Coupled Nitrification Denitrification Research Articles

Responses of N2O production pathways and related functional microbes to temperature across greenhouse vegetable field soils

Greenhouse vegetable field is one of the main sources of agricultural nitrous oxide (N2O) emissions via multiple pathways regulated by different microbes. However, the relative contributions of nitrifier nitrification, nitrifier denitrification, nitrification-coupled denitrification and heterotrophic denitrification to N2O production with temperature change in different greenhouse vegetable soils are poorly understood. In this study, combined approaches of the 15N–18O labeling technique and transcriptome analyses were applied to investigate the pathways of N2O production (nitrifier nitrification, nitrifier denitrification, nitrification-coupled denitrification and heterotrophic denitrification) under 15, 25 and 35 °C in six greenhouse vegetable field soils in mainland China. The results show that heterotrophic denitrification was an important source of N2O in acidic greenhouse vegetable soils, whereas nitrifier nitrification was the dominant source of N2O in alkaline greenhouse vegetable soils. In addition, the contribution of nitrifier denitrification and nitrification-coupled denitrification to N2O production in alkaline greenhouse vegetable soils were higher than in acidic greenhouse vegetable soils. The contribution of nitrification-coupled denitrification increased with temperature at 15–35 °C, whereas nitrifier denitrification increased with temperature at 15–25 °C. Overall, high temperature decreased the contribution of nitrifier nitrification to N2O production. These results have important implications for understanding the temperature-dependent N2O production pathways and for mitigating agricultural N2O emissions under different soil and temperature conditions.

Nitrogen along the Hydrological Gradient of Marsh Sediments in a Subtropical Estuary: Pools, Processes, and Fluxes.

Knowledge on the distribution of nitrogen (N) pools, processes, and fluxes along hydrological gradients provides a comprehensive perspective to understand the underlying causal mechanisms in intertidal flats, and thus improve predictions and climate adaptation strategies. We used a space-for-time substitution method to quantify N pools, processes, and fluxes along a hydrological gradient. Further, we linked N pools and processes and investigated not only surface but also subsurface sediments. Our results showed a gradual decrease in total N (TN) and mineralization rates (PNmin), but an increase in potential rates of nitrification (PNR) and denitrification (PDNR) under an elevated hydrological gradient, except for TN and PNmin in the subsurface sediment, which accumulated on the interaction zone between the high and middle tidal flats. Most sedimentary ammonium N (NH4+) and nitrate N (NO3−) concentrations were similar; however, NH4+ accumulated on the subsurface of the middle tidal flat. NO3− fluxes (from −0.54 to −0.35 mmol m−2 h−1) were uptake fluxes in the intertidal flats, but NH4+ fluxes (−2.48–3.54 mmol m−2 h−1) changed from uptake to efflux in the seaward direction. Structural equation modeling of the effects of inundation frequency, underground biomass, total carbon (TC), electrical conductivity (EC), and clay proportion on the N processes revealed that these accounted for 67%, 82%, and 17% of the variance of PDNR, PNmin, and PNR, respectively. Inundation frequency, underground biomass, TC, EC, and PNmin effects on N pools accounted for 53%, 69%, and 98% of the variance of NH4+, NO3−, and TN, respectively. This suggests that future sea level rise may decrease N storage due to increase in coupled nitrification–denitrification and decrease in N mineralization, and the NH4+ flux may change from sink to source in intertidal ecosystems.

Relative Contributions of DNRA and Denitrification to Nitrate Reduction in Thalassia testudinum Seagrass Beds in Coastal Florida (USA)

