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Soil water is known to be a key factor for controlling N2O emissions. The relationship between N2O emissions and water-filled pore space (WFPS) at hydric equilibrium is often described as exponential, starting at 60% WFPS. Because water in soil is dynamic, however, N2O emissions should be seen as a dynamic process. In this study, we investigated the role of the soil hydric history on N2O emissions by varying the intensity and speed of soil drying. We performed a laboratory experiment under non-steady-state equilibrium to control the bottom boundary condition of undisturbed soil cylinders. Three treatments of stepwise drying were performed across the range 0 to −100 cm pressure head in two wetting–drying cycles. Two types of N2O peaks of the same order of magnitude were detected. A peak appeared within 2 d after rewetting and was related to microbial production processes. The second type of peak was detected within the first two steps of the decreasing pressure head. They occurred during a brief period, an average of 1.6 h after the change in hydric equilibrium, during the phase of steep decline in the soil matric potential. These peaks were induced by diffusion processes, and their intensities were correlated with the amount of water drained. Entrapment of N2O during the wetting phase and rapid displacement during the drying phase may have occurred. Our results showed that WFPS alone is not able to consistently predict N2O emissions by soils because production and emission phases must be distinguished. [ABSTRACT FROM AUTHOR]

Aims: Integrated weed management, which allows reducing the reliance of cropping systems on herbicides, is based on the use of specific combinations of innovative agricultural practices. However the impact of the introduction of these practices in cropping systems may influence soil functioning such as biogeochemical cycling. Here, we investigated N 2O emissions and the abundances of N-cycling microorganisms in 11-year old cropping systems (i.e. conventional reference and integrated weed management) in order to estimate the environmental side-effects of long-term integrated weed management. Methods: N 2O emissions were continuously measured using automated chambers coupled with infrared analysers. Abundances of ammonia oxidizers and denitrifiers together with total bacteria and archaea were determined monthly from 0 to 10 and 10–30 cm soil layer samples by quantitative Polymerase Chain Reaction (qPCR). The relationship between N 2O emissions and microbial abundances during the study were investigated as were their relationships with soil physicochemical parameters and climatic conditions. Results: Over 7 months, the system with integrated weed management emitted significantly more N 2O with cumulated measured emissions of 240 and 544 g N-N 2O ha −1 for conventional and integrated systems, respectively. Abundances of microbial guilds varied slightly between systems, although ammonia-oxidizing bacteria were more abundant in the reference system (1.7 10 6 gene copies g −1 dry weight soil) compared to the integrated system (1.0 10 6 gene copies g −1 dry weight soil). These differences revealed both the long-term modification of soil biogeochemical background and the functioning of microbial processes due to 11 years of alternative field management, and the short-term impacts of the agricultural practices introduced as part of weed management during the cropping year. Conclusions: The abundances of the different microbial communities involved in N cycling and the intensity of N 2O emissions were not related, punctual high N 2O emissions being more dependent on favourable soil conditions for nitrifying and denitrifying activities. Future studies will be performed to check these findings for other pedoclimatic conditions and to examine the impact of such cropping systems. [ABSTRACT FROM AUTHOR]

Approximately 65% of anthropogenic emissions of N2O, a potent greenhouse gas, originate from soils at global scale, and particularly after N fertilisation of the main crops in Europe. Thanks to their capacity to fix atmospheric N2 through biological fixation, legumes allow to reduce N fertilizer use, and possibly N2O emission. Nevertheless, the decomposition of crop organic matter during the crop cycle and during the residue decomposition, and possibly the N fixation process itself, could lead to N2O emissions. The objective of this study was to quantify N2O emissions from a dry pea crop (Pisum sativum, harvested at maturity) and from the subsequent crops in comparison with N2O emissions from wheat and oilseed-rape crops, fertilized or not, in various rotations. A field experiment was conducted during 4 consecutive years, aiming at comparing the emissions during the pea crop, in comparison with those during the wheat (fertilized or not) or oilseed rape crops, and after the pea crop, in comparison with other preceding crops. N2O fluxes were measured using static chambers. In spite of low N2O fluxes, mainly linked with the site soil characteristics, fluxes during the crop were significantly lower for pea and unfertilized wheat than for fertilized wheat and oilseed rape. The effect of the preceding crop was not significant, while soil mineral N at harvest was higher after pea. These results, combined with the emission reduction allowed by the production and transport of the N fertiliser not applied on the pea crop, should be confirmed in a larger range of soil types. Nevertheless, they demonstrate the absence of N2O emission linked to the symbiotic N fixation process, and allow us to estimate the decrease of N2O emissions to 20-25% by including one pea crop in a three-year rotation. At a larger scale, this reduction of GHG emissions at field level has to be cumulated with the reduction of GHG emissions linked with the lower level of production and transport of the N fertiliser not applied on the pea crop. [ABSTRACT FROM AUTHOR]

