Evaluating the role of high N2O affinity complete denitrifiers and non-denitrifying N2O reducing bacteria in reducing N2O emissions in river.

Freshwater rivers are hotspots of N2O greenhouse gas emissions. Dissolved organic carbon (DOC) is the dominant electron donor for microbial N2O reduction, which can reduce N2O emission through enriching high N2O affinity denitrifiers or enriching non-denitrifying N2O-reducing bacteria (N2ORB), but the primary regulatory pathway remains unclear. Here, field study indicated that high DOC concentration in rivers enhanced denitrification rate but reduced N2O flux by improving nosZ gene abundance. Then, four N2O-fed membrane aeration biofilm reactors inoculated with river sediments from river channel, estuary, adjacent lake, and a mixture were continuously performed for 360 days, including low, high, and mixed DOC stages. During enrichment stages, the (nirS+nirK)/nosZ ratio showed no significant difference, but the community structure of denitrifiers and N2ORB changed significantly (p < 0.05). In addition, N2ORB strains isolated from different enrichment stages positioned in different branches of the phylogenetic tree. N2ORB strains isolated during high DOC stage showed significant higher maximum N2O-reducing capability (Vmax: 0.6 ± 0.4 ×10-4 pmol h-1 cell-1) and N2O affinity (a0: 7.8 ± 7.7 ×10-12 L cell-1 h-1) than strains isolated during low (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.7 ± 0.4 ×10-12 L cell-1 h-1) and mixed DOC stages (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.9 ± 0.9 ×10-12 L cell-1 h-1) (p < 0.05). Hence, under high DOC concentration conditions, the primary factor in reducing N2O emissions in rivers is the enrichment of complete denitrifiers with high N2O affinity, rather than non-denitrifying N2ORB.

Significant diurnal variations in nitrous oxide (N2O) emissions from two contrasting habitats in a large eutrophic lake (Lake Taihu, China).

Algae and macrophytes in lake ecosystems regulate nitrous oxide (N2O) emissions from eutrophic lakes. However, knowledge of diurnal N2O emission patterns from different habitats remains limited. To understand the diurnal patterns and driving mechanisms of N2O emissions from contrasting habitats, continuous in situ observations (72 h) of N2O fluxes from an algae-dominated zone (ADZ) and reed-dominated zone (RDZ) in Lake Taihu were conducted using the Floating Chamber method. The results showed average N2O emission fluxes of 0.15 ± 0.06 and 0.02 ± 0.04 µmol m-2 h-1 in the ADZ and RDZ in autumn, respectively. The significantly higher (p < 0.05) N2O fluxes in the ADZ were mainly attributed to differences in nitrogen (N) levels. The results also showed significant diurnal differences (p < 0.05) in the N2O emission fluxes within the ADZ and RDZ, and daytime fluxes were significantly higher (p < 0.05) than nighttime fluxes. The statistical results indicated that N2O emissions from the ADZ were mainly driven by diurnal variations in N loading and the dissolved oxygen (DO) concentration, and those from the RDZ were more influenced by DO, redox potential, and pH. Finally, we determined the proper time for routine monitoring of N2O flux in the two habitats. Our results highlight the importance of considering diverse habitats and diurnal variations when estimating N2O budgets at a whole-lake scale.

Understanding the impacts of coastal deoxygenation in nitrogen dynamics: an observational analysis.