Seagrass beds are vulnerable to eutrophication (nutrient loading) and declining worldwide. To quantify the fate of nitrogen (N) inputs, intact sediment cores were incubated in a continuous-flow system with 15N enrichments to compare N consumption and efflux pathways within and near seagrass (Thalassia testudinum) beds in St. Joseph Bay, Florida. Sediment oxygen demand and total ammonium (NH4+) efflux were greater (p < 0.001) in vegetated versus unvegetated sediments, suggesting that seagrasses enhance organic matter remineralization. Denitrification rates were 2–20× greater than estimates of potential dissimilatory nitrate reduction to ammonium (DNRA). The effect of vegetation on denitrification rates was inconsistent. Direct denitrification of overlying water nitrate and N fixation rates were low, suggesting tight coupling between remineralization, nitrification, and denitrification. DNRA rates were lower than denitrification and consistently greater in vegetated sediments. DNRA rate measurements are conservative if sediment cation exchange decreases the fraction of 15NH4+ reaching overlying water, especially in vegetated sediments, where rates of nitrate-induced ammonium fluxes were observed. Coupled nitrification-denitrification was the major N loss pathway in this system, as evidenced by the lack of 15N-labeled N2 production in isotope-enriched cores. Using measured sediment oxygen demand and NH4+ fluxes as an indicator of organic matter quality and quantity, these results are consistent with previous work showing that labile organic matter helps regulate the balance between N removal and internal recycling pathways in seagrass systems, which has implications for coastal management strategies to address eutrophication.

Salinization lowers nutrient availability in formerly brackish freshwater wetlands; unexpected results from a long-term field experiment

Worldwide, coastal freshwater wetlands are facing salinization at an increasing rate due to large-scale land use change, freshwater extraction, climate-driven sea level rise, droughts and land subsidence. Although it is known that increased surface water salinity does influence wetland functioning, effects on nutrient dynamics reported in literature are contradictory and evidence from controlled, long-term field experiments is scarce. We therefore tested the effects of 4 levels of increased surface water salinity, from oligohaline to mesohaline conditions (0.9, 2.25, 4.5, 9 PSU), on biogeochemical and physicochemical processes in the sediment of a formerly brackish freshwater wetland. For this, we used 16 enclosures in a controlled, 5-year field experiment. Salinization unexpectedly led to a dose dependent decreased availability of nitrogen and phosphorus in the sediment, both in the short and in the long term, even though sulfate reduction rates increased. Decreased phosphorus availability was probably caused by co-precipitation with calcium that was mobilized from sediment adsorption sites. Mobilization of ammonium from the sediment and coupled nitrification–denitrification most probably explained decreased nitrogen availability. Increasing sulfate concentrations associated with increased salinity shifted the dominant mineralization process from methanogenesis to sulfate reduction, also in the long term. We show surface water salinization to have major short-term and long-term consequences for the ecological and biogeochemical functioning of coastal freshwater wetlands.

Effects of sliver nanoparticles on the coupled nitrification-denitrification in suspended sediments from Taihu Lake

The widespread use of commercial silver nanoparticles (Ag NPs) inevitably results in their increasing release into natural waters and potentially threaten the freshwater ecosystems. During their migration, Ag NPs would interact with suspended sediments (SPS) and negatively affect the microbial communities on SPS, which play an important role in the nutrient cycling in aquatic environments. However, there is limited research focusing on the potential effects of Ag NPs on nitrogen cycling in SPS. In this study,15N isotope tracer technique (added15NH4+) was adopted to explore the influences of different concentrations of Ag NPs on coupled nitrification-denitrification (CND) in SPS systems by determining the variation of added NH4+ and the final production of CND (15N-N2). The obtained results indicated that 10 and 5 mg/L Ag NPs inhibited the15N-N2 production of CND by decreasing the utilization of NH4+ in SPS, while 0.5 and 2 mg/L Ag NPs increased the activity of CND in SPS due to the hormesis effects. The study demonstrated the negative effects of Ag NPs on N-cycling in SPS systems and provided more evidences for evaluating the risks of Ag NPs in aquatic ecosystems.

Nitrogen removal by tropical floodplain wetlands through denitrification

Excess nitrogen (N) leading to the eutrophication of water and impacts on ecosystems is a serious environmental challenge. Wetlands can remove significant amounts of N from the water, primarily through the process of denitrification. Most of our knowledge on wetland denitrification is from temperate climates; studies in natural tropical wetlands are very scarce. We measured denitrification rates during a dry and a wet season in five floodplain forests dominated by Melaleuca spp., a coastal freshwater wetland of tropical Australia. We hypothesised that the denitrification potential of these wetlands would be high throughout the year and would be limited by N and carbon (C) availability. Mean potential denitrification rates (Dt) were 5.0±1.7mgm2h–1, and were within the reported ranges for other tropical and temperate wetlands. The rates of Dt were similar between the dry and the wet seasons. From the total unamended denitrification rates (Dw, 3.1±1.7mgm2h–1), 64% was derived from NO3– of the water column and the rest from coupled nitrification–denitrification. The factor most closely associated with denitrification was background water NO3–-N concentrations. Improved management and protection of wetlands could play an important role in improving water quality in tropical catchments.

