Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts

Gaseous Emissions from the Composting Process: Controlling Parameters and Strategies of Mitigation

Organic waste generation, collection, and management have become a crucial problem in modern and developing societies. Among the technologies proposed in a circular economy and sustainability framework, composting has reached a strong relevance in terms of clean technology that permits reintroducing organic matter to the systems. However, composting has also negative environmental impacts, some of them of social concern. This is the case of composting atmospheric emissions, especially in the case of greenhouse gases (GHG) and certain families of volatile organic compounds (VOC). They should be taken into account in any environmental assessment of composting as organic waste management technology. This review presents the relationship between composting operation and composting gaseous emissions, in addition to typical emission values for the main organic wastes that are being composted. Some novel mitigation technologies to reduce gaseous emissions from composting are also presented (use of biochar), although it is evident that a unique solution does not exist, given the variability of exhaust gases from composting.

Interacting drivers and their tradeoffs for predicting denitrification potential across a strong urban to rural gradient within heterogeneous landscapes

Denitrification is a significant regulator of nitrogen pollution in diverse landscapes but is difficult to quantify. We examined relationships between denitrification potential and soil and landscape properties to develop a model that predicts denitrification potential at a landscape level. Denitrification potential, ancillary soil variables, and physical landscape attributes were measured at study sites within urban, suburban, and forested environments in the Gwynns Falls watershed in Baltimore, Maryland in a series of studies between 1998 and 2014. Data from these studies were used to develop a statistical model for denitrification potential using a subset of the samples (N = 188). The remaining measurements (N = 150) were used to validate the model. Soil moisture, soil respiration, and total soil nitrogen were the best predictors of denitrification potential (R²_adj = 0.35), and the model was validated by regressing observed vs. predicted values. Our results suggest that soil denitrification potential can be modeled successfully using these three parameters, and that this model performs well across a variety of natural and developed land uses. This model provides a framework for predicting nitrogen dynamics in varying land use contexts. We also outline approaches to develop appropriate landscape-scale proxies for the key model inputs, including soil moisture, respiration, and soil nitrogen.

Effects of biochar on N2O emission in denitrification pathway from paddy soil: A drying incubation study

N₂O emission from paddy soil is a potential environmental risk, especially when the soil moisture content of paddy soil changes and excessive nitrogen retention occurs. Biochar is known to have a positive effect on reducing N₂O emissions. However, the influence of different types of biochar on N₂O emission with varying soil moisture contents is unclear. The objective of this study was to investigate the effects of biochar made from different feedstocks and at different pyrolysis temperatures on the release of N₂O during drying process of paddy soil. An incubation experiment with four kinds of biochar (rice straw and rice husk biochar pyrolyzed at 400 °C and 700 °C, respectively) applied at 1% (w/w) was conducted on paddy soil with the same initial moisture content (105% water-filled pore space). The emission rate of N₂O, concentrations of ammonium and nitrate, and the abundance of N₂O related microbial functional genes (narG and nosZ) were monitored throughout the incubation period. Biochar amendments reduced cumulative N₂O emissions by 56.8-90.1% compared to the control. Low-temperature rice straw biochar decreased nosZ gene abundance, downregulated the denitrification pathway, and reduced nitrogen loss and N₂O emission. The low-temperature pyrolysis rice husk biochar and the control showed similar trends in narG and nosZ gene abundance and N₂O emission. The high-temperature pyrolysis of rice straw and rice husk biochar showed opposite trends in narG gene abundance, but both increased nosZ gene abundance at the later incubation period. Different feedback on denitrification-derived N₂O emission in biochar application was revealed in this study by establishing a link between biotic and abiotic factors, showing that caution should be exercised when considering the use of biochar to mitigate N₂O emission under drying soil conditions.

Effects of biological nitrification inhibitors on nitrogen use efficiency and greenhouse gas emissions in agricultural soils: A review

To maintain and increase crop yields, large amounts of nitrogen fertilizers have been applied to farmland. However, the nitrogen use efficiency (NUE) of chemical fertilizer remains very low, which may lead to serious environmental problems, including nitrate pollution, air quality degradation and greenhouse gas (GHG) emissions. Nitrification inhibitors can alleviate nitrogen loss by inhibiting nitrification; thus, biological nitrification inhibition by plants has gradually attracted increasing attention due to its low cost and environmental friendliness. Research progress on BNI is reviewed in this article, including the source, mechanisms, influencing factors and application of BNIs. In addition, the impact of BNI on agriculture and GHG emissions is summarized from the perspective of agricultural production and environmental protection, and the key future research prospects of BNIs are also noted.

Biochar and urease inhibitor mitigate NH3 and N2O emissions and improve wheat yield in a urea fertilized alkaline soil

In this study, we explored the role of biochar (BC) and/or urease inhibitor (UI) in mitigating ammonia (NH₃) and nitrous oxide (N₂O) discharge from urea fertilized wheat cultivated fields in Pakistan (34.01°N, 71.71°E). The experiment included five treatments [control, urea (150 kg N ha^-1), BC (10 Mg ha^-1), urea + BC and urea + BC + UI (1 L ton^-1)], which were all repeated four times and were carried out in a randomized complete block design. Urea supplementation along with BC and BC + UI reduced soil NH₃ emissions by 27% and 69%, respectively, compared to sole urea application. Nitrous oxide emissions from urea fertilized plots were also reduced by 24% and 53% applying BC and BC + UI, respectively, compared to urea alone. Application of BC with urea improved the grain yield, shoot biomass, and total N uptake of wheat by 13%, 24%, and 12%, respectively, compared to urea alone. Moreover, UI further promoted biomass and grain yield, and N assimilation in wheat by 38%, 22% and 27%, respectively, over sole urea application. In conclusion, application of BC and/or UI can mitigate NH₃ and N₂O emissions from urea fertilized soil, improve N use efficiency (NUE) and overall crop productivity.

Short-Term Assessment of Nitrous Oxide and Methane Emissions on a Crop Yield Basis in Response to Different Organic Amendment Types in Sichuan Basin

Agriculture’s goal to meet the needs of the increasing world population while reducing the environmental impacts of nitrogen (N) fertilizer use without compromising output has proven to be a challenge. Manure and composts have displayed the potential to increase soil fertility. However, their potential effects on nitrous oxide (N2O) and methane (CH4) emissions have not been properly understood. Using field-scaled lysimeter experiments, we conducted a one-year study to investigate N2O and CH4 emissions, their combined global warming potential (GWP: N2O + CH4) and yield-scaled GWP in a wheat-maize system. One control and six different organic fertilizer treatments receiving different types but equal amounts of N fertilization were used: synthetic N fertilizer (NPK), 30% pig manure + 70% synthetic N fertilizer (PM30), 50% pig manure + 50% synthetic N fertilizer (PM50), 70% pig manure + 30% synthetic N fertilizer (PM70), 100% pig manure (PM100), 50% cow manure-crop residue compost + 50% synthetic N fertilizer (CMRC), and 50% pig manure-crop residue compost + 50% synthetic N fertilizer (PMRC). Seasonal cumulative N2O emissions ranged from 0.39 kg N ha−1 for the PMRC treatment to 0.93 kg N ha−1 for the NPK treatment. Similar CH4 uptakes were recorded across all treatments, with values ranging from −0.68 kg C ha−1 for the PM50 treatment to −0.52 kg C ha−1 for the PM30 treatment. Compared to the NPK treatment, all the organic-amended treatments significantly decreased N2O emission by 32–58% and GWP by 30–61%. However, among the manure-amended treatments, only treatments that consisted of inorganic N with lower or equal proportions of organic manure N treatments were found to reduce N2O emissions while maintaining crop yields at high levels. Moreover, of all the organic-amended treatments, PMRC had the lowest yield-scaled GWP, owing to its ability to significantly reduce N2O emissions while maintaining high crop yields, highlighting it as the most suitable organic fertilization treatment in Sichuan basin wheat-maize systems.

