Potential Denitrification Activity Research Articles

Genetic variation in Zea mays influences microbial nitrification and denitrification in conventional agroecosystems

Abstract Background and Aims Nitrogenous fertilizers provide a short-lived benefit to crops in agroecosystems, but stimulate nitrification and denitrification, processes that result in nitrate pollution, N2O production, and reduced soil fertility. Recent advances in plant microbiome science suggest that genetic variation in plants can modulate the composition and activity of rhizosphere N-cycling microorganisms. Here we attempted to determine whether genetic variation exists in Zea mays for the ability to influence the rhizosphere nitrifier and denitrifier microbiome under “real-world” conventional agricultural conditions. Methods To capture an extensive amount of genetic diversity within maize we grew and sampled the rhizosphere microbiome of a diversity panel of germplasm that included ex-PVP inbreds (Z. mays ssp. mays), ex-PVP hybrids (Z. mays ssp. mays), and teosinte (Z. mays ssp. mexicana and Z. mays ssp. parviglumis). From these samples, we characterized the microbiome, a suite of microbial genes involved in nitrification and denitrification and carried out N-cycling potential assays. Results Here we are showing that populations/genotypes of a single species can vary in their ecological interaction with denitrifers and nitrifers. Some hybrid and teosinte genotypes supported microbial communities with lower potential nitrification and potential denitrification activity in the rhizosphere, while inbred genotypes stimulated/did not inhibit these N-cycling activities. These potential differences translated to functional differences in N2O fluxes, with teosinte plots producing less GHG than maize plots. Conclusion Taken together, these results suggest that Zea genetic variation can lead to changes in N-cycling processes that result in N leaching and N2O production, and thereby are selectable targets for crop improvement. Understanding the underlying genetic variation contributing to belowground microbiome N-cycling into our conventional agricultural system could be useful for sustainability.

Nitrogen fertilization promoted microbial growth and N2O emissions by increasing the abundance of nirS and nosZ denitrifiers in semiarid maize field.

Nitrous oxide (N2O) emissions are a major source of gaseous nitrogen loss, causing environmental pollution. The low organic content in the Loess Plateau region, coupled with the high fertilizer demand of maize, further exacerbates these N losses. N fertilizers play a primary role in N2O emissions by influencing soil denitrifying bacteria, however, the underlying microbial mechanisms that contribute to N2O emissions have not been fully explored. Therefore, the research aimed to gain insights into the intricate relationships between N fertilization, soil denitrification, N2O emissions, potential denitrification activity (PDA), and maize nitrogen use efficiency (NUE) in semi-arid regions. Four nitrogen (N) fertilizer rates, namely N0, N1, N2, and N3 (representing 0, 100, 200, and 300 kg ha-1 yr.-1, respectively) were applied to maize field. The cumulative N2O emissions were 32 and 33% higher under N2 and 37 and 39% higher under N3 in the 2020 and 2021, respectively, than the N0 treatment. N fertilization rates impacted the abundance, composition, and network of soil denitrifying communities (nirS and nosZ) in the bulk and rhizosphere soil. Additionally, within the nirS community, the genera Cupriavidus and Rhodanobacter were associated with N2O emissions. Conversely, in the nosZ denitrifier, the genera Azospirillum, Mesorhizobium, and Microvirga in the bulk and rhizosphere soil reduced N2O emissions. Further analysis using both random forest and structural equation model (SEM) revealed that specific soil properties (pH, NO3--N, SOC, SWC, and DON), and the presence of nirS-harboring denitrification, were positively associated with PDA activities, respectively, and exhibited a significant association to N2O emissions and PDA activities but expressed a negative effect on maize NUE. However, nosZ-harboring denitrification showed an opposite trend, suggesting different effects on these variables. Our findings suggest that N fertilization promoted microbial growth and N2O emissions by increasing the abundance of nirS and nosZ denitrifiers and altering the composition of their communities. This study provides new insights into the relationships among soil microbiome, maize productivity, NUE, and soil N2O emissions in semi-arid regions.

