Soil erosion rates under intensive vegetable production on clay loam, strongly structured soils at Pukekohe, New Zealand

Abstract

Rates of soil redistribution by water and tillage erosion were determined in 3 fields under long-term continuous vegetable production on clay loam, strongly structured soils derived from volcanic ash at Pukekohe, South Auckland, New Zealand. Erosion and deposition rates were estimated using a mass balance model to convert variation in ¹³⁷Cs areal activity within the fields to estimates of erosion and deposition.

Caesium-137 areal activity in cropped fields ranged from 171 to 2144 Bq/m², compared with a reference value under permanent pasture of 774 Bq/m². There was a characteristic pattern of ¹³⁷Cs distribution within each field, with differences between the fields related to variation in topography. Lowest values of ¹³⁷Cs were found in the upper parts of each field and highest values towards the base of each field. In all 3 fields there was a net loss of ¹³⁷Cs, ranging from 13 to 32%, with an average over the 3 fields of 18%. Net rates of soil loss from the fields were 30, 11, and 7 t/ha.year. However, within the fields there was a much wider range of both erosion (up to 92 t/ha.year) and deposition (up to 100 t/ha.year) rates. Most of the soil redistribution is caused by water erosion, with tillage erosion accounting for 10-20% of the soil redistribution. The soil redistribution rates were 2 orders of magnitude higher than sediment export measured at small catchment scale. Soil erosion rates are not reflected in variation in topsoil depth, because frequent tillage and incorporation of organic residues maintains a uniform topsoil depth, but soil deposition rates are closely related to topsoil depth.