Soil nitrogen dynamics at a regional scale along a precipitation gradient in secondary grassland of China

The availability of soil inorganic nitrogen (N) is primarily regulated by the rates of soil N transformation, including mineralization, ammonification, nitrification, and denitrification, and are sensitive to climate, plant, and soil factors. However, the interactive effects among these factors regulating soil N transformation rates in ecosystems across large spatial scales remain unclear. Here, we investigated the spatial patterns of the potential N mineralization, nitrification, ammonification, and denitrification rates in relation to plant traits and soil edaphic conditions across a 600-km precipitation gradient in secondary grasslands of South China. The soil potential N mineralization and nitrification rates significantly increased with increasing precipitation. However, the soil potential N ammonification and denitrification rates did not significantly vary with precipitation. Moreover, the soil potential N nitrification and denitrification rates significantly increased with increasing soil pH, whereas the potential N mineralization and ammonification rates decreased with increasing soil pH. The soil potential N mineralization rate was positively correlated with soil labile N but negatively correlated with soil recalcitrant C and N contents. Our results revealed that changes in soil NH₄⁺-N and pH along precipitation gradients primarily controlled the potential N mineralization, nitrification, and ammonification rates. In contrast, soil NO₃^--N, soil pH, and plant N inputs predominantly regulated the potential N denitrification rate. Overall, our results reveal that soil N transformation varies along the precipitation gradient, and these results need to be considered when studying the effects of climate change on N cycling in grassland ecosystems across diverse environments.

The inhibition effect of tea polyphenols on soil nitrification is greater than denitrification in tea garden soil

Tea polyphenols are the most widely distributed class of secondary metabolites (Camellia sinensis) and account for a considerable proportion of the pruning residues of tea. A large amount of tea polyphenols have fallen down over soil with prunning residues every year. However, the effect of tea polyphenols on soil nitrogen cycle, especially the denitrification process and its related microbial communities, remains unclear. Epigallocatechin gallate (EGCG), the most abundant component of tea polyphenols, was selected to simulate the effects of tea polyphenols on soil nitrification, denitrification, related functional genes and microbial community. The results indicated that addition of EGCG can significantly (p < 0.05) inhibit soil nitrification. Copy numbers of bacterial and archaeal ammonia monooxygenase genes (amoA) decreased as EGCG concentration increased. Further, the ammonia oxidisers exhibited a significantly (p < 0.05) greater niche differentiation under the effect of EGCG compared with the control treatment (no EGCG addition). However, the inhibition effect of EGCG over soil denitrification was most significant at 34 and 36 day of incubation period, and such inhibitory effect was more apparent on nitrification compared with denitrification. EGCG addition increased the diversity of bacterial community. The composition of bacterial community was significantly altered and community variation was primary explained by EGCG, NH₄⁺-N, NO₃^--N, soil organic carbon contents and potential denitrification rates. EGCG addition significantly increased relative abundance of Proteobacteria and Bacteroidetes phyla whereas decreased Actinobacteria. Overall, tea polyphenols can inhibit soil nitrification to a larger extent than denitrification by reducing the abundance of microorganisms carrying the related functional genes. Our results can serve as important basis of reducing the nitrogen pollution risk in tea orchards and could be considered as a powerful natural nitrification inhibitor to reduce the environmental risks caused by unreasonable nitrogen fertiliser adaptation.

Efficiency and mechanism of nitrogen removal from piggery wastewater in an improved microaerobic process

Characterized by high ammonium (NH₄⁺ - N) and low ratio of chemical oxygen demand (COD) to total nitrogen (COD/TN), discharge of piggery wastewater has been identified as a primary pollution source resulting in water eutrophication. An improved microaerobic reactor, internal aerating microaerobic reactor (IAMR), was constructed to treat manure-free piggery wastewater without effluent recycle at dissolved oxygen of 0.3 mg/L and 32 °C. A removal rate of COD, NH₄⁺ - N and TN averaged 77.9%, 94.6% and 82.6% was obtained in the reactor, with the concentration of 258.5, 235.5 and 335.2 mg/L in influent, respectively. 16S rDNA amplicon sequencing, carbon and nitrogen mass balance and stoichiometry indicated that heterotrophic nitrification-anammox was the dominant approach to nitrogen removal. Microbiome phenotypes showed that aerobic bacteria were the dominant microorganisms, and the microbiome oxidative stress tolerance was intensified along with the continuous operation of the IAMR, resulting in the survival of various facultative and anaerobic bacteria for nutrients removal. With the good nutrients removal, less energy consumption, and high tolerance to influent fluctuation, the improved IAMR was confirmed as a promising process for treating wastewater with high NH₄⁺ - N and low COD/TN.

Shift in nitrogen transformation in peatland soil by nitrogen inputs

Inputs of nitrogen (N) to peatlands in the form of fertilizers have rapidly increased due to the intensification of agricultural systems, impacting ecological processes, and the carbon storage function of peatland. However, detailed information on the impacts of long-term N inputs on the individual steps of N transformation processes in peatland soils still needs to be fully understood. We investigated N mineralization and nitrification rates as well as nitrite dependent anaerobic methane oxidation (n-damo), anaerobic ammonium oxidation (anammox), denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in a peatland affected by N inputs for >50 years, using isotope tracing technique and quantitative PCR. Based on the results, N inputs increased N mineralization and nitrification rates by 77 and 43%, respectively. Notably, the contributions of n-damo and anammox to N₂ production were enhanced by 242 and 170%, accounting for 30 and 12%, respectively. The contributions of denitrification and DNRA to N₂ production decreased by 27 and 52%, accounting for 48 and 10% of N₂ production, respectively. Nitrifier abundance increased significantly, with AOA being the dominant prokaryote (from 696 to 1090 copies g^-1), but AOB responded more strongly to N inputs (from 5 to 68 copies g^-1). The N inputs also promoted the growth of n-damo and anammox bacteria, whose abundances increased by 3.7% (from 565 to 586 copies g^-1) and 85.7% (from 305 to 567 copies g^-1), respectively, while denitrifier abundance was significantly reduced, with nirK and nirS abundances decreasing by 58% (from 738 to 308 copies g^-1) and 50% (from 218 to 109 copies g^-1), respectively. Soil pH was the key environmental factor influencing N transformations. We show that n-damo plays important roles in N cycling in peatland subjected to N inputs, providing a scientific basis for improved peatland management.

The Dynamic Response of Nitrogen Transformation to the Dissolved Oxygen Variations in the Simulated Biofilm Reactor

Lab-scale simulated biofilm reactors, including aerated reactors disturbed by short-term aeration interruption (AE-D) and non-aerated reactors disturbed by short-term aeration (AN-D), were established to study the stable-state (SS) formation and recovery after disturbance for nitrogen transformation in terms of dissolved oxygen (DO), removal efficiency (RE) of NH₄⁺-N and NO₃⁻-N and activity of key nitrogen-cycle functional genes amoA and nirS (RNA level abundance, per ball). SS formation and recovery of DO were completed in 0.56–7.75 h after transition between aeration (Ae) and aeration stop (As). In terms of pollutant REs, new temporary SS formation required 30.7–52.3 h after Ae and As interruptions, and seven-day Ae/As interruptions required 5.0% to 115.5% longer recovery times compared to one-day interruptions in AE-D and AN-D systems. According to amoA activity, 60.8 h were required in AE-D systems to establish new temporary SS after As interruptions, and RNA amoA copies (copy number/microliter) decreased 88.5%, while 287.2 h were required in AN-D systems, and RNA amoA copies (copy number/microliter) increased 36.4 times. For nirS activity, 75.2–85.8 h were required to establish new SSs after Ae and As interruptions. The results suggested that new temporary SS formation and recovery in terms of DO, pollutant REs and amoA and nirS gene activities could be modelled by logistic functions. It is concluded that temporary SS formation and recovery after Ae and As interruptions occurred at asynchronous rates in terms of DO, pollutant REs and amoA and nirS gene activities. Because of DO fluctuations, the quantitative relationship between gene activity and pollutant RE remains a challenge.

