Nitrous oxide production in soils cropped to corn with varying N fertilization

Abstract

Mineral N fertilizers may contribute to N gas emissions to the atmosphere. Soil cores were collected in 1993 and 1994, in a sandy loam and a sandy clay cropped with an early-maturing corn (Zea mays L.) hybrid and fertilized with ammonium nitrate at rates of 10, 60, 120 or 180 kg N ha −1 . Denitrification and N 1 O production rates, air-filled porosity (AFP), water-soluble mineral N (WSMN) and water-soluble organic C (WSOC) were measured. Denitrification and N 2 O production rates were generally small, but values >2 µg N 2 O-N kg −1 h −1 were measured (i) when WSMN contents exceeded 5 mg kg −1 , and (ii) when AFP was <50 to 55% in the sandy loam, and <40 to 45% in the sandy clay. For most sampling dates, N 2 O production and denitrification rates increased with N fertilizer level. In 1993, AFP was relatively high and variable in soil cores, and regression analyses revealed that denitrification rates were closely related to AFP. In 1994, AFP was relatively low in soil cores, and regression analyses showed that denitrification and N 2 O production rates were positively related to WSMN and negatively to WSOC. It is suggested that provided AFP was low, N fertilization may have had either a direct effect on denitrification and N 2 O production rates by determining WSMN availability to microorganisms, or an indirect effect by affecting WSOC metabolism in soil. Depending on the year and soil type, mean denitrification rates were 40 to 130% greater in the soil with 180 than with 120 kg N ha −1 . Corresponding N 2 O production rates were 50 to 200% higher in the 180 than in the 120 kg N ha −1 treatment. It appears that limiting N fertilizer to 120 kg ha −1 , under early-maturing corn production, may prevent excessive gaseous N losses due to denitrification. Key words: Denitrification, nitrous oxide, N fertilizers, available N, grain corn