Denitrification Ratio Research Articles

Effects of aged biochar additions at different addition ratios on soil greenhouse gas emissions

Biochar addition is effective in reducing soil greenhouse gas (GHG) emissions, but it's essential to evaluate whether aged biochar retains this capability as its properties change over time. However, research comparing the effects of fresh and aged biochar on soil GHG emissions is limited. Moreover, exploring the priming effect of biochar on native soil organic carbon (SOC) mineralization is crucial for revealing the effect mechanism on soil CO2 emission. However, research investigating the priming effects of aged biochar is limited. In this study, the effects of aged biochar addition on soil physicochemical properties, GHG emissions, and global warming potential (GWP) were examined through an incubation experiment with three treatments: (1) soil only (CK), (2) 1 % aged maize straw biochar addition (HBC1) and (3) 4 % aged maize straw biochar addition (HBC4), and then their effects were compared with those of fresh biochar from our previous research. 13C tracer technology was used to assess the priming effect of aged biochar on native SOC mineralization. Results showed that aged biochar improved soil physicochemical properties. Compared to CK, HBC1 and HBC4 reduced CO2 emissions by 28.02 % and 20.15 %, respectively, and reduced N2O emissions by 61.54 % and 66.39 %. HBC4 significantly increased CH4 emission, whereas HBC1 reduced it. HBC1 and HBC4 reduced GWP by 29.01 % and 21.41 %, respectively. Overall, aged biochar demonstrated a greater reduction effect compared to fresh biochar at the 1 % addition ratio. The CO2 reduction is attributed to the negative priming effect of aged biochar on native SOC mineralization. The reduction in N2O emissions is attributed to aged biochar promoting microbial nitrogen fixation and reducing the ratio of denitrification to nitrification. The variation in CH4 emissions reflects differing dominant factors influencing CH4 emission across varying addition ratios. In conclusion, 1 % aged biochar addition demonstrates a more favorable long-term effect on mitigating GHG emissions.

Effect of agricultural management system (“cash crop”, “livestock” and “climate optimized”) on nitrous oxide and ammonia emissions

AbstractThe study aimed to measure soil-atmosphere N2O fluxes and their controlling factors, as well as NH3 emissions and yields for two soils (silt loam and clay loam) in three management systems over two years under subsequent wheat and maize cultivation. The management systems were characterized as follows: (1) cash crop (C) with mineral fertilizer and conventional tillage; (2) livestock (L) with biogas residue fertilization and its incorporation prior to sowing in maize and reduced tillage; and (3) climate optimized (O) with minimum tillage, 8-year crop rotation, with biogas residue fertilization, in maize without incorporation in clay loam soil or incorporation by strip-tillage prior to seeding in silt loam soil. Stable isotope ratios of N2O and mineral N were determined to identify N2O processes. Within the organically fertilized maize treatments, cumulative N2O fluxes were highest in the O-system treatments of both sites (4.0 to 9.4 kg N ha− 1 a− 1), i.e. more than twice as high as in the L-system (1.5 to 3.1 kg N ha− 1 a− 1). Below root-strip till fertilizer application did not enhance N2O fluxes. Fluxes with mineral fertilization of wheat (1.1 to 3.1 kg N ha− 1 a− 1) were not different from those with organic fertilization. Isotopic values of emitted N2O revealed that bacterial denitrification dominated most of the peak flux events, while the N2O/(N2 + N2O) ratio of denitrification was mostly between 0.1 and 0.5. It can be concluded that, contrary to the intention to lower greenhouse gas fluxes by the O-system management, the highest N2O fluxes occurred in the O-system without biogas digestate incorporation in maize. With respect to NH3 fluxes, we could confirm that the application of digestate application in growing crops without incorporation or late incorporation in fertilization before sowing induces high fluxes. The beneficial aspects of the O-system including more stable soil structure and resource conservation, are thus potentially counteracted by increased N2O and NH3 emissions.

