A short-term investigation of trace gas emissions following tillage and no-tillage of agroforestry residues in western Kenya

Improved-fallow agroforestry systems are increasingly being adopted in the humid tropics for soil fertility management. However, there is little information on trace gas emissions after residue application in these systems, or on the effect of tillage practice on emissions from tropical agricultural systems. Here, we report a short-term experiment in which the effects of tillage practice (no-tillage versus tillage to 15 cm depth) and residue quality on emissions of N₂O, CO₂ and CH₄ were determined in an improved-fallow agroforestry system in western Kenya. Emissions were increased following tillage of Tephrosia candida (2.1 g N₂O-N ha^?1 kg N applied^?1; 759 kg CO₂-C ha^?1 t C applied^?1; 30 g CH₄-C ha^?1 t C applied^?1) and Crotalaria paulina residues (2.8 g N₂O-N ha^?1 kg N applied^?1; 967 kg CO₂-C ha^?1 t C applied^?1; 146 g CH₄-C ha^?1 t C applied^?1) and were higher than from tillage of natural-fallow residues (1.0 g N₂O-N ha^?1 kg N applied^?1; 432 kg CO₂-C ha^?1 t C applied^?1; 14.7 g CH₄-C ha^?1 t C applied^?1) or from continuous maize cropping systems. Emissions from these fallow treatments were positively correlated with residue N content (r = 0.62–0.97; P < 0.05) and negatively correlated with residue lignin content (r = ?0.56, N₂O; r = ?0.92, CH₄; P < 0.05). No-tillage of surface applied Tephrosia residues lowered the total N₂O and CO₂ emitted over 99 days by 0.33 g N₂O-N ha^?1 kg N applied^?1 and 124 kg CO₂-C ha^?1 t C applied^?1, respectively; estimated to provide a reduction in global warming potential of 41 g CO₂ equivalents. However, emissions were increased from this treatment over the first 2 weeks. The responses to tillage practice and residue quality reported here need to be verified in longer term experiments before they can be used to suggest mitigation strategies appropriate for all three greenhouse gases.