Using the Nitrification Inhibitor Nitrapyrin in Dairy Farm Effluents Does Not Improve Yield-Scaled Nitrous Oxide and Ammonia Emissions but Reduces Methane Flux

The application of dairy farm effluents (DFE) without previous treatment in paddocks was intensified due to the approval of this practice in Costa Rican legislation since 2012. Applying DFE instead of synthetic N fertilizer in grasslands is an opportunity to reach a circular economy; however, this practice increases the risk of emissions of nitrous oxide (N2O), methane (CH4), and ammonia (NH3), which contribute to global warming. A field experiment was carried out using a permanent grassland (90% Star grass and 10% Kikuyo grass) to simultaneously assess the effect of nitrapyrin on yield-scaled emissions of NH3, CH4, and N2O. The experiment lasted for 5 months in 2017, based on a randomized complete block design, including three treatments of control (CK) without N application, surface application of DFE with nitrapyrin (SNI), and without nitrapyrin (S). Total N applied was 149 ± 12 kg N ha−1 for both S and SNI treatments split into five applications. CH4 emissions from S, SNI, and CK showed a high temporal variation. Daily fluxes of CH4 from SNI were significantly lower than those of S in August (P &lt; 0.05). Cumulative emissions of CH4, the majority produced in the soil, ranged from 4 to 168 g ha−1 for S, and from −13 to 88 g ha−1 for SNI. The ratio between the N2O cumulative emissions and the N applied as DFE were 1.6 ± 0.5 and 1.7 ± 0.2% for S and SNI, respectively. NH3 volatilization potential was very low (i.e., 0.6 ± 0.2% of the N applied). Under the prevailing experimental conditions, no significant difference between yield-scaled NH3 and N2O emissions were found between S and SNI, suggesting that nitrapyrin may not be a viable mitigation option for gaseous N losses from DFE application in Costa Rican grasslands in rainy season.

Coordination chemistry of the CuZ site in nitrous oxide reductase and its synthetic mimics

Atmospheric nitrous oxide (N2O) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic N2O emissions occur primarily through agricultural disruption of nitrogen homeostasis causing N2O to build up in the atmosphere. The enzyme responsible for N2O fixation within the geochemical nitrogen cycle is nitrous oxide reductase (N2OR), which catalyzes 2H+/2e- reduction of N2O to N2 and H2O at a tetranuclear active site, CuZ. In this review, the coordination chemistry of CuZ is reviewed. Recent advances in the understanding of biological CuZ coordination chemistry is discussed, as are significant breakthroughs in synthetic modeling of CuZ that have emerged in recent years. The latter topic includes both structurally faithful, synthetic [Cu4(µ4-S)] clusters that are able to reduce N2O, as well as dicopper motifs that shed light on reaction pathways available to the critical CuI-CuIV cluster edge of CuZ. Collectively, these advances in metalloenzyme studies and synthetic model systems provide meaningful knowledge about the physiologically relevant coordination chemistry of CuZ but also open new questions that will pose challenges in the near future.

Nitrogen isotopic signatures and fluxes of N2O in response to land-use change on naturally occurring saline-alkaline soil

The conversion of natural grassland to semi-natural or artificial ecosystems is a large-scale land-use change (LUC) commonly occurring to saline–alkaline land. Conversion of natural to artificial ecosystems, with addition of anthropogenic nitrogen (N) fertilizer, influences N availability in the soil that may result in higher N₂O emission along with depletion of ¹⁵N, while converting from natural to semi-natural the influence may be small. So, this study assesses the impact of LUC on N₂O emission and ¹⁵N in N₂O emitted from naturally occurring saline–alkaline soil when changing from natural grassland (Phragmites australis) to semi-natural [Tamarix chinensis (Tamarix)] and to cropland (Gossypium spp.). The grassland and Tamarix ecosystems were not subject to any management practice, while the cropland received fertilizer and irrigation. Overall, median N₂O flux was significantly different among the ecosystems with the highest from the cropland (25.3 N₂O-N µg m⁻² h⁻¹), intermediate (8.2 N₂O-N µg m⁻² h⁻¹) from the Tamarix and the lowest (4.0 N₂O-N µg m⁻² h⁻¹) from the grassland ecosystem. The ¹⁵N isotopic signatures in N₂O emitted from the soil were also significantly affected by the LUC with more depleted from cropland (− 25.3 ‰) and less depleted from grassland (− 0.18 ‰). Our results suggested that the conversion of native saline–alkaline grassland with low N to Tamarix or cropland is likely to result in increased soil N₂O emission and also contributes significantly to the depletion of the ¹⁵N in atmospheric N₂O, and the contribution of anthropogenic N addition was found more significant than any other processes.

Contrasting subsurface denitrification characteristics under temperate pasture lands and its implications for nutrient management in agricultural catchments

Subsurface denitrification plays a key role in the reduction or 'attenuation' of nitrate contamination of groundwater and surface waters. We investigated subsurface denitrification characteristics in the vadose zone and shallow groundwater at four sites under pastoral farming in the Manawatū River catchment, located in the lower part of North Island, New Zealand. The denitrification potential of the vadose zone was determined by the laboratory incubation assays measuring the denitrifying enzyme activity (DEA) in soil samples collected from different soil horizons (up to 2.1 m below ground surface), whereas denitrification rates in shallow groundwaters were measured in situ by single-well, push-pull tests conducted in piezometers installed at multiple depths at the study sites. Soils and underlying geology, defining hydrogeologic settings, appear to influence the spatial variability of subsurface denitrification characteristics at the study sites. Where the vadose zone is thin and composed of coarse-textured soils, percolation of nitrate was evident in observed high nitrate-nitrogen concentrations (>5 mg L^-1) in oxic and young shallow groundwaters, but low nitrate-nitrogen concentrations (<0.05 mg L^-1) were observed in the reduced shallow groundwater found underneath the fine textured soils and/or a thick vadose zone. This was confirmed by the push-pull tests measuring denitrification rates from 0.08 to 1.07 mg N L^-1 h^-1 in the reduced shallow groundwaters (dissolved oxygen or DO < 0.5 mg L^-1), while negligible in the oxic groundwaters (DO > 5 mg L^-1) found at the study sites. These contrasting subsurface denitrification characteristics determine the ultimate delivery of nitrate losses from agricultural soils to receiving waters, where the fine textured thick vadose zone and reducing groundwater conditions offer nitrate reduction in the subsurface environment.

Edaphic factors and plants influence denitrification in soils from a long-term arable experiment

Factors influencing production of greenhouse gases nitrous oxide (N₂O) and nitrogen (N₂) in arable soils include high nitrate, moisture and plants; we investigate how differences in the soil microbiome due to antecedent soil treatment additionally influence denitrification. Microbial communities, denitrification gene abundance and gas production in soils from tilled arable plots with contrasting fertilizer inputs (no N, mineral N, FYM) and regenerated woodland in the long-term Broadbalk field experiment were investigated. Soil was transferred to pots, kept bare or planted with wheat and after 6 weeks, transferred to sealed chambers with or without K¹⁵NO₃ fertilizer for 4 days; N₂O and N₂ were measured daily. Concentrations of N₂O were higher when fertilizer was added, lower in the presence of plants, whilst N₂ increased over time and with plants. Prior soil treatment but not exposure to N-fertiliser or plants during the experiment influenced denitrification gene (nirK, nirS, nosZI, nosZII) relative abundance. Under our experimental conditions, denitrification generated mostly N₂; N₂O was around 2% of total gaseous N₂ + N₂O. Prior long-term soil management influenced the soil microbiome and abundance of denitrification genes. The production of N₂O was driven by nitrate availability and N₂ generation increased in the presence of plants.

Arbuscular mycorrhiza has little influence on N2O potential emissions compared to plant diversity in experimental plant communities

Denitrification is an ecosystem process linked to ongoing climate change, because it releases nitrous oxide (N2O) into the atmosphere. To date, the literature covers mostly how aboveground (i.e. plant community structure) and belowground (i.e. plant-associated soil microbes) biota separately influence denitrification in isolation of each other. We here present a mesocosm experiment where we combine a manipulation of belowground biota (i.e. addition of Rhizophagus irregularis propagules to the indigenous mycorrhizal community) with a realized gradient in plant diversity. We used a seed mix containing plant species representative of mesophytic European grasslands and by stochastic differences in species establishment across the sixteen replicates per treatment level a spontaneously established gradient in plant diversity. We address mycorrhizal-induced and plant-diversity mediated changes on denitrification potential parameters and how these differ from the existing literature that studies them independently of each other. We show that unlike denitrification potential, N2O potential emissions do not change with mycorrhiza and depend instead on realized plant diversity. By linking mycorrhizal ecology to an N-cycling process, we present a comprehensive assessment of terrestrial denitrification dynamics when diverse plants co-occur.