Diluted pyroligneous vinegar promoted Rhododendron growth by changing functional genes involved in N cycling in the rhizosphere

Excessive chemical fertilization in protected horticulture has caused nutrient accumulation, reduction of soil biodiversity, and subsequently soil degradation. Pyroligneous vinegar is a byproduct of pyrolysis for biochar production. While there has been an increase in interest in using pyroligneous vinegar as an amendment to improve soil quality, a paucity of data exists of its effect on the growth of acidophilous plants such as azalea (Rhododendron). To address this, we investigated the changes in the growth of azalea and soil microbial abundance, specifically those involved in nitrogen cycling in the rhizosphere with amendment of conventional inorganic NPK fertilizer (NPK) and four dilutions of pyroligneous vinegar each with matched NPK content. The results showed that the treatments of pyroligneous vinegar increased plant biomass (51%-99%), root length (29%-81%) and surface area (63%-151%), and nitrogen use efficiency (83%-299%) compared to NPK alone. The pyroligneous vinegar treatments decreased soil pH and increased the contents of rhizosphere soil dissolved organic carbon and nitrate, and changed potential denitrification activity. Compared with NPK, pyroligneous vinegar increased archaeal and bacterial amoA genes (AOA and AOB) abundances (45% − 79% and 39% − 75%, respectively) in nitrification, but inhibited the denitrification gene abundances (i.e. nirS and nirK abundance up to 21% lower). Pyroligneous vinegar increased the ratio of nosZ/(nirS + nirK), suggesting an increased potential to lower nitrous oxide emissions.Correlation analysis indicated that nitrogen use efficiency correlated positively with AOA and AOB, but negatively with nirS and nirK abundance. Therefore, our study suggests that pyroligneous vinegar could improve the growth of acidophilous plants and increase their nitrogen use efficiency by changing nitrogen cycling related functional genes, and could be a potential soil amendment in protected horticulture.

Deepened snow in combination with summer warming increases growing season nitrous oxide emissions in dry tundra, but not in wet tundra

Impacts of increased winter snowfall and warmer summer air temperatures on nitrous oxide (N2O) dynamics in arctic tundra are uncertain. Here we evaluate surface N2O dynamics in both wet and dry tundra in West Greenland, subjected to field manipulations with deepened winter snow and summer warming. The potential denitrification activity (PDA) and potential net N2O production (N2Onet) were measured to assess denitrification and N2O consumption potential. The surface N2O fluxes averaged 0.49 ± 0.42 and 2.6 ± 0.84 μg N2O–N m−2 h−1, and total emissions were 212 ± 151 and 114 ± 63 g N2O–N scaled to the entire study area of 0.15 km2, at the dry and wet tundra, respectively. The experimental summer warming, and in combination with deepened snow, significantly increased N2O emissions at the dry tundra, but not at the wet tundra. The deepened snow increased winter soil temperatures and growing season soil N availability (DON, NH4+-N or NO3−-N), but no main effect of deepened snow on N2O fluxes was found at either tundra ecosystem. The mean PDA was 5- and 121-fold higher than the N2Onet at the dry and wet tundra, respectively, suggesting that N2O might be reduced and emitted as dinitrogen (N2). Overall, this study reveals modest but evident surface N2O fluxes from tundra ecosystems in Western Greenland, and suggests that projected increases in winter precipitation and summer air temperatures may increase N2O emissions, particularly at the dry tundra dominating in this region.

