Acetylene Inhibition Research Articles

Unveiling ammonium concentration ranges that determine competition for mineral nitrogen among soil nitrogen transformations under increased carbon availability

Globally, approximately 50% of nitrogen (N) fertilizer applied in agricultural practices escapes into the environment, resulting in water and air pollution and ozone depletion. Soil N transformation processes determine the chemical form of N and the amount of the various N forms, controlling where N fertilizer goes and how much of it is lost. Therefore, a comprehensive understanding of soil N transformation responses to N fertilizer is critical for developing effective management strategies to improve soil N retention capacity and minimize soil N losses. Using 15N tracing techniques with acetylene inhibition, three ranges for ammonium concentration in fertilizer were identified (I: 14–16, II: 46–59, and III: 90–115 mg N kg−1, depending on type of organic materials added) that determine competition for mineral N among N transformation processes under increased carbon availability in an agricultural soil. Increasing ammonium concentration caused a shift from a nitrate assimilation-dominated (<I) to an equally ammonium assimilation- and nitrification-dominated (I-II) to nitrification-dominated (II-III), and finally to nitrification-predominant (>III) period in organic amendment addition treatments. Structural equation modeling revealed that ammonium addition inhibited soil nitrate assimilation by enhancing nitrification and ammonium assimilation, resulting in the shift. Consequently, the ratios of nitrification to ammonium assimilation and to gross N assimilation (nitrate assimilation + ammonium assimilation) increased significantly in response to elevated ammonium concentration with organic amendment addition, indicating lower N retention capacity and higher potential risks of N loss. Overall, we present a comprehensive picture of how concurrent gross N transformation processes interact to compete for mineral N in response to ammonium-forming fertilizer application, and demonstrate that nitrification and ammonium assimilation weaken the stimulating effect of organic amendment on nitrate assimilation with increasing ammonium concentration.

Trade-offs between nitrogen and phosphorus removal with floodplain remediation in agricultural streams

To improve water quality and reduce instream erosion, floodplain remediation along agricultural streams can provide multiple ecosystem services through biogeochemical and fluvial processes. During floodplain inundation, longer water residence time and periodic anoxic conditions can lead to increased nitrogen (N) removal through denitrification but also mobilization of phosphorus (P), impeding overall water quality improvements. To investigate the capacity for N and P processing in remediated streams, we measured potential denitrification and nitrous oxide production and yields together with potential P desorption and P fractions in floodplain and stream sediments in ten catchments in Sweden. Sediment P desorption was measured as equilibrium P concentration, using P isotherm incubations. Denitrification rates were measured with the acetylene inhibition method. Sediment nutrient process rates were combined with hydrochemical monitoring along remediated streams and their paired upstream control reaches of trapezoidal shape to determine the impact of floodplains on water quality. The correlation between floodplain denitrification rates and P desorption (r = 0.53, p = 0.02) revealed a trade-off between soluble reactive P (SRP) and nitrate removal, driven by stream water connectivity to floodplains. Nitrous oxide production was not affected by differences in P processing, but nitrous oxide yields decreased with higher denitrification and P desorption. The release of SRP from floodplains (0.03 ± 0.41 mg P kg−1 day−1) was significantly lower than from trapezoidal stream banks (0.38 ± 0.37 mg P kg−1 day−1), predicted by long-term SRP concentrations in stream water and floodplain inundation frequency. The overall impact of SRP release from floodplains on stream SRP concentrations in remediated reaches was limited. However, the remediated reaches showing increased stream SRP concentrations were also frequently inundated and had higher labile P content and coarse soil texture in floodplain sediments. To fully realize the potential for water quality improvements with constructed floodplains in agricultural streams, the promotion of denitrification through increased inundation should be balanced against the risk of P release from sediments, particularly in streams with high SRP inputs.

