Removing Hydrogen Sulfide Contamination in Biogas Produced from Animal Wastes.

Hydrogen sulfide (HS) contamination in biogas produced from animal wastes limits its use to cooking and precludes it from being used for heating, lighting, or electricity generation. This limitation results in the release to the atmosphere of between 3 and 51% of total biogas produced. Biogas contains 50 to 70% methane (CH), a potent greenhouse gas that contributes to global warming. This study aimed to develop a cost-effective HS filtering system using local materials rich in iron as iron oxide (FeO), which reacts readily with HS and forms adsorbed iron sulfide (FeS) when gas is passed through it. Here we tested the performance of seven New Zealand soils and sand, each at five different gas flow rates (59, 74, 94, 129, and 189 mL min). We found that three materials (allophanic soil, brown soil, and black sand) had stable HS removal efficiencies close to 100% at all gas flow rates, followed by typic sand (89-99%), raw sand (76-99%), acidic sand (48-89%), and podzol soil (58-87%). These results show that inexpensive and simple filters to remove HS from biogas can be made using local soils. Used soil in the filters can then be easily regenerated by exposure to the atmosphere and reused to achieve sustained HS removal efficiency.

Assessing the performance of farm soil-based and hybrid biofilters for methane abatement.

Mitigating methane (CH4) emissions using methanotrophs (methane-oxidizing bacteria, MOB), is a simple, energy efficient and cheap technology. The abundance and distribution of MOB in the environmental samples is critical for efficient removal of emitted CH4 from any source. This study evaluated the performance of farm soils without and with cheap, easily accessible bulking materials as sustainable hybrid biofilter media. Soil-only biofilters removed up to 865 ± 19 g CH4 m-3 d-1 with well-drained organic carbon-rich soils compared with 264 ± 14 g CH4 m-3 d-1 for poorly drained soil. The removal efficiency decreased with increasing flow rate (0.16→0.24 L min-1) and subsequent priming could not return soil biofilters to their previous removal rate.Hybrid biofilters using organic, carbon-rich soils and compost removed up to 2698 g CH4 m-3 d-1 (flow rate 0.35 L min-1). Increasing CH4 flow rates also reduced their efficiency, but the hybrid biofilters with compost quickly regained most of their efficiency and removed up to 2262 g CH4 m-3 d-1 (flow rate 0.3 L min-1) after remixing of biofilter media. These results show that hybrid biofilters removed higher CH4 than soil-only biofilters and were also more resilient. The MOB gene abundance results complement the CH4 removal capacity of both soil-only and hybrid biofilter materials used. The more aerobic, carbon-rich soils had more abundant MOB than the poorly drained soil. The most porous hybrid biofilter with compost and more available nutrients to sustain bacterial growth and activity had the highest MOB abundance and removed the most CH4.

Improving the accuracy of nitrous oxide emission factors estimated for hotspots within dairy-grazed farms.

Nitrous oxide (N2O) emissions from dairy-grazing pastures can be dominated by large emissions from small areas ('hotspots') frequently used by grazing dairy cattle (i.e., water troughs and gateways). N2O emissions from these hotspots are quantified by investigating whether N2O emissions and emission factors (% of applied N emitted as N2O, EF3) from potential hotspots are different from non-hotspots. To better characterise N2O emissions from hotspots and non-hotspots of farms to understand their contributions to national agricultural greenhouse gas inventory calculations, a series of measurements were conducted during winter and spring on two NZ typical dairy farms with contrasting soil drainage (poorly versus well drained). Before measurements were taken, the soils either received a cow urine application or remained untreated. The results showed that changes in water-filled pore space (WFPS) and mineral N around water troughs and gateways, due to additional stock movements and disproportionate excreta-N deposition during previous grazing events, affected both background and total N2O emissions. But there was little impact on EF3 values (calculated using IPCC guidelines) from deposited urine between hotspot and pasture areas. These results suggest the same EF3 values can be used for both to calculate emissions from urine deposited on grazed pastures. However, these results raise concerns about higher background emission in hotspots subtracted from measured emissions from urine-N deposition in calculating EF3 values and discounting the effects of disproportionate N inputs in intensive agriculture on increased background emissions (legacy effect). This IPCC inventory method does not account for the legacy effect of N loading prior to the measurements which may underestimate the emissions. Thus, an allowance for higher hotspot background emissions could be included in the Inventory to accurately estimate total emissions from agriculture.

Use of a urease inhibitor to mitigate ammonia emissions from urine patches.

Urine deposition by grazing livestock is the single largest source of ammonia (NH3) volatilisation losses in New Zealand. Urease inhibitors (UI) have been used to mitigate NH3 losses from fertiliser urea and animal urine. In previous trials, the UI effect in reducing NH3 emissions from urine has been measured by applying urine mixed with the UI to the pasture soil thus increasing the chances of better interaction of the UI in inhibiting the urease enzyme. However, these trials do not represent a realistic grazing scenario where only urine is deposited onto the soil. This current research aimed to identify the best time to spray nBTPT (a UI containing 0.025% N-(n-butyl) thiophosphoric triamide) onto pasture soil to reduce NH3 losses from urine patches. The treatments were: a control (without urine and nBTPT), urine alone at 530 kg N ha-1 and urine plus nBTPT. The UI was applied to the chambers and soil plots 5 and 3 days prior to urine deposition, on the same day and 1, 3 and 5 after urine deposition in autumn. Ammonia losses were measured using the dynamic chamber method. The application of the inhibitor prior to urine deposition reduced NH3 losses with reductions of 27.6% and 17.5% achieved for UAgr-5 and UAgr-3, respectively. However, reductions in NH3 emission were 0.6-2.9% for inhibitor applied post urine deposition. There was also a reduction in both soil NH4 +-N concentration and soil pH in comparison with urine alone or with the treatments where nBTPT was applied after urine deposition.

Corrigendum: Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question.

This corrects the article DOI: 10.1038/srep39077.

Fungal denitrification: Bipolaris sorokiniana exclusively denitrifies inorganic nitrogen in the presence and absence of oxygen.

Fungi may play an important role in the production of the greenhouse gas nitrous oxide (N2O). Bipolaris sorokiniana is a ubiquitous saprobe found in soils worldwide, yet denitrification by this fungal strain has not previously been reported. We aimed to test if B. sorokiniana would produce N2O and CO2 in the presence of organic and inorganic forms of nitrogen (N) under microaerobic and anaerobic conditions. Nitrogen source (organic-N, inorganic-N, no-N control) significantly affected N2O and CO2 production both in the presence and absence of oxygen, which contrasts with bacterial denitrification. Inorganic N addition increased denitrification of N2O (from 0 to 0.3 µg N20-N h(-1) g(-1) biomass) and reduced respiration of CO2 (from 0.1 to 0.02 mg CO2 h(-1) g(-1) biomass). Isotope analyses indicated that nitrite, rather than ammonium or glutamine, was transformed to N2O. Results suggest the source of N may play a larger role in fungal N2O production than oxygen status.

Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question.

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N2) gas. The greenhouse gas nitrous oxide (N2O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N2O. However, N2 can be formed from two distinct N sources (known as hybrid N2) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N2 produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N2O was consumed during N2 formation. Experiments with gas chromatography indicated N2 was formed in the presence of live and dead fungi and in the absence of fungi, while N2O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N2 under both anoxic and 20% O2 atmosphere conditions. Our findings question the assumptions that (1) N2O is an intermediate required for N2 formation, (2) production of N2 and N2O requires anaerobiosis, and (3) hybrid N2 is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N2.