Nitrous oxide is a potent greenhouse gas, with a global warming potential 298 times greater than that of CO2. In agricultural soils, N2O emissions are influenced by a large number of environmental characteristics and crop management techniques that are not systematically reported in experiments. Random Forest (RF) is a machine learning method that can handle missing data and ranks input variables on the basis of their importance. We aimed to predict N2O emission on the basis of local information, to rank environmental and crop management variables according to their influence on N2O emission, and to compare the performances of RI: with several regression models. RF outperformed the regression models for predictive purposes, and this approach led to the identification of three important input variables: N fertilization, type of crop, and experiment duration. This method could be used in the future for prediction of N2O emissions from local information. (c) 2013 Elsevier Ltd. All rights reserved.

8 pages | International audience | 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.

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Rice paddy fields are major sources of atmospheric methane (CH₄) and nitrous oxide (N₂O). Rice variety is an important factor affecting CH₄ and N₂O emissions. However, the interactive effects of rice metabolites and microorganisms on CH₄ and N₂O emissions in paddy fields are not clearly understood. In this study, a high greenhouse gas-emitting cultivar (YL 6) and a low greenhouse gas-emitting cultivar (YY 1540) were used as experimental materials. Metabolomics was used to examine the roots, root exudates, and bulk soil metabolites. High-throughput sequencing was used to determine the microbial community composition. YY 1540 had more secondary metabolites (flavonoids and isoflavonoids) in root exudates than YL 6. It was enriched with the uncultured members of the families Gemmatimonadanceae and Rhizobiales_Incertae_Sedis in bulk soil, and genera Burkholderia-Caballeronia-Paraburkholderia, Magnetospirillum, Aeromonas, and Anaeromyxobacter in roots, contributing to increased expression of pmoA and nosZ genes and reducing CH₄ and N₂O emissions. YL 6 roots and root exudates contained higher contents of carbohydrates [e.g., 6-O- acetylarbutin and 2-(3- hydroxyphenyl) ethanol 1′-glucoside] than those of YY 1540. They were enriched with genera RBG-16-58-14 in bulk soil and Exiguobacterium, and uncultured member of the Kineosporiaceae family in roots, which contributed to increased expression of mcrA, ammonia-oxidizing archaea, ammonia-oxidizing bacteria, nirS, and nirK genes and greenhouse gas emissions. In general, these results established a link between metabolites, microorganisms, microbial functional genes, and greenhouse gas emissions. The metabolites of root exudates and roots regulated CH₄ and N₂O emissions by influencing the microbial community composition in bulk soil and roots.

Nitrogen loss via overland flow from agricultural land use is a global threat to waterways. On-farm denitrifying woodchip bioreactors can mitigate NO₃⁻ exports by increasing denitrification capacity. However, denitrification in sub-optimal conditions releases the greenhouse gas nitrous oxide (N₂O), swapping the pollution from aquatic to atmospheric reservoirs. Here, we assess NO₃⁻-N removal and N₂O emissions from a new edge-of-field surface-flow bioreactor during ten rain events on intensive farming land. Nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO₃⁻-N m⁻³ wetted woodchip d⁻¹ with a mean of 30.3 ± 7.3 g NO₃⁻-N m⁻³. The nitrate removal efficiency (NRE) was ∼73% in ideal hydrological conditions and ∼18% in non-ideal conditions. The fraction of NO₃⁻-N converted to N₂O (rN₂O) in the bioreactor was ∼3.3 fold lower than the expected 0.75% IPCC emission factor. We update the global bioreactor estimated Q₁₀ (NRR increase every 10 °C) from a recent meta-analysis with previously unavailable data to >20 °C, yielding a new global Q₁₀ factor of 3.1. Mean N₂O CO₂-eq emissions (431.9 ± 125.4 g CO₂-eq emissions day⁻¹) indicate that the bioreactor was not significantly swapping aquatic NO₃⁻ for N₂O pollution. Our estimated NO₃⁻-N removal from the bioreactor (9.9 kg NO₃⁻-N ha⁻¹ yr⁻¹) costs US$13.14 per kg NO₃⁻-N removed and represents ∼30% NO₃⁻-N removal when incorporating all flow and overflow events. Overall, edge-of-field surface-flow bioreactors seem to be a cost-effective solution to reduce NO₃⁻-N runoff with minor pollution swapping to N₂O.

Natural processes and human activities play a crucial role in changing the nitrogen cycle and increasing nitrous oxide (N₂O) emissions, which are accelerating at an unprecedented rate. N₂O has serious global warming potential (GWP), about 310 times higher than that of carbon dioxide. The food production, transportation, and energy required to sustain a world population of seven billion have required dramatic increases in the consumption of synthetic nitrogen (N) fertilizers and fossil fuels, leading to increased N₂O in air and water. These changes have radically disturbed the nitrogen cycle and reactive nitrogen species, such as nitrous oxide (N₂O), and have impacted the climatic system. Yet, systematic and comprehensive studies on various underlying processes and parameters in the altered nitrogen cycle, and their implications for the climatic system are still lacking. This paper reviews how the nitrogen cycle has been disturbed and altered by anthropogenic activities, with a central focus on potential pathways of N₂O generation. The authors also estimate the N₂O–N emission mainly due to anthropogenic activities will be around 8.316 Tg N₂O–N yr⁻¹ in 2050. In order to minimize and tackle the N₂O emissions and its consequences on the global ecosystem and climate change, holistic mitigation strategies and diverse adaptations, policy reforms, and public awareness are suggested as vital considerations. This study concludes that rapidly increasing anthropogenic perturbations, the identification of new microbial communities, and their role in mediating biogeochemical processes now shape the modern nitrogen cycle.

