Assessment of CH4 and N2O fluxes in a Danish beech (Fagus sylvatica) forest and an adjacent N-fertilised barley (Hordeum vulgare) field: effects of sewage sludge amendments

Fluxes of CH4 and N2O were measured regularly in an agricultural field trea ted with 280 g m(-2) of sewage sludge. In a nearby beech forest N2O and CH4 fluxes were measured in a well-drained (dry) area and in a wet area adjace nt to a drainage canal. We observed brief increases of both CH4 and N2O emi ssions immediately following soil applications of digested sewage sludge. C umulated values for CH4 emissions over the course of 328 days after sludge applications indicated a small net source in sludge treated plots (7.6 mg C m(-2)) whereas sludge-free soil constituted a small sink (-0.9 mg C m(-2)) . The CH4 emission amounted 0.01% of the sludge-C. Extrapolated to current rates of sludge applications in Danish agriculture this amounts to 0.1% of the total agricultural derived CH4. Sludge applications did not affect cumu lated fluxes of N2O showing 312 mg N2O-N m(-2) and 304 mg N m(-2) with and without sludge, respectively. Four months after the sludge applications a s ignificant effect on CO2 and NO emissions was still obvious in the field, t he latter perhaps due to elevated nitrification. Nitrous oxide emission in the beech forest was about six times smaller (45 mg N m(-2)) than in the fi eld and independent of drainage status. Methane oxidation was observed all- year round in the forest cumulating to -225 mg C m(-2) and -84 mg C m(-2) i n dry and wet areas. In a model experiment with incubated soil cores, nitro gen amendment (NH4Cl) and perturbation significantly reduced CH4 oxidation in the forest soil, presumably as a result of increased nitrification activ ity. Sludge also induced net CH4 production in the otherwise strong CH4 oxi dising forest soil. This emphasises the potential for CH4 emissions from se wage sludge applications onto land. The study shows, however, that emission s of N2O and CH4 induced by sewage sludge in the field is of minor importan ce and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH4 and N2O.