Phosphorus inputs and outputs for New Zealand in 2001 at national and regional scales

Abstract

Since 1990, agricultural use of land in New Zealand has intensified and some areas have received increasing loads of phosphorus (P) fertilisers. This has led to increased concentrations of animal stock units on pastures. As both stock units and human populations increase, there is increasing concern about P loss from land and towns to waters. In order to assess the extent of the issue of P loss for New Zealand (267 000 km²) we have developed a first P budget for the season 2001/02 at both national and regional council scales. The average rate of P input is estimated to be 9.9 kg/ha nationally, and this is mainly from fertiliser on pasture. The average rates of P application on pastures range from 7 kg/ha in drier regions to 28 kg/ha in regional councils with intensive dairying. The average rate of output is estimated at 4.3 kg/ha, leaving 5.6 kg/ha stored in soils; this is thought to be stabilised by both aluminium and iron compounds and within soil organic matter, where it becomes less labile with time. The region with the largest P output (17 kg/ha) is Gisborne, which has large areas of erodable land under pasture. The average P load in rivers for New Zealand is 1.6 kg/ha, and sediment‐P is the major component. Dissolved P in runoff from pastures, and effluent from animal stock also add to the P loads in waters; therefore, mitigation practices need to be introduced by land managers.

Keywords:

• erosion
• Overseer®
• particulate‐P
• phosphate budget
• weathering

Notes

Landcare Research, Private Bag 11052, Palmerston North 4442, New Zealand.

GNS Science, PO Box 31312, Lower Hutt 5040, New Zealand.

NIWA, PO Box 11115, Hamilton 3251, New Zealand.