Hot moments of nitrous oxide (N₂O) emissions induced by interactions between weather and management make a major contribution to annual N₂O budgets in agricultural soils. The causes of N₂O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O₂) concentration can be critically important. We conducted high time-resolution field observations of O₂ and N₂O concentrations during hot moments in a dryland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extreme precipitation (Fer.+Pre.), were observed. Soil O₂ and N₂O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O₂ concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O₂ depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. The soil O₂ concentration dropped to as low as 0.2% (with the highest soil N₂O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N₂O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, soil N₂O concentrations were negatively correlated with soil O₂ concentrations (r = −0.5, P < 0.01), showing a quadratic increase as soil O₂ concentrations declined. Our results provide new understanding of the rapid short response of N₂O production to O₂ dynamics driven by changes in soil environmental factors during hot moments. Such understanding helps improve soil management to avoid transitory O₂ depletion and reduce the risk of N₂O production.

It is a common practice to maintain soil fertility based on the paddy-upland rotation with green manure in the subtropical region of China. However, rare studies are known about greenhouse gas (GHG) emissions from the paddy-upland rotation with green manure incorporation. Therefore, we conducted a field experiment of two years to compared with the effect of two kinds of green manure (CV: Chinese milk vetch and OR: Oilseed rape), and two kinds of cropping system (DR: double rice system and PR: paddy-upland rotation) on greenhouse gases emissions. We have found that the annual accumulation of CH₄ of Chinese milk vetch-rice-sweet potato || soybean was significantly reduced by 32.95%∼63.22% compared with other treatments, mainly because Chinese milk vetch reduced the abundance of methanogens by reducing soil C/N ratio. Meanwhile increasing soil permeability resulting from paddy-upland rotation also reduced soil CH₄ emission. However, The annual accumulation of N₂O of Chinese milk vetch-rice-sweet potato || soybean was increased by 17.39%∼870.11% compared with other treatments, mainly attributed to paddy-upland rotation decreased soil pH and nosZ abundance and increased nirK and nirS, thus enhancing N₂O emission, meanwhile the Chinese milk vetch incorporation and its interaction with the paddy-upland rotation has greatly enhanced the contents of NO₃⁻-N and abundance of ammonia-oxidizing archaea (AOA). The area-scaled global warming potential (GWP) and the biomass-scaled greenhouse gas emissions intensity (GHGI) of Chinese milk vetch-rice-sweet potato || soybean was reduced by 19.01%∼50.69% and 5.38%∼35.77% respectively. Thereby, the Chinese milk vetch-rice-sweet potato || soybean cropping system was suitable for agricultural sustainable development.

Hydrochar (HC), an environment-friendly material, enhances soil carbon sequestration and mitigate greenhouse gases (GHGs) emissions in croplands. In this study, the water-washed HC (WW-HC) was applied to paddy soil to investigate effects on nitrous oxide (N₂O) and methane (CH₄) emissions during rice growth period. Four treatments, namely control (without N fertilizer and WW-HC), N fertilizer (WW-HC00), N fertilizer with 0.5 wt% WW-HC (WW-HC05) and N fertilizer with 1.5 wt% WW-HC (WW-HC15), were established. Results showed the WW-HC addition reduced N₂O and CH₄ emissions, global warming potential (GWP) and greenhouse gas intensity (GHGI) during the growing season. Moreover, the WW-HC application reduced N₂O cumulative emission (P < 0.05) (by 28.6% and 23.8% for WW-HC05 and WW-HC15, respectively). It was mainly due to the reduced ratio of (nirK + nirS) to nosZ under WW-HC15 (P < 0.05). Compared with WW-HC00, the WW-HC05 reduced CH₄ cumulative emissions by 14.8%, while the WW-HC15 increased by 9.7%. This might be ascribed to the significantly reduced expression of the methanogenic mcrA gene and ratio of mcrA to pmoA by WW-HC (P < 0.05). The WW-HC05 amendment decreased GWP and GHGI by 18.6% and 32.5%, respectively. Furthermore, the WW-HC application greatly improved nitrogen use efficiency by 116–145% compared with the control. Our study indicates the WW-HC application is a promising GHGs mitigation practice in paddy fields.

Microbial denitrification is a main source of nitrous oxide (N₂O) emissions which have strong greenhouse effect and destroy stratospheric ozone. Though the importance of sulfide driven chemoautotrophic denitrification has been recognized, its contribution to N₂O emissions in nature remains elusive. We built up long-term sulfide-added microcosms with sediments from two freshwater lakes. Chemistry analysis confirmed sulfide could drive nitrate respiration in long term. N₂O accumulated to over 1.5% of nitrate load in both microcosms after long-term sulfide addition, which was up to 12.9 times higher than N₂O accumulation without sulfide addition. Metagenomes were extracted and sequenced during microcosm incubations. 16 S rRNA genes of Thiobacillus and Defluviimonas were gradually enriched. The nitric oxide reductase with c-type cytochromes as electron donors (cNorB) increased in abundance, while the nitric oxide reductase receiving electrons from quinols (qNorB) decreased in abundance. cnorB genes similar to Thiobacillus were enriched in both microcosms. In parallel, enrichment was observed for enzymes involved in sulfur oxidation, which supplied electrons to nitrate respiration, and enzymes involved in Calvin Cycle, which sustained autotrophic cell growth, implying the coupling relationship between carbon, nitrogen and sulfur cycling processes. Our results suggested sulfur pollution considerably increased N₂O emissions in natural environments.