Effects of long-term organic fertilizer substitutions on soil nitrous oxide emissions and nitrogen cycling gene abundance in a greenhouse vegetable field

Intensive vegetable fields, characterized by extremely high nitrogen (N) application rates, emit a large amount of the potent greenhouse gas nitrous oxide (N2O). Short-term substitution of organic fertilizers for chemical fertilizers may mitigate N2O emissions from vegetable fields. However, the long-term impacts, particularly on soil background N2O emissions (excluding effects caused by newly applied fertilizers) in vegetable fields, remain poorly elucidated. Thus, we conducted an 11-year experiment to investigate the long-term fertilization effects on soil background N2O emissions and the abundance of N-cycling genes in a greenhouse vegetable field. The soil capacity for potential denitrification activity (PDA) was further measured to evaluate denitrification potential. The field experiment comprised of four treatments: no fertilization (control, CK), only chemical N fertilizers (CF), 50 % of chemical N fertilizers substituted by organic fertilizers (OL), and 87.5 % of chemical fertilizers substituted by organic fertilizers (OH). The long-term application of chemical fertilizers alone (CF) and organic fertilizer substitutions (OL and OH) increased soil background N2O emissions 30-fold and 10-fold, respectively, with average N2O fluxes of 1.2 ± 3.8, 33.8 ± 5.1, 9.4 ± 2.3 and 10.2 ± 2.7 μg N2O-N m−2 h−1 in the CK, CF, OL and OH treatment, respectively. The CF treatment significantly decreased the abundance of AOA amoA, nirS and nosZ genes, while both the OL and OH treatments increased the abundance of AOB amoA, and the OH treatment also increased nosZ gene abundance. Soil pH was a key determinant of the contrasting responses of N-cycling genes to different fertilizer types. Compared with low fertilizer substitutions (OL), high organic fertilizer substitutions (OH) did not result in more soil background N2O emissions, despite higher potential denitrification activities and soil total N contents. This was due to increased abundance of nosZ gene and likely enhanced N2O consumption activities. Soil NO3−-N concentrations, soil pH, and the abundance of nosZ gene were three main factors in controlling the responses of soil background N2O emissions to long-term fertilization. Overall, this study suggests that in the long-term perspective organic fertilizer substitution is a beneficial practice to mitigate N2O emissions from intensive greenhouse vegetable cropping systems.

Soil texture is an easily overlooked factor affecting the temperature sensitivity of N2O emissions

As a potent greenhouse gas, soil nitrous oxide (N2O) is strongly stimulated by rising temperature, triggering a positive feedback effect of global warming. However, its temperature sensitivity varies greatly among soils with different physical and chemical characteristics, while associated mechanisms remain unknown. Here we performed a meta-analysis of the effect of warming on N2O emission and found distinctions in the response of N2O to temperature increase in soils with different textures. Then, we conducted an incubation experiment on 11 arable soils with varying textures sampled across China. The results show that the temperature sensitivity of N2O emissions was lower as soil texture became more clayey and was consistent with the outcome of meta-analysis. Further analysis was conducted by classifying the soils into clay and loam subgroups. As shown in the clay soil subgroup, N2O emission was significantly correlated with both inorganic nitrogen contents and potential denitrification and nitrification activities. Correlation analysis and partial least square (PLS) path model revealed that temperature mediated N2O emission by regulating nosZ gene abundance indirectly. In loam soils, however, the indirect effect of temperature on N2O production was achieved mainly through nirS gene abundance. Additionally, soil DON content strongly correlated with N2O emission in both subgroups and affected N2O emissions by influencing the abundance of denitrifiers under warming conditions. Our findings suggest that (i) soil texture was an important factor affecting temperature sensitivity of N2O emission and (ii) variable efficacy of warming in soil N2O production might originate from the enriching DON and nitrate content and its different indirect effects on nirS- or nosZ-type denitrifiers.

Neonicotinoid pesticide and nitrate mixture removal and persistence in floating treatment wetlands.

