Managing natural processes in drainage ditches for nonpoint source phosphorus control

Abstract:

Drainage ditches are an integral part of agricultural landscapes in many areas. We evaluated the effect of physical, chemical, and biological properties of three sources of ditch sediments on phosphorus (P) transport in the ditches. Using an indoor “fluvarium,” we found that sediments from ditches draining agricultural areas maintained an appreciably higher dissolved reactive P (DRP) concentration in flowing water (0.374 mg L⁻¹ [0.374 ppm]) than did forested sediments (0.006 mg L⁻¹). The larger release of P from agricultural sediments reflects a greater Mehlich-3 P and equilibrium P concentration (EPC₀) of these sediments. When DRP was added to ditch water (2.60 mg L⁻¹) to simulate surface runoff from soils amended with poultry litter, water equilibrated to 0.194 mg DRP L⁻¹ with agricultural sediments and 2.096 mg DRP L⁻¹ with forest sediments. The larger removal of P by agricultural sediments was related to a greater P sorption maximum and clay content of those sediments. Gamma irradiation of the ditch sediments showed microbial P biomass accounted for 10% to 40% P uptake. Thus, both abiotic and biotic processes are important in drainage ditches and influence whether ditch sediments act as sources or sinks of P transported from these drained areas. Clearly, drainage ditches are reactive conduits in terms of P transport and can influence not only the amount but environmental bioavailability of P exported and thereby biological response of receiving waters.