Alteration of nitrous oxide emissions from floodplain soils by aggregate size, litter accumulation and plant–soil interactions

Abstract. Semi-terrestrial soils such as floodplain soils are considered potential hot spots of nitrous oxide (N2O) emissions. Microhabitats in the soil – such as within and outside of aggregates, in the detritusphere, and/or in the rhizosphere – are considered to promote and preserve specific redox conditions. Yet our understanding of the relative effects of such microhabitats and their interactions on N2O production and consumption in soils is still incomplete. Therefore, we assessed the effect of aggregate size, buried leaf litter, and plant–soil interactions on the occurrence of enhanced N2O emissions under simulated flooding/drying conditions in a mesocosm experiment. We used two model soils with equivalent structure and texture, comprising macroaggregates (4000–250 µm) or microaggregates (&lt;250 µm) from a N-rich floodplain soil. These model soils were planted with basket willow (Salix viminalis L.), mixed with leaf litter or left unamended. After 48 h of flooding, a period of enhanced N2O emissions occurred in all treatments. The unamended model soils with macroaggregates emitted significantly more N2O during this period than those with microaggregates. Litter addition modulated the temporal pattern of the N2O emission, leading to short-term peaks of high N2O fluxes at the beginning of the period of enhanced N2O emission. The presence of S. viminalis strongly suppressed the N2O emission from the macroaggregate model soil, masking any aggregate-size effect. Integration of the flux data with data on soil bulk density, moisture, redox potential and soil solution composition suggest that macroaggregates provided more favourable conditions for spatially coupled nitrification–denitrification, which are particularly conducive to net N2O production. The local increase in organic carbon in the detritusphere appears to first stimulate N2O emissions; but ultimately, respiration of the surplus organic matter shifts the system towards redox conditions where N2O reduction to N2 dominates. Similarly, the low emission rates in the planted soils can be best explained by root exudation of low-molecular-weight organic substances supporting complete denitrification in the anoxic zones, but also by the inhibition of denitrification in the zone, where rhizosphere aeration takes place. Together, our experiments highlight the importance of microhabitat formation in regulating oxygen (O2) content and the completeness of denitrification in soils during drying after saturation. Moreover, they will help to better predict the conditions under which hot spots, and “hot moments”, of enhanced N2O emissions are most likely to occur in hydrologically dynamic soil systems like floodplain soils.

Season‐specific trends and linkages of nitrogen and oxygen cycles in Chesapeake Bay

AbstractA three‐decade time series of solute concentrations was combined with a box‐modeling system to analyze long‐term trends in the concentration, production, and transport of dissolved inorganic nitrogen species along the mainstem axis of Chesapeake Bay. Water‐ and salt‐balance calculations associated with box‐modeling provided regional, seasonal, and interannual estimates of net advective and nonadvective transport and net biogeochemical production rates for oxygen and dissolved nitrogen. The strongest decadal trends were observed for decreasing late‐summer ammonium concentrations in bottom layers from brackish to polyhaline bay regions. Contemporaneous trends of increasing late‐summer bottom‐layer dissolved oxygen (O2) concentration were consistent with the observed NH4+ patterns, suggesting that increasing dissolved O2 levels may also reflect declining bottom respiration and drive nitrogen loss via increased rates of coupled nitrification–denitrification. Significant (but weaker) trends of increasing nitrate plus nitrite (NO2+3−) concentration and net production were consistent with the notion that increased nitrification may be stimulated by increasing dissolved O2 concentrations. Sorting bottom water NH4+ and NO2+3− net production rates into two pools (before and after the year 2000) revealed that general seasonal patterns were similar, but recent NH4+ net production rates were consistently lower and NO2+3− and NO2− rates higher in summer and fall compared to earlier years, especially in the middle Bay regions. We conclude that late‐season replenishment of oxygen associated with declining nutrient loads induced a negative feedback process, whereby decreased hypoxia suppressed NH4+ recycling and created conditions favorable for additional nitrogen loss via coupled nitrification–denitrification.

