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 180kgNha-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±0.08kgNha-1 in V/M versus 0.10±0.05kgNha-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±0.17kgNha-1, while NT exacerbated N2O emissions in the legume-based cropping system (0.80±0.07kgNha-1 in V/M versus -0.07±0.06kgNha-1 in O/M). Maize yield was not affected by tillage, but increased from 2.32Mgha-1 in O/M to 4.44Mgha-1 in V/M. Yield-scaled N2O emissions varied from -33g N2O-NMg-1 grain in NT O/M to 179g N2O-NMg-1 grain in NT V/M, and were intermediate in CT soil (106 and 156g N2O-NMg-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 (67gNMg-1 grain) was half of that after urea-N application (152gNMg-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.