Advanced nitrogen and phosphorus removal in pilot-scale anaerobic/aerobic/anoxic system for municipal wastewater in Northern China

Removing nitrogen and phosphorus from low ratio of chemical oxygen demand to total nitrogen and temperature municipal wastewater stays a challenge. In this study, a pilot-scale anaerobic/aerobic/anoxic sequencing batch reactor (A/O/A-SBR) system first treated 15 m3/d actual municipal wastewater at 8.1–26.4 °C for 224 days. At the temperature of 15.7 °C, total nitrogen in influent and effluent were 45.5 and 10.9 mg/L, and phosphorus in influent and effluent were 3.9 and 0.1 mg/L. 16 s RNA sequencing results showed the relative abundance of Competibacter and Tetrasphaera raised to 1.25 % and 1.52 %. The strategy of excessive, no and normal sludge discharge enriched and balanced the functional bacteria, achieving an endogenous denitrification ratio more than 43.3 %. Sludge reduction and short aerobic time were beneficial to energy saving contrast with a Beijing municipal wastewater treatment. This study has significant implications for the practical application of the AOA-SBR process.

The Effect of Foam-Recycle on Ammonium Removal by Aerobic Denitrification Using Alcaligenes faecalis No. 4

Aerobic denitrifier Alcaligenes faecalis No. 4 removes ammonium-nitrogen to nitrogen gas via denitrification in a single aerobic condition. In our previous studies, factors such as ammonium removal rate, denitrification ratio, and cell growth were tested in various conditions. The removal pathway from ammonium to nitrogen gas still needs to be determined in detail. To clarify this pathway of Alcaligenes faecalis No. 4, we in this study investigated the effects of several factors on ammonium removal, such as foam-recycle, initial pH, initial ammonium concentration, and airflow rate. Denitrification ratio was improved by up to 23% through foam-recycle. The improvement of the denitrification ratio was resulted by the higher enzyme activity of hydroxylamine oxidoreductase (HAO) in the produced foam, which was about 28 times higher than that in the culture broth (i.e., without foam-recycle). The stripped ammonia was significantly high (above pH 9). The initial ammonium concentration and airflow rate also influenced the denitrification ratio.

Effect of calcareous and siliceous amendments on N2O emissions of a grassland soil

AbstractLiming of acidic agricultural soils has been proposed as a strategy to mitigate nitrous oxide (N2O) emissions, as increased soil pH reduces the N2O/N2 product ratio of denitrification. The capacity of different calcareous (calcite and dolomite) and siliceous minerals to increase soil pH and reduce N2O emissions was assessed in a 2‐year grassland field experiment. An associated pot experiment was conducted using homogenized field soils for controlling spatial soil variability. Nitrous oxide emissions were highly episodic with emission peaks in response to freezing–thawing and application of NPK fertilizer. Liming with dolomite caused a pH increase from 5.1 to 6.2 and reduced N2O emissions by 30% and 60% after application of NPK fertilizer and freezing–thawing events, respectively. Over the course of the 2‐year field trial, N2O emissions were significantly lower in dolomite‐limed than non‐limed soil (p < .05), although this effect was variable over time. Unexpectedly, no significant reduction of N2O emission was found in the calcite treatment, despite the largest pH increase in all tested minerals. We tentatively attribute this to increased N2O production by overall increase in nitrogen turnover rates (both nitrification and denitrification) following rapid pH increase in the first year after liming. Siliceous materials showed little pH effect and had no significant effect on N2O emissions probably because of their lower buffering capacity and lower cation content. In the pot experiment using soils taken from the field plots 3 years after liming and exposing them to natural freezing–thawing, both calcite (p < .01) and dolomite (p < .05) significantly reduced cumulative N2O emission by 50% and 30%, respectively, relative to the non‐limed control. These results demonstrate that the overall effect of liming is to reduce N2O emission, although high lime doses may lead to a transiently enhanced emission.