Biological production and outgassing of greenhouse gasses (GHG) in Eastern Boundary Upwelling Systems (EBUS) are vital for fishing productivity and climate regulation. This study examines temporal variability of biogeochemical and oceanographic variables, focusing on dissolved oxygen (DO), nitrate, nitrogen deficit (N deficit), nitrous oxide (N2O) and air-sea N2O flux. This analysis is based on monthly observations from 2000 to 2023 in a region of intense seasonal coastal upwelling off central Chile (36°S). Strong correlations are estimated among N2O concentrations and N deficit in the 30-80 m layer, and N2O air-sea fluxes with the proportion of hypoxic water (4 < DO < 89 µmol L-1) in the water column, suggesting that N2O accumulation and its exchange are mainly associated with partial denitrification. Furthermore, we observe interannual variability in concentrations and inventories in the water column of DO, nitrate, N deficit, as well as air-sea N2O fluxes in both downwelling and upwelling seasons. These variabilities are not associated with El Niño-Southern Oscillation (ENSO) indices but are related to interannual differences in upwelling intensity. The time series reveals significant nitrate removal and N2O accumulation in both mid and bottom layers, occurring at rates of 1.5 µmol L-1 and 2.9 nmol L-1 per decade, respectively. Particularly significant is the increase over the past two decades of air-sea N2O fluxes at a rate of 2.9 µmol m-2 d-1 per decade. These observations suggest that changes in the EBUS, such as intensification of upwelling and the prevalence of hypoxic waters may have implications for N2O emissions and fixed nitrogen loss, potentially influencing coastal productivity and climate.

Nitrogen addition stimulates N2O emissions via changes in denitrification community composition in a subtropical nitrogen-rich forest.

Microbially driven nitrification and denitrification play important roles in regulating soil N availability and N2O emissions. However, how the composition of nitrifying and denitrifying prokaryotic communities respond to long-term N additions and regulate soil N2O emissions in subtropical forests remains unclear. Seven years of field experiment which included three N treatments (+0, +50, +150 kg N ha-1 yr-1; CK, LN, HN) was conducted in a subtropical forest. Soil available nutrients, N2O emissions, net N mineralization, denitrification potential and enzyme activities, and the composition and diversity of nitrifying and denitrifying communities were measured. Soil N2O emissions from the LN and HN treatments increased by 42.37% and 243.32%, respectively, as compared to the CK. Nitrogen addition significantly inhibited nitrification (N mineralization) and significantly increased denitrification potentials and enzymes. Nitrification and denitrification abundances (except nirK) were significantly lower in the HN, than CK treatment and were not significantly correlated with N2O emissions. Nitrogen addition significantly increased nirK abundance while maintaining the positive effects of denitrification and N2O emissions to N deposition, challenging the conventional wisdom that long-term N addition reduces N2O emissions by inhibiting microbial growth. Structural equation modeling showed that the composition, diversity, and abundance of nirS- and nirK-type denitrifying prokaryotic communities had direct effects on N2O emissions. Mechanistic investigations have revealed that denitrifier keystone taxa transitioned from N2O-reducing (complete denitrification) to N2O-producing (incomplete denitrification) with increasing N addition, increasing structural complexity and diversity of the denitrifier co-occurrence network. These results significantly advance current understanding of the relationship between denitrifying community composition and N2O emissions, and highlight the importance of incorporating denitrifying community dynamics and soil environmental factors together in models to accurately predict key ecosystem processes under global change.

Effect of mulching and biochar addition on the distribution and emission characteristics of N2O from furrow-ridge tillage soils.

Mulching and biochar are increasingly used individually in agriculture, but little is known about their combined effects on N2O distribution and dispersion in ridge and furrow profiles. We conducted a 2-year field experiment in northern China to determine soil N2O concentrations using the in situ gas well technique and calculate N2O fluxes from ridge and furrow profiles by the concentration gradient method. The results showed that mulch and biochar increased soil temperature and moisture and altered the mineral nitrogen status, leading to a decrease in the relative abundance of nitrification genes in the furrow area and an increase in the relative abundance of denitrification genes, with denitrification remaining as the main source of N2O production. N2O concentrations in the soil profile increased significantly after fertiliser application, and N2O concentrations in the ridge area of the mulch treatment were much higher than those in the furrow area, where vertical and horizontal diffusion occurred. Biochar addition was effective in reducing N2O concentrations but had no effect on the N2O distribution and diffusion pattern. Soil temperature and moisture, but not soil mineral nitrogen, explained the variation in soil N2O fluxes during the non-fertiliser application period. Compared to furrow-ridge planting (RF), furrow-ridge mulch planting (RFFM), furrow-ridge planting with biochar (RBRF) and furrow-ridge mulch planting with biochar (RFRB) resulted in 9.2%, 11.8% and 20.8% increases in yield per unit area and 1.9%, 26.3% and 27.4% decreases in N2O fluxes per unit of yield, respectively. The interaction between mulching and biochar significantly affected the N2O fluxes per unit of yield. Biochar costs aside, RFRB is very promising for increasing alfalfa yields and reducing N2O fluxes per unit of yield.

