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

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 (NO3-) and soluble reactive P (SRP) concentrations and therefore examine event-scale NO3- and SRP export dynamics. Here we analyzed event NO3- and SRP concentration-discharge hysteresis patterns and yields for >400 events to evaluate how LULC and seasonality influence event NO3- and SRP export dynamics in three low-order watersheds with different primary LULCs (agricultural, forested, and urban). Differences among event NO3- 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 NO3-: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 NO3- and SRP export patterns at each site. Overall these findings suggest LULC and seasonality interact to alter the timing and magnitude of event NO3- and SRP exports, leading to seasonal patterns in event NO3- to SRP stoichiometry that may influence ecological processes, such as productivity, in receiving waters.