Abstract: Microbial transformations of nitrification and denitrification are the main sources of nitrous oxide (N2O) from soils. Relative contributions of both processes to N2O emissions were estimated on an agricultural soil using 15N isotope tracers (15NH4 + or 15NO3 −), for a 10-day batch experiment. Under unsaturated and saturated conditions, both processes were significantly involved in N2O production. Under unsaturated conditions, 60% of N-N2O came from nitrification, while denitrification contributed around 85–90% under saturated conditions. Estimated nitrification rates were not significantly different whatever the soil moisture content, whereas the proportion of nitrified N emitted as N2O changed from 0.13 to 2.32%. In coherence with previous studies, we interpreted this high value as resulting from the decrease in O2 availability through the increase in soil moisture content. It thus appears that, under limiting aeration conditions, some values for N2O emissions through nitrification could be underestimated. [Copyright &y& Elsevier]

This paper presents a new algorithm, Nitrous Oxide Emission (NOE) for simulating the emission of the greenhouse gas N2O from agricultural soils. N2O fluxes are calculated as the result of production through denitrification and nitrification and reduction through the last step of denitrification. Actual denitrification and nitrification rates are calculated from biological parameters and soil water-filled pore space, temperature and mineral nitrogen contents. New suggestions in NOE consisted in introducing (1) biological site-specific parameters of soil N2O reduction and (2) reduction of the N2O produced through nitrification to N2 through denitrification. This paper includes a database of 64 N2O fluxes measured on the field scale with corresponding environmental parameters collected from five agricultural situations in France. This database was used to test the validity of this algorithm. Site per site comparison of simulated N2O fluxes against observed data leads to mixed results. For 80% of the tested points, measured and simulated fluxes are in accordance whereas the others resulted in an important discrepancy. The origin of this discrepancy is discussed. On the other hand, mean annual fluxes measured on each site were strongly correlated to mean simulated annual fluxes. The biological site-specific parameter of soil N2O reduction introduced into NOE appeared particularly useful to discriminate the general level of N2O emissions from site to site. Furthermore, the relevance of NOE was confirmed by comparing measured and simulated N2O fluxes using some data from the US TRAGNET database. We suggest the use of NOE on a regional scale in order to predict mean annual N2O emissions. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a greenhouse gas that mainly originates from soils and agricultural activities. International initiatives require that countries calculate national inventories of their N2O emissions from agricultural soils. Several methodologies can be applied: (i) Tier I Intergovernmental Panel on Climate Change (IPCC) default approach that only takes into account nitrogen (N) input, (ii) Country-specific methodologies (Tier II and Tier III) that account for regional climatic and land use impacts on N2O emission factors, and include several sources. Strategies to mitigate N2O emissions from agricultural soils are based on a rational use of N resource and the stimulation of soil aerobic conditions and biological activity. Management practices to reduce the N2O emissions should be focused on: (i) Avoiding the soil denitrification process by maximizing soil aeration and reducing their acidity, (ii) Improving N fertilization by reducing free N in soil and optimizing N use efficiency in cropping systems, (iii) Direct actions on the microbial processes by limiting the nitrification process and stimulating the last step of the denitrification process (N2O reduction to N2).

Arable soils are a large source of nitrous oxide (N2O) emissions, making up half of the biogenic emissions worldwide. Estimating their source strength requires methods capable of capturing the spatial and temporal variability of N2O emissions, along with the effects of crop management. Here, we applied a process-based model, CERES, with geo-referenced input data on soils, weather, and land use to map N2O emissions from wheat-cropped soils in three agriculturally intensive regions in France. Emissions were mostly controlled by soil type and local climate conditions, and only to a minor extent by the doses of fertilizer nitrogen applied. As a result, the direct emission factors calculated at the regional level were much smaller (ranging from 0.0007 to 0.0033 kg N2O-N kg–1 N) than the value of 0.0125 kg N2O-N kg–1 N currently recommended in the IPCC Tier 1 methodology. Regional emissions were far more sensitive to the soil microbiological parameter s governing denitrification and its fraction evolved as N2O, soil bulk density, and soil initial inorganic N content. Mitigation measures should therefore target a reduction in the amount of soil inorganic N upon sowing of winter crops, and a decrease of the soil N2O production potential itself. From a general perspective, taking into account the spatial variability of soils and climate thereby appears necessary to improve the accuracy of national inventories, and to tailor mitigation strategies to regional characteristics. The methodology and results presented here may easily be transferred to winter oilseed rape, whose has growing cycle and fertilser requirements are similar.