The effect of pH on Marinobacter hydrocarbonoclasticus denitrification pathway and nitrous oxide reductase

Increasing atmospheric concentration of N₂O has been a concern, as it is a potent greenhouse gas and promotes ozone layer destruction. In the N-cycle, release of N₂O is boosted upon a drop of pH in the environment. Here, Marinobacter hydrocarbonoclasticus was grown in batch mode in the presence of nitrate, to study the effect of pH in the denitrification pathway by gene expression profiling, quantification of nitrate and nitrite, and evaluating the ability of whole cells to reduce NO and N₂O. At pH 6.5, accumulation of nitrite in the medium occurs and the cells were unable to reduce N₂O. In addition, the biochemical properties of N₂O reductase isolated from cells grown at pH 6.5, 7.5 and 8.5 were compared for the first time. The amount of this enzyme at acidic pH was lower than that at pH 7.5 and 8.5, pinpointing to a post-transcriptional regulation, though pH did not affect gene expression of N₂O reductase accessory genes. N₂O reductase isolated from cells grown at pH 6.5 has its catalytic center mainly as CuZ(4Cu1S), while that from cells grown at pH 7.5 or 8.5 has it as CuZ(4Cu2S). This study evidences that an in vivo secondary level of regulation is required to maintain N₂O reductase in an active state.

Duration and frequency of drainage and flooding events interactively affect soil biogeochemistry and N flux in subtropical peat soils

With the demand for restoration and future prediction of climate change effects, subtropical peatlands are expected to be subjected to hydrologic regimes with variable duration and frequency of drained and flooded conditions, but knowledge of their interactive effects on soil biogeochemistry and emission of greenhouse gases including nitrous oxide (N₂O) is largely limited. The objective of this study was to investigate how the duration and frequency of drainage and flooding events interactively influence soil biogeochemical properties and denitrification and related net N₂O production rates following rewetting. Surface soils are susceptible to different hydrologic regimes. Significantly higher pH, extractable organic carbon (ext. OC), ammonium (NH₄⁺-N), denitrification enzyme activity (DEA), but lower nitrate (NO₃^--N), microbial biomass C and N were observed when the peat soils were under flooded conditions compared to drained conditions. Two-week and four-week drainage or flooding duration did not result in statistically significant differences in soil biogeochemical properties. A 24-week prolonged drainage led to an accumulation of NO₃^--N and a significantly lower pH. Soil microbial biomass and fungal:bacterial abundance likely increased with the frequency of drainage-flooding cycles. Significant differences in denitrification and net N₂O production rates following reflooding were mainly found in the surface soils. Structural equation modeling indicated that hydroperiod and water-filled pore space (WFPS) prior to reflooding is likely to control denitrification and net N₂O production through its regulation of NO₃^--N and activity of microorganisms involved in denitrification while higher drainage-flooding frequency decreases the availability of organic C and NO₃^--N for denitrification. Our results also suggest high NO₃^--N and low pH within peat soils caused by prolonged drainage likely leads to a significant N₂O emission pulse following reflooding. For peat soils subjected to frequent drainage-flooding cycles, N₂O emission pulses following reflooding would decrease with time, attributing to the loss of substrates for denitrification.

Effect of rice straw and swine manure biochar on N2O emission from paddy soil

We analyzed the effects of rice straw biochar (RSBC) and swine manure biochar (SMBC) on N₂O emission from paddy soil. The biochars were added to soil at the rates of 1% and 5% (w/w), and N₂O emission, soil properties and soil enzyme activities were determined at the elongation, heading and maturation stages of rice growth. The N₂O flux started within 2 h of adding the biochar, and decreased significantly thereafter during the three growth stages. The cumulative N₂O emission was suppressed by 45.14–73.96% following biochar application, and 5% SMBC resulted in the lowest cumulative emission. In addition, biochar application significantly increased soil pH, soil organic carbon (SOC), NO₃⁻ levels and urease activity, and decreased soil NH₄⁺ and nitrate reductase activity. Regression analysis indicated that cumulative N₂O emission was correlated positively to NH₄⁺, and negatively to soil pH, SOC and NO₃⁻. SEM further revealed that biochar application weakened the denitrification process, and the NH₄⁺ level had the most significant impact on N₂O emission. Taken together, RSBC and SMBC regulated the nitrogen cycle in paddy soil and mitigated N₂O emission by increasing soil pH, decreasing nitrate reductase activity and NH₄⁺ content.

Effect of Long-Term Pesticides and Chemical Fertilizers Application on the Microbial Community Specifically Anammox and Denitrifying Bacteria in Rice Field Soil of Jhenaidah and Kushtia District, Bangladesh

In this study, we investigated the effect of long-term pesticides and chemical fertilizers application on the microbial communities specifically anammox and denitrification bacteria in rice field soils. The abundances of microbial communities (16S rDNA), anammox (hszB), and denitrification (narG, nirK, nirS, and nosZ) genes were quantified by q-PCR. 10 pesticides (5 insecticides, 3 fungicides and 2 herbicides) and chemical fertilizers urea, potassium, phosphate, DAP (di-ammonium phosphate), gypsum, and boric acid were used by local farmers. Nitrate, SOC (ammonia, soil organic carbon), N and C content significantly (p < 0.05) decreased in the rice field soils as compared to the upland soils. Abundance of 16S rDNA, hszB, narG, nirK, nirS, and nosZ genes significantly (p < 0.05) decreased in the rice field soils and positively correlated with chemical properties of soils. Our results provide useful information and further maintenance should be instilled to the potential of chemical and biological factors decreased in rice field soils.

Relationship between tillage management and DMPSA nitrification inhibitor efficiency

Agricultural sustainability is compromised by nitrogen (N) losses caused by soil microbial activity. Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) produced as consequence of nitrification and denitrification processes in soils. Nitrification inhibitors (NI) as 3,4-dimethylpyrazole-succinic acid (DMPSA) are useful tools to reduce these N losses from fertilization. The objective of this work was to test the efficiency of DMPSA in two different tillage management systems, conventional tillage (CT) and no-tillage (NT), in a winter wheat crop under Humid Mediterranean conditions. N fertilizer was applied as ammonium sulphate (AS) with or without DMPSA in a single or split application, including an unfertilized treatment. GHG fluxes (N₂O, CO₂ and CH₄) were measured by the closed chamber method. amoA and nosZI genes were quantified by qPCR as indicators of nitrifying and denitrifying populations. Nitrification was inhibited by DMPSA in both CT and NT, while the higher water filled pore space (WFPS) in NT promoted a better efficiency of DMPSA in this system. This higher efficiency might be due to a greater N₂O reduction to N₂ as result of the nosZI gene induction. Consequently, DMPSA was able to reduce N₂O emissions down to the unfertilized levels in NT. Provided that NT reduced CO₂ emissions and maintained crop yield compared to CT, the application DMPSA under NT management is a promising strategy to increase agro-systems sustainability under Humid Mediterranean conditions.

Assessing the effects of manure application rate and timing on nitrous oxide emissions from managed grasslands under contrasting climate in Canada

It is uncertain whether process-based models are currently capable of simulating the complex soil, plant, climate, manure management interactions that influence soil nitrous oxide (N₂O) emissions from perennial cropping systems. The objectives of this study were (1) to calibrate and evaluate the DeNitrification DeComposition (DNDC) model using multi-year datasets of measured nitrous oxide (N₂O) fluxes, soil moisture, soil inorganic nitrogen, biomass and soil temperature from managed grasslands applied with manure slurry in contrasting climates of Canada, and (2) to simulate the impact of different manure management practices on N₂O emissions including slurry application i) rates (for both single vs. split); and ii) timing (e.g., early vs. late spring). DNDC showed "fair" to "excellent" performance in simulating biomass (4.7% ≤ normalized root mean square error (NRMSE) ≤ 29.8%; -9.5% ≤ normalized average relative error (NARE) ≤ 16.1%) and "good" performance in simulating soil temperature (13.2% ≤ NRMSE ≤ 18.1%; -0.7% ≤ NARE ≤ 10.8%) across all treatments and sites. However, the model only showed "acceptable" performances in estimating soil water and inorganic N contents which was partially attributed to the limitation of a cascade water sub-model and inaccuracies in simulating root development/uptake. Although, the DNDC model only demonstrated "fair" performance in simulating daily N₂O fluxes, it generally captured the impact of the timing and rate of slurry application and soil texture (loam vs. sandy loam) on total N₂O emissions. The DNDC model simulated N₂O emissions from spring better than split manure application (fall and spring) at the Manitoba site partially due to the overestimation of available substrates for microbial denitrification from fall application during the wet spring periods. Although DNDC performed adequately for simulating most of the manure management impacts considered in this study we recommend improvements in the simulation of soil freeze-thaw cycles, manure decomposition dynamics, soil water storage, rainfall canopy interception, and microbial denitrification and nitrification activities in grasslands.

