Impacts of management on leaching of nitrate from pastures

Abstract

Clover-based grasslands as used in New Zealand, while considered "clean" relative to grasslands farmed intensively with fertiliser nitrogen (N), are nevertheless polluting. Recent measurements of nitrate-N down the soil profile below heavily stocked (22.5 ee/ha) pastures grazed by sheep, have shown that leaching losses under set stocking are 2-3 times those from rotational grazing (37 vs 16 kg N/ha/yr). As the input of N, its storage, and outgoings in animal products are similar in both systems, this indicates that considerably more urinary N is being lost to the aerial environment from rotationally grazed pastures, with nitrate leaching being the main avenue of loss under set stocking. Differences in sward structure are suggested as the major factor. Lower nitrate leaching from cocksfoot vs ryegrass-based pastures are thought to be associated with lower herbage quality factors reducing urinary N formation. While grazing management will not greatly influence total N losses to the environrnent, it can mfluence the avenues by which N enters the wider environment. Aquatically sensitive catchments may be better protected from leaching of nitrate by modification of the grazing management approaches. Keywords pasture management, nitrate leaching, ammonia volatilisation, pasture structure, herbage quality, groundwater pollution