A novel coupled microbial fuel cell operation for organic and nitrogen removal with simultaneous energy recovery from wastewater

The inherent disadvantage of MFC in wastewater treatment is low energy recovery and is even lower when dealing with nitrogen. This study investigated a novel coupled MFC, where each MFC is designed or configured to accomplish different tasks. The 1st MFC was responsible for dominating the electricity generation from oxidation of organics and the 2nd MFC for nitrogen removal with an adequate minimum amount of carbon. At the 1st MFC, 88.3–98.7 % of organic matter was removed with an average power density of 3432–3847 mW m−3 depending on the initial chemical oxygen demand (COD) concentration. Ammonium was completely oxidized to nitrite and nitrate in the cathode chamber of the 1st MFC. These were then reduced in both chambers of the 2nd MFC helping to achieve a total nitrogen removal of 56.5–79.8 %. The high ratio of autotrophic denitrification demonstrated that bioelectrochemical denitrification is the main pathway in the cathode chamber of 2nd MFC when the remaining organic carbon is minimal. The cathodic nitrification and anodic heterotrophic denitrification processes contributed to pH stability in these chambers. These findings indicated a suitable operating method for a coupled MFC that can efficiently remove nitrogen and maximize the energy recovery from organic matter in wastewater.

Waterlogging effects on N2O and N2 emissions from a Stagnosol cultivated with Silphium perfoliatum and silage maize

European policy recommends that biomass production occur on marginal land, such as poorly draining Stagnosols. Compared to annual cropping, perennial crops may better mitigate N2O emissions at such sites, through more complete denitrification. To test that hypothesis, we compared N2 and N2O fluxes from the soils of a perennial crop (cup plant, Silphium perfoliatum L.) and an annual crop (silage maize, Zea mays L.). Intact soil columns (35 cm height, 14.4 cm diameter) were incubated for 37 days. The soils were fertilized with 60 or 120 kg N ha−1 and exposed to successive phases of waterlogging: free drainage, waterlogging of 1/3-, and waterlogging of 2/3- of the column. Source-specific N2O and N2 fluxes were measured using the 15 N gas flux method. Denitrification was higher in cup plant than maize soil and total N losses from denitrification were dominated by emissions from the third phase. Cup plant soil emitted 33.6 ± 78.1 mg N m−2 and 95.8 ± 64.4 mg N m−2 more N2O than maize soil in the low and high N treatments, respectively. The product ratio of denitrification (N2Oi = N2O/(N2 + N2O)) increased with waterlogging in maize soil, while remaining stable in cup plant soil. Emissions from the top 10 cm dominated the N2Oi rather than N2 fluxes from the saturated soil. This study did not show N2O mitigation in cup plant soil, instead highlighting the complexity of plant-soil effects on denitrification. We clearly showed that the application of a general N2Oi for agricultural soils across annual and perennial cropping is not recommended.

Liming modifies greenhouse gas fluxes from soils: A meta-analysis of biological drivers

Acidic soils cover about 30% of the world’s land. Liming is a management practice applied worldwide to reduce the negative effects of acidification on soil fertility and plant growth. Liming also affects the biotic and abiotic soil properties controlling the production and consumption of the greenhouse gases (GHGs) carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Although our understanding of how liming regulates net GHG emissions is increasing, the impact of liming on soil biological drivers of GHG emissions has not been quantitatively synthesized. Here we conducted a global meta-analysis using 1474 paired observations from 124 studies to explore the responses of GHG emissions to liming, with a focus on soil biological factors. We show that the N2O mitigation capacity of liming could be linked to (i) increases in bacterial abundance of N2O reductase genes (NosZ) and decreases in fungi:bacteria ratio, both contributing to a lower N2O:N2 product ratio of denitrification; and (ii) reductions in soil mineral nitrogen (N) via stimulation of plant N uptake. The limited evidence available indicates that liming reduced CH4 emissions and the abundance of methanogens, but it had no effect on CH4 uptake and abundance of methanotrophs. Liming-induced increases in soil CO2 emissions can be explained by higher heterotrophic and/or autotrophic respiration. The strong coupling between liming effects on GHG emissions and on soil microbial communities involved in GHG production and consumption can be used to identify strategies to reduce GHGs in response to liming, and to improve process-based models for better predictions of soil GHG emissions.