Soil glomalin-related protein affects aggregate N2O fluxes by modulating denitrifier communities in a fertilized soil.

Agricultural ecosystems contribute significantly to atmospheric emissions of soil nitrous oxide (N2O), which exacerbate environmental pollution and contribute to global warming. Glomalin-related soil protein (GRSP) stabilizes soil aggregates and enhances soil carbon and nitrogen storage in agricultural ecosystems. However, the underlying mechanisms and relative importance of GRSP on N2O fluxes within soil aggregate fraction remain largely unclear. We examined the GRSP content, denitrifying bacterial community composition, and potential N2O fluxes across three aggregate-size fractions (2000-250 µm, 250-53 µm, and <53 µm) under a long-term fertilization agricultural ecosystem, subjected to mineral fertilizer or manure and their combination. Our findings indicated that various fertilization treatments have no discernible impact on the size distribution of soil aggregates, paving the way to further research into the impact of soil aggregates on GRSP content, the denitrifying bacterial community composition, and potential N2O fluxes. GRSP content increased with the increase in soil aggregate size. Potential N2O fluxes (including gross N2O production and N2O reduction and net N2O production) among aggregates were highest in microaggregates (250-53 µm), followed by macroaggregates (2000-250 µm) and lowest in silt + clay (<53 µm) fractions. Potential N2O fluxes had a positive response to soil aggregate GRSP fractions. The non-metric multidimensional scaling analysis revealed that soil aggregate size could drive the denitrifying functional microbial community composition, and deterministic processes play more critical roles than stochasticity processes in driving denitrifying functional composition under soil aggregate fractions. Procrustes analysis revealed a significant correlation between denitrifying microbial community, soil aggregate GRSP fractions, and potential N2O fluxes. Our study suggests that soil aggregate GRSP fractions influence potential nitrous oxide fluxes by affecting denitrifying microbial functional composition within soil aggregate.

Effects of drying-rewetting cycles on the fluxes of soil greenhouse gases.

Irregular precipitation caused by climate changes has resulted in frequent events of soil drying-rewetting cycles (DWC), which can strongly affect soil carbon (C) and nitrogen (N) cycling, including the fluxes of greenhouse gases (GHGs). The response of soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes to DWC events may differ among different ecosystem types and vary with experimental settings and soil properties, but these processes were not quantitatively assessed. Here, we evaluated the responses of soil GHG fluxes to DWC, compared with consistent moisture, as well as the associated driving factors with 424 paired observations collected from 47 publications of lab incubation experiments. Results showed that: (1) DWC significantly decreased soil CO2 emissions by an average of 9.7%, but did not affect the emissions and uptakes of soil CH4 and N2O; (2) DWC effects on soil GHG emissions varied significantly among different ecosystem types, with CO2 emissions significantly decreased by 6.8 and 16.3% in croplands and grasslands soils, respectively, and CH4 and N2O emissions significantly decreased and increased in wetlands and forests soils, respectively; (3) the effects of DWC on CO2 emissions were also positively regulated by organic C and N concentrations, pH, clay concentration, and soil depth, but negatively by C:N ratio and silt concentration, while DWC effects on N2O emissions were negatively controlled by C:N ratio, silt concentration, and soil depth. Overall, our results showed that CO2 emissions were significantly decreased by DWC, while the fluxes of CH4 and N2O were not affected, indicating an overall decrease of GHGs in response to DWC. Our results will be useful for a better understanding of global GHG emissions under future climate change scenario.

Large alpine deep lake as a source of greenhouse gases: A case study on Lake Fuxian in Southwestern China.