The ratio of denitrification end-products were influenced by soil pH and clay content across different texture classes in Oklahoma soils

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to stratospheric ozone depletion and global climate change. Soil denitrification has two potential end-products, N2O and dinitrogen (N2), and the ratio of these end-products (N2O:(N2O+N2) or the N2O ratio) is controlled by various factors. This study aims to quantify the influence of soil pH on the ratio of denitrification end-products in Oklahoma soils with different soil textures. Six natural grassland soils encompassing three distinct soil textures were incubated in the laboratory under natural and modified pH with an overall tested pH ranging from 2 to 10. Denitrification end-products were measured in the laboratory using the acetylene inhibition technique and further estimated using a process-based biogeochemical model. Both the laboratory and model results showed that soil pH and texture influenced the ratio of the denitrification end-products. Generally, as soil pH increased the N2O ratio decreased, although both lab and model results indicated that this relationship was not linear. Soil texture may have an indirect effect on the N2O ratio, as two soils of the same texture could have different N2O ratios. However, clay percentage of the soil did show a linear positive correlation with the N2O ratio, suggesting components of soil texture may be more influential than others. Overall, soil pH was a controlling factor in the ratio of denitrification end-products and the newly observed nonlinear relationship warrants further study, particularly when considering its effects in different soil textures.

Awakening soil microbial utilization of nitrate by carbon regulation to lower nitrogen pollution

Soil microbial immobilization of nitrate (NO3−-N, INO3) and its role in mitigating NO3−-N pollution in agricultural ecosystems globally is often neglected because of the tenet that microbes preferentially use ammonium over NO3−-N. Soil INO3 driven by heterotrophic microorganisms is often carbon (C)-limited, and may therefore be stimulated by C sources with high C/nitrogen (N) ratios. Here, using 15NO3−-N labelling coupled with acetylene inhibition, quantitative PCR and Illumina high-throughput sequencing approaches, we demonstrated that INO3 was stimulated from zero to a substantial level by crop residue amendment without any environmental risk; the degree of stimulation depending on residue quality and soil type. The stimulation was predicted well by C mineralization of the added residue. Soil with a lower initial nutrient status had reduced INO3 efficiency due to stronger C constraint and lower return on energy investment. Fast-growing bacterial r-strategists were identified as the key driver of INO3. We estimate that, globally, residue amendment could cause a total NO3−-N loss reduction of about 33.6 Tg N yr−1 through INO3 stimulation. We suggest that soil INO3 driven by addition of exogenous C is an overlooked process that can help curb NO3−-N accumulation in soil and its subsequent release to the environment, the “nitrate time bomb”.

Direct evidence indicates that revegetation improves organic carbon limitation in sediment denitrification in a eutrophic headwater river

Fluvial sediments are one of the hotspots for river nitrogen removal and denitrification is usually considered the main process of nitrogen removal. The availability of nitrogen in headwater rivers is often a limiting factor for sediment denitrification. However, with the increase of N load caused by human activities, we hypothesized that carbon sources might become a key factor controlling the denitrification of sediments in headwater rivers. Here, we measured the denitrification rate using the acetylene inhibition technique and related environmental factors, including both water and sediment features, throughout a year in a subtropical eutrophic river (1st-3rd order streams). The results of our Pearson correlation and multiple regression analyses supported our hypothesis. We found that C sources had a higher explanatory power than N sources, and sediment total carbon (STC) had the greatest influence on the denitrification rate among the different C sources, and that water carbon sources directly affected the sediment denitrification rates according to a structural equation model (SEM). A degraded river reach was selected for revegetation as part of our study, and we found that revegetation increased water DOC from 19.47 mg l−1 to 24.45 mg l−1, reduced water NO3−-N from 0.61 mg l−1 to 0.35 mg l−1, and significantly increased the sediment denitrification rates by an average of 176%. Our study provides direct evidence that revegetation improves organic carbon limitation in sediment denitrification in a eutrophic headwater river.

