Long-Term Observa ons of Vadose Zone and Groundwater Nitrate Concentra ons under Irrigated Agriculture

290 T demand for higher crop yields and the relatively low cost of N fertilizer per unit of yield increase has resulted in increased NO3–N concentrations in the groundwater, especially in areas with coarse-textured soils and shallow groundwater. An increased level of NO3–N in the groundwater is not a locally confi ned problem, however, as NO3–N from agricultural sources in the Midwest has been linked to problems as far reaching as the hypoxic zone in the Gulf of Mexico (Burkart and James, 1999). Excess NO3–N entering the groundwater and surface water from agricultural areas is the result of a number of factors. In the most basic sense, NO3–N concentration increases in the vadose zone and groundwater because there is more N in the system than can be utilized by an actively growing crop or otherwise immobilized. Numerous studies have reported higher NO3–N leaching and higher soil test N with higher N fertilizer application rates compared with reduced input management. Andraski et al. (2000) reported suction cup lysimeter NO3–N concentrations <10 mg L−1 when N application was below the economic optimum N rate (EONR) but >20 mg L−1 when N application was above the EONR. Angle et al. (1993) measured soil N levels of 8.7 mg kg−1 when 260 kg ha−1 was applied compared with only 2.5 mg kg−1 with no fertilizer applied. Martin et al. (1994) found 106 kg ha−1 less NO3 leaching when using a split application of 126 kg ha−1 compared with a 232 kg ha−1 preplant application. Several studies have found that higher than optimum irrigation rates or excessive rainfall after irrigation caused an increase in NO3–N leaching on sandy soils. Errebhi et al. (1998) reported that NO3–N leaching increased by 35% when the fertility rate was increased from 0 to 135 kg N ha−1 in a normal year, compared with an increase of 157% in a year with heavy rains in the growing season. Gehl et al. (2005) found that an irrigation rate of 1.25 times the optimum resulted in 64 kg N ha−1 more leaching than the optimum irrigation rate. Gentry et al. (1998) reported higher tile drain NO3–N losses (64 vs. 38 kg N ha −1) when soil NO3–N was high after a poor corn-growing season, emphasizing the infl uence of the weather on NO3–N leaching. High soil NO3–N levels, leading to increased N losses in subsurface drainage and groundwater, have been shown to result from increased mineralization of soil organic N during periods of favorable soil temperature and moisture conditions. Albus and Knighton (1998) indicated an increase in shallow groundwater NO3–N concentration from 13 to 40 mg L −1 29 mo after irrigation was initiated on a previously unirrigated site, and Cambardella et al. (1999) reported soil NO3–N values of Long-Term Observa ons of Vadose Zone and Groundwater Nitrate Concentra ons under Irrigated Agriculture