Freshwater lakes are recognized as potential sources of greenhouse gases (GHGs) that contribute to global warming. However, the spatiotemporal patterns of GHG emissions have not been adequately quantified in large deep lakes, resulting in substantial uncertainties in the estimated GHG budgets in global lakes. In this study, the spatial and seasonal variability of diffusive GHG (CO2, CH4, and N2O) emissions from Lake Fuxian located on a plateau in Southwestern China were quantified. The results showed that the surface lake water was oversaturated with dissolved GHG concentrations, and the average concentrations were 24.25 µM CO2, 0.044 µM CH4, and 14.28 nM N2O, with diffusive emission rates of 8.82 mmol CO2 m-2 d-1, 31.94 µmol CH4 m-2 d-1, and 4.94 µmol N2O m-2 d-1, respectively. Diffusive CH4 flux exhibited high temporal and spatial variability similar to that in most lakes. In contrast, diffusive CO2 and N2O flux showed distinct seasonal variability and similar spatial patterns, emphasizing the necessity for increasing the temporal resolution in GHG flux measurements for integrated assessments. Water temperature and/or oxygen concentrations were crucial in regulating seasonal variability in GHG emissions. However, no limnological parameter was found to govern the spatial GHG patterns. The frequent advection mixing caused by wind-driven currents might be the reason for the low spatial heterogeneity in GHGs, in which the inconspicuous mechanism requires further research. It was recommended that at least 11 locations were needed for representative whole lake flux estimates at each sampling campaign. In addition, the maximum peak of CH4 in the oxycline from Lake Fuxian indicated that low CH4 oxidation occurred in oxic waters. Overall, this study suggests that, compared to other tropical and temperate lakes, this alpine deep lake is a minor CO2 and CH4 source, but a moderate N2O source, which are horizontally uniform.

Reducing nitrous oxide emissions from irrigated maize by using urea fertilizer in combination with nitrapyrin under different tillage methods.

The aim of this study was to evaluate the effectiveness of nitrification inhibitor (nitrapyrin; NI) as a mitigation option for yield-scaled emissions of nitrous oxide (N2O) under tillage management and urea fertilization in the irrigated maize fields in northern Iran. A split-plot experiment was performed based on a randomized completed blocks design with three replicates. The main plots were the levels of tillage practices (conventional tillage (CT) and minimum tillage (MT), and the subplots were the fertilizer treatments (control, urea, and urea + NI). The gas samples for measuring N2O emissions were collected during the maize growing season from June to September, using opaque manual circular static chambers. Soil samples were taken at 0-10 cm to determine water-filled pore space, ammonium (NH4+), and nitrate (NO3-) concentrations in the soil. When the crop reached physiological maturity, maize was harvested to measure grain yield, biomass production, N uptake of aboveground, and nitrogen use efficiency (NUE). The results showed that the applying NI in combination with urea reduced the total N2O emissions by up to 58% and 64% in MT and CT, respectively. In the urea + NI treatment, mean soil concentrations of NH4+ and NO3- were significantly higher (20%) and lower (23.5%), respectively, compared with other treatments. The NI reduced the yield-scaled N2O-N emission up to 79% and 55% for CT and MT, respectively. Furthermore, compared to treatment with urea alone, the application of NI increased the NUE of the MT and CT systems by an average of 55% and 46%, respectively. This study emphasized that the application of nitrapyrin should be encouraged in irrigated maize fields, in order to minimize N2O emissions and improve NUE and biomass production.

Nitrous oxide flux observed with tall-tower eddy covariance over a heterogeneous rice cultivation landscape.

Although croplands are known to be strong sources of anthropogenic N2O, large uncertainties still exist regarding their emission factors, that is, the proportion of N in fertilizer application that escapes to the atmosphere as N2O. In this study, we report the results of an experiment on the N2O flux in a landscape dominated by rice cultivation in the Yangtze River Delta, China. The observation was made with a closed-path eddy covariance system on a 70-m tall tower from October 2018 to December 2020 (27 months). Temperature and precipitation explained 78% of the seasonal and interannual variability in the observed N2O flux. The growing season (May to October) mean flux (1.14 nmol m-2 s-1) was much higher than the median flux found in the literature for rice paddies. The mean N2O flux during the observational period was 0.90 ± 0.71 nmol m-2 s-1, and the annual cumulative N2O emission was 7.6 and 9.1 kg N2O-N ha-1 during 2019 and 2020, respectively. The corresponding landscape emission factor was 3.8% and 4.6%, respectively, which were much higher than the IPCC default direct (0.3%) and indirect emission factors (0.75%) for rice paddies.

