Long‐Term Nitrate Leaching Under Vegetable Production with Cover Crops in the Pacific Northwest

Abstract

Nitrate (NO3−–N) lost through leaching contaminates the groundwater in many agricultural areas. Understanding the relationship between leaching and climate is an important consideration for developing nutrient management strategies. Soil water was sampled for 11 yr under plots with summer vegetables grown in the Willamette Valley, Oregon, with and without winter cover crops. A randomized complete‐block, split‐plot design was employed with three N fertilizer application rates (N0 for no fertilizer, N1 for an intermediate input, and N2 for a full, recommended N input). Fall‐seeded cover crops included cereal rye (Secale cereale L. var. Wheeler), triticale (Triticosecale X L. var. Celia), and a common vetch (Vicia sativa)/triticale mix. Twenty‐six, 0.26‐m2 passive capillary samplers (PCAPS), which are capable of sampling in unsaturated and saturated conditions, allowed for the uninterrupted measurement of soil water and solute flux at a depth of 1.2 m. Eleven‐year flow‐weighted average NO3−–N concentrations (mg L−1) for fallow (F) and cover cropped (CC) treatments were 16.7 and 11.9 for the N2 fertilizer treatment, 9.9 and 6.4 for the N1 fertilizer treatment, and 7.0 and 4.1 for the N0 treatment. Annual average NO3−–N mass losses (kg ha−1) for F and CC treatments were 76 and 46 for the N2 fertilizer treatment, 40 and 30 for the N1 fertilizer treatment, and 29 and 17 for the N0 treatment. Fully fertilized CC and F treatments consistently exceeded the EPA's 10 mg L−1 drinking water standard set for NO3−–N. The timing and quantity of yearly rainfall, particularly from October to December, was a dominant contributor to the fluctuations of NO3−–N concentrations and mass losses. Soil NO3−–N and bromide soil concentration profiles suggested that high leaching losses in leaching Year 2001 were related to the previous drier‐than‐average winter and the early arrival of rainfall following the vegetable harvest. The long‐term water quality benefits observed under the CC treatments suggest that an increase in organically bound N is occurring.