Seasonal Dynamics in Runoff Generation, Flowpaths and Phosphorus Mobilization From Reduced-till Agricultural Fields in Ontario, Canada

Reduced tillage systems used in agriculture have been shown to decrease losses of particulate phosphorus (P), but may increase the risk of dissolved P transport in some landscapes. Most of our knowledge of P losses from agricultural systems is based on observations made during the frost-free season and little is known about winter processes. Given the magnitude of the spring freshet in many regions, it is important to characterize P dynamics during this period. Discharge and P transport in overland flow and subsurface (tile) drainage were monitored at three reduced-till fields in southern Ontario, Canada for 18 months to (1) quantify runoff and P loads from fields; (2) characterize seasonality in the relative contributions of tile drainage and overland flow to runoff and P loads, and (3) demonstrate variable responses among different event types. Transport pathways were active throughout the non-growing season (NGS) and this period accounted for the majority of annual P loads over the study period. Drainage tiles were the dominant hydrologic pathway from fields throughout the study period, but were a small source of P when compared to P loss in overland flow. Overland flow was predominantly observed during winter thaws when ground frost was present. However, the magnitude and speciation of P losses during individual winter events were variable, and, were governed by a combination of antecedent conditions and precipitation characteristics. Given the importance of the NGS to annual P losses, we suggest that management steps should be taken to minimize the risk of losses during this period.