Effect of tillage state of paddy soils with heavy metal pollution on the nosZ gene of N2O reductase

Paddy soils are an important source of atmospheric nitrous oxide (N2O). However, numerous studies have focused on N2O production during the soil tillage period, neglecting the N2O production during the dry fallow period. In this study, we conducted an incubation experiment using the acetylene inhibition technique to investigate N2O emission and reduction rates of paddy soil profiles (0-1 m) from Guangdong Province and Jinlin Province in China, with different heavy-metal pollution levels. The abundance and community structures of denitrifying bacteria were determined via quantitative-PCR and Illumina MiSeq sequencing of nosZ, nirK, and nirS genes. Our results showed that the potential N2O emission rate, N2O production rate, and denitrification rate have decreased with increasing soil vertical depth and heavy-metal pollution. More importantly, we found that the functional gene type of N2O reductase switched with the tillage state of paddy soils, which clade Ⅱ nosZ genes were the dominant gene during the tillage period, while clade Ⅰ nosZ genes were the dominant gene during the dry fallow period. The heavy-metal pollution has less effect on the niche differentiation of the nosZ gene. The N2O emission rate was significantly regulated by the genus Bradyhizobium, which contains both N2O reductase and nitrite reductase genes. Our findings suggests that the nosZ gene of N2O reductase can significantly impact the N2O emission from paddy soils.

Plants Mitigate Nitrous Oxide Emissions from Antibiotic-Contaminated Agricultural Soils

Vegetable production systems are hotspots of nitrous oxide (N2O) emissions and antibiotic pollution. However, little is known about the interconnections among N2O emissions, vegetable growth, and antibiotic contamination. To understand how plants regulate N2O emissions from enrofloxacin (ENR)-contaminated soils, in situ N2O emissions were measured in pot experiments with cherry radish and pakchoi. Gross N2O production and consumption processes were discriminated based on an acetylene inhibition experiment. Results indicated that vegetable growth decreased the cumulative N2O flux from 0.71 to -0.29 kg ha-1 and mitigated the ENR-induced increase in N2O emissions. Radish displayed better mitigation of N2O emissions than pakchoi. By combining the analysis of N2O flux with soil physicochemical and microbiological properties, we demonstrated that growing vegetables could either promote gross N2O consumption or decrease gross N2O production, primarily by interacting with soil nitrate, clade II nosZ (nosZII)-carrying bacteria, and Deinococcus-Thermus. ENR inhibited N2O consumption more than N2O production, with the nosZII-carrying bacteria, represented by Gemmatimonadetes, as the main inhibition target. However, increasing nosZII-carrying bacteria by growing radish offsets the inhibitory effect of ENR. These findings provide new insights into N2O emissions and antibiotic pollution in vegetable-soil ecosystems and broaden the options for mitigating N2O emissions.

The influence of soil acidification on N2O emissions derived from fungal and bacterial denitrification using dual isotopocule mapping and acetylene inhibition

Denitrification, as both origins and sinks of N2O, occurs extensively, and is of critical importance for regulating N2O emissions in acidified soils. However, whether soil acidification stimulates N2O emissions, and if so for what reason contributes to stimulate the emissions is uncertain and how the N2O fractions from fungal (ffD) and bacterial (fbD) denitrification change with soil pH is unclear. Thus, a pH gradient (6.2, 7.1, 8.7) was set via manipulating cropland soils (initial pH 8.7) in North China to illustrate the effect of soil acidification on fungal and bacterial denitrification after the addition of KNO3 and glucose. For source partitioning, we used and compared SP/δ18O mapping approach (SP/δ18O MAP) and acetylene inhibition technique combined isotope two endmember mixing model (AIT-IEM). The results showed significantly higher N2O emissions in the acidified soils (pH 6.2 and pH 7.1) compared with the initial soil (pH 8.7). The cumulative N2O emissions during the whole incubation period (15 days) ranged from 7.1 mg N kg−1 for pH 8.7–18.9 mg N kg−1 for pH 6.2. With the addition of glucose, relative to treatments without glucose, this emission also increased with the decrement of pH values, and were significantly stimulated. Similarly, the highest N2O emissions and N2O/(N2O + N2) ratios (rN2O) were observed in the pH 6.2 treatment. But the difference was the highest cumulative N2O + N2 emissions, which were recorded in the pH 7.1 treatment based on SP/δ18O MAP. Based on both approaches, ffD values slightly increased with the acidification of soil, and bacterial denitrification was the dominant pathway in all treatments. The SP/δ18O MAP data indicated that both the rN2O and ffD were lower compared to AIT-IEM. It has been known for long that low pH may lead to high rN2O of denitrification and ffD, but our documentation of a pervasive pH-control of rN2O and ffD by utilizing combined SP/δ18O MAP and AIT-IEM is new. The results of the evaluated N2O emissions by acidified soils are finely explained by high rN2O and enhanced ffD. We argue that soil pH management should be high on the agenda for mitigating N2O emissions in the future, particularly for regions where long-term excessive nitrogen fertilizer is likely to acidify the soils.

