Effect of three contrasting onion (Allium cepa L.) production systems on nitrous oxide emissions from soil

N2O emissions were measured from three contrasting onion (Allium cepa L.) p roduction systems over an 8.5-month period. One system was established on s oil where a clover sward had 3 months earlier been ploughed in (ploughed cl over site). This production system followed conventional production managem ent practices. The other two systems were established on soil where a mixed herb ley had 3 months earlier been either ploughed or rotovated. These las t two production systems followed the guidelines of the International Feder ation of Organic Agriculture Movements (IFOAM). Cumulative N2O emissions we re significantly greater from the ploughed clover site compared to the plou ghed ley site (3.8 and 1.6 kg N2O-N ha(-1), respectively), while cumulative N2O emissions from the ploughed ley and rotovated ley sites were not signi ficantly different from each other. Emissions from all sites were dominated by episodes of high N2O flux activity following seedbed preparation and dr illing, when soil water suction (SWS) was shown to be the rate-controlling variable. The decline in the N2O fluxes after these peak emissions followed clear exponential relationships of the form F=Ae(-kt) (r greater than or e qual to 0.91), where F is the daily flux and A is the y-intercept. First-or der decay constants (k) during these periods of declining N2O fluxes (corre sponding to half-lives of 2.6-3.0 days) were not significantly different in magnitude from the first-order rate constants that characterised the incre asing SWS. Gross differences in cumulative emissions between the clover and ley sites were attributed to the influence of differing soil pHs at the tw o sites on the N2O:(N2O + N-2) ratio in the denitrification products. It al so appeared that fertiliser applications to the clover site had both direct and indirect effects on N2O emissions by: (1) enhancing N2O emissions via potential nitrification, (2) increasing the NO3- supply for enhanced N2O em issions via denitrification, and (3) influencing the N2O:(N2O + N-2) ratio by lowering soil pH and increasing NO3- concentrations. Onion crop yields w ere greater at the clover site, mainly due to the higher density of plantin g made possible under a conventional production philosophy. Expressing the yield on the basis of net N2O emissions, 23 t onions kg(-1) N2O-N was obtai ned from the ploughed clover, which was double that obtained for the two sy stems based on the ley site. However, when the N2O emissions from the culti vation of the soils prior to the sowing of the onions was included, all thr ee systems produced a similar yield per kilogram of N2O-N emitted, averagin g 10 t kg(-1).