Effects of Manure and Tillage on Edge-of-Field Phosphorus Loss in Seasonally Frozen Landscapes.

Environmental conditions and management practices affect nutrient losses in surface runoff, but their relative impacts on phosphorus (P) loss during frozen and nonfrozen ground periods have not been well quantified. More specifically, the relative importance of manure application, tillage, and soil-test P (STP) has not been assessed at the field scale. In this study, we compiled a dataset composed of 125 site-years of data from 26 fields that were continually monitored for edge-of-field P loss during snowmelt and storm events. Regression tree analyses were performed to rank the level of influence each environmental and management factor had on nutrient loads. Dissolved P (DP) was the majority of the total P (TP) during frozen conditions, but a small portion of TP during nonfrozen conditions. Manure application had a greater influence on the flow-weighted mean concentrations (FWMCs) of TP and DP during frozen conditions than during nonfrozen conditions. No-till resulted in greater TP and DP FWMCs during frozen conditions than conventional tillage, whereas the opposite effect for TP FWMC was seen during nonfrozen conditions. However, regression tree analysis revealed that STP (0- to 5-cm depth) was the most important factor in predicting DP and TP FWMCs during frozen conditions and DP FWMC during nonfrozen conditions. Extremely high STP values were associated with late-frozen manure applications and grazed pastures. Reducing surface P loss in seasonally frozen landscapes will require prioritizing management strategies that avoid manure application through early- and late-frozen conditions and lead to a drawdown of STP, particularly in the top 5 cm.

Dynamics of Measured and Simulated Dissolved Phosphorus in Runoff from Winter-Applied Dairy Manure.

Agricultural P loss from fields is an issue due to water quality degradation. Better information is needed on the P loss in runoff from dairy manure applied in winter and the ability to reliably simulate P loss by computer models. We monitored P in runoff during two winters from chisel-tilled and no-till field plots that had liquid dairy manure applied in December or January. Runoff total P was dominated by nondissolved forms when soils were bare and unfrozen. Runoff from snow-covered, frozen soils had much less sediment and sediment-related P, and much more dissolved P. Transport of manure solids was greatest when manure was applied on top of snow and runoff shortly after application was caused by snowmelt. Dissolved P concentrations in runoff were greater when manure was applied on top of snow because manure liquid remained in the snowpack and allowed more P to be available for loss. Dissolved runoff P also increased as the amount of rain or snowmelt that became runoff (runoff ratio) increased. The SurPhos manure P runoff model reliably simulated these processes to provide realistic predictions of dissolved P in runoff from surface manure. Overall, for liquid dairy manure applied in winter, dissolved P concentrations in runoff can be decreased if manure is applied onto bare, unfrozen soil, or if runoff ratio can be reduced, perhaps through greater soil surface roughness from fall tillage. Both management approaches will allow more manure P to infiltrate into soil and less move in runoff. SurPhos is a tool that can reliably evaluate P loss for different management and policy scenarios for winter manure application.