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

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 cla y cropped with an early-maturing corn (Zea mays L.) hybrid and fertilized w ith ammonium nitrate at rates of 10, 60, 120 or 180 kg N ha(-1). Denitrific ation and N2O production rates, air-filled porosity (AFP), water-soluble mi neral N (WSMN) and water-soluble organic C (WSOC) were measured Denitrifica tion and N2O production rates were generally small, but values >2 mu g N2O- N kg(-1) h(-1) were measured (i) when WSMN contents exceeded 5 mg kg(-1) an d(ii) when AFP was <50 to 55% in the sandy loam, and <40 to 45% in the sand y clay. For most sampling dates, N2O production and denitrification rates i ncreased with N fertilizer level. In 1993, AFP was relatively high and vari able in soil cores, and regression analyses revealed that denitrification r ates were closely related to AFP. In 1994, AFP was relatively low in soil c ores, and regression analyses showed that denitrification and N2O productio n rates were positively related to WSMN and negatively to WSOC. It is sugge sted that provided AFP was low, N fertilization may have had either a direc t effect on denitrification and N2O production rates by determining WSMN av ailability to microorganisms, or an indirect effect by affecting WSOC metab olism in soil. Depending on the year and soil type, mean denitrification ra tes were 40 to 130% greater in the soil with 180 than with 120 kg N ha(-1). Corresponding N2O 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.