Mitigating phosphorus loss in runoff from agricultural soils: the role of cover crops under conditions of freeze-thaw cycles
Eutrophication of surface waters caused by excessive phosphorus (P) loading is of water quality concern because of its promotion of algal blooms. Cover crops reduce total P (TP) loads in runoff and erosion but have been reported as potential source of dissolved reactive P (DRP) to runoff during winter due to lysis of cover crop cells by freeze-thaw cycles (FTC). However, the effects of cover crops on runoff P could be influenced by other factors such as species and age of cover crop, herbicide termination of cover crop, and soil test P (STP). Experiments were conducted to assess P release from cover crops -- oat (Avena sativa), cereal rye (Secale cereale), annual ryegrass (Lolium multiflorum), red clover (Trifolium pratense) and mustard (Brassica nigra) to runoff DRP. Freeze-thaw cycles elevated P release from cover crops but their impact was reduced when mature cover crops acclimatized at cooler temperatures in the field prior to laboratory extractions. The amounts of DRP contributing to runoff by cover crops was low relative to their tissue P pools and water extractable P (WEP) release under laboratory FTC conditions. Since red clover and mustard typically had less biomass than oat and cereal rye, they contributed minimally or to no detectable DRP amounts to runoff P. Particulate P (PP) and Total (TP) in runoff differed by cover crop species while herbicide application increased runoff DRP contribution by some species of cover crops. Runoff DRP and TP were higher in fall than spring whether cover crops were killed with herbicide or left to grow over winter. Generally, the STP range of these studies, 11 to 32 mg Olsen P kg-1 soil, did not impact cover crop biomass size nor their contribution to runoff DRP despite the high levels (45 to 63 mg kg-1) of WEP released from cover crops. The results of this study suggest that cover crop species selection, management of cover crop biomass and termination of cover crop, either by cutting late in the fall or by leaving it on the field over winter, are essential practices that should be adopted to minimize environmental P risk management.