Short- and medium-term effects of NH4+ on CH4 and N2O fluxes in arable soils with a different texture

The short- (24 h) and medium-term (30 d) effect of NH4+ on the CH4 and N2O fluxes from two arable soils with different textures have been tested. Soil A had a medium (loam) texture and a cation exchange capacity (CEC) of 8.4 cmol(+) kg−1. Soil B had a coarse (loamy sand) texture and a CEC of 4.7 cmol(+) kg−1. During the short-term experiment, both soils showed a decreasing CH4-oxidising capacity upon increasing NH4+ application rates. These CH4 oxidation rates were inversely related to the N2O emissions. The two soils showed slightly different CH4 oxidation kinetics. The ki value has been determined as the NH4+ concentration of the soil resulting in a reduction by 50% of the initial (no NH4+ added) CH4 oxidation rate constant (k1). The ki value in soil A (22.2 mg NH4+–N kg−1) was twice as high as that of soil B (12.4 mg NH4+–N kg−1). Similarly, the CEC of soil A was about twice as high as the CEC of soil B. This could, however, not explain the different N2O fluxes in soil A and B. Therefore, it was suggested that differences in the microbial community in both soils might be even more important in controlling the CH4 and N2O fluxes. This suggestion could also be made from the results obtained in the medium-term experiment. Herein, soil A showed a transient recovery of the CH4 oxidation rate upon NH4+ addition and a peak emission of N2O when the CH4 oxidation was inhibited. Soil B, however, gave a persistent inhibition of the CH4 oxidation upon NH4+ addition, a low N2O flux and a lag phase of 4 d of the nitrification activity. It is hypothesised that the microbial communities of each soil responded or developed differently to the CH4, NH4+, NO3− or NO2− concentrations. Therefore, it is suggested that identification of the microbial structure, response or diversity might be as important as soil physical-chemical parameters in explaining CH4 and N2O fluxes from aerobic soils.