The potential importance of soil denitrification as a major N loss pathway in intensive greenhouse vegetable production systems

BackgroundAbout 30 % of vegetables in China are produced in intensively managed greenhouses comprising flood irrigation and extreme rates of nitrogen fertilizers. Little is known about denitrification N losses.MethodsSoil denitrification rates were measured by the acetylene inhibition technique applied to anaerobically incubated soil samples. Four different greenhouse management systems were differentiated: Conventional flood irrigation and over-fertilization (CIF, 800 kg N ha−1, 460 mm); CIF plus straw incorporation (CIF+S, 889 kg N ha−1, 460 mm); Drip fertigation with reduced fertilizer application rates (DIF, 314 kg N ha−1, 190 mm); DIF plus straw incorporation (DIF+S, 403 kg N ha−1, 190 mm). Soil denitrification was measured on nine sampling dates during the growing season (Feb 2019-May 2019) for the top-/ subsoil (0 – 20/ 20- 40 cm) and on three sampling dates for deep soils (40-60/ 80-100 cm). Data was used to constrain N-input-output balances of the different vegetable production systems.ResultsRates of denitrification were at least one magnitude higher in topsoil than in sub- and deep soils. Total seasonal denitrification N losses for the 0 – 40 cm soil layer ranged from 76 (DIF) to 422 kg N ha−1 (CIF+S). Straw addition stimulated soil denitrification in top- and subsoil, but not in deep soil layers. Integrating our denitrification data (0-100 cm) with additional data on N leaching, N2O emissions, plant N uptake, and NH3 volatilization showed, that on average 50 % of added N fertilizers are lost due to denitrification.ConclusionsDenitrification is likely the dominant environmental N loss pathway in greenhouse vegetable production systems. Reducing irrigation and fertilizer application rates while incorporating straw in soils allows the reduction of accumulated nitrate.

Factors limiting microbial N2O and CO2 production in a cultivated peatland overlying an acid sulphate subsoil derived from black schist

Factors limiting the production of the greenhouse gases nitrous oxide (N2O) and carbon dioxide (CO2) were investigated in three incubation experiments conducted with soil from top- and subsoil horizons of a peatland which had an acid sulphate mineral subsoil derived from black schists. The effect of moisture was investigated by equilibrating undisturbed soil samples from three horizons (H2, Cg and Cr) at −10, −60 or −100 cm matric potential and measuring the gas production. In the second experiment, the effects of temperature and various substrates were studied by incubating disturbed soil samples in aerobic conditions at 5 or 20 °C, and measuring basal respiration and N2O production before and after adding water, glucose or ammonium into the soil. In the third experiment, the effects of added glucose and/or nitrate on the denitrification in soil samples from four horizons (H1, H2, Cg and Cr were investigated by acetylene inhibition and monitoring of N2O production during a 48-h anaerobic incubation. The production of CO2 in the topmost peat horizon was largest at −10 cm matric potential, and it was larger than those in the mineral subsoil also at −60 and −100 cm potentials. In contrast, drainage seemed to increase N2O production, whereas in the wettest condition the production of N2O in the mineral subsoil was small and the peat horizon was a sink of N2O. Lowering of temperature (from 20 °C to 5 °C) decreased CO2 production, as expected, but it had almost no role in the production of N2O in aerobic conditions. Glucose addition increased the aerobic production of CO2 in peat, but it had a minor effect in the mineral horizons. Lack of C source (glucose) was limiting anaerobic N2O production in the uppermost peat horizon, while in all other horizons, nitrate proved to be the most limiting factor. It is concluded that peatlands with black schist derived acid sulphate subsoil horizons, such as in this study, have high microbial activity in the peaty topsoil horizons but little microbial activity in the mineral subsoil. These findings are contrary to previous results obtained in sediment-derived acid sulphate soils.