Nitrification and denitrification in estuarine sediments with tube-dwelling benthic animals

Burrowing benthic animals belonging to the same functional group may produce species-specific effects on microbially mediated nitrogen (N) processes depending upon different ecological traits. We investigated the effects of two tube-dwelling organisms, amphipods (Corophium insidiosum) and chironomid larvae (Chironomus plumosus), on benthic N cycling in bioturbated estuarine sediments. Aims of this work were to analyze the interactions among burrowers and N-related microbial processes in two distinct sedimentary environments colonized by benthic animals with different ecological traits. We hypothesized higher rates of nitrification and higher coupled nitrification–denitrification in sediments with C. insidiosum due to continuous ventilation rates. We expected higher denitrification of water column nitrate in sediments with C. plumosus due to lower and intermittent ventilation activity and lower oxygen levels in burrows. To this purpose, we combined process–specific (nitrification and denitrification) with net N flux measurements in intact and reconstructed sediments. Sediments with C. insidiosum had higher rates of oxygen demand and of potential nitrification and higher concentration of pore water NH4+ as compared to sediments with C. plumosus. Sediments with both species displayed comparable net N2 fluxes, mostly sustained by respiration of water column NO3− in sediments with chironomid larvae and by NO3− produced within sediments in sediments with corophiid amphipods. Corophium insidiosum stimulated nitrification nearly 15-fold more as compared to C. plumosus. Overall, our results demonstrate that sediments with burrowing fauna may display similar rates of denitrification, but underlying mechanisms may deeply vary and be species-specific.

The influence of an invasive plant on denitrification in an urban wetland

Abstract Wetlands are often biogeochemical hotspots, and they can remove excess N via denitrification and assimilatory uptake. Wetlands are also susceptible to plant invasions, but the effect of invasive plants on denitrification in freshwater wetland sediments is not well‐studied. Two distinct mechanisms suggest the potential for invasive plants to alter denitrification. First, invasive plants often produce more biomass than non‐invasive species, thus potentially providing additional carbon (C) for denitrifiers. Second, some invasive wetland plants funnel more oxygen into the root zone than non‐invasive plants, potentially stimulating coupled nitrification–denitrification. Using the push–pull isotope pairing technique, we measured denitrification and coupled nitrification–denitrification in the sediments beneath monoculture plots of Phragmites australis and Typha domingensis, and beneath unvegetated sediments, in an urban wetland in Melbourne, Australia. We also measured pore water nutrient concentrations and calculated the diffusive flux of nutrients from the sediments into the overlying water column. We hypothesised that plots containing P. australis would have the highest denitrification and coupled nitrification–denitrification rates, followed by plots containing T. domingensis, with the lowest rates in the unvegetated plots, as a result of higher C and oxygen availability. Instead, we found that denitrification and coupled nitrification–denitrification rates were highly variable, with no difference among plot type. However, we did find that diffusive flux of ammonium from the sediments into the water column was lower in the vegetated plots than in the unvegetated plots, suggesting that vegetation enhances wetland N retention via plant assimilatory uptake.