Conversion from rice fields to vegetable fields alters product stoichiometry of denitrification and increases N2O emission

Information about effects of conversion from rice fields to vegetable fields on denitrification process is still limited. In this study, denitrification rate and product ratio (i.e., N2O/(N2O + N2) ratio) were investigated by soil-core incubation based N2/Ar technique in one rice paddy field (RP) and two vegetable fields (VF4 and VF7, 4 and 7 years vegetable cultivating after conversion from rice fields, respectively). Genes related to denitrification and bacterial community composition were quantified to investigate the microbial mechanisms behind the effects of land-use conversion. The results showed that conversion of rice fields to vegetable fields did not significantly change denitrification rate although the abundance of denitrification related genes was significantly reduced by 79.22%–99.84% in the vegetable soils. Whereas, compared with the RP soil, N2O emission rate was significantly (P < 0.05) increased by 53.5 and 1.6 times in the VF4 and VF7 soils, respectively. Correspondingly, the N2O/(N2O + N2) ratio increased from 0.18% (RP soil) to 5.65% and 0.65% in the VF4 and VF7 soils, respectively. These changes were mainly attributed to the lower pH, higher nitrate content, and the altered bacterial community composition in the vegetable soils. Overall, our results showed that conversion of rice fields to vegetable fields increased the N2O emission rate and altered the product ratio of denitrification. This may increase the contribution of land-use conversion to global warming and stratospheric ozone depletion.

Treatment of DEHP-rich PVC waste in subcritical urine wastewater: Efficient dechlorination, denitrification, plasticizer decomposition, and preparation of high-purity phthalic acid crystals

It is difficult to dispose diethylhexyl phthalate-rich polyvinyl chloride (DEHP-rich PVC) waste due to the high level of chlorine and plasticizer. On the other hand, the denitrification of urine wastewater with high nitrogen content also faces great challenges. In this study, a synergistic treatment strategy was developed for the DEHP-rich PVC waste and urine wastewater by a subcritical water process. Subcritical urine wastewater (SUW) was used as a reaction medium in the synergistic treatment. PVC dechlorination, DEHP decomposition, and denitrification of urine wastewater were synchronously achieved in the one pot SUW. Under the optimal conditions (300 °C, 15 min, 1:5 g/mL), the PVC dechlorination ratio, urine wastewater denitrification ratio and DEHP decomposition ratio could reach 98.4%, 64.9%, and 99.2%, respectively. The decomposition of DEHP mainly included hydrolysis, nucleophilic substitution, and acylation. DEHP could be converted into phthalic acid crystal at 220 °C with a yield of 66.25% due to the efficient hydrolysis action of SUW. All the removed Cl was transferred from PVC matrix to aqueous phase. Hydroxyl nucleophilic substitution is the principal dechlorination path of PVC. The reactions between N-containing species and DEHP in SUW resulted in the high-efficiency denitrification of urine wastewater, and the N element was fixed in solid residue or transferred to oil phase as amides compounds. It is believed that the proposed SUW process is a promising technology for the synergistic treatment of DEHP-rich PVC waste and urine wastewater.

Recovery of denitrification and dissimilatory nitrate reduction to ammonium following reoxygenation of sediments from a periodically hypoxic temperate lagoon

