Phosphorus and nitrate run‐off in hill pasture and forest catchments, Taita, New Zealand

Abstract

Phosphorus and nitrogen were measured in stream run‐off from the four catchments of the Taita Experimental Basin (41° 11′ S, 174° 58′ E). The land is used as exotic conifer forest, native forest, and hill pasture. Multiple regression analysis was used to estimate chemical losses per unit area in floods and at low flows.

At low flows, the hill pasture (fertilised with lime at 630 kg·ba⁻¹·y⁻¹, and superphosphate at 380 kg·ha⁻¹·y⁻¹) tended to lose more phosphorus and nitrate than the forested land, but differences were small, and not always significant. During large floods, the hill pasture (No. 5 Catchment) lost about 3 times as much reactive phosphate and 2–5 times as much total phosphorus as the forested land, and 130–190 times as much nitrate as land in the Exotic Forest and Native Forest 2 Catchments. Nitrate losses from land in the No. 4 Catchment (mainly native forest) were as high as those from the hill pasture, so high nitrate loss is not associated solely with agriculture.

Losses of total phosphorus via the catchment streams were estimated as: No. 5 Catchment (hill pasture), 293 g·ha⁻¹·y⁻¹; Native Forest 2 Catchment, 201 g·ha⁻¹·y⁻¹; No. 4 Catchment, 124 g·ha⁻¹·y⁻¹; Exotic Forest Catchment, 71 g·ha⁻¹.y⁻¹. Nitrate‐N losses were estimated to have been 1356 g·ha⁻¹·y⁻¹, 11.5 g·ha⁻¹·y⁻¹, 1436 g·ha⁻¹·y⁻¹, and 44 g·ha⁻¹·y⁻¹ respectively. Phosphorus and nitrate concentrations were similar in the Exotic Forest and Native Forest 2 streams, but the Exotic Forest tended to lose smaller amounts because it yielded about 50% less water per unit area.

Over the 2‐y study, an estimated 47–70% of phosphorus losses and up to 83% nitrate losses occurred in large floods; 31% and 48% respectively were apparently lost from the hill pasture catchment in a single flood. Less than 20% of estimated phosphorus losses and as little as 1% of nitrate losses occurred at low flows.

Run‐off of phosphorus and nitrate was spasmodic, and this should be considered in assessing the impact of surface run‐off on the biology and chemistry of receiving waters.