Resistance to cropping pressure of two New Zealand soils with contrasting mineralogy

Changes in soil properties in response to cropping pressure were measured in 2 mineralogically contrasting New Zealand topsoils. Waiareka clay (Vertic Haplustoll) with >65% smectite clay was compared with Wakanui silt loam (Aquic Haplustept), which contains about 20% mixed or illite clays. Soil properties (0-10 cm) were assessed by changes in organic carbon (C) and nitrogen (N), pH, cation exchange capacity, Olsen P, microbial C, soil respiration, microbial functional evenness, mineralisable N, bulk density, unsaturated hydraulic conductivity, readily available and total available water, macro porosity and total porosity, particle density, and particle size distribution. Additional samples of the Waiareka clay soil were taken at 10-20 cm depth. Cropping pressure was estimated using a scale based on the number of years in crop v. those in pasture. The organic and biological status of the Waiareka pasture soil (0-10 cm) was greater than that of the Wakanui pasture, with contents of 5.1% organic C, 0.43% total N, 1429 microbial C (mu g/cm(3)), and 123 mineralisable N (mu g/cm(3)), compared with values of 4.0%, 0.31%, 795 mu g/cm(3), and 89 mu g/cm(3), respectively. Total and macroporosity, and total and readily available water, were also greater on the Waiareka pasture soil compared with the Wakanui pasture soil. Under cropping pressure, the Waiareka soil had marked declines in organic matter C and N, microbial C and soil respiration, microbial functional evenness, macroporosity, and available water. In contrast, the Wakanui silt loam showed little response to cropping pressure; mineralisable N declined slightly and particle density increased under cropping. We concluded that the initial properties of a soil were not good predictors of subsequent resistance to cropping pressure; and that soils with initially high status have the potential for proportionally greater decline.