AbstractEutrophication and stratification events have increased the incidences of hypoxia and anoxia in coastal environments. The depletion of oxygen in these environments can lead to a shift in biogeochemical processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The balance between denitrification and DNRA is critical because denitrification leads to a loss of bioavailable nitrogen and DNRA maintains it within the system. This study examined the effects of reoxygenation on denitrification and DNRA in sediments collected from a periodically hypoxic system (Gippsland Lakes, Australia) and subjected to experimental hypoxia using a modified 15N‐isotope pairing technique. For freshly collected sediments, the ratio of denitrification : DNRA was generally &lt; 1 indicating a dominance of DNRA. After 3–4 weeks of oxygenation, denitrification increased relative to DNRA resulting in a denitrification : DNRA ratio &gt; 1. Profiles of 15N‐N2 and accumulation in these experiments showed denitrification and DNRA generally only took place in the surface ~ 2 cm of sediment. Prolonged experiments with reoxygenated sediments showed a decrease in the denitrification : DNRA to &lt; 1 after 56 d coincident with deep accumulation of within the sediment (2–5 cm) suggesting faunal irrigation enhanced DNRA deep in the sediment. An experiment with bromide tracer confirmed the occurrence of DNRA deep within the sediment at 2–6 cm was coincident with tracer penetration, and there was a significant positive relationship between bioirrigation and DNRA. We suggest DNRA deep within the sediment was enhanced by increased availability of organic carbon, reduced solutes, and possibly exposure of reworked sediment to .

Nitrate removal from a nitrate-rich reverse osmosis concentrate: Superior efficiency using the bioaugmentation of an Acinetobacter biofilm

Highly effective nitrate removal from a nitrate-rich reverse osmosis (RO) concentrate was achieved by the bioaugmentation of a new Acinetobacter isolate. The anoxic denitrifying capabilities of the isolate were studied in a real RO concentrate solution (containing 230 mg/L nitrate, 14,120 mg/L total dissolved solids, and 18 mg/L biological oxygen demand) obtained from a brackish, nitrate-rich groundwater desalination facility. The treatment of the real RO concentrate with a biofilm afforded a nitrate removal efficiency up to four times greater than that of a bioaugmented bacterial suspension of the Acinetobacter isolate in both batch and continuous bioreactors (nitrate removal rates: 1.34 and 1.37 kg-N/m3•d, respectively). These values are the highest rates of groundwater and groundwater RO concentrate denitrification ever reported. Additionally, the developed method provides the highest denitrification ratio (C:N = 3:1) between biological nitrate assimilation and denitrification. Lastly, both the isolate suspension and the biofilm cultures were able to consume all the organic matter supplemented to the concentrate, which is important since that the organic matter must be removed prior to chlorination and release. This bacterium, therefore, has great potential for successful nitrate removal during RO concentrate treatment.

Long-term treatment of municipal wastewater using a mesh rotating biological reactor and changes in the biofilm community

The effects of temperature and hydraulic retention time on the sewage treatment performance of mesh rotating biological reactor (MRBR) were investigated. The MRBR contained net-like 3D plastic mesh disks instead of conventional rotating disks. A laboratory-scale MRBR was used to treat real sewage for over 1 year. The soluble chemical oxygen demand concentrations for the effluent were 48–64 mg/L whether the hydraulic retention time was 2 or 4 h and when the influent sewage temperatures was between 12.2 and 24.5 °C. The sewage temperature strongly affected the NH 4 + -N removal rete, which decreased by 0.1 kg N m − 3 day − 1 for each 12 °C decrease in temperature. Even though the NH 4 + -N removal rate was low, simultaneous nitrification and denitrification occurred at a removal efficiency of 95%. The microbial community structure in the biofilm attached to the MRBR disks was different to the microbial community structure of conventional activated sludge that had been used to treat the same sewage. The microbial community structure of the MRBR biofilm sludge was stable throughout winter and summer regardless of the hydraulic retention time. This suggested that the microbial community was initially controlled by the influent sewage but then became to specific to the MRBR. This was attributed to the high degree of simultaneous nitrification and denitrification that occurred. • Sewage treatment performance of a mesh rotating biological reactor (MRBR) evaluated. • BOD removal of a MRBR were 76%–90% during a year at HRT of 2 h. • High simultaneous nitrification and denitrification ratio was observed. • Microbial community of a MRBR biofilm was similar between in winter and summer. • MRBR biofilm have larger proportion of β - and γ -proteobacteria than activated sludge.