Nitrate leaching and nitrous oxide emissions from maize after grass-clover on a coarse sandy soil: Mitigation potentials of 3,4-dimethylpyrazole phosphate (DMPP)

Cropping of maize (Zea mays L.) on sandy soil in wet climates involves a significant risk for nitrogen (N) losses, since nitrate added in fertilizers or produced from residues and manure may be lost outside the period with active crop N uptake. This one-year lysimeter experiment investigated the potential of Vizura®, a formulation for liquid manure (slurry) with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP), to mitigate nitrous oxide (N₂O) emissions and nitrate (NO₃^-) leaching from a coarse sandy soil cropped with maize. Maize followed grass-clover (Lolium perenne L.-Trifolium pratense L.) with spring incorporation and was fertilised with cattle slurry. A total of 12 treatments in triplicate were included in a factorial experiment with 1 m² large and 1.4 m deep lysimeters: 1) with or without spraying the above-ground biomass of grass-clover with DMPP before incorporation; 2) application of cattle manure with or without DMPP, or no fertilization; and 3) natural rainfall or extra rain events to represent wet spring conditions, which were simulated with an automated and programmable irrigation system. Around 20 kg N ha^-1 was returned to the soil in grass-clover above-ground biomass, and 145 kg N ha^-1 in cattle manure. Cumulative annual N₂O emissions ranged from 0.4 to 1.3 kg N ha^-1, with between 49 and 86% of emissions occurring during spring. Manure application increased N₂O emissions, while extra rainfall had no effect. The mitigation of N₂O emissions by DMPP ranged from 46 to 67% under natural, and from 44 to 48% under high rainfall conditions. Total annual NO₃^- leaching ranged from 65 to 162 kg N ha^-1. The extent of NO₃^- leaching to 1.4 m depth during spring was low, and instead most (72-83%) of total annual NO₃^--N leaching was recorded during autumn before harvest. The extra rainfall during spring increased NO₃^--N leaching in the pre-harvest period, but it is not clear to what extent this was associated with the N in grass-clover residues or manure applied in spring, or from N mineralisation below the root zone. Despite evidence for a reduction of NO₃^- leaching in three of four scenarios, overall this effect was not significant. No DMPP was detected in leachates. In conclusion, DMPP significantly reduced N₂O emissions from cattle manure on this sandy loam soil independent of rainfall, while there was no significant effect on NO₃^- leaching. The results indicate that N₂O emissions and NO₃^--N leaching were partly derived from below-ground sources of N not affected by DMPP, which should be further investigated to better predict the mitigation potential of nitrification inhibitors.

Changes in soil greenhouse gas fluxes by land use change from primary forest

Primary forest conversion is a worldwide serious problem associated with human disturbance and climate change. Land use change from primary forest to plantation, grassland or agricultural land may lead to profound alteration in the emission of soil greenhouse gases (GHG). Here, we conducted a global meta-analysis concerning the effects of primary forest conversion on soil GHG emissions and explored the potential mechanisms from 101 studies. Our results showed that conversion of primary forest significantly decreased soil CO2 efflux and increased soil CH4 efflux, but had no effect on soil N2 O efflux. However, the effect of primary forest conversion on soil GHG emissions was not consistent across different types of land use change. For example, soil CO2 efflux did not respond to the conversion from primary forest to grassland. Soil N2 O efflux showed a prominent increase within the initial stage after conversion of primary forest and then decreased over time while the responses of soil CO2 and CH4 effluxes were consistently negative or positive across different elapsed time intervals. Moreover, either within or across all types of primary forest conversion, the response of soil CO2 efflux was mainly moderated by changes in soil microbial biomass carbon and root biomass while the responses of soil N2 O and CH4 effluxes were related to the changes in soil nitrate and soil aeration-related factors (soil water content and bulk density), respectively. Collectively, our findings highlight the significant effects of primary forest conversion on soil GHG emissions, enhance our knowledge on the potential mechanisms driving these effects and improve future models of soil GHG emissions after land use change from primary forest.

NO and N2 O transformations of diverse fungi in hypoxia: evidence for anaerobic respiration only in Fusarium strains

Fungal denitrification is claimed to produce non-negligible amounts of N₂ O in soils, but few tested species have shown significant activity. We hypothesized that denitrifying fungi would be found among those with assimilatory nitrate reductase, and tested 20 such batch cultures for their respiratory metabolism, including two positive controls, Fusarium oxysporum and Fusarium lichenicola, throughout the transition from oxic to anoxic conditions in media supplemented with NO 2 - . Enzymatic reduction of NO 2 - (NIR) and NO (NOR) was assessed by correcting measured NO- and N₂ O-kinetics for abiotic NO- and N₂ O-production (sterile controls). Significant anaerobic respiration was only confirmed for the positive controls and for two of three Fusarium solani cultures. The NO kinetics in six cultures showed NIR but not NOR activity, observed through the accumulation of NO. Others had NOR but not NIR activity, thus reducing abiotically produced NO to N₂ O. The presence of candidate genes (nirK and p450nor) was confirmed in the positive controls, but not in some of the NO or N₂ O accumulating cultures. Based on our results, we conclude that only the Fusarium cultures were able to sustain anaerobic respiration and produced low amounts of N₂ O as a response to an abiotic NO production from the medium.

Terrestrial N2 O emissions and related functional genes under climate change: A global meta-analysis

Nitrous oxide (N₂ O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N₂ O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N₂ O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N₂ O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N₂ O emissions (most effectively induced by open-top chambers). Whole-day or whole-year warming treatment significantly enhanced N₂ O emissions, but daytime, nighttime or short-season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N₂ O emission by an average of 55%, while decreased precipitation suppressed N₂ O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U-shape relationship with soil moisture; further insight into biotic mechanisms underlying N₂ O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N₂ O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.

Enhanced efficiency fertilisers reduce nitrous oxide emissions and improve fertiliser 15N recovery in a Southern Australian pasture

The effects of reducing nitrogen (N) rates or using enhanced efficiency fertilisers (EEFs) (i.e. urease and nitrification inhibitors, and controlled release fertilisers) on nitrous oxide (N₂O) emissions and nitrogen use efficiency (NUE) are not well understood in temperate, rainfed pastures. A field experiment on rainfed ryegrass pasture in southern Australia examined the effect of granular urea N rate (0, 250 and 420 kg N ha^-1 over 6 months) and EEF use (at 250 kg N ha^-1 with NBPT, DMPP, or polymer coating (PCU)) on N₂O emissions, NUE and fertiliser N recovery (using ¹⁵N techniques). Cumulative net-N₂O emissions increased with N rate from 308 g N₂O-N ha^-1 (250 kg N ha^-1) to 514 g N₂O-N ha^-1 (420 kg N ha^-1). Using EEFs reduced N₂O emissions by 22% (NBPT), 44% (DMPP) and 56% (PCU) compared to urea. The emission factor (EF) (kg net N₂O-N per kg N applied) was 0.12 for both N rates (250 and 420 kg N ha^-1) but reduced with the EEFs to 0.10 (NBPT) and 0.07 (DMPP and PCU) compared to urea. EEF use had no significant impact on biomass or apparent NUE but led to a greater recovery of N in the soil after one month (44.8% (DMPP) and 45.9% (NBPT) compared to urea (33.7%)). Within one month 42% of the N applied as urea (U50) was lost from the plant-soil system, which was reduced with DMPP (32% loss) and NBPT (33% loss). After six months, 52% (U50) to 59% (U84) of the urea N applied was lost. The positive effect of the EEFs on N₂O emissions, and the increased recovery of N in the soil-plant system with the EEFs over one month indicates they could provide longer term benefits though soil N storage, and could be applied at lower N rates to achieve NUE benefit.

How does high C load affect nitrous oxide emission of subsurface wastewater infiltration system?

