Methane oxidation potentials and fluxes in agricultural soil: Effects of fertilisation and soil compaction

We have studied the inhibiting effect of fertilisation and soil compaction on CH4 oxidation by measuring gas fluxes and soil mineral N dynamics in the field, and CH4 oxidation rates in laboratory-incubated soil samples. The fertilisation and soil compaction field experiment was established in 1985, and the gas fluxes were measured from 1992 to 1994. Methane oxidation was consistently lower in fertilised than in unfertilised soil, but there apparently was no effect of repeated fertiliser additions on the fertilised plots. The measured mineral N in fertilised and unfertilised soil showed large differences in NH4+ concentrations just after fertilisation, but the levels rapidly converged because of plant uptake and nitrification. The CH4 oxidation rate did not reflect these contrasting mineral N patterns, suggesting that the CH4 oxidation capacity remaining in the soil that had been fertilised since 1985 was largely insensitive to ammonia in the new fertiliser. Thus, competitive inhibition by ammonia may have been involved in the early stage of the field fertiliser experiment, but the CH4 oxidation remaining after 7 to 9 years of continued fertilisation seems not to have been affected by ammonia. The substrate affinity of the CH4-oxidizing microflora appeared to be the same in both the fertilised soil and the unfertilised control, as judged from the response to elevated CH4 concentrations (52 mu l l(-1)) in laboratory incubations. Soil compaction resulted in a persistent reduction of CH4 influx, also seen in laboratory incubations with sieved (4-mm mesh) soil samples. Since the sieving presumably removes diffusion barriers created by the soil compaction, the fact that compaction effects persisted through the sieving may indicate that soil compaction has affected the biological potential for CH4 oxidation in the soil.