Antecedent conditions, hydrological connectivity and anthropogenic inputs: Factors affecting nitrate and phosphorus transfers to agricultural headwater streams

Abstract

This paper examines relationships between rainfall–runoff, catchment connectivity, antecedent moisture conditions and fertiliser application with nitrate-N and total phosphorus (TP) fluxes in an arable headwater catchment over three hydrological years (2012–2014). Annual precipitation totals did not vary substantially between years, yet the timing of rainfall strongly influenced runoff generation and subsequent nitrate-N and TP fluxes. The greatest nitrate-N (>250kgNday−1) and TP (>10kgTPday−1) fluxes only occurred when shallow groundwater was within 0.6m of the ground surface and runoff coefficients were greater than 0.1. These thresholds were reached less frequently in 2012 due to drought recovery resulting in lower annual nitrate-N (7.4kgNha−1) and TP (0.12kgPha−1) fluxes in comparison with 2013 (15.1kgNha−1; 0.21kgPha−1). The wet winter of 2013 with elevated shallow groundwater levels led to more frequent activation of sub-surface pathways and tile drain flow. Throughout the period, dry antecedent conditions had a temporary effect in elevating TP loads. Evidence of TP source exhaustion after consecutive storm events can be attributed to the repeated depletion of temporarily connected critical source areas to the river network via impermeable road surfaces. Fertiliser application varied considerably across three years due to differences in crop rotation between farms, with annual N and P fertiliser inputs varying by up to 21% and 41%, respectively. Proportional reductions in annual riverine nitrate-N and TP loadings were not observed at the sub-catchment outlet as loadings were largely influenced by annual runoff. Nitrate loadings were slightly higher during fertiliser application, but there was little relationship between P fertiliser application and riverine TP load. These data indicate that this intensive arable catchment may be in a state of biogeochemical stationarity, whereby legacy stores of nutrients buffer against changes in contemporary nutrient inputs.