Nitrous oxide fluxes from long-term limed soils following P and glucose addition: Nonlinear response to liming rates and interaction from added P.

Liming of acidic soils to regulate pH for crop growth may decrease emissions of nitrous oxide (N2O) due to direct effects of pH on the synthesis of N2O reductases by denitrifying bacteria. However, liming also changes general pH-dependent soil properties, including availability of phosphorus (P), with a feedback on N2O fluxes that remains largely unknown. Here we used a mesocosm approach to study the combined role of liming and P in regulating N2O fluxes from denitrification in an arable coarse sandy soil where N2O emissions under field condition coincided with rainfall events and irrigation, which facilitated anoxia. Soils from three long-term liming treatments (0, 4, and 12 Mg ha-1) with resulting pH(CaCl2) of 3.6, 4.7 and 6.3 were incubated at original bulk density first at 60% water filled pore space (WFPS) and successively at 75% WFPS with added nitrate, inorganic P (0 and 10 µg P g-1 soil) and glucose as labile carbon. N2O fluxes were measured during 28 days and were supplemented with measurements of CO2 fluxes, microbial biomass, potential denitrification, and acid phosphatase activity. The results showed a nonlinear response of N2O fluxes to liming rates, with highest fluxes at the intermediate liming level (4 Mg ha-1). Furthermore, inorganic P stimulated N2O fluxes only at the intermediate liming level. Assays of potential denitrification indicated that the N2O/(N2O + N2) product ratio decreased consistently with increasing liming rates, but total N2O fluxes responded nonlinearly likely due to combined effects on N2O/(N2O + N2) product ratios and total denitrification rates. The results suggest that liming and P addition interact on microbial properties and N2O emissions from acidic arable soils and may not follow linear trends. This makes it uncertain to predict and model the resulting net effect, which may depend on the actual pH range and P availability from the unlimed to the limed treatments.

The driving effect of nitrogen-related functional microorganisms under water and nitrogen addition on N2O emission in a temperate desert.

Nitrous oxide (N2O) is an important greenhouse gas and a precursor of ozone depletion in the upper atmosphere, thus contributing to climate change and biological safety. The mechanisms and response characteristics of N2O emission in desert soils to precipitation and nitrogen (N) deposition are still unclear. To further elucidate this, an in-situ experiment was conducted in the Gurbantunggut Desert, a temperate desert in China, between June and September 2015 and 2016. The response in N2O flux to water addition (equivalent to 5 mm precipitation) was very transient in summer, only lasting one to two days. This was attributed to the rapid decrease in soil moisture following the water addition, due to the high temperature and drought conditions, and there was no significant change in N2O emission or in the abundance of N-related key functional genes. In contrast, N2O emissions increased significantly in response to N addition. This was associated with an increase in functional gene abundances of amoA (ammonia oxidizing bacteria (AOB)) and ammonia-oxidizing archaea (AOA), which responded positively to increasing soil NH4+-N content, but were inhibited by increasing soil NO3--N content. The abundance of the nirS (nitrate reductase) gene was significantly increased by increasing soil NO3--N content. Interestingly, the indirect effect of increased soil moisture in enhancing N2O emission by increasing the abundance of AOA was offset by a direct effect of soil moisture in inhibiting soil N2O emission. Overall, N2O emissions were mainly controlled by AOA rather than AOB in summer, and were more sensitive to soil available N than to soil moisture in this temperate desert.

A 4-year field measurement of N2O emissions from a maize-wheat rotation system as influenced by partial organic substitution for synthetic fertilizer.

