Impact of P inputs on the source-sink P dynamics of sediment along an agricultural ditch network

Authors: EZZATI, GOLNAZ - Teagasc (AGRICULTURE AND FOOD DEVELOPMENT AUTHORITY); FENTON, OWEN - Teagasc (AGRICULTURE AND FOOD DEVELOPMENT AUTHORITY); HEALY, MARK - National University Of Ireland; CHRISTIANSON, LAURA - University Of Illinois; Feyereisen, Gary; THORNTON, STEVE - University Of Sheffield; CHEN, QING - China Agricultural University; FAN, BINGQIAN - China Agricultural University; DING, JIAHUI - China Agricultural University; DALY, KAREN - Teagasc (AGRICULTURE AND FOOD DEVELOPMENT AUTHORITY)

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2020
Publication Date: 3/1/2020
Citation: Ezzati, G., Fenton, O., Healy, M.G., Christianson, L.E., Feyereisen, G.W., Thornton, S., Chen, Q., Fan, B., Ding, J., Daly, K. 2020. Impact of P inputs on the source-sink P dynamics of sediment along an agricultural ditch network. Journal of Environmental Management. 257. https://doi.org/10.1016/j.jenvman.2019.109988.
DOI: https://doi.org/10.1016/j.jenvman.2019.109988

Interpretive Summary: Phosphorus (P) losses from intensive dairy farms through tile drainage to ditches and nearby streams is a threat to water quality. There is a potential for P mitigation along the ditch bank given the right location and favorable chemistry dynamics for removing P from the water stream. In this field study, a point source of P from a nearby dairy yard increased dissolved P concentration in the ditch water. The P input resulted in P accumulation along the ditch bank downstream from the source and also in P accumulation in ditch sediments. The sediment that moved downstream actually became a secondary source of P to the water. The study showed that to mitigate P in ditch water, intervention measures must both remove P from the water and also remediate sediments. In-ditch measures will require some lag time before improvements in water quality will be observed.

Technical Abstract: Phosphorus (P) loss from intensive dairy farms is a pressure on water quality in agricultural catchments. At farm scale, P sources can enter in-field drains and open ditches, resulting in transfer along ditch networks and delivery into nearby streams. Open ditches could be a potential location for P mitigation if the right location were identified, depending on P sources entering the ditch and the source-sink dynamics at the sediment-water interface. The objective of this study was to identify the right location along a ditch to mitigate P losses on an intensive dairy farm. High spatial resolution grab samples for water quality, along with sediment and bankside samples, were collected along an open ditch network to characterize the P dynamics within the ditch. Phosphorus inputs to the ditch adversely affected water quality, and a step change in P concentrations (increase in mean dissolved reactive phosphorus (DRP) from 0.054 to 0.228 mg L-1) midway along the section of the ditch sampled, signaled the influence of a point source entering the ditch. Phosphorus inputs altered sediment P sorption properties as P accumulated along the length of the ditch. Accumulation of bankside and sediment labile extractable P, Mehlich 3 P (M3P) (from 13 to 97 mg kg-1) resulted in a decrease in P binding energies (k) to < 1 L mg-1 at downstream points and raised the equilibrium P concentrations (EPC0) from 0.07 to 4.61 mg L-1 along the ditch. The increase in EPC0 was in line with increasing dissolved and total P in water, demonstrating the role of sediment downstream in this ditch as a secondary source of P to water. Implementation of intervention measures are needed to both mitigate P loss and remediate sediment to restore the sink properties. In-ditch measures need to account for a physicochemical lag time before improvements in water quality will be observed.