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 treated 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 adjacent to a drainage canal. We observed brief increases of both CH4 and N2O emissions immediately following soil applications of digested sewage sludge. Cumulated 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 cumulated 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 significant effect on CO2 and NO emissions was still obvious in the field, the 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 field 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) in dry and wet areas. In a model experiment with incubated soil cores, nitrogen amendment (NH4Cl) and perturbation significantly reduced CH4 oxidation in the forest soil, presumably as a result of increased nitrification activity. Sludge also induced net CH4 production in the otherwise strong CH4 oxidising forest soil. This emphasises the potential for CH4 emissions from sewage sludge applications onto land. The study shows, however, that emissions of N2O and CH4 induced by sewage sludge in the field is of minor importance and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH4 and N2O.