Soil denitrifying enzyme activity and its influencing factors in a bamboo forest riparian zone in the upper reaches of the Taihu Lake Basin, China

Soil denitrifying enzyme activity (DEA) was measured by acetylene inhibition technique, along with exploration of factors influencing DEA in a bamboo forest riparian zone in the upper reaches of the Taihu Lake Basin during summer. Our aim was to provide important insights into the assessment of ecological functions of bamboo forest riparian zone on reducing nitrogen pollution in rivers. The results showed that the riparian soil DEA ranged from 6.32 to 23.22 μg N·kg-1·h-1, with a mean value of 14.65 μg N·kg-1·h-1. The vertical distribution (0-40 cm soil profile) of DEA was affected by several factors, such as soil organic carbon (SOC), total nitrogen (TN), nitrate nitrogen (NO3--N), soil water content, and activity of carbon and nitrogen hydrolase, which resulted in decreased DEA with increasing soil depth. The horizontal changes in DEA (at the same soil depth but at different distances from river) was mainly governed by the variation in SOC concentration. In this area, the concentration of soil dissolved organic carbon was relatively low, which might inhibit the soil DEA during summer.

Organic amendment enhanced microbial nitrate immobilization with negligible denitrification nitrogen loss in an upland soil

Both soil microbial nitrate (NO3−-N) immobilization and denitrification are carbon (C)-limited; however, to what extent organic C addition may increase NO3−-N immobilization while stimulate denitrification nitrogen (N) loss remains unclear. Here, 15N tracing coupled with acetylene inhibition methods were used to assess the effect of adding glucose, wheat straw and peanut straw on NO3−-N immobilization and denitrification under aerobic conditions in an upland soil, in which NO3−-N immobilization has been previously demonstrated to be negligible. The organic C sources (5 g C kg−1 soil) were added in a factorial experiment with 100, 500, and 1000 mg N kg−1 soil (as K15NO3) in a 12-d laboratory incubation. Microbial NO3−-N immobilization in the 12-d incubation in the three N treatments was 5.5, 7.7, and 8.2 mg N kg−1 d−1, respectively, in the glucose-amended soil, 5.9, 4.2, and 2.4 mg N kg−1 d−1, respectively, in the wheat straw-amended soil, and 4.9, 5.1 and 4.4 mg N kg−1 d−1, respectively, in the peanut straw-amended soil. Therefore, under sufficient NO3−-N substrate, the higher microbial NO3−-N immobilization in the glucose than in the crop residue treatments was likely due to the slow decomposition of the latter that provided low available C. The 15N recovery in the N2O + N2 pool over the12-day incubation was <2% for all treatments, indicating negligible denitrification N loss due to low denitrification rates in the aerobic incubation in spite of increasing C availability. We conclude that external C addition can enhance microbial NO3−-N immobilization without causing large N losses through denitrification. This has significant implications for reducing soil NO3−-N accumulation by enhancing microbial NO3−-N immobilization through increasing C inputs using organic materials and subsequently mitigating nitrate pollution of water bodies.

