Nitrous oxide emission and methane production and consumption by arable agriculture

This study examined the influence of different arable management practices in Canterbury, New Zealand, on nitrous oxide (N₂O) and methane (CH₄) fluxes from soils. These agricultural practices included: contrasting temporary pastures used for restoring soil structure, different cultivation techniques, nitrogen (N) fertilizer applications, contrasting onion production systems and contrasting straw residue management regimes. The data collected from these field experiments were used to develop a mechanistic model that describes N₂O emissions from arable soils. Nitrous oxide emissions were greatly enhanced by autumn ploughing of short-term grass/legume swards, with fluxes reaching 245 g N₂O-N ha⁻¹ day⁻¹. Cumulative emissions, measured from April until July, were greater following the ploughing of a clover sward ('ploughed clover') than emissions following the ploughing or rotovating of a mixed herb ley sward ('ploughed ley' and 'rotovated ley', respectively). During spring and summer, the three cultivated sites described above ('ploughed clover', 'ploughed ley; and 'rotovated ley') were used for the production-of onion crops. Nitrous oxide and CH₄ fluxes were determined throughout the cropping period until onion harvesting in the following March. One onion production system, using the 'ploughed clover' soil, followed conventional onion production guidelines, whilst the other two systems, based on the 'ploughed ley' and 'rotovated ley' soils, followed organic onion production guidelines. Cumulative N₂O emissions were significantly greater from the 'ploughed clover' site (3.8 kg N₂O -N ha⁻¹) compared to the 'ploughed ley' and 'rotovated ley' sites (1.6 and 2.4 kg N₂O-N ha⁻¹). Cultivation and seed drilling enhanced N₂O fluxes when soil conditions were moist (water filled pore space greater than 60%). The 'ploughed clover' soil exhibited the largest N₂O fluxes, reaching 189 g N₂O-N ha⁻¹ day⁻¹ within one week of drilling the onion crop. In contrast, the 'ploughed ley' and 'rotovated ley' soils exhibited peak N₂O emissions of 27 and 115 g N₂O-N ha⁻¹ day⁻¹, respectively, also within one week of drilling their onion crops. The most important driving variables controlling the N₂O emissions were considered to be mineral N, soil pH and soil moisture, expressed as either water filled pore space or soil water suction. Comparison of the onion yield on a per kg N₂O-N emitted basis showed that the crop grown at the 'ploughed clover' site produced 23.1 t onions kg-1 N₂O-N, which was twice that obtained from the 'ploughed ley' and 'rotovated ley' treatments. This was calculated for the period of onion production. But when the N₂O emissions associated with preplanting cultivation were included, the yield per quantity of N₂O emitted from all three systems (ploughed clover, ploughed and rotovated ley) was similar, averaging 10.0 t kg⁻¹ N₂O-N. Three contrasting straw residue management treatments (burning, incorporation or removal of residues) produced similar, low N₂O fluxes (1 to 2 g N₂O-N ha⁻¹ d⁻¹) throughout a 2-month monitoring period. These low fluxes were due to the low soil moisture content limiting both the rates of nitrification and denitrification. There were no significant differences between the 3 treatments. Application of 40 kg ammonium-N ha⁻¹ fertilizer to the three straw management treatments had no effect on N₂O emissions when compared to treatments receiving no fertilizer. A strong negative relationship between soil pH and the N₂O:( N₂O +N₂) ratio of the denitrification products (r = - 0.62; P < 0.01) was revealed in a series of denitrification enzyme activity (DEA) assays, conducted on soils from all field experiments. Manipulation of soil pH was identified as a potential mitigation option for N₂O emissions from agricultural soil. It is suggested that the maintenance of soil pH at about 6.5 might help to maintain a low mole fraction of N₂O emitted via denitrification without maximising nitrification activity. Results from the DEA assays also revealed a positive relationship between potential denitrification activity (PDA) and soil moisture content (r = 0.67; P < 0.01). It is proposed that soil water content or volumetric water content could be used as a predictor of PDA. A microscale mechanistic model, NOPAS II, for predicting N₂O fluxes from non-grazed pasture and arable soils was developed using some of the data collected during the cultivation and crop production experiments. This model relies on Michaelis-Menten kinetics to describe N₂O fluxes via nitrification and denitrification. Validation of the model, using seven independent datasets extracted from the field experiments described in this study, showed no significant lack of fit (P < 0.05; r = 0.987) between measured and predicted cumulative N₂O emissions. Methane uptake by soil was low (averaging 1.5 g CH₄-C ha⁻¹ day⁻¹) throughout all experiments conducted in this study. During the production of the onion crop, CH₄ uptake was greater at the ley site compared to the clover site, while, within the clover site, the non-cropped control treatment exhibited greater uptake. These observations were consistent with the higher soil ammonium content at the ploughed clover site inhibiting methane monooxygenase enzyme activity, thus resulting in lower uptake rates.