Mesocosm and microcosm experiments were conducted to explore the applicability of floating treatment wetlands (FTWs), an ecologically based management technology, to remove neonicotinoid insecticides and nitrate from surface water. The mesocosm experiment evaluated three treatments in triplicate over a 21-d period. Floating treatment wetland mesocosms completely removed nitrate-N over the course of the experiment even when neonicotinoid insecticides were present. At the completion of the experiment, 79.6% of imidacloprid and degradation byproducts and 68.3% of thiamethoxam and degradation byproducts were accounted for in the water column. Approximately 3% of imidacloprid and degradation byproducts and 5.0% of thiamethoxam and degradation byproducts were observed in above-surface biomass, while ∼24% of imidacloprid and degradation byproducts, particularly desnitro imidacloprid, and<0.1% of thiamethoxam and degradation byproducts were found in the below surface biomass. Further, 1 yr after the experiments, imidacloprid, thiamethoxam, and degradation byproducts persisted in biomass but at lower concentrations in both the above- and below-surface biomass. Comparing the microbial communities of mature FTWs grown in the presence and absence of neonicotinoids, water column samples had similar low abundances of nitrifying Archaeal and bacterial amoA genes (below detection to 104 ml-1 ) and denitrifying bacterial nirK, nirS, and nosZ genes (below detection to 105 ml-1 ). Follow-up laboratory incubations found the highest denitrification potential activities in FTW plant roots compared with water column samples, and there was no effect of neonicotinoid addition (100 ng L-1 ) on potential denitrification activity. Based on these findings, (a) FTWs remove neonicotinoids from surface water through biomass incorporation, (b) neonicotinoids persist in biomass long-term(>1 yr after exposure), and (c) neonicotinoids do not adversely affect nitrate-N removal via microbial denitrification.

Grazing Intensity Has More Effect on the Potential Nitrification Activity Than the Potential Denitrification Activity in An Alpine Meadow

On the Qinghai–Tibet Plateau, nitrogen (N) cycling, such as nitrification and denitrification, in the alpine meadow soils have been considerably affected by grazing, with possible consequences for nitrous oxide (N2O) emissions. However, there is a lack of understanding about how the potential nitrification activity (PNA) and the potential denitrification activity (PDA) might be affected by the grazing intensity. We collected the soil samples in alpine meadow in the east of the Qinghai–Tibet Plateau that was grazed at different intensities from 2015 in peak growing season 2021. We determined the soil physical and chemical properties, the functional gene abundances of nitrifiers and denitrifiers, and the soil PNA and PDA to explore the relationships between a range of abiotic and biotic factors and the PNA and PDA. We found that the PNA and the nitrifiers were significantly affected by the grazing intensity but that the PDA and the denitrifiers were not. The ammonia-oxidizing archaea (AOA) abundance was highest but the ammonia-oxidizing bacteria (AOB)abundance was lower than the control significantly at the highest grazing intensity. The AOA abundance and the soil NH4+-N explained most of the variation in the PNA. The pH was the main predictor of the PDA and controlled the nirS abundance but not the nirK and nosZ abundances. Overall, the PNA was more responsive to the grazing intensity than the PDA. These findings can improve estimations of the nitrification and denitrification process and N2O emissions in alpine meadow.

Impact of soil amendments on nitrous oxide emissions and the associated denitrifying communities in a semi-arid environment.

Denitrifying bacteria produce and utilize nitrous oxide (N2O), a potent greenhouse gas. However, there is little information on how organic fertilization treatments affect the denitrifying communities and N2O emissions in the semi-arid Loess Plateau. Here, we evaluated how the denitrifying communities are responsible for potential denitrification activity (PDA) and N2O emissions. A field experiment was conducted with five fertilization treatments, including no fertilization (CK), mineral fertilizer (MF), mineral fertilizer plus commercial organic fertilizer (MOF), commercial organic fertilizer (OFP), and maize straw (MSP). Our result showed that soil pH, soil organic carbon (SOC), and dissolved organic nitrogen (DON) were significantly increased under MSP treatment compared to MF treatment, while nitrate nitrogen (NO3−−N) followed the opposite trend. Organic fertilization treatments (MOF, OFP, and MSP treatments) significantly increased the abundance and diversity of nirS- and nosZ-harboring denitrifiers, and modified the community structure compared to CK treatment. The identified potential keystone taxa within the denitrifying bacterial networks belonged to the distinct genera. Denitrification potentials were significantly positively correlated with the abundance of nirS-harboring denitrifiers, rather than that of nirK- and nosZ-harboring denitrifiers. Random forest modeling and structural equation modeling consistently determined that the abundance, community composition, and network module I of nirS-harboring denitrifiers may contribute significantly to PDA and N2O emissions. Collectively, our findings highlight the ecological importance of the denitrifying communities in mediating denitrification potentials and the stimulatory impact of organic fertilization treatments on nitrogen dynamics in the semi-arid Loess Plateau.