A missing link in the estuarine nitrogen cycle?: Coupled nitrification-denitrification mediated by suspended particulate matter

In estuarine and coastal ecosystems, the majority of previous studies have considered coupled nitrification-denitrification (CND) processes to be exclusively sediment based, with little focus on suspended particulate matter (SPM) in the water column. Here, we present evidence of CND processes in the water column of Hangzhou Bay, one of the largest macrotidal embayments in the world. Spearman’s correlation analysis showed that SPM was negatively correlated with nitrate (rho = −0.372, P = 0.018) and marker genes for nitrification and denitrification in the water column were detected by quantitative PCR analysis. The results showed that amoA and nir gene abundances strongly correlated with SPM (all P < 0.01) and the ratio of amoA/nir strongly correlated with nitrate (rho = −0.454, P = 0.003). Furthermore, aggregates consisting of nitrifiers and denitrifiers on SPM were also detected by fluorescence in situ hybridization. Illumina MiSeq sequencing further showed that ammonia oxidizers mainly belonged to the genus Nitrosomonas, while the potential denitrifying genera Bradyrhizobium, Comamonas, Thauera, Stenotrophomonas, Acinetobacter, Anaeromyxobacter, Sulfurimonas, Paenibacillus and Sphingobacterium showed significant correlations with SPM (all P < 0.01). This study suggests that SPM may provide a niche for CND processes to occur, which has largely been missing from our understanding of nitrogen cycling in estuarine waters.

Recovering Nitrogen from Farm-Scale Drainage: Mechanism and Economics

Abstract. A two-year study evaluated the nitrogen (N) fluxes, processing, and treatment efficiency (TE) of a 14.85 ha stormwater detention area (SDA) receiving drainage from a vegetable farm in subtropical Florida. The TE was 62% and 89% in years 1 and 2, respectively. Seepage N losses are often ignored in estimating stormwater treatment. Approximately 11% and 20% of the incoming N left the SDA through seepage, reducing the TE to 51% and 67% in years 1 and 2, respectively, indicating the importance of subsurface N losses for downstream water quality. Rainfall variability controlled the timing and volume of the inflow drainage and surface water levels inside the SDA. Variable water levels controlled the aerobic and anoxic conditions inside the SDA, thus controlling the N processing and treatment. Coupled nitrification-denitrification, as a result of frequent wetting-drying cycles, was the main N treatment pathway during year 1. Drought conditions in year 2 led to 89% less surface outflow compared to year 1, resulting in water volume retention being the main process for N retention. The N TE could be increased from 68% to 86% if about two-thirds (63%) of aboveground biomass in the SDA area is harvested annually during the dry season. A payment for environmental services (PES) framework, with the state as buyer and the SDA owner as seller of N treatment services, was evaluated with a 20-year net present worth (NPW) of biomass harvesting for enhanced N treatment. The economic analysis included the benefit from composting the harvested biomass for on-farm use. A positive NPW ($835,000) indicated the economic feasibility of the project, predicting an annual benefit of $42,000 year-1 for the 112 ha farm. Scale-up of the PES approach can offer additional N treatment and C sequestration services as well as increased farm productivity. Keywords: Best management practices, Impoundments, Nutrient treatment, Payment for environmental services, Phytoremediation, Stormwater.

Microphytobenthos and chironomid larvae attenuate nutrient recycling in shallow‐water sediments

Abstract In shallow‐water sediments, the combined action of microphytobenthos and bioturbating fauna may differentially affect benthic nutrient fluxes and exert a bottom‐up control of pelagic primary production. In many cases, the effects of microphytobenthos and macrofauna on nutrient cycling were studied separately, ignoring potential synergistic effects. We measured the combined effects of microphytobenthos and chironomid larvae on sediment–water fluxes of gas (O2, TCO2 and N2) and nutrients (NH4+, NO3−, NO2−, and SiO2) in shallow‐water sediments of a hypertrophic freshwater lagoon. Fluxes were measured in the light and in the dark in reconstructed sediments with low (L = 600 ind/m2), high (H = 1,800 ind/m2) and no (C) addition of chironomid larvae, after 3 weeks of pre‐incubation under light/dark regime to allow for microalgal growth. Besides flux measurements, pore water nutrient (NH4+, and SiO2) and dissolved metal concentrations (Fe2+ and Mn2+) were analysed and diffusive fluxes were calculated. Chironomid larvae increased sediment heterotrophy, by augmenting benthic O2 demand and TCO2 and N2 dark production. However, on a daily basis, treatments C and L were net O2 producing and N2 sinks while treatment H was net O2 consuming and N2 producing. All treatments were net C sink regardless of chironomid density. Microphytobenthos always affected benthic nutrient exchange, as significantly higher uptake or lower efflux was measured in the light compared with dark incubations. Theoretical inorganic N, P and Si demand by benthic microalgae largely exceeded both dark effluxes of NH4+, and SiO2 and their net uptake in the light, suggesting the relevance of N‐fixation, water column NO3− and solid‐phase associated P and Si as nutrient sources to benthic algae. Chironomid larvae had a minor effect on inorganic N and P fluxes while they significantly stimulated inorganic Si regeneration. Their bioturbation activity significantly altered pore water chemistry, with a major reduction in nutrient (highest for NH4+ and lowest for SiO2) and metal concentration. Underlying mechanisms are combinations of burrow ventilation and bioirrigation with stimulation of element‐specific processes as coupled nitrification–denitrification, co‐precipitation and inhibition of anaerobic paths such as Fe3+ or Mn4+ reduction or re‐oxidation of their end products. The combined activity of benthic algae and chironomid larvae may significantly attenuate internal nutrient recycling in shallow eutrophic ecosystems, and contribute to the control of pelagic primary production.

