Characterisation of soil emissions of nitric oxide at field and laboratory scale using high resolution method
Résumé
Agricultural soils may account for 10% of anthropogenic emissions of NO, a precursor of tropospheric ozone with potential impacts on air quality and global warming. However, the estimation of this biogenic source strength and its relationships to crop management is still challenging because of the spatial and temporal variability of the NO fluxes. Here, we present a combination of new laboratory- and field-scale methods to characterise NO emissions and single out the effects of environmental drivers. First, NO fluxes were continuously monitored over the growing season of a maize-cropped field located near Paris (France), using 6 automatic chambers. Mineral fertilizer nitrogen was applied from May to October 2005. An additional field experiment was carried out in October to test the effects of N fertilizer form on the NO emissions. The automatic chambers were designed to measure simultaneously the NO and N2O gases. Laboratory measurements were carried out in parallel using soil cores sampled at same site to test the response of NO fluxes to varying soil N-NH4 and water contents, and temperatures. The effects of soil core thickness were also analysed. The highest NO fluxes occurred during the first 5 weeks following fertilizer application. The cumulative loss of NO-N over the growing season was estimated at 1.5 kg N ha-1, i.e. 1.1% of the N fertilizer dose (140 kg N ha-1). All rainfall events induced NO peak fluxes, whose magnitude decreased over time in relation to the decline of soil inorganic N. In October, NO emissions were enhanced with ammonium forms of fertilizer N. Conversely, the application of nitrate-based fertilizers did not significantly increase NO emissions compared to an unfertilized control. The results of the subsequent laboratory experiments were in accordance with the field observations in magnitude and time variations. NO emissions were maximum with a water soil content of 15% (w w-1), and with a NH4-N content of 180 mg NH4-N kg soil-1. The response of NO fluxes to soil temperature was fitted with two exponential functions, involving a Q10 of 2.0 below 20 °C and a Q10 of 1.4 above. Field and laboratory experiments indicated that most of the NO fluxes originated from the top 10 cm of soil. The characterisation of this layer in terms of mean temperature, NH4 and water contents is thus paramount to explaining the variations of NO fluxes.