Identifying the origin of nitrous oxide dissolved in deep ocean by concentration and isotopocule analyses

Dissolved N2O concentrations in oil palm plantation drainage in a peat swamp of Malaysia

Oil palm plantations in Southeast Asia are the largest supplier of palm oil products and have been rapidly expanding in the last three decades even in peat-swamp areas. Oil palm plantations on peat ecosystems have a unique water management system that lowers the water table and, thus, may yield indirect N₂O emissions from the peat drainage system. We conducted two seasons of spatial monitoring for the dissolved N₂O concentrations in the drainage and adjacent rivers of palm oil plantations on peat swamps in Sarawak, Malaysia, to evaluate the magnitude of indirect N₂O emissions from this ecosystem. In both the dry and wet seasons, the mean and median dissolved N₂O concentrations exhibited over-saturation in the drainage water, i.e., the oil palm plantation drainage may be a source of N₂O to the atmosphere. In the wet season, the spatial distribution of dissolved N₂O showed bimodal peaks in both the unsaturated and over-saturated concentrations. The bulk δ¹⁵N of dissolved N₂O was higher than the source of inorganic N in the oil palm plantation (i.e., N fertilizer and soil organic nitrogen) during both seasons. An isotopocule analysis of the dissolved N₂O suggested that denitrification was a major source of N₂O, followed by N₂O reduction processes that occurred in the drainage water. The δ¹⁵N and site preference mapping analysis in dissolved N₂O revealed that a significant proportion of the N₂O produced in peat and drainage is reduced to N₂ before being released into the atmosphere.

N2O production and emission pathways in anammox biofilter for treating wastewater with low nitrogen concentrations

Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (N₂O) production will greatly reduce the advantages of this process. It is important to identify the N₂O emission pathways and then reduce the N₂O production in anammox system. To date, very limited research has been done to investigate the N₂O production and N₂O emission pathways in anammox biofilter. In this study, N₂O production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and N₂O emission pathways were analyzed with batch tests using N₂O microsensor and stable isotope mass spectrometry. The results showed N₂O production increased with the increase of filtration rates in anammox biofilter, where the N₂O emission factor increased from 0.012 % at 1.0 m/h to 0.496 % at 3.0 m/h. And the optimal operation condition was at filtration rate of 1.5 m/h, where NH₄⁺-N and NO₂^--N removal efficiencies reached 99 % and N₂O concentration was the lowest. qPCR showed that anammox bacteria, nitrifying bacteria and denitrifying bacteria were all present in anammox biofilter, with anammox bacteria in the highest abundance. And nitrifying bacteria and denitrifying bacteria provided the possibility of N₂O production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were N₂O production pathways in aerobic zone and anoxic zone of anammox biofilter, respectively. In addition, batch tests under different conditions showed no oxygen environment could reduce N₂O production. Therefore, the production of N₂O in anammox system is a problem that cannot be ignored and should be paid more attention to.

Isotopic assessment of soil N2O emission from a sub-tropical agricultural soil under varying N-inputs

Nitrogen fertilizers result in high crop productivity but also enhance the emission of N₂O, an environmentally harmful greenhouse gas. Only approximately a half of the applied nitrogen is utilized by crops and the rest is either vaporized, leached, or lost as NO, N₂O and N₂ via soil microbial activity. Thus, improving the nitrogen use efficiency of cropping systems has become a global concern. Factors such as types and rates of fertilizer application, soil texture, moisture level, pH, and microbial activity/diversity play important roles in N₂O production. Here, we report the results of N₂O production from a set of chamber experiments on an acidic sandy-loam agricultural soil under varying levels of an inorganic N-fertilizer, urea. Stable isotope technique was employed to determine the effect of increasing N-fertilizer levels on N₂O emissions and identify the microbial processes involved in fertilizer N-transformation that give rise to N₂O. We monitored the isotopic changes in both substrate (ammonium and nitrate) and the product N₂O during the entire course of the incubation experiments. Peak N₂O emissions of 122 ± 98 μg N₂O-N m^-2 h^-1, 338 ± 49 μg N₂O-N m^-2 h^-1 and 739 ± 296 μg N₂O-N m^-2 h^-1 were observed for urea application rate of 40, 80, and 120 μg N g^-1. The duration of emissions also increased with urea levels. The concentration and isotopic compositions of the substrates and product showed time-bound variation. Combining the observations of isotopic effects in δ¹⁵N, δ¹⁸O, and ¹⁵N site preference, we inferred co-occurrence of several microbial N₂O production pathways with nitrification and/or fungal denitrification as the dominant processes responsible for N₂O emissions. Besides this, dominant signatures of bacterial denitrification were observed in a second N₂O emission pulse in intermediate urea-N levels. Signature of N₂O consumption by reduction could be traced during declining emissions in treatment with high urea level.

What can we learn from N2 O isotope data? - Analytics, processes and modelling

The isotopic composition of nitrous oxide (N2 O) provides useful information for evaluating N2 O sources and budgets. Due to the co-occurrence of multiple N2 O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N2 O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N2 O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N2 O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N2 O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N2 O production and reduction processes. More recently, process-based N2 O isotopic models have been developed for natural abundance and 15 N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N2 O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N2 O isotope community will continue to advance our understanding of N2 O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.