Subsurface wastewater infiltration systems (SWISs) have drawn much attention due to the lower operating costs, lower energy demands and absence of secondary pollutants requiring further treatment. The process of denitrification involves reduction of nitrate to nitrous oxide (N₂O) and dinitrogen gas (N₂). Though removal of nitrate is advantageous from a water quality perspective, N₂O may contribute to adverse environmental effects. This study evaluated N₂O emission at high C loading regimes. The results revealed that as the C load increased, N₂O emission increased first and then decreased, indicating that carbon source was a limiting factor for the release of N₂O from the denitrification process. Denitrification was the dominant process for the release of N₂O at any of the given C loads. When the influent carbon load was in the range of 220-460 mg/L, the contribution of denitrification to N₂O emissions came to 69.77-83.11% (feeding period) and 67.07-79.53% (rest period), respectively.

Nitrogen purification potential limited by nitrite reduction process in coastal eutrophic wetlands

Coastal wetlands accumulate enormous quantities of nitrogen due to their position at the interface between land and sea and high trapping capacity. Fortunately, they have high nitrogen (N) purifying (removal) capacity, which means that they likely play an important role in mitigating against coastal eutrophication. However studies that empirically measure the degree to which wetlands purify nitrogen and their removal pathways (e.g. denitrification, anammox, plant uptake, microbial immobilization, etc.) are rare. In this study, the N purification potential (denitrification and anammox) and enzyme activities related to denitrification in different subtropical wetlands types were conducted in nitrogen-enriched wetlands of Daya Bay, Southern China. We found the average N purification rate was 11.4 μmol N·kg^-1·h^-1, with denitrification accounting for 84.2%-100% of the total N₂ production in the wetlands of Daya Bay. The N purification potential in the wet season, subtidal areas and mangrove forests were generally observed to be higher than that in the dry season, high and low tidal areas, barren and estuary habitats, respectively. Correspondingly, these differences were mainly driven by the temperature, Eh and NH₄-N, respectively. Additionally, the nitrate reductase (Nar) and nitrite reductase (Nir) activities tended to be similar among different seasons and tidal areas, however, Nir activity in mangrove forest was 1.5-fold and 2-fold of the estuarine and barren areas, respectively. Meanwhile, Nir showed a positive correlation with denitrification rate. These results indicate that NO₂-N reduction, the key control mechanism for N purification, should be the rate-limiting step of the denitrification process in Daya Bay wetlands. Notably, mangroves could improve N removal rates by 48.0% compared to other wetlands. Therefore, protecting and restoring mangrove ecosystems could be an effective way to reduce the risk of coastal eutrophication.

Practicing anammox in a novel hybrid anaerobic-aerobic baffled reactor for treating high-strength ammonium piggery wastewater with low COD/TN ratio

A novel hybrid anaerobic-aerobic baffled reactor (HAOBR) with four compartments was constructed to treat manure-free piggery wastewater with an average COD/TN ratio as low as 0.98, without any supplement of external carbon source. Inoculated with aerobic activated sludge and operated at hydraulic retention time 36 h, 32 °C and reflux ratio 2.0, the reactor could perform steadily within 24 days. The removal of COD, NH₄⁺-N and TN within the 21-days steady phase averaged 87.0%, 100% and 91.3%, respectively. Analysis of stoichiometry and results of high-throughput pyrosequencing revealed that the excellent nitrogen removal in the HAOBR was achieved by the cooperation of heterotrophic and autotrophic denitrification with anammox as the dominant approach. Compared with the previously developed microaerobic treatment processes and the recently reported modified A/O process, the HAOBR was more cost-efficient in treating manure-free piggery wastewater because of the less energy consumption, rapid startup process and efficient nutrients removal.

Nitrogen Removal from Domestic Wastewater and the Development of Tropical Ornamental Plants in Partially Saturated Mesocosm-Scale Constructed Wetlands

Vertical partially saturated (VPS) constructed wetlands (CWs) are a novel wastewater treatment system for which little information is known about its design parameters and performance under tropical climates. The objective of this study is to evaluate the nitrogen removal process from domestic wastewater and the production of tropical ornamental plants (Canna hybrids and Zantedeschia aethiopica) in VPS CWs at a mesocosms scale. Nine VPS CWs, with a free-flow zone of 16 cm and a saturated zone of 16 cm, were used as experimental units. Three units were planted with Canna hybrids., and three, with Zantedeschia aethiopica (one plant per unit); the remaining three units were established as controls without vegetation. They were fed with domestic wastewater intermittently and evaluated for the elimination of COD, N-NH₄, N-NO₃, Norg, NT, and PT. The results showed an increase in the removal for some pollutants in the vegetated systems, i.e., N-NH₄ (35%), Norg (16%), TN (25%), and TP (47%) in comparison to the unvegetated systems. While N-NO₃ removal showed better removal in 10% of the systems without vegetation, no significant differences were found (p > 0.05) for COD removal. The aerobic and anaerobic conditions in the VPS CWs favor the elimination of pollutants in the systems, and also the development of the tropical species evaluated in this study; good development was exhibited by a high growth rate and biomass production.

Biological removal of nitrogen oxides by microalgae, a promising strategy from nitrogen oxides to protein production

Nitrogen oxides (NO_x) are the components of fossil flue gases that give rise to serious environmental and health hazards. Among the available techniques for NO_x removal, microalgae-based biological removal of NO_x (BioDeNO_x) is a promising and competent technology with eco-friendly path of low energy and low-cost solution for the pollution. In this review article, current biological technologies including bacteria-based and microalgae-related BioDeNO_x are discussed. Comparing to direct BioDeNO_x approach, indirect BioDeNO_x by microalgae is more promising since it is more stable, reliable and efficient. By transforming inorganic nitrogen nutrients to organic nitrogen, microalgae can potentially play an important role in converting NO_x into high-value added products. The microalgae-based BioDeNO_x process displays an attractive prospect for flue gas treatment to reduce environmental NO_x pollution and potentially supply protein products, establishing an efficient circular-economy strategy.

Strategic grazing management and nitrous oxide fluxes from pasture soils in tropical dairy systems

Greenhouse gases emissions are considered one of the most important environmental issues of dairy farming systems. Nitrous oxide (N₂O) has particular importance owing to its global warming potential and stratospheric ozone depletion. The objective of this study was to investigate the influence of two rotational grazing strategies characterized by two pre-grazing targets (95% and maximum canopy light interception; LI_95% and LI_Max, respectively) on milk production efficiency and N₂O fluxes from soil in a tropical dairy farming system based on elephant grass (Pennisetum purpureum Schum. cv. Cameroon). Results indicated that LI_95% pre-grazing target provided more frequent defoliations than LI_Max. Water-filled pore space, soil and chamber temperatures were affected by sampling periods (P₁ and P₂). There was a significant pre-grazing target treatment × sampling period interaction effect on soil NH₄⁺ concentration, which was most likely associated with urinary-N discharge. During P₁, there was a greater urinary-N discharge for LI_95% than LI_Max (26.3 vs. 20.9 kg of urinary-N/paddock) caused by higher stocking rate, which resulted in greater N₂O fluxes for LI_95%. Inversely, during P₂, the soil NH₄⁺ and N₂O fluxes were greater for LI_Max than LI_95%. During this period, the greater urinary-N discharge (46.8 vs. 44.8 kg of urinary-N/paddock) was likely associated with longer stocking period for LI_Max relative to LI_95%, since both treatments had similar stocking rate. Converting hourly N₂O fluxes to daily basis and relating to milk production efficiency, LI_95% was 40% more efficient than LI_Max (0.34 vs. 0.57 g N-N₂O/kg milk·ha). In addition, LI_95% pre-grazing target decreased urea-N loading per milk production by 34%. Strategic grazing management represented by the LI_95% pre-grazing target allows for intensification of tropical pasture-based dairy systems, enhanced milk production efficiency and decreased N-N₂O emission intensity.