Minimizing tillage modifies fungal denitrifier communities, increases denitrification rates and enhances the genetic potential for fungal, relative to bacterial, denitrification

Nitrous oxide (N2O) emissions from arable soils are predominantly caused by denitrifying microbes, of which fungal denitrifiers are of particular interest, as fungi, in contrast to bacteria, terminate denitrification with N2O. Reduced tillage has been shown to increase gaseous nitrogen losses from soil, but knowledge of how varying tillage regimes and associated soil physical and chemical alterations affect fungal denitrifiers is limited. Based on results from a long-term (>40 years) tillage experiment, we show that non-inversion tillage resulted in increased potential denitrification activity in the upper soil layers, compared to annual or occasional (every 4–5 years) conventional inversion tillage. Using sequence-corrected abundance of the fungal nirK gene, we further identified an increased genetic potential for fungal denitrification, compared to that caused by bacteria, with decreasing tillage intensity. Differences in the composition and diversity of the fungal nirK community imply that different tillage regimes select for distinct fungal denitrifiers with differing functional capabilities and lifestyles, predominantly by altering carbon and nitrogen related niches. Our findings suggest that the creation of organic hotspots through stratification by non-inversion tillage increases the diversity and abundance of fungal denitrifier communities and modifies their composition, and thus their overall relevance for N2O production by denitrification, in arable soils.

Long-term metal pollution shifts microbial functional profiles of nitrification and denitrification in agricultural soils

The increasing contamination of heavy metals in agricultural soils and its impact on the nitrogen (N) cycle and N use efficiency have attracted considerable attention in recent years. In this study, agricultural soils neighboring the Dabaoshan copper mining area (DBS) and Qingyuan electronic-waste recycling area (QY), in Guangdong, China, were sampled to study the interaction between heavy metals and nitrification/denitrification processes, especially the related microbial functional profiles. Results showed that the contamination of heavy metals affected nitrifiers and denitrifiers differently. The potential nitrification activity was about four times lower in metal-polluted soils compared with the unpolluted ones, with a significant decrease in the abundance of amoA and nxrB (p < 0.05) in the polluted samples. On the other hand, the potential denitrification activity was more metal-resistant, which attributed to its complex species composition as shown by a slightly higher α-diversity index, and was slightly higher (p > 0.05) in the polluted samples. Among the five denitrifying genes tested, nosZ gene had the highest increase and the nirK gene the most decrease in numbers and in the polluted soils. The metal-polluted soils had fewer correlations among N functional genes based on the co-occurrence network analysis. In addition, the core taxa of the whole bacterial community changed from copiotrophic to oligotrophic bacteria in the presence of heavy metals. Mantel test indicated that heavy metals were the dominant factors determining N-related genes while the bacterial community composition was due to a combination of heavy metal presence and soil properties such as TOC, NO2−, and pH. It is concluded that long-term heavy metals pollution potentially affected nitrifiers and denitrifiers differently as indicated by the shift in N functional genes and the change in nitrification/denitrification processes.

Potential denitrification activity response to long-term nitrogen fertilization - A global meta-analysis

