Effect of NH4Cl addition on methane oxidation by paddy soils

Abstract Methane emission from flooded rice fields is a net product of CH 4 production and oxidation. The ability of paddy soils to oxidize CH 4 produced endogenously is an important natural feature for mitigating CH 4 emission to the atmosphere. We conducted a series of studies on CH 4 oxidation by two fresh paddy soils (Wuxi soil and Yingtan soil). The soils were amended with NH 4 Cl at rates equivalent to 0, 10 and 50 μg N g −1 soil, respectively, and incubated over five consecutive periods at elevated CH 4 concentrations of ∼500, ∼1000, ∼1500 or ∼2000 μl l −1 in the headspace, respectively, at 25°C. NH 4 Cl addition inhibited the rates of CH 4 oxidation by the paddy soils at all initial CH 4 concentrations during the first incubation. The inhibitory effect was strengthened with increasing NH 4 Cl addition and alleviated with increasing initial CH 4 concentration, i.e. there was an interaction of NH 4 Cl and initial CH 4 concentration on the inhibitory effect. If the initial CH 4 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, NH 4 Cl addition stimulated the CH 4 oxidation rate. If the initial CH 4 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 CH 4 concentration in paddy soils when they are flooded is generally higher than ∼2000 μl l −1 , we conclude that the initial effect of NH 4 Cl addition on CH 4 oxidation in paddy soils is temporary.