Agronomic and economic implications of cover crop and phosphorus fertilizer management practices for water quality improvement.

Phosphorus (P) fertilization practices and winter cover crops are promoted to protect water quality yet can potentially influence crop yield and profitability. This study examined the impacts of three P fertilizer management practices (no P, fall broadcast P, and spring injected P) and winter annual cover crop use on yields, net returns, and water quality in a no-till corn-soybean rotation. Treatments were replicated in a 4-yr field study where sediment, total P, and dissolved reactive P (DRP) losses in edge-of-field surface runoff were continuously monitored. Production budgets were developed for each treatment and coupled with water quality data to identify optimal practices and opportunity costs for improving water quality. Applying P fertilizers increased crop yield regardless of application method or cover crop, but the response was more pronounced in corn than soybean. The cover crop reduced corn grain yield in 1 yr with high temperature and low precipitation but did not impact grain yield in the other corn year or either year of soybean. The most profitable treatment was fall broadcast P fertilizer with no cover crop, which also had the greatest total P and DRP losses and near greatest sediment loss. The lowest-cost methods of reducing total P, DRP, and sediment losses were, respectively, no P fertilizer without a cover crop ($47.56 kg-1 total P), spring injected P fertilizer without a cover crop ($56.47 kg-1 DRP), and spring injected P fertilizer with a cover crop ($0.16 kg-1 sediment). Widespread adoption of these practices will likely require monetary incentives.

Cover crop and phosphorus fertilizer management impacts on surface water quality from a no-till corn-soybean rotation.

Best management practices that reduce potential phosphorus (P) loss and provide flexibility in P fertilizer management are needed to help producers protect water quality while maintaining crop yield. This study examined the impacts of P fertilizer management (no P, fall broadcast P, and spring injected P) and cover crop use on annual concentrations and loads of sediment, total P, and dissolved reactive P (DRP) in edge-of-field runoff from a no-till corn (Zea mays)-soybean (Glycine max) rotation in the Central Great Plains, USA, from September 2015 through September 2019. The spring injected P fertilizer treatment generally had 19% less total P and 33% less DRP loss compared to the fall broadcast treatment, confirming the importance of P fertilizer management as a practice for reducing P loss. The addition of a cover crop had an inconsistent effect on total P loss, with no effect in 2016 and 2017, increasing loss in 2018 by 56%, and decreasing it in 2019 by 40%. The inconsistent impact of cover crops on total P loss was related to cover crop effects on sediment loss. Although cover crop impacts on total P losses were inconsistent, the addition of a cover crop increased DRP loss in three of four years. Cover crop use consistently reduced sediment loss, with greater sediment reduction when P fertilizer was applied. Results from this study highlight the benefit of cover crops for reducing sediment loss and the continued need for proper fertilizer management to reduce P loss from agricultural fields.

Species and termination method effects on phosphorus loss from plant tissue.

Cover crops are often recommended as a best management practice to reduce erosion, weed pressure, and nutrient loss. However, cover crops may be sources of phosphorus (P) to runoff water after termination. Two greenhouse trials were conducted to determine the effects of cover crop species, termination method, and time after termination on water-extractable P (WEP) release from crop biomass. Treatments were structured in a 3 × 3 × 3 factorial and arranged in a randomized complete block design with six replicates. Treatments included three cover crop species (triticale [× Triticosecale; Triticum × Secale 'Trical'], rapeseed [Brassica napus L. 'Winfred'], and crimson clover [Trifolium incarnatum L.]); three termination methods (clipping, freezing, and herbicide); and three WEP extraction times (1, 7, and 14 d after termination). Rapeseed consistently resulted in the least WEP when exposed to the same method of termination and at the same extraction time as the other species. For both trials, terminating crop tissue via freezing increased concentrations of WEP compared with other termination methods. The WEP release from cover crop tissue increased as the time after extraction increased, but the effect was greater for herbicide- and freeze-terminated cover crops and less for clipping-terminated cover crops. Future studies on WEP release from cover crops should pay close attention to the effects of extraction timing. Producers may be able to reduce P loss from cover crop tissue by selecting cover crop species with low WEP and minimizing the amount of biomass exposed to freezing conditions.