Denitrification is one of the most important biological process by which reactive N is removed in the biosphere, but the responses of potential denitrification activity (PDA) and associated gene abundances to long-term N fertilization are poorly understood. We compiled data from 1041 observations across 62 studies and quantified the effects of long-term N input on PDA, the denitrification N2O/(N2O+N2) product ratio, and abundances of denitrifying communities using meta-analysis. Boosted regression tree (BRT) analysis showed that soil pH was the most important explanatory variables among which explained the change of PDA, followed by soil organic carbon (SOC) and duration of fertilization. Long-term N fertilization significantly increased potential denitrification activity by 75.9% and increased the relative abundances of nirK, nirS, and nosZ gene copies by 60.4%, 77.0%, and 25.1%, respectively. Further, long-term N loading increased the denitrification N2O/(N2O+N2) ratio by 22.1% and nir(K+S)/nosZ ratios by 27.7%, respectively. The effect of long-term N fertilization on potential denitrification activity was positively correlated with SOC (R2 = 0.11; n = 132; P < 0.001), while long-term N loading increased the SOC by 29.0% compared to the unfertilized control. The responses of nirK, nirS, and nosZ abundances to long-term N fertilization were positively correlated with soil pH, whereas N loading decreased the soil pH by 4.6%. Thus, we postulate that long-term N fertilization might have a double effect on the activities and populations of denitrifying communities due to the increased SOC and decreased soil pH.

Biological Indicators of Soil Condition on the Kabanyolo Experimental Field, Uganda

Soil biological activity is an integral characteristic reflecting the state of soil fertility, biodiversity, and the activity of soil processes carried out by soil organisms. In Africa, studies of soil biological properties are few compared to the agrochemical research. In this paper, we present an assessment of multiple biochemical and microbiological properties of soil from an agricultural field located in the African tropical savanna. We determined basal respiration, substrate-induced respiration, C of microbial biomass, the potential activity of denitrification, nitrogen fixation activity, and estimated prokaryotic components in the soil microbial complex by quantitative PCR. Basal respiration of soils ranged from 0.77 ± 0.04 to 1.90 ± 0.23 μg C-CO2·g−1·h−1, and substrate-induced respiration ranged from 3.31 ± 0.17 to 7.84 ± 1.04 μg C-CO2·g−1·h−1. The C reserves of microbial biomass averaged 403.7 ± 121.6 μg C·g−1 of soil. The N2O emission from the upper layer on average amounted to 2.79 ng N-N2O·g−1·day−1, and the potential denitrification activity reached 745 ± 98 ng N-N2O·g−1·h−1. The number of copies of bacterial genes varied from (0.19 ± 0.02) × 108 to (3.52 ± 0.8) × 108 copies·g−1, and of archaea—from (0.10 ± 0.01) × 107 to (0.29 ± 0.01) × 107 copies·g−1 of soil. These results were in good agreement with the studies in other seasonally wet tropical regions: the biological activity was relatively low. The difference between biological indicators of the experimental field and the reference profile were insignificant except for nitrogen loss, which was higher in the ploughed field. Biological indicators strongly varied in space; we explained their heterogeneity by non-uniform management practices in the course of agrochemical field experiments in the past. The use of organic fertilisers may cause the release of climatically active gases due to intensive microbial respiration and denitrification, but the intensity of emission would strongly depend on the cultivation and management method.

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.

Characterization of nirS- and nirK-containing communities and potential denitrification activity in paddy soil from eastern China

Denitrification is important for nitrogen balance in agricultural ecosystems. It is well established that the abundance and structure of denitrifier are strongly influenced by environmental factors. However, knowledge of how nirS- and nirK-harboring microbial communities vary in paddy soils of different climatic regions is lacking, along with how these microbes are associated with denitrification potential. In this study, forty paddy soils from tropical, sub-tropical, warm-temperate, and mid-temperate climate zones were collected. The results showed that soils from the sub-tropical zone had the highest denitrification rates. The abundances of nirS and nirK had a strong negative association with C/N ratio and clearly varied among the four climate zones. Variation in nirS- and nirK-containing communities existed across sampling sites from each climate zone in terms of α- and β-diversity. Soil pH and climate factors significantly affected community diversity. Network analysis revealed that different climate regions had similar keystone taxa like Azospira, and Achromobacter, which were significantly related to denitrification rates. Structural equation modeling indicated that the differences in denitrifying enzyme activity among the four climate zones were mostly explained by climate factors, soil pH, and nirS biodiversity. Specifically, the biodiversity of nirS was more important than that of nirK in regulating potential denitrification activity in paddy soil, suggesting that nirS-type denitrifiers may have high activity under anaerobic conditions. Our results allow a deeper insight into the relative contribution of nirS- and nirK- containing communities to the soil denitrification activity in paddy soils across climate zones. This study highlighted the need of manipulation experiment to explain how denitrifier biodiversity affect potential denitrification activity in the future.

