Nitrate-nitrogen in effluent from agricultural tile drains in California

Subsurface drainage water of agricultural lands collected in tile lines may contain nitrate-nitrogen which serves to pollute surface waters. The objective of the research was to determine quantities and concentrations of nitrate-nitrogen in tile drainage and their relationship to soil, fertilizer input, crop, and time with respect to water applicaiton. The first research phase consisted of extensive monitoring of tile drainage effluents collected in important agricultural areas in California. More intensive investigation of nitrate-nitrogen within a few soil profiles was done during the second phase. Nitrate-nitrogen concentrations were not well-correlated with any measured parameter. The amount of nitrate-nitrogen contained in tile effluent was quite well-correlated with total water discharge and fertilizer nitrogen application. Higher amounts of nitrate-nitrogen were removed from coarse-textured soil profiles as compared to profiles containing a layer of fine-textured material. Some alluvial soils on the west side of the San Joaquin Valley apparently contain significant native nitrogen which contributes both to high nitrate-nitrogen concentrations and total mass emissions in tile effluent. Implications of the research findings to nitrogen and water management to achieve a desired water quality standard are discussed. It appears that the most positive approach to controlling nitrate-nitrogen in irrigated agriculture is through control of leaching.