Characterizing nitrogen outflow from pre-harvest rice field drain events

• Continuous and discrete data characterize nitrogen leaving rice fields at pre-harvest drain.
• Diel patterns of nitrate were seen within rice paddies and tailwater recovery systems.
• Continuous sampling showed advantages over discrete sampling.
• No environmental impact is seen from nitrogen outflow during rice field drain.

Tailwater recovery systems (TWR) provide an excellent testbed for examining nutrient loading from agriculture non-point sources, such as pre-harvest rice (Oryza sativa L.) field drains, on receiving waters. In this study, the focus was to use continuous sampling of nitrate-nitrogen (NO3−-N) concentration, paired with discrete grab samples of water which were analyzed for total nitrogen and inorganic nitrogen species to (1) assess if rice paddies are a source of nitrogen loading to downstream systems; (2) monitor the diel cycles in NO3−-N of rice paddies and TWR; and (3) describe the nitrogen capture capacity of TWR during these events. Five rice paddies within the Mississippi Delta with adjoining TWR were selected as case study locations. Both paddy and TWR were instrumented to continuously monitor nitrate, pH, dissolved oxygen, specific conductivity, and water temperature; discrete grab samples of water were also taken at deployment and collection. During the study, most TWR had total nitrogen concentrations <1 mg L−1; the majority of nitrogen present at drain was organic (>51% for paddies, and >88% for TWR). The percent change in total nitrogen concentrations between draining paddies and post-drain TWR ranged from −14 to +178%; the percent of organic nitrogen increased between 5 and 24% in TWR following rice drains. Both nitrogen accumulation and dilution in TWR were observed during drain events. Diel cycles were apparent and were in phase with dissolved oxygen (average values between 3.7 and 13.2 mg L−1). The average peak-to-peak amplitude in TWR was 0.101 mg NO3−-N L−1. Total nitrogen captured by TWR ranged from 0.009 to 0.610 kg ha−1. Increases in NO3−-N concentrations were observed in several TWR during drain events, but concentrations remained low and loads were determined to be of little consequence; this suggests limited detrimental impact of rice drains downstream.