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NO2− plays a crucial role in regulating N2O formation from the soil, while how it affects the production of soil N2O is still not well understood. In this study, N2O and NO emissions from an agricultural field of the North China Plain (NCP) were comparatively investigated under five different fertilizer treatments (NH4+, NO3−, NO2−, NH4+ + NO2− and NO3− + NO2−). Additionally, soil NH4+, NO2− and NO3− concentrations and the abundance of functional genes associated with nitrogen cycling were also analyzed in the incubation experiment. The results showed that the N2O average fluxes from the complex treatments of NO2− + NO3- were 1.4–2.4 times the sum of those from the separate treatments of NO2- and NO3- whereas from the complex treatments of NO2- + NH4+ were a factor of 1-1.4 larger than those from the separate treatments of NO2− and NH4+, indicating the coupling interaction of NO2− with NH4+ or NO3− makes a remarkable contribution to N2O emission from the soil. Significant reduction of the activity of N2O reductase was found in the soil with the addition of NO2−, which favored the accumulation of N2O formed through nitrification of NH4+ and denitrification of NO2−, resulting in relatively high N2O emissions from the complex treatments. As the intermediate product of nitrification and denitrification, NO2− produced is also expected to interact with NH4+ or NO3- to promote N2O emission from the soil, especially during fertilization events when NO2− is easily accumulated due to the acceleration of the nitrification and denitrification processes.

In paddy ecosystems, the feedback of N2O emissions on climate change as well as the effects of biochar on N2O emissions will have a profound impact on global warming and rice production. In order to investigate the effect of biochar amendment on N2O emissions from paddy soils under elevated atmospheric CO2 concentration (700 ppm) and air temperature (ambient +3 °C), a microcosm study was performed in plant growth chambers with rice plant and rice-straw-derived biochar. N2O emissions from urea-fertilized paddy soils had a significant increase with the biochar amendment during midseason drainage under ambient CO2 concentration and air temperature. However, N2O emissions was suppressed under simultaneous elevated CO2 concentration and air temperature, regardless of whether biochar was amended; under this condition the stimulating effect of biochar was attenuated. Reduced mineral N concentrations and increased DOC concentrations could inhibit N2O emission at simultaneously elevated CO2 concentration and air temperature. Increased soil pH and variation in the abundance of archaeal and bacterial amoA genes indicated that biochar amendment could stimulate ammonia oxidation-induced N2O emission during nitrification. Moreover, the liming effect of biochar was lessened under elevated CO2 concentration and temperature, which may contribute to the attenuated stimulating effect of biochar on N2O emissions. More attention should to be paid to the effect of soil pH-induced changes in ammonia oxidation, when mitigating N2O emissions in urea-fertilized paddy soils with biochar amendment at elevated CO2 concentrations and air temperatures.

Narasin is an antibiotic administered to broiler chickens to prevent coccidiosis. After storage, excreta containing parent narasin compounds is commonly spread as nitrogen fertilizer, yielding initial soil concentrations in the low μg·kg−1 range. In soil, antibiotics have been found to modify one or more pathways in the biogeochemical nitrogen cycle. The concentrations tested are often too high to be considered environmentally relevant, despite evidence that sub-therapeutic doses may also be disruptive. We conducted soil mesocosm experiments to determine the overall impact of trace narasin on major nitrogen pools and fluxes in soils treated with 0, 1, 10, 100, or 1000 ng·kg−1 narasin. Water content in the mesocosms varied from 40% to 80% water-filled pore space (WFPS), simulating a range of different redox conditions. Under aerobic conditions (40% WFPS), exposure to narasin inhibited nitrification, yielding increases in soil ammonium by up to 76%, perhaps by targeting nitrifying fungi. Under the same conditions, narasin caused soil nitrate concentrations to decline 17–39%. When the soil was near saturation (80% WFPS), nitrate increased by an average of 30%. Mass balances and isotopic enrichment of N2O indicate that NAR may also affect anammox and the rate of nitrifier nitrification/denitrification. In aerobic soils, N2O flux increased with antibiotic dose and the rise in flux strongly correlates to the N2O:N2 product ratio from dentification. This relationship suggests that N2O flux may increase in soils exposed to narasin even when total denitrification is modestly inhibited. We conclude that trace concentrations of narasin can significantly modify biogeochemical activities in soil on short timescales. Our results indicate the potential for extremely low concentrations of antibiotics to impact agricultural productivity, terrestrial N2O flux, and non-point source nitrogen pollution.