Understanding Phosphorus, Nitrogen and Cadmium Transfer through a Young Stony Soil

In Canterbury land-use intensification, particularly irrigated dairy expansion, is occurring on stony soils. Concerns exist about the ability of these soils to sustain intensified land-use, while maintaining nutrient leaching within discharge limits. Environmental models consistently predict stony soils as having a high vulnerability to leaching under intensive land use, but there is little experimental research to validate model predictions. This paper presents scoping experiments to quantify the degree of leaching vulnerability of young stony sand soils, and to determine likely key drivers for leaching. Barrel lysimeters (460 mm in diameter by 700 mm deep) of intact soil columns were collected of a young stony sand soil from alluvium. Four lysimeters were used to study the preferential leaching of nitrogen (N), phosphorus (P), and cadmium (Cd) from a pulse (25 mm depth) of dairy shed effluent (DSE) followed by continuous artificial rainfall – both applied at 5 mm/h. A further two lysimeters were used to study the preferential leaching of N, P, Cd, and carbon (C) under simulated periodic irrigation (12–18 mm depth applied every 3–4 days). Sequential treatments of superphosphate, cow urine, and DSE were applied, with intervals of at least one pore volume (>200 mm) of drainage between each treatment. The constant-rate experiment demonstrated that these soils have the potential for rapid leaching of N, P and Cd from an application of DSE. In contrast, no unequivocal increase in P, N and Cd leaching occurred after the DSE treatment in the simulated irrigation experiment, indicating strong sensitivity of these soils to application depth and rate of DSE. In the periodic-irrigation experiment the urine treatment resulted in rapid leaching of N, P, C, and Cd starting within 15–60 mm of drainage following the urine application. This indicates that under irrigated dairy grazing, urine will be a key driver for leaching of a range of possible contaminants. Given the intensification of agricultural development on these vulnerable soils, the results of this scoping study confirm predictions that young stony sand soils have high potential leaching vulnerability, and we argue that these results urgently justify further research. Introduction There are large areas of stony soils in the eastern plains of both the South and North Island of New Zealand, with 1.68 million hectares occurring on land of <15° slope, for which there is potential for intensive irrigated land use (Carrick et al. 2013). In Canterbury there is clear evidence of increasing land-use intensification on stony soils, particularly irrigated dairy expansion. However, there is concern about the ability of stony soils to sustain intensified land use, while maintaining nutrient leaching within discharge limits (PCE 2013). Environmental models consistently predict stony soils as having a high vulnerability to