Nitrous oxide emissions following application of residues and fertiliser under zero and conventional tillage

Emissions of N2O were measured following combined applications of inorganic N fertiliser and crop residues to a silt loam soil in S. E. England, UK. Effects of cultivation technique and residue application on N2O emissions were examined over 2 years. N2O emissions were increased in the presence of residues and were further increased where NH4NO3 fertiliser (200 kg N ha(-1)) was applied. Large fluxes of N2O were measured from the zero till treatments after residue and fertiliser application, with 2.5 kg N2O- N ha(-1) measured over the first 23 days after application of fertiliser in combination with rye ( Secale cereale) residues under zero tillage. CO2 emissions were larger in the zero till than in the conventional till treatments. A significant tillage/residue interaction was found. Highest emissions were measured from the conventionally tilled bean ( Vicia faba) (1.0 kg N2O- N ha(-1) emitted over 65 days) and zero tilled rye (3.5 kg N2O-N ha(-1) over 65 days) treatments. This was attributed to rapid release of N following incorporation of bean residues in the conventionally tilled treatments, and availability of readily degradable C from the rye in the presence of anaerobic conditions under the mulch in the zero tilled treatments. Measurement of N-15-N2O emission following application of N-15-labelled fertiliser to microplots indicated that surface mulching of residues in zero till treatments resulted in a greater proportion of fertiliser N being lost as N2O than with incorporation of residues. Combined applications of N-15 fertiliser and bean residues resulted in higher or lower emissions, depending on cultivation technique, when compared with the sum of N2O from single applications. Such interactions have important implications for mitigation of N2O from agricultural soils.