Soil N2O emissions depend on the status of stoichiometric balance between organic C and inorganic N. As a beneficial management practice to sustain soil fertility and crop productivity, partial substitution of organic fertilizers (OFs) for synthetic fertilizers (SFs) can directly affect this balance status and regulate N2O emissions. However, no multi-year field studies of N2O emissions under different ratios of OFS to SFs have been performed. We conducted a 4-year experiment to measure N2O emissions in a maize-wheat rotation in central China. Six treatments were included: total SF (TS), total OF, no N fertilizer, and ratios of to SF with 1: 2 (LO), 1: 1 (MO), and 2: 1 (HO), based on N content. Two incubation experiments were performed to further interpret the field data. In the first year, cumulative N2O emissions (kg N ha-1) in LO, MO, and HO were 4.59, 4.68, and 3.59, respectively, significantly lower than in TS (6.67). However, from the second year onwards, organic substitution did not reduce N2O emissions and even significantly enhanced them in the fourth year relative to TS. Soil respiration under OF-amended soils increased over the course of the experiment. From the second year onwards, there was no marked difference in mineral N concentrations between OF- and SF-amended soils. OF caused a drop in soil pH. Cumulative N2O was negatively correlated with pH. Long-term organic substitution enhanced N2O emissions produced via denitrification rather than nitrification and resulted in higher temperature sensitivity of N2O emissions than TS. The enhanced N2O emissions from the OF-treated soils were mainly attributable to accelerated OF decomposition, increased denitrification-N2O emissions, and lessened N2O reduction due to lower pH and greater NO3-. These results indicate that OF substitution can reduce N2O emissions in the first year, but in the long-term it can increase emissions, especially as soils warm.

Spatial and seasonal variability of nitrous oxide in a large freshwater lake in the lower reaches of the Yangtze River, China.

Aquatic ecosystems are recognized as a source of N2O in accordance with the flux estimations of rivers and estuaries; however, limited research has been conducted on large lakes. In this study, we report the annual N2O dynamics of a large eutrophic freshwater lake located in the subtropical zone of East China. The dissolved N2O concentrations in Lake Chaohu were observed to be between 8.5 and 92.3 nmol L-1 with emission rates between 0.3 and 53.6 µmol m-2 d-1, exhibiting considerable spatiotemporal variability. The average seasonal N2O concentrations were obtained, with the highest value of 23.4 nmol L-1 found in winter and the lowest value of 12.7 nmol L-1 found in summer. In contrast to the N2O concentrations observed, the highest N2O emission rates occurred during summer, while the lowest emission rates occurred in autumn. The emissions of N2O were substantially high in the western part of the lake, which suffers from serious eutrophication. In addition, the hotspots of N2O emissions have been found around the inflowing mouth of the Nanfei River, which transports large amounts of nutrients into the lake. The results suggest that anthropogenically enhanced nutrient inputs may have a significant role in the production and emission of N2O. However, the negative relationship between the surface water temperature and the N2O concentration suggests that, N2O fluxes might be influenced by other inconspicuous mechanisms. In the future the nitrogen dynamics of water and sediment in the lake should be collated to reveal mechanisms controlling N2O emissions. In summary, Lake Chaohu acts as a source of N2O with its most eutrophic part contributing 54.9% of the total N2O emissions of the whole lake.

Effects of warming and elevated O3 concentrations on N2O emission and soil nitrification and denitrification rates in a wheat-soybean rotation cropland.

The effects of warming and elevated ozone (O3) concentrations on nitrous oxide (N2O) emission from cropland has received increasing attention; however, the small number of studies on this topic impedes understanding. A field experiment was performed to explore the role of warming and elevated O3 concentrations on N2O emission from wheat-soybean rotation cropland from 2012 to 2013 using open-top chambers (OTCs). Experimental treatments included ambient temperature (control), elevated temperature (+2 °C), elevated O3 (100 ppb), and combined elevated temperature (+2 °C) and O3 (100 ppb). Results demonstrate that warming significantly increased the accumulative amount of N2O (AAN) emitted from the soil-winter wheat system due to enhanced nitrification rates in the wheat farmland and nitrate reductase activity in wheat leaves. However, elevated O3 concentrations significantly decreased AAN emission from the soil-soybean system owing to reduced nitrification rates in the soybean farmland. The combined treatment of warming and elevated O3 inhibited the emission of N2O from the soybean farmland. Additionally, both the warming and combined treatments significantly increased soil nitrification rates in winter wheat and soybean croplands and decreased denitrification rates in the winter wheat cropping system. Our results suggest that global warming and elevated O3 concentrations will strongly affect N2O emission from wheat-soybean rotation croplands.

