Land Use and Season Influence Event‐Scale Nitrate and Soluble Reactive Phosphorus Exports and Export Stoichiometry from Headwater Catchments

Abstract

Catchment nutrient export, especially during high flow events, can influence ecological processes in receiving waters by altering nitrogen (N) and phosphorus (P) concentrations and relative amounts (stoichiometry). Event-scale N and P export dynamics may be significantly altered by land use/land cover (LULC) and season. Consequently, to manage water resources, it is important to understand how LULC and season interact to influence event N and P export. In situ, high-frequency spectrophotometers allowed us to continuously and concurrently monitor nitrate (NO 3 −) and soluble reactive P (SRP) concentrations and therefore examine event-scale NO 3 − and SRP export dynamics. Here we analyzed event NO 3 − and SRP concentration-discharge hysteresis patterns and yields for >400 events to evaluate how LULC and seasonality influence event NO 3 − and SRP export dynamics in three low-order watersheds with different primary LULCs (agricultural, forested, and urban). Differences among event NO 3 − and SRP hysteresis patterns suggest these nutrients have different source areas and dominant transport pathways that were impacted by both LULC and seasonality. Unexpectedly, we observed similar seasonal patterns in event NO 3 − :SRP stoichiometry among LULCs, with the most N-enriched events occurring in spring, and event stoichiometry approaching Redfield N:P ratios in the fall. However, seasonal stoichiometry patterns were driven by unique seasonal NO 3 − and SRP export patterns at each site. Overall these findings suggest LULC and seasonality interact to alter the timing and magnitude of event NO 3 − and SRP exports, leading to seasonal patterns in event NO 3 − to SRP stoichiometry that may influence ecological processes, such as productivity, in receiving waters. Plain Language Summary High flow events transport relatively large quantities of nitrogen (N) and phosphorus (P) to streams and downstream waterbodies where they may stimulate algal blooms and degrade water quality. We evaluated how land uses and seasons alter event nutrient transport. We monitored >400 events with sensors in streams with contrasting land uses. Event N and P concentration patterns differed from each other suggesting dissolved N and P were transported from different locations in the landscape. Further, the agricultural and urban streams received more dissolved N and P than the forested stream. This likely results from fertilizer applications in excess of crop (agricultural and lawn grass) needs and landscape modifications, such as drainage systems and impervious surfaces, that limit soils and vegetation from removing nutrients from runoff. Lastly, season influenced the ratio of dissolved N to P delivery, with spring events transporting the most N relative to P and fall events transporting the least. Overall, land use and season uniquely influenced event nutrient transport. Management strategies to reduce algal blooms in downstream waterbodies must consider interactions among land use, nutrient type, and season. However, ratios of N to P may change seasona...

Nitrogen dynamics and nitrogen‐to‐phosphorus stoichiometry in cold region agricultural streams

“…Larger summer rainfall events tend to drive N/P ratios down (Correll et al., 1999; Green et al., 2007; Green & Finlay, 2010; Kincaid et al., 2020), which is consistent with our finding that the extreme rainfall‐runoff event of 2014 had the lowest N/P ratios in each stream. In a detailed study of the 2014 extreme rainfall event, Wilson et al.…”

“…Larger summer rainfall events tend to drive N/P ratios down (Correll et al, 1999;Green et al, 2007;Green & Finlay, 2010;Kincaid et al, 2020), which is consistent with our finding that the extreme rainfall-runoff event of 2014 had the lowest N/P ratios in each stream. In a detailed study of the 2014 extreme rainfall event, Wilson et al (2019) attribute low N/P ratios (mean = 7.1, SD = 2.1) to the efficient transport of P but not N from the watershed to the stream network via surface drainage ditches.…”

“…The N/P ratios responded more strongly to high‐flow conditions in streams from well‐drained catchments (WCE and WCW) than a poorly drained catchment (OR), which may be due to differences in hydrological connectivity (Rose et al., 2018; Kincaid et al., 2020) that arise from the differences among catchment characteristics such as effective drainage area, proportion of wetland, and catchment size. Stronger hydrological connectivity coupled with the high proportion of conservation tillage practiced in this region (41.7%) (Statistics Canada, 2021b) may explain the lowered N/P ratios observed in WCE and WCW during the spring (Tiessen et al., 2010).…”

Hydrologic regimes drive nitrate export behavior in human-impacted watersheds

“…We note that this classification scheme differs from traditional baseflow separation techniques that use graphical, geochemical, or isotopic approaches to identify and separate the proportion of the hydrograph that is comprised of baseflow and stormflow (Hooper and Shoemaker, 1986;Klaus and McDonnell, 2013). Baseflow separation techniques have shown that a large fraction of event water is derived from baseflow (e.g., Schilling and Zhang, 2004).…”

The Case for Studying Highly Modified Agricultural Streams: Farming for Biogeochemical Insights

“…Storm export data modified from S. L. Speir unpubl. and Kincaid et al (2020).…”