Estimating and modelling the risk of redox-sensitive phosphorus loss from saturated soils using different soil tests

Phosphorus (P) loss from agricultural soils can negatively affect water quality. Models and management to decrease losses increasingly focus on P that is available and transported from areas in the landscape that are regularly saturated – and periodically anaerobic. Current models use soil tests conducted in oxic conditions, which do not represent anaerobic conditions. This could lead to inaccurate P loss predictions in wetter areas, where runoff and P loss would be generated. In the present study, anoxic water extractable P test (anoxic WEP) and sodium bicarbonate-dithionite extractable P (Dithionite-P) tests were first developed using samples from New Zealand and Ireland, and were used to predict P that is available under anoxic conditions in the short- and long-term. Using archived soils from New Zealand and Ireland, it was confirmed that present testing was under-predicting losses, which could have implications for modelling and correlations with agriculture and water quality. Anoxic WEP and Dithionite-P varied by soil order and land use and that anoxic WEP was greater than oxic WEP, which showed the short-term impact of soil anoxia on P release. Fluvisols, Gleysols and Luvisols were found to be particularly enriched in anoxic WEP and dithionite-P, owing to their periodic saturation. Therefore, P loss prediction from these soils could be significantly under-estimated during periods of saturation. Models predicting anoxic WEP and Dithionite-P (R² = 54%) at the 1:50,000 scale in New Zealand found relatively small proportions of agricultural land were enriched in Dithionite-P (31% >85 mg/kg) or anoxic WEP (3% >0–0.291 mg/L). The data shows that a uniform test under-predicts P losses in anoxic conditions. Although poorly-drained and saturated areas make up a small proportion of agricultural landscapes, these areas are important in terms of overall load and the P being lost from these parts is under-predicted. The new tests should be deployed in these areas and the other tests can still be used for other areas. This finer-scale data will help improve the isolation of critical source areas (CSAs) of P loss. The present study shows the importance of creating bespoke techniques for saturated areas, which will become important in a changing climate where the extent and duration of saturation in land is changing.