Effects of straw returning and feeding on greenhouse gas emissions from integrated rice-crayfish farming in Jianghan Plain, China.

Great efforts have been devoted to assessing the effects of straw managements on greenhouse gas (GHG) emissions, global warming potential (GWP), and net economic budget in rice monoculture (RM). However, few studies have evaluated the effects of straw managements on GHG emissions and net ecosystem economic budget (NEEB) in integrated rice-crayfish farming (RC). Here, a randomized block field experiment was performed to comprehensively evaluate the effects of aquatic breeding practices (feeding or no feeding of forage) and straw managements (rice straw returning or removal) on soil NH4+-N and NO-3-N contents, redox potential (Eh), CH4 and N2O emissions, GWP, and NEEB of fluvo-aquic paddy soil in a rice-crayfish co-culture system in Jianghan Plain of China. We also compared the differences in CH4 and N2O emissions, GWP, and NEEB between RM and RC. Straw returning significantly increased CH4 and N2O emissions by 34.9-46.1% and 6.2-23.1% respectively compared with straw removal. Feeding of forage decreased CH4 emissions by 13.9-18.7% but enhanced N2O emissions by 24.4-33.2% relative to no feeding. Compared with RM treatment, RC treatment decreased CH4 emissions by 18.1-19.6% but increased N2O emissions by 16.8-21.0%. Moreover, RC treatment decreased GWP by 16.8-22.0% while increased NEEB by 26.9-75.6% relative to RM treatment, suggesting that the RC model may be a promising option for mitigating GWP and increasing economic benefits of paddy fields. However, the RC model resulted in a lower grain yield compared with the RM model, indicating that more efforts are needed to simultaneously increase grain yield and NEEB and decrease GWP under RC model.

[Effect of Nitrification on N2O Emissions and Their Environmental Factors in Saline-alkali Wetlands].

Wetlands are important sources and sinks for N2O. Exploring the role of N2O emissions in saline-alkali wetlands has great significance in understanding the nitrification mechanism of N2O production and assessing the role of saline-alkali wetlands in the greenhouse effect. The present study examined the N2O fluxes and environmental factors of a typical Zhalong reed wetland during the growing season. The results suggested that the N2O fluxes tended to decrease in volatility, with the highest value in mid-July. The mean flux of N2O was (37.49±15.75) µg·(m2·h)-1, indicating that the typical Zhalong reed wetland was a source of N2O. The N2O fluxes exhibited a significantly positive correlation with soil temperature at different depths (P<0.05), and the impact of the upper soil temperature on N2O flux was higher than that of deep soil. In the flooding period, the relationship between N2O fluxes and water table depth was negatively correlated (P<0.05). Meanwhile, the TOC and TN contents were lower, and the N2O flux was significantly positively correlated with the NH4+-N content in the 0-40 cm soil layer (P<0.05), but it was not related to NO3--N content. Nitrification was stronger than denitrification. There was a significant positive correlation between ammonia-oxidizing bacterial activity and soil temperature in 0-20 cm layer (P<0.01). Additionally, the activity of ammonia-oxidizing bacteria also presented significantly positive linear correlation with the N2O fluxes (P<0.001), which indicated that the release of N2O in saline-alkali wetlands was greatly affected by nitrification.

[Characteristics of N2O Release and Influencing Factors in Grass-type and Algae-type Zones of Taihu Lake During Summer].

