Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events

Authors: Hanrahan, Brittany; King, Kevin; Williams, Mark

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2020
Publication Date: 8/27/2020
Citation: Hanrahan, B.R., King, K.W., Williams, M.R. 2020. Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events. Science of the Total Environment. 754. Article 142047. https://doi.org/10.1016/j.scitotenv.2020.142047.
DOI: https://doi.org/10.1016/j.scitotenv.2020.142047

Interpretive Summary: The amount of nitrogen (N) and phosphorus (P) lost from agricultural fields in subsurface tile drainage is influenced by weather, landscape, and management characteristics. Precipitation characteristics, including rainfall totals, event intensity, event duration, and antecedent moisture conditions, can also impact N and P losses by influencing or modifying other important controlling variables (e.g., discharge). The goal of this study was to determine controls of flow and nutrient loss from tile drains induced by precipitation events (rainfall > ~0.25 in) observed across agricultural fields (n = 40) in the edge-of-field network. We also assessed the modifying influence of event precipitation (i.e., total rainfall), duration, and intensity, and previous 7-day precipitation. Tile discharge was strongly and positively influenced by previous 7-day precipitation and total rainfall and negatively influenced by daily temperature and tile spacing. Both tile NO3- and DRP loads were positively influenced by transport and source variables, including event discharge and total fertilizer applied as well as soil test P (STP) in the case of tile DRP load; factors with the strongest negative influence on tile NO3- and DRP loads were related to time of year. The strength and direction of both positive and negative controls also varied with precipitation characteristics. For example, the positive influence of event discharge on nutrient loads lessened as event duration, event intensity, and previous 7-day precipitation increased, while the positive influence of N and P sources strengthened, particularly in response to extreme (or maximum) event. Results here demonstrate the predominant role of transport and source controls while accounting for interactive effects among site-specific characteristics and underscore the importance of storm dynamics when managing N and P loss from agricultural fields.

Technical Abstract: The magnitude of nitrogen (N) and phosphorus (P) exported from agricultural fields via subsurface tile drainage systems is often determined by site-specific interactions between weather, landscape, and management factors. Factors related to transport (e.g., tile discharge) and source (e.g., fertilizer application) are important controls of N and P loss that vary with precipitation characteristics, such as the duration and intensity of events, and antecedent moisture conditions. Here, we used multiple regression analyses to determine controls of event-driven discharge, nitrate (NO3-) load, and dissolved reactive P (DRP) load leveraging a unique dataset of ~7000 precipitation events observed across 40 agricultural fields instrumented to collect continuous water quality samples. We also calculated marginal effects of significant controls and assessed the modifying influence of event precipitation (i.e., total rainfall), duration, and intensity, and previous 7-day precipitation. Tile discharge was strongly and positively influenced by previous 7-day precipitation and total rainfall and negatively influenced by daily temperature and tile spacing. Both tile NO3- and DRP loads were positively influenced by transport and source variables, including event discharge and total fertilizer applied as well as soil test P (STP) in the case of tile DRP load; factors with the strongest negative influence on tile NO3- and DRP loads were related to time of year. The strength and direction of both positive and negative controls also varied with precipitation characteristics. For example, the positive influence of event discharge on nutrient loads lessened as event duration, event intensity, and previous 7-day precipitation increased, while the positive influence of N and P sources strengthened, particularly in response to extreme (or maximum) event. Results here demonstrate the predominant role of transport and source controls while accounting for interactive effects among site-specific characteristics and underscore the importance of storm dynamics when managing N and P loss from agricultural fields.