An integrated assessment of nitrogen source, transformation and fate within an intensive dairy system to inform management change

From an environmental perspective optimised dairy systems, which follow current regulations, still have low nitrogen (N) use efficiency, high N surplus (kg N ha-1) and enable ad-hoc delivery of direct and indirect reactive N losses to water and the atmosphere. The objective of the present study was to divide an intensive dairy farm into N attenuation capacity areas based on this ad-hoc delivery. Historical and current spatial and temporal multi-level datasets (stable isotope and dissolved gas) were combined and interpreted. Results showed that the farm had four distinct attenuation areas: high N attenuation: characterised by ammonium-N (NH4+-N) below 0.23 mg NH4+-N l-1 and nitrate (NO3--N) below 5.65 mg NO3--N l-1 in surface, drainage and groundwater, located on imperfectly to moderately-well drained soils with high denitrification potential and low nitrous oxide (N2O) emissions (av. 0.0032 mg N2O-N l-1); moderate N attenuation: characterised by low NO3--N concentration in drainage water but high N2O production (0.0317 mg N2O-N l-1) and denitrification potential lower than group 1 (av. δ15N-NO3-: 16.4‰, av. δ18O-NO3-: 9.2‰), on well to moderately drained soils; low N attenuation-area 1: characterised by high NO3--N (av. 6.90 mg NO3--N l-1) in drainage water from well to moderately-well drained soils, with low denitrification potential (av. δ15N-NO3-: 9.5‰, av. δ18O-NO3-: 5.9‰) and high N2O emissions (0.0319 mg N2O l-1); and low N attenuation-area 2: characterised by high NH4+-N (av. 3.93 mg NH4+-N l-1 and high N2O emissions (av. 0.0521 mg N2O l-1) from well to imperfectly drained soil. N loads on site should be moved away from low attenuation areas and emissions to air and water should be assessed.

Changes in nitrogen functional genes in soil profiles of grassland under long-term grazing prohibition in a semiarid area

Grazing prohibition has been used to restore degraded grassland ecosystems in semiarid areas; however, the impact of this measure on soil nitrogen (N) cycling is poorly understood. Furthermore, recent studies have tended to focus on the topsoil and ignored a steep gradient of nutrient accumulation with soil depth. Here, we investigated changes in N functional genes (NFGs) involved in organic N decomposition (chiA), archaeal and bacterial ammonia oxidation (amoA-AOA and amoA-AOB), respectively, denitrification (nirK and nirS), and N fixation (nifH) in soil profiles from a chronosequence of grazing prohibition (0, 10, 15, 25, and 35 years) in the semiarid grasslands of the Loess Plateau, China. The abundance of all the investigated NFGs in grassland soils after 35 years' grazing prohibition was higher than in grazed grassland. This result suggests that microbial N turnover potential is facilitated by grazing prohibition, probably through enhanced biomass production via increases in nutrient input into the soil. The higher ratio of (chiA + nifH)/(amoA-AOA + amoA-AOB) and values of (nirK + nirS) in grazing-prohibited grasslands than in grazed grassland suggest that prohibition of grazing not only improved microbial N storage potential but also increased N gas emission potential. The abundances of NFGs varied along the soil profiles and responded differently to environmental factors. The chiA and nifH abundances decreased with soil depth and were associated with variation in aboveground biomass, NH₄⁺-N, and organic carbon, while amoA-AOA, nirK, and nirS genes increased with depth and were more affected by soil organic carbon, moisture, and bulk density. Multivariate regression tree analysis demonstrated that aboveground biomass was the best explanatory variable for the changes in NFGs in grazed grassland, while soil organic carbon was the best in the grazing-prohibited grasslands. Our results provide new insight into the soil N cycling potential of degraded and restored semiarid grassland ecosystems.

Spatial and Seasonal Variations in the Abundance of Nitrogen-Transforming Genes and the Microbial Community Structure in Freshwater Lakes with Different Trophic Statuses

Identifying nitrogen-transforming genes and the microbial community in the lacustrine sedimentary environment is critical for revealing nitrogen cycle processes in eutrophic lakes. In this study, we examined the diversity and abundance of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), denitrifying bacteria (DNB), and anammox bacteria (AAOB) in different trophic status regions of Lake Taihu using the amoA, Arch-amoA, nirS, and hzo genes as functional markers. Quantitative Polymerase Chain Reaction (qPCR) results indicated that the abundance of the nirS gene was the highest, while the amoA gene had the lowest abundance in all regions. Except for the primary inflow area of Lake Taihu, Arch-amoA gene abundance was higher than the hzo gene in three lake bays, and the abundance of the nirS gene increased with decreasing trophic status. The opposite pattern was observed for the amoA, Arch-amoA, and hzo genes. Phylogenetic analyses showed that the predominant AOB and AOA were Nitrosomonas and Nitrosopumilus maritimus, respectively, and the proportion of Nitrosomonas in the eutrophic region (87.9%) was higher than that in the mesotrophic region (71.1%). Brocadia and Anammoxoglobus were the two predominant AAOB in Lake Taihu. Five novel unknown phylotypes of AAOB were observed, and Cluster AAOB-B was only observed in the inflow area with a proportion of 32%. In the DNB community, Flavobacterium occurred at a higher proportion (22.6–38.2%) in all regions, the proportion of Arthrobacter in the mesotrophic region (3.6%) was significantly lower than that in the eutrophic region (15.6%), and the proportions of Cluster DNB-E in the inflow area (24.5%) was significantly higher than that in the lake bay (7.3%). The canonical correspondence analysis demonstrated that the substrate concentration in sedimentary environments, such as NO_x^--N in the sediment, NH₄⁺-N in the pore water, and the total organic matter, were the key factors that determined the nitrogen-transforming microbial community. However, the temperature was also a predominant factor affecting the AOA and AAOB communities.

Nitrous oxide emissions decrease with plant diversity but increase with grassland primary productivity

Nitrous oxide (N₂O), a main greenhouse gas that contributes to ozone layer depletion, is released from soils. Even when it has been argued that agriculture is the main cause of its increase in the atmosphere, natural ecosystems are also an important source of N₂O. However, the impacts of human activities on N₂O emissions through biodiversity loss or primary productivity changes in natural ecosystems have rarely been assessed. Here, we analyzed the effects of vegetation attributes such as plant diversity and production, as drivers of N₂O emission rates, in addition to environmental factors. We measured N₂O emissions monthly during 1 year in 12 sites covering a large portion of the Rio de la Plata grasslands, Argentina, and related these emissions with climate, soil and vegetation attributes. We performed spatial and temporal models of N₂O emissions separately, to evaluate which drivers control N₂O in space and over time independently. Our results showed that in the spatial model, N₂O emissions decreased with increments in plant species richness, with concomitant reductions in soil [Formula: see text] whereas N₂O emissions increased with primary productivity. By contrast, in the temporal model, monthly precipitation and monthly temperature were the main drivers of N₂O emissions, with positive correlations, showing important differences with the spatial model. Overall, our results show that biological drivers may exert substantial control of N₂O emissions at large spatial scales, together with climate and soil variables. Our results suggest that biodiversity conservation of natural grasslands may reduce regional greenhouse gas emissions, besides maintaining other important ecosystem services.

Gross N transformation rates and related N2O emissions in Chinese and UK agricultural soils

The properties of agricultural soils in various regions of the world are variable and can have a significant but poorly understood impact on soil nitrogen (N) transformations and nitrous oxide (N₂O) emissions. For this reason, we undertook a study of gross N transformations and related N₂O emissions in contrasting agricultural soils from China and the UK. Seven Chinese and three UK agricultural soils were collected for study using a ¹⁵N tracing approach. The soil pH ranged from 5.4 to 8.7, with three acidic soils collected from Jinjing, Lishu and Boghall; one neutral soil collected from Changshu, and the other six alkaline soils collected from Quzhou, Zhangye, Changwu, Jinzhong, Boxworth and Stetchworth. Our results showed that the main N transformation processes were oxidation of ammonium (NH₄⁺) to nitrate (NO₃^-) (O_NH4), and mineralization of organic N to NH₄⁺. The gross autotrophic nitrification rates calculated in the three acidic soils were between 0.25 and 4.15 mg N kg^-1 d^-1, which were significantly lower (p < 0.05) than those in the remaining neutral and alkaline soils ranging from 6.94 to 14.43 mg N kg^-1 d^-1. Generally, soil pH was positively correlated (p < 0.001) with gross autotrophic nitrification rate and cumulative N₂O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N₂O emissions. There was also a significant positive correlation between the gross autotrophic nitrification rate and cumulative N₂O emissions, highlighting the importance of this process for producing N₂O emissions in these agricultural soils under aerobic conditions. Gross NH₄⁺ immobilization rates were very low in most soils except for the Jinjing soil with the lowest pH. In conclusion, the gross autotrophic nitrification rates and related N₂O emissions were controlled by soil pH irrespectively of the soil's origin in these agricultural soils.

Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852-2014): From traditional agriculture to conventional intensive systems

France was a traditionally agricultural country until the first half of the 20th century. Today, it is the first European cereal producer, with cereal crops accounting for 40% of the agricultural surface area used, and is also a major country for livestock breeding with 25% of the European cattle livestock. This major socioecological transition, with rapid intensification and specialisation in an open global market, has been accompanied by deep environmental changes. To explore the changes in agricultural GHG emissions over the long term (1852-2014), we analysed the emission factors of N₂O from field experiments covering major land uses, in a gradient of fertilisation and within a range of temperature and rainfall, and used CH₄ emission coefficients for livestock categories, in terms of enteric and manure management, considering the historical changes in animal excretion rates. We also estimated indirect CO₂ emissions, rarely accounted for in agricultural emissions, using coefficients found in the literature for the dominant energy consumption items (fertiliser production, field work and machinery, and feed import). From GHG emissions of ~30,000 ktons CO₂ Eq yr^-1 in 1852, reaching 54,000 ktons CO₂ Eq yr^-1 in 1955, emissions more than doubled during the 'Glorious thirties' (1950-1980), and peaked around 120,000 ktons CO₂ Eq yr^-1 in the early 2000s. For the 2010-2014 period, French agriculture GHG emissions stabilised at ~114,000 ktons CO₂ Eq yr^-1, distributed into 49% methane (CH₄), 22% carbon dioxide (CO₂) and 29% nitrous oxide (N₂O). A regional approach through 33 regions in France shows a diversity of agriculture reflecting the hydro-ecoregion distribution and the agricultural specialisation of local areas. Exploring contrasting scenarios at the 2040 horizon suggests that only deep changes in the structure of the agro-food system would double the reduction of GHG emissions by the agricultural sector.

Improved isotopic model based on 15 N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N2 O emissions in a clay grassland soil

RATIONALE

Isotopic signatures of N2 O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2 O emissions. The contribution of each source to N2 O emissions after N-application is difficult to determine. Here, isotopologue signatures of emitted N2 O are used in an improved isotopic model based on Rayleigh-type equations.

METHODS

The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2 O, N2 and CO2 over a 12-day incubation period. To determine the source of N2 O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2 O and those from the 15 N-tracing technique.

RESULTS

The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The 15 N-enrichment analysis shows that initially ~70% of the emitted N2 O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N-pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh-type model applied to N2 O isotopocules data (δ15 Nbulk -N2 O values) shows poor agreement with the measurements for the original one-pool model for treatment 1c; the two-pool models gives better results when using a third-order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches.

CONCLUSIONS

The importance of N2 O emissions from different N-pools in soil for the interpretation of N2 O isotopocules data was demonstrated using a Rayleigh-type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.

Improvement of the 15 N gas flux method for in situ measurement of soil denitrification and its product stoichiometry

RATIONALE

Field measurement of denitrification in agricultural ecosystems using the ¹⁵ N gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low ¹⁵ N₂ fluxes in the presence of a high atmospheric N₂ background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N₂ -depleted atmospheres.

METHODS

We developed a new procedure to conduct the ¹⁵ N gas flux method suitable for field application using an artificially N₂ -depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without ¹⁵ N-labelling and using different flushing strategies were conducted to develop a suitable field method. Subsequently, this method was tested in the field and results were compared with those obtained from the conventional ¹⁵ N gas flux method.

RESULTS

Results of the two methods were in close agreement showing that the denitrification rates determined were not biased by the flushing procedure. Best sensitivity for N₂  + N₂ O fluxes was 10 ppb, which was 80-fold better than that of the reference method. Further improvement can be achieved by lowering the N₂ background concentration below the values established in the present study.

CONCLUSIONS

In view of this progress in sensitivity, the new method will be suitable to measure denitrification dynamics in the field beyond peak events.

Nutrient removal from high ammonium swine wastewater in upflow microaerobic biofilm reactor suffered high hydraulic load

To understand the ability of an upflow microaerobic biofilm reactor (UMBR) to remove nutrient from manure-free swine wastewater rich in NH₄⁺ with a COD/TN ratio less than 1.00, effect of hydraulic loading rate (HLR) on the microaerobic process was evaluated with a constant reflux ratio of 25 at 25 °C. The results showed that changes in HLR had a remarkable effect on the performance of the UMBR in nutrient removal from the wastewater. With the favorable HLR 3.0 m³/(m³·d) (Hydraulic Retention Time (HRT) 8 h), average removal of COD, NH₄⁺ and TN in the microaerobic process reached 59.3%, 87.7% and 84.7%, respectively, though the COD/TN ratio was as low as 0.84. With an over HLR of 4.0 m³/(m³·d) (HRT decreased to 6 h), bad performance of the UMBR was observed with an average removal of COD, NH₄⁺ and TN as low as 45.0%, 59.0% and 57.5%, respectively. Since the HLR was decreased to 2.4 m³/(m³·d) (HRT 10 h), the microaerobic process regained the efficiency in nutrient removal with a removal of COD, NH₄⁺ and TN averaged 59.0%, 95.3% and 87.8%, respectively. The microaerobic condition allowed anammox bacteria, ammonia-oxidizing bacteria and archaea, nitrite-oxidizing bacteria and denitrifiers to all thrive in the UMBR, resulting in the efficient synchronous removal of organic carbon and nitrogen. As the dominant approach to nitrogen removal, anaerobic ammonium oxidation (anammox) pathway contributing to the TN removal in the microaerobic process exceeded 59.5% at HLR 3.0 m³/(m³·d). The results demonstrated that the UMBR can remove nitrogen and carbon from swine wastewater, with a suitable HLR.

Remarkable N2O emissions by draining fallow paddy soil and close link to the ammonium-oxidizing archaea communities

Fallow paddies experience natural flooding and draining water status due to rainfall and evaporation, which could induce considerable nitrous oxide (N₂O) emissions and need to be studied specially. In this study, intact soil columns were collected from a fallow paddy field and the flooding-draining process was simulated in a microcosm experiment. The results showed that both N₂O concentrations in the soil and N₂O emission rates were negligible during flooding period, which were greatly elevated by draining the fallow paddy soil. The remarkable N₂O concentrations in the soil and N₂O emission/h during draining both had significant relationships with the Arch-amoA gene (P < 0.01) but not the Bac-amoA, narG, nirK, nirS, and nosZ genes, indicating that the ammonium-oxidizing archaea (AOA) might be the important players in soil N₂O net production and emissions after draining. Moreover, we observed that N₂O concentrations in the upper soil layers (0-10 cm) were not significantly different from that in the 10-20 cm layer under draining condition (P > 0.05). However, the number of AOA and the nitrification substrate (NH₄⁺-N) in the 0-10 cm layer were significantly higher than in the 10-20 cm layer (P < 0.01), indicating N₂O production in the 0-10 cm layer might be higher than the measured concentration and would contribute considerably to N₂O emissions as shorter distance of gas diffusion to the soil surface.

Biochar reduces the efficiency of nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) mitigating N2O emissions

Among strategies suggested to decrease agricultural soil N₂O losses, the use of nitrification inhibitors such as DMPP (3,4-dimethylpyrazole phosphate) has been proposed. However, the efficiency of DMPP might be affected by soil amendments, such as biochar, which has been shown to reduce N₂O emissions. This study evaluated the synergic effect of a woody biochar applied with DMPP on soil N₂O emissions. A incubation study was conducted with a silt loam soil and a biochar obtained from Pinus taeda at 500 °C. Two biochar rates (0 and 2% (w/w)) and three different nitrogen treatments (unfertilized, fertilized and fertilized + DMPP) were assayed under two contrasting soil water content levels (40% and 80% of water filled pore space (WFPS)) over a 163 day incubation period. Results showed that DMPP reduced N₂O emissions by reducing ammonia-oxidizing bacteria (AOB) populations and promoting the last step of denitrification (measured by the ratio nosZI + nosZII/nirS + nirK genes). Biochar mitigated N₂O emissions only at 40% WFPS due to a reduction in AOB population. However, when DMPP was applied to the biochar amended soil, a counteracting effect was observed, since the N₂O mitigation induced by DMPP was lower than in control soil, demonstrating that this biochar diminishes the efficiency of the DMPP both at low and high soil water contents.