Nitrous oxide emission as affected by changes in soil water content and nitrogen fertilization

Nitrous oxide emission was measured in laboratory incubations of an alluvia l soil (58% clay, pH 7.4). The soil was amended with 40 mg N kg(-1) as NaNO 3 or NH4Cl, or with NaCl as a control. Each fertilization treatment was adj usted to three different water contents: constant 60% WHC (water-holding ca pacity), constant 120% WHC, and water content alternating between 60 and 12 0% WHC. During an 8-day incubation period N2O emission rates and inorganic nitrogen concentrations in soil (NH4+, NO2-, NO3-) were determined at regul ar intervals. In the control and after nitrate application small N2O emissi on rates occurred with only minor variations over time, and no differences between the water treatments. In contrast, with ammonium application N2O em ission rates were much higher during the first two days of incubation, with peaks in the constant 60% WHC and 120% WHC at day 1 and in the changing-wa ter treatment at day 2, when the first wet period (120% WHC) was completed. This N2O peak in the changing-water treatment was 4 to 9 times higher than with constant WHC and occurred when both, NH4+ and NO2- concentrations dec lined sharply. Thus, this N2O emission flush can be attributed to nitrifier denitrification, After the second rewetting of the NH4+-amended soil no fu rther N2O emission peak was observed, being in accordance with small NH4+ a nd NO2- concentrations in soil at that time. The unexpectedly small N2O flu xes in the constant 120% WHC treatment after nitrate application were proba bly caused by the reduction of N2O to N-2 under the prevailing conditions. It can be concluded that continuous wetting or flooding of a soil is an eff ective measure to reduce N2O emissions immediately after the :application o f NH4+ fertilizers.