Impacts of Soil Phosphorus Drawdown on Snowmelt and Rainfall Runoff Water Quality

Abstract

Managing P export from agricultural land is critical to address freshwater eutrophication. However, soil P management, and options to draw down soil P have received little attention in snowmelt‐dominated regions because of limited interaction between soil and snowmelt. Here, we assessed the impacts of soil P drawdown (reducing fertilizer P inputs combined with harvest removal) on soil Olsen P dynamics, runoff P concentrations, and crop yields from 1997 to 2014 in paired fields in Manitoba, Canada. We observed that Olsen P concentrations in the 0‐ to 5‐cm soil layer were negatively correlated with the cumulative P depletion and declined rapidly at the onset of the drawdown practice (3.1 to 5.4 mg kg −1 yr −1 during 2007–2010). In both snowmelt runoff and rainfall runoff, concentrations of total dissolved P (TDP) were positively correlated with the concentrations of soil Olsen P. Soil P drawdown to low to moderate fertility levels significantly decreased mean annual flow‐weighted TDP concentrations in snowmelt runoff from 0.60 to 0.30 mg L −1 in the field with high initial soil P and from 1.17 to 0.42 mg L −1 in the field with very high initial soil P. Declines in TDP concentration in rainfall runoff were greater. Critically, yields of wheat ( Triticum spp.) and canola ( Brassica napus L.) were not affected by soil P depletion. In conclusion, we demonstrate that relatively rapid reductions in P loads are achievable at the field scale via managing P inputs and soil P pools, highlighting a management opportunity that can maintain food security while improving water security in cold regions. Core Ideas Reducing or ceasing soil P input decreased soil Olsen P rapidly. Soil Olsen P concentrations were negatively correlated with cumulative P depletion. Declining soil P led to decreases in flow‐weighted mean dissolved P concentrations. Improved water quality can be achieved with management of soil P. Agricultural productivity can also be maintained with lower soil P.