KWAME AGYEI FRIMPONG, ELIZABETH M. BAGGS
University of Aberdeen, Institute of Biological and Environmental Sciences, United Kingdom
Emissions of N2O were measured following addition of 15N-labelled residues of Vigna unguiculata (cowpea), Mucuna pruriens and Leucaena leucocephala) to a Ferric Luvisol from Ghana at a rate of 100 mg N kg-1 soil under controlled environment conditions. Residues were also applied in different ratio combinations with inorganic N fertiliser, each combination applied at a total rate of 100 mg N kg-1 soil. N2O emissions were increased after addition of residues, and further increased with combined applications of residues and inorganic N fertiliser. However, 15N-N2O production was low and short-lived in all treatments, suggesting that most of the measured N2O-N was derived from the applied fertiliser or native soil mineral N pools. There was no consistent trend in magnitude of emissions with increasing proportion of inorganic fertiliser in the application. The positive interactive effect between residue- and fertiliser-N sources was most pronounced in the 25:75 Leucena:fertiliser treatment where 22.5 g N2O-N m-2 kg biomass-1 was emitted over 30 days. N2O (loge) emission over the 30 day experiment from all residue amended treatments was positively correlated with residue C:N ratio (r=0.63; p < 0.05), and negatively correlated with residue polyphenol content (r=-0.59; p < 0.05), polyphenol:N ratio (r=-0.61; p < 0.05) and (lignin+polyphenol):N ratio (r=-0.61; p < 0.05), indicating the role of residue chemical composition, or quality, in regulating emissions even when combined with inorganic fertiliser. Under our controlled experimental conditions the 75:25 residue: fertiliser ratio appeared to offer the best compromise between release of N and management of N2O emission, but the positive interactive effect even in this treatment means that it is unlikely that combined applications of residues and inorganic fertiliser can lower N2O emissions, unless the residue is of very low quality promoting strong immobilisation of soil mineral N.
Keywords: Crop residues, inorganic fertiliser, nitrous oxide, stable isotopes, tropical soil