Conventional and Conservation Tillage: Effect On Sediment and Nutrient Losses within the Cold, Semi-Arid, Canadian Prairies.

Conservation tillage is a management practice that has been widely promoted to reduce sediment and nutrient transport from agricultural fields. However, in cold-climate regions, snowmelt runoff often exceeds rainfall runoff during the course of a year, and the effect of conservation tillage on nutrient export during the spring snowmelt period is not well known. Therefore, a long-term paired watershed study was used to compare snowmelt and rainfall-induced runoff, sediment and nutrient losses from conventional and conservation tilled watersheds in the Northern Great Plains region of western Canada. 

Runoff and dissolved nutrient concentrations were typically greatest during the snowmelt period, while concentrations of sediment and particulate nutrients were greatest during rainfall events. However, since > 80 % of total runoff occurred as snowmelt, the majority of sediments, dissolved and particulate nutrients were exported during the snowmelt period. Overall, conservation tillage reduced total export of sediment by 65 % and nitrogen (N) by 68 %, relative to conventional tillage. Surprisingly, losses of phosphorus (P) actually increased by 12 % on conversion to conservation tillage. This occurred because the conservation tillage system was more susceptible to losses of soluble P in snowmelt runoff (likely due to the stratification of P at the soil surface and the leaching of P from crop residues), and these losses were not offset by reductions in particulate P during the summer. It appears that management practices designed to improve water quality by reducing sediment and sediment-bound nutrient export (in particular for P) from agricultural fields and watersheds will be less effective in cold, semi-arid regions where nutrient export is snowmelt-driven and primarily in the dissolved form. In these situations, it may be most practical to implement management practices that reduce the accumulation of nutrients in crop residues and surface soil.