Long-term effects of cropping system on N2O emission potential

The potential for N2O emissions outside the main growing season may be influenced by long-term effects of cropping system. This was investigated by collecting intact soil cores (100 cm(3), 0-4 cm depth) under winter wheat in three organic cropping systems and a conventional reference within a long-term crop rotation experiment. Average annual inputs of C in crop residues and manure ranged from 1.7 to 3.3 Mg ha(-1). A simulated freeze-thaw cycle resulted in a flush of CO2 during the first 48 h, which could be mainly from microbial sources. Other samples were adjusted to approximately -10, -30 or -100 hPa and amended with excess (NO(3) over bar)-N-15 prior to freezing and thawing. Denitrification was the main source of N2O during a 72-h incubation at 22 degrees C, as judged from N2O and total N-15 evolution. Although the input of C in the conventionally managed cropping system was significantly less than in the organic cropping systems, it showed higher N2O evolution at all three matric potentials. Estimates of relative gas diffusivity (D-p/D-0) in soil from the four cropping systems indicated that C input affected soil aeration. Soil from the two cropping systems with highest C input showed N2O evolution at D-p/D-0 in excess of 0.02, which is normally considered a threshold for development of anaerobic sites in the soil, presumably because the oxygen demand was also high. The study shows that cropping system affects both soil gas diffusivity and C availability, and that both characteristics significantly influence the N2O emission potential. (C) 2012 Elsevier Ltd. All rights reserved.