Greenhouse estimates of CO2 and N2O emissions following surface application of grass mulch: importance of indigenous microflora of mulch

The effects of surface-applied, fresh grass mulch on CO2 and N2O emissions were determined for 7 weeks in a greenhouse microcosm study under aerated conditions with daily irrigation (1.8 mm day−1). Dynamics of CO2 and N2O emissions were determined by automated, continuous flux measurements. Grass mulch was added (C input: 100 g C m−2, N input: 9.2 g N m−2) to soil columns (Ap horizon of a cambisol) and quartz sand columns to assess the importance of indigenous microflora of grass residues for mulch decomposition and N2O production. About 76% of the grass mulch carbon was respired within 50 days, regardless of whether the grass mulch was added to soil or quartz sand. Total N2O–N emissions caused by the surface application of grass mulch was equivalent to 0.3% (application on soil) and 0.2% (application on quartz sand) of the N applied in the grass mulch. As much as 89% of the diurnal variability in N2O emission and 80% of the diurnal variability in CO2 release from the decomposing grass mulch could be explained by changes in the temperature. The results show that indigenous microflora present on the plant residues determine both the decomposition rate of the surface-applied grass mulch and N-mineralisation and denitrification of mulch N.