Nitrogen removal through sediment denitrification in the Yangtze Estuary and its adjacent East China Sea: A nitrate limited process during summertime

Nitrogen nutrient surplus is the main cause of a series of environmental problems in the Yangtze Estuary and its adjacent East China Sea (ECS). Denitrification plays an important role in controlling nitrate dynamics and fate in estuarine and coastal environments. We investigated the natural and potential rates of denitrification in the sediments of the Yangtze Estuary and ECS via slurry incubation experiments combined with acetylene inhibition techniques to reveal its contributions to total nitrogen reduction in this hypereutrophic continental shelf area. Key environmental factors, such as the sediment grain size, sediment extractable inorganic nitrogen (NH4+, NO3− and NO2−), sediment organic carbon (SOC), total nitrogen (TN), isotopic compositions (δ13C and δ15N), etc., were also investigated to determine the main factors controlling the denitrification processes. The measured rates of denitrification ranged from 0.39 to 28.49 ng N g−1·h−1. The total nitrogen removed by denitrification in the study area was 3.7 × 1010 g during August. In total, at least 3.3% of the external inorganic nitrogen transported annually into the estuary could be removed by the denitrification processes in the study area. The sediment denitrification rates correlated significantly with the extractable ammonium and δ15N values of surface sediments, indicating that coupled nitrification-denitrification processes may play an important role in nitrogen removal. Almost undetectable levels of nitrate in the sediment further revealed that nitrate supply, regardless of diffusion from the overlying water or production by sediment nitrification processes, is the bottleneck for denitrification.

Denitrifier abundance and community composition linked to denitrification potential in river sediments.

Denitrification in river sediments plays a very important role in removing nitrogen in aquatic ecosystem. To gain insight into the key factors driving denitrification at large spatial scales, a total of 135 sediment samples were collected from Huaihe River and its branches located in the northern of Anhui province. Bacterial community composition and denitrifying functional genes (nirS, nirK, and nosZ) were measured by high-throughput sequencing and real-time PCR approaches. Potential denitrification rate (PDR) was measured by acetylene inhibition method, which varied from 0.01 to 15.69 μg N g-1 h-1. The sequencing results based on 16S rRNA gene found that the main denitrification bacterial taxa included Bacillus, Thiobacillus, Acinetobacter, Halomonas, Denitratisoma, Pseudomonas, Rhodanobacter, and Thauera. Therein, Thiobacillus might play key roles in the denitrification. Total nitrogen and N:P ratio were the only chemical factors related with all denitrification genes. Furthermore, nirS gene abundance could be more susceptible to environmental parameters compared with nirK and nosZ genes. Canonical correspondence analysis indicated that NO3-, NO2-, NH4+ and IP had the significant impacts on the nirS-encoding bacterial community and spatial distributions. There was a significantly positive correlation between Thiobacillus and nirS gene. We considered that higher numbers of nosZ appeared in nutrient rich sediments. More strikingly, PDR was positively correlated with the abundance of three functional genes. Random forest analysis showed that NH4+ was the most powerful predictor of PDR. These findings can yield practical and important reference for the bioremediation or evaluation of wetland systems.

Nitrosospira cluster 3 lineage of AOB and nirK of Rhizobiales respectively dominated N2O emissions from nitrification and denitrification in organic and chemical N fertilizer treated soils