Deepened snow enhances gross nitrogen cycling among Pan-Arctic tundra soils during both winter and summer

Many Arctic regions currently experience an increase in winter snowfall as a result of climate change. Deepened snow can enhance thermal insulation of the underlying soil during winter, resulting in warmer soil temperatures that promote soil microbial nitrogen (N)-cycle processes and the availability of N and other nutrients. We conducted an ex situ study comparing the effects of deepened snow (using snow fences that have been installed for 3–13 years) on microbial N-cycle processes in late summer (late growing season) and winter (late snow-covered season) among five tundra sites in three different geographic locations across the Arctic (Greenland (dry and wet tundra), Canada (mesic tundra), and Svalbard, Norway (heath and meadow tundra)). Soil gross N cycling rates (mineralization, nitrification, immobilization of ammonium (NH4+) and nitrate (NO3−), and denitrification) were determined using a15N pool dilution. Potential denitrification activity (PDA) and nitrous oxide reductase activity (N2OR) were measured to assess denitrifying enzyme activities.The deepened snow treatment across all sites had a significant effect of the potential soil capacity of accelerating N cycling rates in late winter, including quadrupled gross nitrification, tripled NO3−-N immobilization, and doubled denitrification as well as significantly enhanced late summer gross N mineralization, denitrification (two-fold) and NH4+-N availability. The increase in gross N mineralization and nitrification rates were primarily driven by the availability of dissolved organic carbon (DOC) and nitrogen (DON) across sites. The largest increases in winter DOC and DON concentrations due to deepened snow were observed at the two wetter sites (wet and mesic tundra), and N cycling rates were also more strongly affected by deepened snow at these two sites than at the three other drier sites. Together, these results suggest that the potential effects of deepened winter snow in stimulating microbial N-cycling activities will be most pronounced in relatively moist tundra ecosystems. Hence, this study provides support to prior observations that growing season biogeochemical cycles in the Arctic are sensitive to snow depth with altered nutrient availability for microorganisms and vegetation. It can be speculated that on the one hand growing season N availability will increase and promote plant growth, but on the other hand foster increased water- and gaseous (e.g. N2 and N2O) N-losses with implications for overall nutrient status.

Nitrous oxide surface fluxes in a low Arctic heath: Effects of experimental warming along a natural snowmelt gradient

Climate change is profound in the Arctic where increased snowfall during winter and warmer growing season temperatures may accelerate soil nitrogen (N) turnover and increase inorganic N availability. Nitrous oxide (N2O) is a potent greenhouse gas formed by soil microbes and in the Arctic, the production is seen as limited mainly by low inorganic N availability. Hence, it can be hypothesized that climate change in the Arctic may increase total N2O emissions, yet this topic remains understudied. We investigated the combined effects of variable snow depths and experimental warming on soil N cycling in a factorial field study established along a natural snowmelt gradient in a low Arctic heath ecosystem. The study assessed N2O surface fluxes, gross N mineralization and nitrification rates, potential denitrification activity, and the pools of soil microbial, soil organic and soil inorganic N, carbon (C) and phosphorus (P) during two growing seasons. The net fluxes of N2O averaged 1.7 μg N2O–N m−2 h−1 (range −3.6 to 10.5 μg N2O–N m−2 h−1), and generally increased from ambient (1 m) to moderate (2–3 m) snow depths. At the greatest snow depth (4 m) where snowmelt was profoundly later, N2O fluxes decreased, likely caused by combined negative effects of low summer temperatures and high soil moisture. Positive correlations between N2O and nitrate (NO3−) and dissolved organic N (DON) suggested that the availability of N was the main controlling variable along the snowmelt gradient. The maximum N2O fluxes were observed in the second half of August associated with high NO3− concentrations. The effect of growing season experimental warming on N2O surface flux varied along the snowmelt gradient and with time. Generally, the experimental warming stimulated N2O fluxes under conditions with increased concentrations of inorganic N. In contrast, warming reduced N2O fluxes when inorganic N was low. Experimental warming had no clear effects on soil inorganic N. The study suggests that if increased winter precipitation leads to a deeper snow cover and a later snowmelt, total emissions of N2O from low Arctic heath ecosystems may be enhanced in the future and, dependent on dissolved N availability, summer warming may stimulate or reduce total emissions.

