Enhanced ammonium sources to reduce nitrate leaching

One approach to reduce NO3 movement to groundwater is increasing the proportion of N supplied to the crop as NH4-N. Nitrification inhibitors (NI's) can be used to enhance NH4-N supply, but most studies have focused on yield response, with little attention given to environmental impacts. To determine the effect of enhanced NH4 sources on corn grain yield, N uptake and NO3 movement to groundwater, three sidedress materials were compared during three different growing seasons. Application of anhydrous ammonia (AA) and addition of the NI, dicyandiamide (DCD) to urea-ammonium nitrate (UAN) both reduced NO3 leaching losses relative to that incurred with UAN. With AA and UAN + DCD (as compared with UAN) subsoil solution NO3 concentrations were reduced by an average of: 1.1 mg NO3-N kg(-1) soil following (fall 1993) a dry growing season; 2.4 mg NO3-N kg(-1) soil during (spring and summer 1994) and 1.4 mg NO3-N kg(-1) soil after (fall 1994) a wet growing season; and 0.5 mg NO3-N kg(-1) soil following (fall 1995) a growing season with intermediate rainfall. Based on average solution NO3 concentrations and approximate drainage after harvest, estimated N losses between harvest and freeze-up were 43, 22 and 19 kg N ha(-1) with UAN, UAN + DCD and AA, respectively (average of 3 years). Grain yields and aboveground N uptake were greater with AA and UAN + DCD than with UAN, and residual fertilizer N (applied N less aboveground N uptake) was 18, 6 and -2 kg N ha(-1) with UAN, UAN + DCD and AA, respectively (average of 3 years). As is often observed, the trend for greater yield with addition of the NI was not large or consistent enough to meet registration criteria. Data demonstrating reduced NO3 leaching are also relevant, and positive environmental impacts should be a criterion for registration. For growers who are reluctant to use AA, this would provide an alternative source to maximize yield while minimizing NO3 movement to groundwater.