Nutrients in Runoff from Cotton Production in the Lower Mississippi River Basin: An On‐Farm Study

Core Ideas Edge‐of‐field water quality monitoring quantifies conservation efficacy for cotton. Cumulative nutrient losses are related to cumulative runoff volume. Nutrient runoff in growing season is a small fraction of that applied as fertilizer. Nutrient loss is greater with runoff generated by rainfall than furrow irrigation. Edge‐of‐field monitoring of runoff from fields in a cotton (Gossypium hirsutum L.)–corn (Zea mays L.) rotation on a commercial farm was conducted from 2013 to 2017. The objectives were to quantify the differences in nitrate + nitrite‐nitrogen (NO3–N), total nitrogen (TN), soluble reactive phosphorus (SRP), and total phosphorus (TP) losses for individual discharge events generated from irrigation as compared with rainfall and losses during the growing season and non‐growing season with and without cover crops. Cereal rye (Secale cereal L.), as a cover crop, was planted in two of the fields each fall. Median runoff volumes ranged from 0.61 to 1.06 cm across the four fields. Median NO3–N loss ranged from 0.03 to 0.1 kg ha−1, and TN ranged from 0.11 to 0.23 kg ha−1. Soluble reactive P and TP loss ranged from 0.006 to 0.025 kg ha−1 and from 0.025 to 0.049 kg ha−1, respectively. Mean concentrations of NO3–N and TN were significantly greater during the growing season in one of two fields with cover crops. Losses of P in runoff were 3 to 10 times greater during rainfall‐induced events than during irrigation runoff. Total N and TP loss rates from planting to harvest were 3.7 and 4.0% of N and P applied as fertilizer, respectively. Total nutrient loss increased linearly, with an increase in runoff volume from planting to harvest. This work provides needed data on nutrient losses in runoff during cotton production and will provide cotton producers and conservation professionals information in support of cost‐effective conservation and mitigation measures.