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 NH ₄NO₃ 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 flint 23 days after application of fertiliser in combination with rye (Secale cereale) residues under zero tillage. CO ₂ 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 ¹⁵N-N₂O emission following application of ¹⁵N-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 ¹⁵N 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.