Oxygen-depletion by rapid ammonia oxidation regulates kinetics of N2O, NO and N2 production in an ammonium fertilised agricultural soil

Oxygen (O2) is a key factor driving the expression of N-cycle-related functional genes and regulating nitrogenous gas production in the soil. However, how and to what extent the associated gene transcription and corresponding gas kinetics interact with the transition of soil O2 status caused by N fertilisation, remains poorly understood. In this context, we conducted a robotized incubation experiment using a He/O2 atmosphere with different amounts of ammonium-based fertiliser (0 (control), 60 (AS60) and 200 mg N kg−1 (NH4)2SO4 (AS200)) applied to an agricultural soil with a strong nitrification potential under three different initial O2 levels (oxic 21%, sub-oxic 3%, and anoxic 0%) over 14 days. Through repeated measurements of N2O, NO, and N2 concentrations and kinetics of mineral N following the decline of O2 concentrations, we found that higher ammonium addition (200 vs. 60 mg N kg−1) caused faster O2 consumption in the headspace and induced up to 238 times higher net accumulation of N2O under initially oxic headspace condition. We speculate that this was due to: 1) the rapid transition of O2 status from oxic to anoxic due to vigorous ammonia oxidation; and 2) the increased N2O/(N2O + NO + N2) ratio of denitrification with higher N addition. The amoA and nosZ gene transcript numbers changed significantly in response to ammonium addition and decreasing O2 concentration, whereas high-throughput sequencing revealed a significant structural alteration of the soil microbiota along with the transition of O2 status. Our results highlight that the vigorousness of oxygen depletion in the soil matrix driven by rapid ammonia oxidation is the proximal factor that regulates gas kinetics in high nitrification-potential soil when O2 diffusion is limited. This implies that practices which reduce hotspots of ammonia oxidation have the potential to mitigate N2O emissions from nitrifier denitrification and denitrification in agricultural soil.

Simultaneous partial nitrification and denitrifying phosphorus removal (PNDPR) in a sequencing batch reactor process operated at low DO and high SRT for carbon and energy reduction

This study reports on a novel partial nitrification-denitrifying phosphorus sequencing batch reactor (SBR) system enriched with non-conventional denitrifying phosphorus accumulating organisms (DPAOs), treating carbon limited synthetic wastewater over 175 days. The non-conventional operating conditions, such as low DO (0.25 mg/L–0.35 mg/L) and relatively long solids retention time (SRT) of 15 d, favored the enrichment of a wide variety of denitrifying phosphorus accumulating organisms (DPAOs), such as Rhodocyclus, Dechloromonas, and Cytophaga. In contrast to the Accumulibacter, these microorganisms can sustain a very low DO environment and simultaneously perform denitrification and enhanced biological phosphorus removal (EBPR) using oxygen, nitrite, and nitrate as electron acceptors. The low DO also favored the washout of nitrite oxidizing bacteria (NOB), leading to simultaneous partial nitrification-denitrifying phosphorus removal (PNDPR). Partial nitrification at a high nitrite accumulation ratio (NAR ~ 80%) was also unfavourable for the growth of denitrifying glycogen accumulating organisms (DGAOs) in the PNDPR system. Simultaneous nitritation-denitritation (SND), nitrogen, and phosphorus removal efficiencies were maintained above 90%. Online cyclic tests and batch studies showed that nitrite shunt was achieved, and denitrification was predominantly controlled by DPAOs rather than ordinary heterotrophs (OHO). Mass balances showed that the COD/NOX−-N ratio for denitrification was 3.8. Compared to the conventional enhanced biological phosphorus removal (EBPR) process, the low DO-PNDPR process achieves reductions in carbon and aeration energy demands by approximately 47% relative to conventional biological nutrient removal (BNR) plants, and 49%, relative to conventional nitrification/denitrification systems, respectively.

