Aeration effects on CO2, N2O, and CH4 emission and leachate composition of a forest soil

The availability of 02 is one of the most important factors controlling the chemical and biological reactions in soils. In this study, the effects of different aeration conditions on the dynamics of the emission of trace gases (CO2, N2O, CH4) and the leachate composition (NO3-, DOC, Mn, Fe) were determined. The experiment was conducted with naturally structured soil columns (silty clay, Vertisol) from a well aerated forest site. The soil monoliths were incubated in a microcosm system at different 02 concentrations (0, 0.001, 0.005, 0.01, 0.05, and 0.205 m(3) m(-3) in the air flow through the headspace of the microcosms) for 85 days. Reduced O-2 availability resulted in a decreased CO2 release but in increased N2O emission rates. The greatest cumulative N2O emissions (= 1.6 g N2O-N m(-2)) were observed at intermediate O-2 concentrations (0.005 and 0.01 m(3) m(-3)) when both nitrification and denitrification occurred simultaneously in the soil. Cumulative N2O emissions were smallest (= 0.05 g N2O-N m(-2)) for the aeration with ambient air (O-2 concentration: 0.205 m(3) m(-3)), although nitrate availability was greatest in this treatment. The emission of CH4 and leaching of Mn and Fe were restricted to the soil columns incubated under completely anoxic conditions. The sequence of the reduction processes under completely anoxic conditions complied with the thermodynamic theory: soil nitrate was reduced first, followed by the reduction of Mn(IV) and Fe(III) and finally CO2 was reduced to CH4. The re-aeration of the soil columns after 85 days of anoxic incubation terminated the production of CH4 and dissolved Fe and Mn in the soil but strongly increased the emission rates of CO2 and N2O and the leaching of NO3- probably because of the accumulation of DOC and NH4+ during the previous anoxic period.