Effect of NH4Cl addition on methane oxidation by paddy soils

Methane emission from flooded rice fields is a net product of CH4 production and oxidation. The ability of paddy soils to oxidize CH4 produced endogenously is an important natural feature for mitigating CH4 emission to the atmosphere. We conducted a series of studies on CH4 oxidation by two fresh paddy soils (Wuxi soil and Yingtan soil). The soils were amended with NH4Cl at rates equivalent to 0, 10 and 50 μg N g−1 soil, respectively, and incubated over five consecutive periods at elevated CH4 concentrations of ∼500, ∼1000, ∼1500 or ∼2000 μl l−1 in the headspace, respectively, at 25°C. NH4Cl addition inhibited the rates of CH4 oxidation by the paddy soils at all initial CH4 concentrations during the first incubation. The inhibitory effect was strengthened with increasing NH4Cl addition and alleviated with increasing initial CH4 concentration, i.e. there was an interaction of NH4Cl and initial CH4 concentration on the inhibitory effect. If the initial CH4 concentration was sufficiently high (above ∼2000 μl l−1 for Wuxi soil and ∼1000 μl l−1 for Yingtan soil), the initial inhibitory effect was alleviated and then eliminated during subsequent incubations. Eventually, NH4Cl addition stimulated the CH4 oxidation rate. If the initial CH4 concentration was not sufficiently high (less than 500 μl l−1 in the headspace for both soils), the inhibitory effect was maintained during subsequent incubations. Considering the fact that the CH4 concentration in paddy soils when they are flooded is generally higher than ∼2000 μl l−1, we conclude that the initial effect of NH4Cl addition on CH4 oxidation in paddy soils is temporary.