Phosphorus Loss in Adjacent Agricultural Watersheds Under Different Phosphorus Fertilizer Management

Abstract

Intensive agricultural practices are linked to non-point source pollution of aquatic ecosystems around the world. In recent decades, there has been great impetus to investigate fertilizer management practices that contribute to crop productivity and protect water quality. A six-year study (2016-2021) was conducted in the School Branch watershed (Indiana, USA) to compare phosphorus (P) export from two adjacent fields receiving P fertilizer at different application rates. The fields were under corn-soybean rotation in a no-tillage and cover crop management. During the first years (2016-2017) of the study, or the “tri-state phase”, both fields received P fertilizer at an average rate of 78.5 kg P/ha/yr. During the “precision phase” (2018-2021), the West field received P at tri-state recommended rates, whereas the East field was switched to liquid injection of P fertilizer at an average rate of 22.4 kg P/ha/yr (or 70% less than tri-state rate). Water flow and P loss via surface and sub-surface pathways were monitored. Results showed no effect of P fertilizer rate on crop yield. However, despite the indication of soil P enrichment due to the initial high P input, reduction in P fertilizer input resulted in a substantial decrease (relative to an average loss of 2.9 kg P/ha/yr during the “tri-state phase”) in the amount of both DRP (25%) and TDP (72%) exported from the East field. The presence of a cover crop had variable effects on seasonal DRP export; in comparison to “no cover crop”, DRP loss tended to be higher (2 to 5-fold) in the fall and lower (2 to 4-fold) in the spring. Among the cover crops planted, winter wheat led to a marked reduction in water discharge and TDP export, underscoring the potential water quality benefits of this harvestable cover crop. Future studies should examine the underlying mechanisms of how different species of the cover crop affect seasonal P export.