Impact of the Rate of Rewetting of a Dried Soil on the Concentration of Phosphorus In Leachate

The drying and rewetting of soils has long been known to result in the increased mobilisation of nutrients which potentially can transfer from the soil via leachate and contaminate surface waters. There are still many gaps in our knowledge of the processes, variables and sources involved in the regulation of the quantities of nutrients mobilised in this way. In the laboratory we tested the hypothesis that the rate of rewetting of a dried soil affects the solubilisation and ultimately the concentrations of phosphorus (P) present in leachate. We prepared a grassland pelostagnogley soil by passing it through a 2mm sieve and removing all visible live floral and faunal material. Half the soil was dried at 35°C and half was maintained at approximately 40% water holding capacity. Both dried and moist soils were loosely packed into funnels plugged with glass wool and 25ml of deionised water was added gradually over periods of 0, 2, 4 , 24 and 48 hours. The leachate was collected and analysed for dissolved and particulate reactive P and total P. In all cases P concentrations were significantly higher in the leachate from the dried soil treatments than from the moist soil treatments. Dissolved P concentrations were highest in the leachate from the 2 hour treatment, while for particulate P, concentrations were highest in the 0 hour treatment. These probably reflect variations in factors such as the energy of the rewetting water available for mobilisation and transfer of particulates, suitability of conditions for solubilisation of P and opportunities for recycling of released P within the soil. These results suggest that changes in patterns of rainfall as a result of climate change could significantly affect the quantities of P leached from soils.