The denitrification paradox: The role of O2 in sediment N2O production

We designed a novel laboratory sediment flux chamber in which we maintained the headspace O2 partial pressure at preselected values, allowing us to experimentally regulate “in-situ” O2 to evaluate its role in net N2O production by an intertidal estuarine sediment (Tyne, UK). In short-term (30 h) incubations with 10 L of overlying estuarine water (∼3 cm depth) and headspace O2 regulation (headspace: sediment/water ratio ∼9:1), net N2O production was highest at 1.2% O2 (sub-oxic; 32.3 nmol N2O m−2 d−1), an order of magnitude higher than at either 0.0% (anoxic; 2.5 N2O nmol m−2 d−1) or 20.85% (ambient; 2.3 nmol N2O m−2 d−1) O2. In a longer-term sealed incubation (∼490 h) without O2 control, time-dependent behaviour of N2O in the tank headspace was highly non-linear with time, showing distinct phases: (i) an initial period of no or little change in O2 or N2O up to ∼ 100 h; (ii) a quasi-linear, inverse correlation between O2 and N2O to ∼360 h, in which O2 declined to ∼2.1% and N2O rose to ∼7800 natm; (iii) over the following 50 h a slower O2 decline, to ∼1.1%, and a more rapid N2O increase, to ∼12000 natm; (iv) over the next 24 h a slowed O2 decline towards undetectable levels and a sharp fall in N2O to ∼4600 natm; (iv) a continued N2O decrease at zero O2, to ∼3000 natm by ∼ 490 h. These results show clearly that rapid N2O consumption (∼115 nmol m−2 d−1), presumably via heterotrophic denitrification (HD), occurs under fully anoxic conditions and therefore that N2O production, which was optimal for sub-oxic O2, results from other nitrogen transformation processes. In experiments in which we amended sediment overlying water to either 1 mM NH4+ or 1 mM NO3−, N2O production rates were 2–134 nmol N2O m−2 d−1 (NH4+ addition) and 0.4–2.2 nmol N2O m−2 d−1 (NO3− addition). We conclude that processes involving NH4+ oxidation (nitrifier nitrification; nitrifier denitrification; nitrification-coupled denitrification) are principally responsible for N2O production in Tyne sediments. Highest N2O production occurred under sub-oxic headspace (∼1.2 O2%) incubations. Anoxic sediments where HD was isolated acted as periodic N2O sinks or relatively small sources of N2O. Our experimental approach thus gives valuable insight into the O2 effect on N2O fluxes from intertidal sediments.

Fate of 15N-labelled ammonium nitrate with or without the new nitrification inhibitor DMPSA in an irrigated maize crop

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N2O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using 15N single-labelled ammonium nitrate (15NH4NO3 or NH415NO3) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize (Zea mays L.) crop. Gaseous fluxes, i.e. N2O, 15N2O, 15N2, methane (CH4), and carbon dioxide (CO2) were measured, as well as soil mineral N, yield components and 15N recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N2O emissions (118 g N ha−1) compared to ammonium nitrate without inhibitor (231 g N ha−1). The 15N analyses revealed that most N2O losses (particularly during the emission peak) came from 15NH4NO3 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than NH415NO3 in this calcareous low C-content soil. As expected, DMPSA decreased N2O losses from 15NH4+ oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N2O losses coming from 15NO3−. No significant effect of DMPSA on CH4 oxidation or 15N2 fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15–30 cm and 30–45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N2O emissions from irrigated crops in semi-arid areas.

