pastoral hill country soil.