Date of this Version
Nitrate leaching from agricultural soils can increase groundwater nitrate concentrations. The objectives of the study were to assess the accumulation and movement of nitrate in the soil profile over a 2-year period under def¬icit irrigation conditions following a one time application of N in cattle feedlot manure and commercial fertilizer to corn at rates to achieve yield goals expected under conditions of full irrigation. Cattle manure and ammonium nitrate were applied in 2002 at the University of Nebraska recommended rate (1M and 1F, respectively) and cat¬tle manure was applied at twice the recommended rate (2M) for N for the 2002 corn (Zea mays L.) crop. The rec¬ommended rate was based on expected yields under full irrigation. The manure N treatments were applied to percolation lysimeters and adjacent plots on a Cozad silt loam soil. Ammonium nitrate was applied only to the percolation lysimeters. Leachate from the lysimeters was extracted from a depth of 2.1 m and soil samples were collected from field plots in 0.3 m depth increments to 2.1 m on a periodic basis. Water available to the crop was sufficient to meet 89 and 79% of the potential crop ET in 2002 and 2003, respectively. When averaged over the ma¬nure N treatments, reduced ET resulted in grain yields that were approximately 2.1 and 2.7 Mg ha-1 less than ex¬pected in 2002 and 2003. Under deficit water inputs there was leachate movement below the root zone. Leachate depths averaged over N treatments were, however, reduced by 15% (33 mm) in 2002 and 47% (102 mm) in 2003 compared with those reported under full irrigation. The average nitrate-N (NO3−-N) concentrations in leachate were higher under the 2M treatment (41 mg L-1) compared to the 1M treatment (17 mg L-1). The average NO3−-N concentrations in leachate from the 1F treatment (28 mg L-1) was not different than the 1 or 2M treatments. There were trends for greater NO3−-N mass losses in leachate averaged over all treatments in 2003 compared to 2002, indicating that NO3−-N derived from the 2002 application leached to at least 2.4 m below the soil surface. There were no mass loss differences in leachate due to the 2001 crop in 2002. In 2003, mass of NO3--N in lysimeters cropped to soybean in 2001 were significantly higher (144 kg NO3--N ha-1) than the mass in lysimeters cropped to corn in 2001 (51 kg NO3--N ha-1). Nitrate-N mass increased in the 0.9–2.1 m soil depth 12–13 months after ma-nure N treatment applications. The 2M treatment had greater soil NO3−-N mass than the 1M treatment for most sampling dates in the surface 0.9 m. This research shows that there can be significant nitrate losses under deficit irrigation when manure N is over applied. These losses are likely related to water initially stored below the root zone and preferential flow of water from irrigation and precipitation. Determining accurate yield expectations under deficit irrigation conditions, correct scheduling of irrigation, and the use current best management prac¬tices for N management can help minimize nitrate losses in leachate.