A comparison of nutrient losses to waters following pasture renewal by cultivation or direct-drilling

Abstract

Agricultural soils enriched in phosphorus (P) have been linked to increases in P losses and declining water quality. Cultivation of pastures near sensitive waterways lowers surface soil P and therefore the concentration of P in surface runoff. However, such a practise can increase the mineralisation of organic nitrogen (N), negating this as a mitigation option for N sensitive catchments. A field trial was set up to compare pasture renewal using either direct-drilling or cultivation (which mixed the plough layer to 150 mm) on hydraulically-isolated runoff plots at Tussock Creek near Invercargill, Southland. The trial ran from March to December 2015. Soil P concentrations (0-150 mm depth) decreased in the cultivated plots by about 20%, 8 months after cultivation. Concentrations of nitrate-N in surface runoff and shallow drainage, and P in shallow drainage only, were not different between directdrilling and cultivation treatments. Cultivation reduced filterable reactive P (FRP) losses in surface runoff by 69% (P=0.047) compared to direct-drilled plots. This study showed that mixing P within the plough layer decreased P loss over the 10 months of measurement. Conventional tillage that inverts the plough layer may result in even larger decreases in soil P concentrations. Tillage can therefore be used as a strategy to decrease P loss in small areas where surface runoff is likely, with little risk of increasing catchment-scale N loss. This includes near stream areas, but care must be taken to leave an adequate buffer strip to prevent sediment (and P) loss via erosion of bare soil. Keywords: phosphorus-enriched soil, cultivation, direct-drilling, runoff, drainage