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