Contributions of freshwater mussels (Unionidae) to nutrient cycling in an urban river: filtration, recycling, storage, and removal

Consumers contribute to nutrient cycling in aquatic ecosystems by nutrient retention in tissues, metabolic transformations and excretion, and promoting microbial processes that remove nutrients (i.e., nitrogen (N) loss via denitrification). Freshwater mussels (Unionidae) form dense assemblages in rivers, and affect nutrient transformations through feeding, biodeposition, and bioturbation. However, the effects of Unionid mussels on N and phosphorus (P) retention are not commonly measured. We quantified rates of filtration, retention, and biodeposition of carbon (C), N, and P for two Unionid species: Lasmigona complanata and Pyganodon grandis. We used continuous-flow cores with 15N tracers to measure denitrification in sediments alone and with a single individual of each species. We conducted measurements in an urban river near Chicago, IL, USA that is a target for Unionid restoration. Both Unionid species showed high rates of P-specific feeding and retention, but low N-specific rates. This was in agreement with high N:P ratio in the water column. Each species significantly increased denitrification relative to sediment alone. 15N tracers suggested direct denitrification of nitrate increased denitrification, although enhanced coupled nitrification–denitrification likely also contributed to denitrification. Finally, denitrification rates with and without mussels were used to estimate the value of N loss under different scenarios for mussel restoration at the river scale. Overall, restored Unionid populations may enhance P retention in soft tissues and shells and N loss via denitrification. Ecosystem managers may find greater support for restoration of Unionid populations with careful calculations of their ecosystem role in nutrient retention and removal.

Nitrifier-induced denitrification is an important source of soil nitrous oxide and can be inhibited by a nitrification inhibitor 3,4-dimethylpyrazole phosphate.

Soil ecosystem represents the largest contributor to global nitrous oxide (N2 O) production, which is regulated by a wide variety of microbial communities in multiple biological pathways. A mechanistic understanding of these N2 O production biological pathways in complex soil environment is essential for improving model performance and developing innovative mitigation strategies. Here, combined approaches of the 15 N-18 O labelling technique, transcriptome analysis, and Illumina MiSeq sequencing were used to identify the relative contributions of four N2 O pathways including nitrification, nitrifier-induced denitrification (nitrifier denitrification and nitrification-coupled denitrification) and heterotrophic denitrification in six soils (alkaline vs. acid soils). In alkaline soils, nitrification and nitrifier-induced denitrification were the dominant pathways of N2 O production, and application of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) significantly reduced the N2 O production from these pathways; this is probably due to the observed reduction in the expression of the amoA gene in ammonia-oxidizing bacteria (AOB) in the DMPP-amended treatments. In acid soils, however, heterotrophic denitrification was the main source for N2 O production, and was not impacted by the application of DMPP. Our results provide robust evidence that the nitrification inhibitor DMPP can inhibit the N2 O production from nitrifier-induced denitrification, a potential significant source of N2 O production in agricultural soils.

Coupled Nitrification-Denitrification Caused by Suspended Sediment (SPS) in Rivers: Importance of SPS Size and Composition.

