Nitrous oxide emission as affected by liming an acidic mineral soil used for arable agriculture

The effect of liming an acidic mineral soil (Dystric Nitosol from southern China), used for arable agriculture, on N2O emission was studied in an incubation experiment. After the soil pH had been raised from pH 4.4 to 5.2, 6.7 and 8.1, soil samples were either amended with NH4+ and incubated aerobically, favoring nitrification or, after application of NO3−, the incubation took place under anaerobic conditions, favoring denitrification. Gas sampling for N2O determination and soil analyses were performed at regular intervals up to 13 days. Under nitrification conditions only small N2O emission rates were observed (max. 6 μg N kg−1 d−1) with significant differences between high and low pH values during the first 2 days of incubation. The nitrifying activity was low, even with high pH, and this, together with good aeration conditions, could partly explain the small N2O evolution. During denitrification, however, cumulative N2O emissions reached much higher values (1600 μg N kg−1 in comparison to 40 μg N kg−1 under nitrification conditions). N2O emission during denitrification was significantly enhanced by increasing soil pH. Under alkaline conditions (pH 8.1) a large nitrite accumulation occurred, which was in line with the highest nitrate reductase activity determined in this treatment. The limited availability of organic carbon is probably the main reason for the absence of further reduction of NO2− to N2O or N2. At pH 6.7 the total N2O emission was slightly higher than at pH 8.1, although the start of pronounced emissions was retarded and only small amounts of NO2− accumulated. Acid soil conditions caused either negligible (pH 4.4) or only small (pH 5.2) N2O emissions. It can be concluded that these kinds of soil, used alternatively for production of upland crops or paddy rice, are prone to high N2O emissions after flooding, particularly under neutral to alkaline conditions. In order to avoid major N2O evolution and accumulation of nitrite, which can be leached into groundwater, the pH should not be raised to values above 5.5–6.