Phosphorus retention and release from agricultural ditch networks

• Main Author: Li, Shijie
• Published: Ulster University 2006
• Subjects: 628
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.646851

The potential of agricultural drainage ditch sediments to release and retain phosphorus (P) was investigated at six sites in two sub-catchments of the River Blackwater in Counties Monaghan and Armagh, Ireland. The bed sediment was sampled on a fortnightly basis between April 2008 and March 2009: the sediment (top 1 cm) was analysed for total P, individual P fractions (with a modified Psenner sequential extraction scheme), equilibrium P concentration (EPCo), iron (Fe) and manganese (Mn) concentrations; sediment pore water (top 5 cm) and stream water were analysed for soluble reactive P (SRP). In relation to variation in stream discharge, sediment total P (TP) decreased rapidly during the largest storms, followed by calmer hydrological conditions and recovery in sediment TP. This provides field evidence of sediment TP depletion during storms, demonstrating the physical basis of the sediment's net P release in this study. This is also consistent with the interpretation of storm P-discharge hysteresis, where stream P concentration does not follow the rising and falling limbs of the hydrograph, that high discharge events can mobilise P retained in bed sediment. In terms of sediment P fractionation, the reductant-soluble bicarbonate-dithionite (BO) fraction was by far the largest, in terms of concentration and contribution ratio to TP (medians: 41 % - 62%), and the most sensitive to change in discharge, which suggests that the redox-sensitive binding of P to Fe and Mn hydroxides in the BO fraction was the dominant chemical process controlling sediment P. In addition, an inverse relationship was observed between Fe/P ratio in the BO fraction and sediment TP, but the BO-Fe data has been shown to be compromised due to analytical delays and inadequate sample preservation. This observation would have further indicated the chemical basis to sediment P retention during the recovery from storms: a decrease in Fe/P ratio points to the uptake of P by the reductant-soluble amorphous Fe hydroxide in this fraction. The comparison between sediment EPCo (medians: 0.005 - 0.041 mg P L-1 ) and stream P concentration (medians: 0.023 - 0.129 mg P L-1 ) generally indicated potential for sediment P retention. This was in spite of consistently reducing conditions in the bed sediment and the presence of high pore water P concentration at P-enriched sites (highest median: 2.133 mg P L-1 ). No evidence was found that supports the hypothesis that potential sediment P release contributed to the stream water SRP concentration during the stable low flow periods of the summer. This indicates that rural point sources are more likely to be responsible. Therefore, the ditches acted as temporary storage of P from catchment sources, chemically retaining P during low flow periods, before physically releasing the accumulated P load during episodic high discharge events. Under non-storm conditions, the bed sediment was clearly a P sink, mitigating nutrient enrichment in situ and downstream, by removing in-stream P and delaying downstream transport of P loading from catchment sources. There is a need to understand better the potential P release upon physical disturbance by storm flow - in particular the environmental implications of the dominance of the BD-TP fraction and the reductant-soluble Fe-P binding mechanism in the mobilised bed sediment.