Agricultural soils are known as nitrous oxide (N2O) sources due to the input of nitrogen (N) fertilizer and livestock faecal excretion. However, few studies have explored the responses of N2O microbial emission pathways and their related functional guilds to long-term soil fertilization regimes. This study examined the effects of four 38-year long-term fertilization treatments (control with no-fertilizer (CK); inorganic N fertilizer (N); manure (M); inorganic N fertilizer with manure (MN)) on the N2O levels generated from nitrification and denitrification, as well as on the nitrifiers and heterotrophic denitrifiers. The acetylene inhibition method was used to determine the microbial pathways of the N2O emissions and molecular biological techniques (cloning, high-throughput sequencing and quantitative PCR) were performed to investigate the nitrifier and denitrifier responses. The organic fertilization demonstrably increased the N2O emissions from nitrification and the abundance of canonical ammonia oxidizers and complete ammonia oxidizers (clade B), but distinctly decreased the abundance of clade A. Combined with the results that strong and positive relationships were detected between the abundance of bacterial amoA genes and N2O content produced from nitrification. We proposed that organic fertilizer applications stimulated the generation of N2O from nitrification dominated by the Nitrosospira cluster 3 lineage of AOB. In contrast, the amount of N2O produced by denitrification was appreciably stimulated by chemical N fertilization, compared to organic fertilization. Strong and positive relationships were detected between the nirK-type denitrifying gene abundances and the N2O emissions generated from denitrification. This suggested that single N fertilizer additions appreciably stimulated N2O production from denitrification dominated by the nirK lineage of the Rhizobiales. These results indicated that chemical N and organic fertilizers stimulated N2O emissions from denitrification and nitrification, respectively, which were dominated by the Nitrosospira cluster 3 lineage of AOB and the nirK lineage of Rhizobiales, respectively.

Microbial process-oriented understanding of stimulation of soil N2O emission following the input of organic materials

Although crop residue return increases upland soil emissions of nitrous oxide (N2O), a potent greenhouse gas, the mechanisms responsible for the increase remain unclear. Here, we investigate N2O emission pathways, gross nitrogen (N)-cycling rates, and associated N-cycling gene abundances in an upland soil following the addition of various organic material under aerobic incubation using a combination of 15N tracing technique, acetylene (C2H2) inhibition, and real-time PCR (qPCR) methods. Increased total N2O emissions following organic material amendment was attributed to both increased nitrification-derived N2O emissions, following increased ammonia-oxidizing bacteria (AOB)-amoA abundance, and denitrification-derived N2O emissions, following increased nirS and decreased nosZ abundance. Increasing plant residue carbon (C)/N ratio decreased total N2O emissions by decreasing the contribution of denitrification to N2O emissions, potentially due to higher proportions of denitrified N emitted as N2O than nitrified N emitted as N2O. We further propose a novel conceptual framework for organic material input effects on denitrification-derived N2O emissions based on the decomposable characteristics of the added organic material. For slowly decomposing organic materials (e.g., plant residue) with insufficient available C, NO3−-N immobilization surpassed denitrification, resulting in gradual decrease in denitrification-derived N2O emissions with an increase in mineralization of plant residue C losses. In contrast, available C provided by readily available C sources (e.g., glucose) seemed sufficient to support the co-occurrence of NO3−-N immobilization and denitrification. Overall, for the first time, we offer a microbial process perspective of N2O emissions following organic material input. The findings could facilitate the improvement of process-orientated models of N2O emissions and the formulation of appropriate N2O mitigation strategies for crop residue-amended soils.

Contribution of Particulate and Mineral-Associated Organic Matter to Potential Denitrification of Agricultural Soils

Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.

Roles of bulk and rhizosphere denitrifying bacteria in denitrification from paddy soils under straw return condition

Roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification under straw return conditions are rarely discriminated, limiting our understanding on nitrogen biogeochemistry in soils amended with straw. The objective of this study was to explore the microbial mechanisms accounting for the altered rhizosphere and bulk soil denitrification with straw amendment. In this study, straw was added into two representative paddy soils (0.5% w/w) from Yixing (YX) and Taizhou (TZ) in Jiangsu city, China, for rice growth over a 37-day period. Denitrification potentials (net N2O emission, total denitrification, and N2O reduction rates) of rhizosphere and bulk soils at the end of rice growth were measured using an acetylene inhibition technique. In addition, denitrifying bacterial community compositions and functional gene abundances were analyzed using Illumina sequencing and quantitative real-time PCR. For control treatment without straw addition, total denitrification and N2O reduction rates were significantly higher in the rhizosphere than that in bulk soils; however, net N2O emission potentials were similar between rhizosphere and bulk for TZ (22.6–53.3 vs. 9.44–46.6 mg N kg−1) and YX (946 ± 126 vs. 699 ± 350 mg N kg−1). Under straw return condition, N2O emission potentials from the bulk of both soils were significantly elevated, corresponding to the increase in the relative abundance of some taxa of denitrifying bacteria (unnamed environmental samples and unclassified proteobacteria) and increase in nirK, nirS, and nosZ genes. This indicates a positive response of bulk soil denitrifying bacteria to straw addition. In contrast, straw addition produced inhibitive effects on the growth of denitrifying bacteria with decreased nirK, nirS, and nosZ gene abundances in the rhizosphere. This led to decreased N2O emission potentials at the end of incubation for rhizosphere soils, which were 98.2% and 25.2% lower for TZ and YX compared to those of bulk soils. These results suggested that the roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification shifted with straw addition, which might inspire further studies to target the denitrification hotspots to effectively mitigate greenhouse emissions at the early period of rice growth.

