Downslope movement of nutrients in hill pasture, Taita, New Zealand

Abstract

The quantities of nitrogen (N), phosphorus (P), calcium (Ca) , magnesium (Mg) , sodium (Na) , potassium (K), total solids (TS) , and organic matter (OM) moving downslope at the surface of hill pasture during rain events were estimated for one year. About 60–80% of the year's totals of the nitrate N, Ca, Mg, Na, TS, and OM, and about 30–50% of the Kjeldahl N, total P, and K moved downslope in winter during 3 overland flow events accounting for about 75% of the year's water runoff. The remainder of nutrients moved downslope in rain events with no observed overland flow. About 45% of the year's total of Kjeldahl N and about 30% of the K moved downslope in rain events after grazing. About 34% of the year's total P movement took place in 2 rain events after superphosphate application to recently grazed pasture, but under these conditions only about 0.2% of the year's total moved in topdressed pasture with long grass. Postgrazing and post-fertiliser events accounted for less than 4% of the year's downslope water movement. Estimated downslope losses in surface runoff were 0.59 kg/ha/year for Kjeldahl Nand 0.11 kg/ha/year for total P. With less than 1.2% of the rainfall collected as surface runoff, the amounts of nutrients lost downslope were less than total atmospheric inputs. With the exception of overland flow events in which runoff moved relatively long distances it is suggested that these losses did not reach streams directly, and the area studied was not normally a direct ",ource area for stream nutrients. It is suggested that management to benefit stream water quality in similar catchments should aim at reducing overland flow over the catchment as a whole, and at reducing the effects of grazing and fertiliser application in nutrient source-areas.