Contingent Effects of Liming on N2O-Emissions Driven by Autotrophic Nitrification

Liming acidic soils is often found to reduce their N2O emission due to lowered N2O/(N2O + N2) product ratio of denitrification. Some field experiments have shown the opposite effect, however, and the reason for this could be that liming stimulates nitrification-driven N2O production by enhancing nitrification rates, and by favoring ammonia oxidizing bacteria (AOB) over ammonia oxidizing archaea (AOA). AOB produce more N2O than AOA, and high nitrification rates induce transient/local hypoxia, thereby stimulating heterotrophic denitrification. To study these phenomena, we investigated nitrification and denitrification kinetics and the abundance of AOB and AOA in soils sampled from a field experiment 2–3 years after liming. The field trial compared traditional liming (carbonates) with powdered siliceous rocks. As expected, the N2O/(N2O + N2) product ratio of heterotrophic denitrification declined with increasing pH, and the potential nitrification rate and its N2O yield (YN2O: N2O-N/NO3–-N), as measured in fully oxic soil slurries, increased with pH, and both correlated strongly with the AOB/AOA gene abundance ratio. Soil microcosm experiments were monitored for nitrification, its O2-consumption and N2O emissions, as induced by ammonium fertilization. Here we observed a conspicuous dependency on water filled pore space (WFPS): at 60 and 70% WFPS, YN2O was 0.03-0.06% and 0.06–0.15%, respectively, increasing with increasing pH, as in the aerobic soil slurries. At 85% WFPS, however, YN2O was more than two orders of magnitude higher, and decreased with increasing pH. A plausible interpretation is that O2 consumption by fertilizer-induced nitrification cause hypoxia in wet soils, hence induce heterotrophic nitrification, whose YN2O decline with increasing pH. We conclude that while low emissions from nitrification in well-drained soils may be enhanced by liming, the spikes of high N2O emission induced by ammonium fertilization at high soil moisture may be reduced by liming, because the heterotrophic N2O reduction is enhanced by high pH.

In silico assessment of the potential of basalt amendments to reduce N2O emissions from bioenergy crops

AbstractThe potential of large‐scale deployment of basalt to reduce N2O emissions from cultivated soils may contribute to climate stabilization beyond the CO2‐removal effect from enhanced weathering. We used 3 years of field observations from maize (Zea mays) and miscanthus (Miscanthus × giganteus) to improve the nitrogen (N) module of the DayCent model and evaluate the potential of basalt amendments to reduce N losses and increase yields from two bioenergy crops. We found 20%–60% improvement in our N2O flux estimates over previous model descriptions. Model results predict that the application of basalt would reduce N2O emissions by 16% in maize and 9% in miscanthus. Lower N2O emissions responded to increases in the N2:N2O ratio of denitrification with basalt‐induced increases in soil pH, with minor contributions from the impact of P additions (a minor component of some basalts) on N immobilization. The larger reduction of N2O emissions in maize than in miscanthus was likely explained by a synergistic effect between soil pH and N content, leading to a higher sensitivity of the N2:N2O ratio to changes in pH in heavily fertilized maize. Basalt amendments led to modest increases in modeled yields and the nitrogen use efficiency (i.e., fertilizer‐N recover in crop production) of maize but did not affect the productivity of miscanthus. However, enhanced soil P availability maintained the long‐term productivity of crops with high nutrient requirements. The alleviation of plant P limitation led to enhanced plant N uptake, thereby contributing to lower microbial N availability and N2O emissions from crops with high nutrient requirements. Our results from the improved model suggest that the large‐scale deployment of basalt, by reducing N2O fluxes of cropping systems, could contribute to the sustainable intensification of agriculture and enhance the climate mitigation potential of bioenergy with carbon capture and storage strategies.

