Soil nitrous oxide emissions as affected by long-term tillage, cropping systems and nitrogen fertilization in Southern Brazil

• Greater N2O fluxes occurred in legume- than in grass-based cropping system.
• Legume cover-crop exacerbate soil N2O emissions only in no-tillage.
• Both nitrification and denitrification processes were involved in N2O production.
• Yield-scaled N2O emission was 2-fold greater with fertilizer-N than biologically fixed N.
• Partial replacement of fertilizer-N by legume cover-crops-N helps mitigate N2O.

Soil nitrous oxide (N2O) emissions are affected by management practices, but little information is available on the interactive effects of tillage, cropping systems and N sources in tropical and subtropical soils. In an 18-yr old experiment located in a subtropical Acrisol of Southern Brazil we conducted a sequence of two trials. The 1-year trial (October 2003–2004) was set to evaluate the long-term effects of tillage [CT: conventional; and NT: no-tillage] and cropping systems [O/M: black oat (Avena strigosa Schreb.)/maize (Zea mays L.); and V/M: vetch (Vicia sativa L.)/maize] on soil N2O emissions, either in the post-management period (45 days after desiccation and knife-rolling of winter cover crops) or in the whole year. The second and short-term trial (October–November 2004) was carried out to compare the impact of N sources [urea (mineral) and legume-residue of vetch (biologically fixed), both at 180 kg N ha−1] on soil N2O emissions during 53 days after cover-crop management. Air sampling was carried out by static chambers and N2O analysis by gas chromatography. In the 45-day post-management period of the 1-year trial, soil N2O emissions were practically not affected by tillage systems, but increased 4 times due to vetch residues (average of 0.40 ± 0.08 kg N ha−1 in V/M versus 0.10 ± 0.05 kg N ha−1 in O/M) and related with soil contents of NO3−-N, NH4+-N, and dissolved organic C (DOC). Over the whole year, soil N2O emissions under CT were similar for grass- and legume-based cropping systems and averaged 0.43 ± 0.17 kg N ha−1, while NT exacerbated N2O emissions in the legume-based cropping system (0.80 ± 0.07 kg N ha−1 in V/M versus −0.07 ± 0.06 kg N ha−1 in O/M). Maize yield was not affected by tillage, but increased from 2.32 Mg ha−1 in O/M to 4.44 Mg ha−1 in V/M. Yield-scaled N2O emissions varied from −33 g N2O-N Mg−1 grain in NT O/M to 179 g N2O-N Mg−1 grain in NT V/M, and were intermediate in CT soil (106 and 156 g N2O-N Mg−1grain in V/M and O/M cropping systems, respectively). In the short-term trial, the N2O emitted in excess relative to the control treatment (O/M without N fertilizer) was at least 3 times greater with urea-N (0.44% of applied N) than with legume-residue-Nsource (0.13% of applied N). Yield-scaled N2O emission after vetch residues management (67 g N Mg−1 grain) was half of that after urea-N application (152 g N Mg−1 grain). Partially supplying the maize N requirements with winter legume cover-crops may be a feasible strategy to mitigate soil N2O emissions in the subtropical conservation agriculture.