Nitrogen inputs best predict farm field nitrate leaching in the Willamette Valley, Oregon

Nitrate leaching is an important yet difficult to manage contribution to groundwater and surface water contamination in agricultural areas. We examined 14 farm fields over a four year period (2014-2017) in the southern Willamette Valley, providing 53 sets of annual, field-level agricultural performance metrics related to nitrogen (N), including fertilizer inputs, crop harvest outputs, N use efficiency (NUE), nitrate-N leaching and surplus N. Crop-specific nitrate-N leaching varied widely from 10 kg N ha(-1) yr(-1) in hazelnuts to > 200 kg N ha(-1) yr(-1) in peppermint. Averaging across all sites and years, most leaching occurred during fall (60%) and winter (32%). Overall NUE was 57%. We used a graphical approach to explore the relationships between N inputs, surplus, crop N harvest removal and NUE by crop type. The blueberry site had high inputs and surplus, peppermint had high inputs but also high crop N removal and NUE and thus lower surplus, and most wheat crops had high NUE and evidence of using soil N. Annual N surplus was not well correlated with leaching, and leaching varied more by crop type and inputs. Grass seed and hazelnuts, which are dominant crop types in the southern Willamette Valley, were intermediate in terms of NUE, leaching and surplus. Of all performance metrics, N input was most closely aligned with field-level crop N harvest and nitrate leaching, therefore optimizing N inputs may well inform local efforts to reduce groundwater nitrate contamination.

Automated summary

Nitrate leaching is a significant issue contributing to groundwater and surface water contamination in agricultural areas, with notable variations in crop-specific leaching rates. The study in the southern Willamette Valley revealed that most leaching occurred during fall and winter, with an average NUE of 57% across all sites and years. Optimizing N inputs could help in reducing groundwater nitrate contamination.