Suspended sediment (SPS) is ubiquitous in rivers, and SPS with different particle sizes and compositions may affect coupled nitrification-denitrification (CND) occurring on SPS significantly. However, there is no related research report. In this work, 15N isotope tracer technique was adopted to explore the CND in systems containing SPS (8 g L-1 and 1 g L-1) collected from the Yellow River with various particle sizes, including <2, 2-20, 20-50, 50-100, and 100-200 μm. The results showed that the CND occurred on SPS and the CND rate was negatively related to particle size; both nitrification and denitrification rate constants increased with decreasing SPS particle size. For instance, SPS (8 g L-1) with a particle size below 2 μm had the highest 15N2 emission rate of 1.15 mg-N/(m3·d), which was 2.9 times that of 100-200 μm. This is because SPS with a smaller particle size had a larger specific surface area and a higher organic carbon content, which is beneficial for bacteria growth. Both the nitrifying and denitrifying bacteria population were positively correlated with CND rate (p < 0.05). Different from the 15N2 production, 15N2O emission rate did not decrease with increasing SPS particle size. For the system containing 8 g L-1 SPS, 15N2O emission rate reached the highest of 1.05 μg-N/(m3·d) in the 50-100 μm SPS system, which was 17.5 times that of 100-200 μm. Similar results could be found from the system with 1 g L-1 SPS. This is due to the fact that the oxygen concentration at the SPS-water interface increased with SPS particle size, and the oxygen conditions might be most suitable for the production of N2O in the 50-100 μm system. This study suggests that SPS size and composition play an important role in nitrogen cycle of river systems, especially for the production of N2O.

Nitrogen uptake and coupled nitrification–denitrification in riverine sediments with benthic microalgae and rooted macrophytes

Riverine ecosystems receive nitrogen loads from point and diffuse sources that are transferred downstream. Such loads may undergo poorly explored retention and dissipation processes, varying along gradients of nitrogen availability due to different interactions among primary producers and microbial communities. We measured carbon uptake and nitrogen fluxes in microcosms containing riverine sediments with benthic algae and submerged macrophytes (Vallisneria spiralis L.). Coupled nitrification–denitrification rates were determined via 15N–NH4 + injection in the pore water and quantification of the produced 29N2 and 30N2. Two sites with different N–NO3 − concentration and sediment organic content were investigated. We hypothesized that: (1) N–NO3 − availability promotes water column N uptake and attenuates primary producers-bacteria competition; (2) coupled nitrification–denitrification is stimulated by radial oxygen loss from roots; (3) macrophyte meadows favour both temporary nitrogen retention and permanent removal. Sediments with V. spiralis were mostly inorganic C and N sinks and always displayed higher coupled nitrification–denitrification rates compared to sediments with microphytobenthos. Highest rates, up to 100 µmol N m−2 h−1, were measured at the more eutrophic site and in the light. This is likely due to a shift from root to leaf uptake at the N–NO3 − rich site, attenuating plant-bacteria competition, and to increased radial oxygen loss in organic-rich sediments, stimulating nitrification. High rates of N uptake and loss in lotic sediments represent natural buffers preventing N transport downstream and stress the need to preserve aquatic vegetation and its ecosystem services.

Mechanical insights into the effect of fluctuation in soil moisture on nitrous oxide emissions from paddy soil

Paddy fields are subjected to fluctuating water regimes as a result of the alternate drying and wetting water management, which often incurs a sensitive change in N2O emissions from paddy soils. However, how the soil moisture regulates the emission of N2O from paddy soil remains uncertain. In this study, three incubation experiments were designed to study the effects of constant and fluctuating soil moisture on N2O emission and the sources of N2O emission from paddy soil. Results showed that the N2O emission from paddy soil at 100 % WHC (water-holding capacity) was higher than that at 40, 65, 80, 120, and 160 % WHC, indicating that 100 % WHC was the optimum soil moisture content for N2O emission under the incubation experiment. Small peak of N2O flux appeared when the soil moisture content from 250 % WHC decreased near to 100 % WHC, lower than that triggered by nitrogen (N) fertilization, which was mainly owing to the low NH4 + concentration at this period. Nitrification dominated the emissions of N2O from paddy soil at 250 % WHC (54.96 %), higher than that of nitrification-coupled denitrification (6.74 %) and denitrification (38.3 %). The contribution of denitrification to N2O emissions (44.10 %) was equivalent to that of nitrification (44.45 %) in soil at 100 % WHC, which was higher than that of 250 % WHC treatment. In conclusion, the finding suggested that the peak of N2O in paddy soils during midseason aeration could be attributed to the occurrence of optimum soil moisture under sufficient N availability, favorable for the production and accumulation of N2O.