Spatial heterogeneity of N2O generation and emissions in multi-ecotype lakes limited the accurate estimation of the N2O fluxes in lakes, but few studies on the characteristics of N2O generation and emissions have been conducted. In this study, N2O flux at the water-gas interface, dissolved N2O concentration in the water column, and N2O flux at the sediment-water interface in typical grass-type and algal-type zones of Taihu Lake were analyzed during summer, and indoor micro-environment experiments were conducted to illustrate the main factors affecting the generation and emissions of N2O. The results showed that the N2O fluxes at the water-gas interface, dissolved N2O concentration, and N2O fluxes at the sediment-water interface of the emergent macrophyte type area was higher than the algae-type area and submerged macrophyte area during the summer., with N2O fluxes at the water-gas interface of (115.807±7.583), (79.768±1.842), and (3.685±0.295) µmol ·(m2 ·h)-1, respectively. The dissolved N2O concentration in the water column were (0.051±0), (0.029±0.001), and (0.018±0) µmol ·L-1, respectively; and the N2O fluxes at the sediment-water interface were (178.275±3.666), (160.685±0.642), and (75.665±1.016) µmol ·(m2 ·h)-1, respectively. The spatial difference could be attributed to dominant plants and the concentration of inorganic nitrogen in the water column. The results of micro-environment experiments showed that nitrate and organic carbon sources could significantly increase the N2O production potential of sediments, the high concentration of NH4+-N in the water column might inhibit the N2O production in sediments, and the production rates of N2O in the sediment increased remarkably when the incubation temperature increased, suggesting that the generation and emissions of N2O were mainly restricted by nitrate, organic carbon, and temperature in summer.

Microbial adaptation to long-term N supply prevents large responses in N dynamics and N losses of a subtropical forest.

Atmospherically-deposited nitrogen (N) can stimulate complex soil N metabolisms and accumulations over time. Whether long-term (decadal) N deposition effects on soil N transformations and functional microbes differ from the short-term (annual) effects has rarely been assessed. Here we conducted a laboratory 15N tracing study with soil samples from a short-term (one year) N addition site and a long-term (12 years) site in a subtropical forest. The effects of simulated N deposition on soil N2O emissions, N transformation rates and microbial nitrifying and denitrifying genes were determined. Our results showed that: (1) long-term N addition did not change soil N2O fluxes significantly in comparison to the short-term N addition. Denitrification, heterotrophic nitrification and autotrophic nitrification contributed 53%, 28% and 18% to total N2O emissions, respectively. (2) Autotrophic nitrification was the dominant N transformation process, except for the high-N treatment at the long-term site. The magnitude of soil N transformation rates was significantly different among N addition treatments but not between short- and long-term N addition sites. However, long-term N addition changed the responses of specific N transformation rates to N addition markedly, especially for the rates of nitrification, organic N mineralization to NH4+, NO3- immobilization and dissimilatory NO3- reduction to NH4+ (DNRA). (3) Responses of ammonia oxidizing archaea and bacteria (AOA and AOB) were more variable than those of denitrifying N2O-producers (nirK) and denitrifying N2O-reducers (nosZ), particularly at the long-term site. (4) The close correlations among N2O flux, functional genes and soil properties observed at the short-term site were weakened at the long-term site, posing a decreased risk for N losses in the acid subtropical forest soil. There is evidence for an adaptation of functional microbial communities to the prevailing soil conditions and in response to long-term natural and anthropogenic N depositions.

Effects of warming on N2O fluxes in a boreal peatland of Permafrost region, Northeast China.

Climate warming is expected to increasingly influence boreal peatlands and alter their greenhouse gases emissions. However, the effects of warming on N2O fluxes and the N2O budgets were ignored in boreal peatlands. Therefore, in a boreal peatland of permafrost zone in Northeast China, a simulated warming experiment was conducted to investigate the effects of warming on N2O fluxes in Betula. Fruticosa community (B. Fruticosa) and Ledum. palustre community (L. palustre) during the growing seasons from 2013 to 2015. Results showed that warming treatment increased air temperature at 1.5m aboveground and soil temperature at 5cm depth by 0.6°C and 2°C, respectively. The average seasonal N2O fluxes ranged from 6.62 to 9.34µgm-2h-1 in the warming plot and ranged from 0.41 to 4.55µgm-2h-1 in the control plots. Warming treatment increased N2O fluxes by 147% and transformed the boreal peatlands from a N2O sink to a source. The primary driving factors for N2O fluxes were soil temperature and active layer depth, whereas soil moisture showed a weak correlation with N2O fluxes. The results indicated that warming promoted N2O fluxes by increasing soil temperature and active layer depth in a boreal peatland of permafrost zone in Northeast China. Moreover, elevated N2O fluxes persisted in this region will potentially drive a noncarbon feedback to ongoing climate change.