Performance evaluation and microbial community structure of mesh rotating biological reactor treating sewage

The fundamental properties and performance of sewage treatment by a mesh rotating biological reactor (MRBR) were investigated. MRBR is an improved type of rotating biological contactor (RBC) reactor. Municipal wastewater treatment with a MRBR exhibited 67 % soluble chemical oxygen demand (sCOD) removal at the surface area loading rate of 22 g sCOD·m−2·day−1, and an average of 41 % ammonium nitrogen removal at a ammonium nitrogen surface area loading rate of 3.5 g-N·m−2·day−1. During a continuous experiment, there was no increase in nitrate nitrogen or nitrite nitrogen corresponding to a decrease in ammonium nitrogen, so simultaneous nitrification and denitrification was occurring in the MRBR. The volumetric oxygen transfer coefficient (KLa) of MRBR was 49.0 d−1 at a rotational speed of 5.0 rpm, comparable to the KLa of other RBCs even with the use of only one mesh disk. The biomass in the MRBR was 18,000 g TVS·m−3, which is the first report of the sludge concentration on the MRBR mesh disks. The high sewage treatment performance of the MRBR was attributed to the fundamental properties of the reactor, such as KLa and sludge amount. Microbial community analysis revealed that the MRBR attached biomass harbored a broad range and high abundance of nitrifying and denitrifying bacteria, indicating the development of an optimal microbial community related to the relatively high simultaneous nitrification and denitrification ratio.

Electron transfer pathways and kinetic analysis of cathodic simultaneous nitrification and denitrification process in microbial fuel cell system

Microbial fuel cell (MFC) is an innovative bioconversion technology for wastewater treatment accompanied with electricity recovery. In this study, a kinetic model was developed base on Activated Sludge Model No.1 (ASM1) to describe electron transfer pathways during the simultaneous nitrification and denitrification (SND) process in the biocathode system of a dual-chamber MFC. The batch running of the dual-chamber MFC system showed that it produced a power density up to 2.96 W m−3 within 48 h, the achieved SND efficiency and autotrophic denitrification ratio in the cathodic denitrification process were up to 87.3 ± 0.8% and 69.5 ± 6.6%, respectively. Meanwhile, by integrating nitrification, autotrophic denitrification, heterotrophic denitrification, organic carbon oxidation, and oxygen reduction in the cathode, the model was able to precisely fit the concentration variations of NH3–N, dissolved oxygen (DO) and chemical oxygen demand (COD) during the cathodic SND process (R2 ≥ 0.9876). The cathode electrons tended to be completely utilized with the increase of autotrophic denitrification ratio in the cathodic denitrification process. When the nitrification rate was enhanced, the autotrophic denitrification would prevail in the competition with the heterotrophic denitrification. In summary, the developed model was confirmed to be effective and reliable for describing the electron transfer pathways and predicting the performance of the nitrogen removal reactions during the cathodic SND process in a double-chamber MFC.

Soil incubation study showed biogas digestate to cause higher and more variable short‐term N2O and N2 fluxes than mineral‐N

AbstractToday, a large share of mineral fertilizer is substituted by biogas digestates. Biogas digestates are known to promote N2O production, compared to mineral fertilizer. In particular, the initial phase following fertilizer application is crucial for the N gas release as N2O and also N2. However, this period impact has been rarely investigated, especially not across various field sites. Thus, undisturbed soil cores from two fertilizer types (biogas digestate vs. mineral fertilizer) at five sites with different site characteristics were investigated in a short‐term laboratory experiment under N2‐free helium–oxygen incubation atmosphere. Across sites, biogas digestate soil cores showed significantly higher absolute N2O fluxes compared to mineral fertilizer soil cores, even though this effect was dominated by samples from one site (Dornburg with the highest biogas digestate fertilization rate). Also relative N2O fluxes showed a similar tendency. On average, absolute and relative N2 fluxes differed between the two fertilizer types, while N2 fluxes were highest at the Dornburg site. A N2O/(N2O+N2) ratio of denitrification below or equal to 0.5 clearly highlighted the importance of N2O reduction to N2 for three of five the biogas digestate soil cores. Soil characteristics like bulk density and water‐filled pore space as proxies for gas diffusivity in soil, as well as N availability ( , ), significantly affected the N2O and N2 fluxes from the biogas digestate soil cores. While this study presents data on short‐term N2O and N2 fluxes, there is a need for further studies in order to investigate the dynamics, the duration of the observed effects and their significance at the field scale.