Lagged response of nutrient leaching to reduced surpluses at the field and catchment scales.

Lag times in reduced nutrient leaching after improved agricultural practices must be assessed to inform management decisions and evaluate their cost-effectiveness. This study estimated the leaching of phosphorus (P) and nitrogen (N) at two scales, a 33.8-ha nested field (data from 1973 to 2011) and an 820-ha agricultural catchment (data from 1992 to 2010). Field management was evaluated along with long-term nutrient trends. Estimates including and excluding nutrient concentrations in the stream under dry catchment conditions (mean 2.5 months year⁻¹) revealed that wastewater contributed 10% of the total P (TP) catchment leaching, mostly in dissolved reactive (DRP) form. More recent flow-proportional sampling at the two scales demonstrated 45% higher particulate P leaching from the field than with discrete water sampling, whereas a past discrete sampling strategy may have overestimated DRP contributions to the stream. The accumulated field P balance was increasingly negative after 1987, whereas crop yields increased. From 1998, the measured accumulated TP leaching from the field was lower than the mean accumulated leaching, considering the estimated downward trend (−10%) over the 38-year study period ( p < 0.05). The average total N (TN) field leaching of 28 kg ha⁻¹ year⁻¹ (1973-2011) was 10% higher than the catchment leaching (1992-2010). The rates and timing of fields N and P fertilization became better adapted to crop requirements over time, and thereafter, a 14% reduction in TN leaching was estimated. No significant trends were estimated for catchment leaching for either TP or TN over the 18-year period, demonstrating an extended lag period for reduced nutrient leaching and the need for long-term monitoring of the effects of pollution mitigation. Copyright © 2014 John Wiley & Sons, Ltd. Highlights Time lag in P and N leaching responses to reduce nutrient surpluses, Close relationship with N export at nested scale, Scale effects in Q indices and PP transport, Point sources of pollution prolonging the time lag for DRP leaching mitigation [ABSTRACT FROM AUTHOR]