Residual effects of four-year amendments of organic material on N2O production driven by ammonia-oxidizing archaea and bacteria in a tropical vegetable soil

Organic material (OM) applied to cropland not only enhances soil fertility but also profoundly affects soil nitrogen cycling. However, little is known about the relative contributions of soil ammonia-oxidizing archaea (AOA) and bacteria (AOB) to nitrous oxide (N2O) production during ammonia oxidation in response to the additions of diverse types of OMs in the tropical soil for vegetable production. Herein, the soils were sampled from a tropical vegetable field subjected to 4-year consecutive amendments of straw or manure. All the soils were amended with ammonium sulfate ((NH4)2SO4, applied at a dose of 150 mg N kg−1) and incubated aerobically for four weeks under 50% water holding capacity. 1-octyne or acetylene inhibition technique was used to differentiate the relative contributions of AOA and AOB to N2O production. Results showed that AOA dominated N2O production in soil managements of unfertilized control (CK), chemical fertilization (NPK), and NPK with straw (NPKS), whereas AOB contributed more in soil under NPK with manure (NPKM). Straw addition stimulated AOA-dependent N2O production by 94.8% despite the decreased AOA-amoA abundance. Moreover, manure incorporation triggered both AOA- and AOB-dependent N2O production by 147.2% and 233.7%, respectively, accompanied with increased AOA and AOB abundances. Those stimulating effects were stronger for AOB, owing to its sensitivity to the alleviated soil acidification and decreased soil C/N ratio. Our findings highlight the stimulated N2O emissions during ammonia oxidation by historical OM amendments in tropical vegetable soil, with the magnitude of those priming effects dependent on the types of OM, and appropriate measures need to be taken to counter this challenge in tropical agriculture ecosystems.

Microcystis blooms aggravate the diurnal alternation of nitrification and nitrate reduction in the water column in Lake Taihu

To explore the effects of Microcystis blooms on nitrogen (N) cycling in the water column, the community structures of the Microcystis-attached and free-living bacteria in Lake Taihu were assessed and a mesocosm experiment was further conducted on the shore of Lake Taihu. The bacterial communities of Microcystis-attached and free-living bacteria were dominated by heterotrophic bacteria, such as Pseudomonas and Massilia, while the relative abundances of the genera related to traditional autotrophic nitrification were surprisingly low. However, the dramatic increase in nitrate (NO3−) levels at the daytime suggested that in the mesocosms nitrification did occur, during which the heterotrophic nitrifiers played a predominant role as revealed by the acetylene inhibition experiment. The ammonium (NH4+) concentrations were always maintained at a low level, indicating that most of the substrates for daytime nitrification originated from organic N. The total N being removed during the experiment was much less than the sum of daily NO3− reduction, while the decrease in NO3− concentration was much higher than the increase in NH4+ concentration during the night, indicating that assimilation was the main explanation for nocturnal NO3− reduction. Thus, the cycling of organic N (remineralization) – heterotrophic nitrification – NO3− assimilation (reduction) promoted by Microcystis blooms aggravates the diurnal variation of NO3− in the water column.