No-Till and Solid Digestate Amendment Selectively Affect the Potential Denitrification Activity in Two Mediterranean Orchard Soils

Improved soil managements that include reduced soil disturbance and organic amendment incorporation represent valuable strategies to counteract soil degradation processes that affect Mediterranean tree cultivations. However, changes induced by these practices can promote soil N loss through denitrification. Our research aimed to investigate the short-term effects of no-tillage and organic amendment with solid anaerobic digestate on the potential denitrification in two Mediterranean orchard soils showing contrasting properties in terms of texture and pH. Denitrifying enzyme activity (DEA) and selected soil variables (available C and N, microbial biomass C, basal respiration) were monitored in olive and orange tree orchard soils over a five-month period. Our results showed that the application of both practices increased soil DEA, with dynamics that varied according to the soil type. Increased bulk density, lowered soil aeration, and a promoting effect on soil microbial community growth were the main DEA triggers under no-tillage. Conversely, addition of digestate promoted DEA by increasing readily available C and N with a shorter effect in the olive grove soil, due to greater sorption and higher microbial efficiency, and a long-lasting consequence in the orange orchard soil related to a larger release of soluble substrates and their lower microbial use efficiency.

Long term combined fertilization and soil aggregate size on the denitrification and community of denitrifiers

Balanced fertilization strategies that combine the use of organic and inorganic fertilizers have been adopted to increase soil fertility and crop yield and to safeguard environment. However, knowledge on the influence of these fertilization strategies on microbe-mediated nitrogen cycling in the agricultural ecosystems remains limited. We analyzed the denitrifiers with nitrite reductase genes (nirK and nirS) in three soil aggregate classes (large macroaggregates, >2 mm; small macroaggregates, 0.25–2 mm; and microaggregates, <0.25 mm) in response to long-term application of inorganic fertilizers (NPK), NPK plus straw (NPKS), and NPK plus manure (NPKM) in the fluvo-aquic soil of North China Plain. Both fertilization and aggregate size had significant effects on the nirK and nirS gene abundances and soil potential denitrification activity (PDA). PDA in the combined fertilizer-treated soils (NPKS and NPKM treatments) was two times more than that in inorganic NPK-treated soil. PDA dramatically increased in small macroaggregates and microaggregates of NPKS treatment. The nirS gene abundance was significantly higher than that of the nirK gene abundance, and NPKS and NPKM treatments increased the nirS to nirK ratio in bulk soils and in microaggregates compared with NPK treatment and control. Correlation analysis and automatic linear modeling revealed a primary correlation between PDA and abundance of nirS-type denitrifiers. Soil properties, such as SOC, pH and TN, significantly affected the community structure of nirK- and nirS-type denitrifiers while only aggregate size significantly influenced the composition of nirK-type denitrifiers. Taken together, our results suggest that long-term combined fertilization increased the denitrification potential in small macroaggregate and microaggregate classes, and the increased gene abundance and the shift community structures of nirS- and nirK-type denitrifiers were potentially responsible for an increase in the promoted denitrification.

Contribution of co-denitrification to nitrous oxide production from subsurface wastewater infiltration system

Sewage treatment is one of the important sources of nitrous oxide (N2O) production, understanding its microbial process is fundamental for reducing N2O emissions. In this study, the stable isotope tracer method was used to reveal the contribution of co-denitrification to the release of N2O in addition to nitrification and denitrification in subsurface wastewater infiltration system (SWIS). The effects of nitrate addition modes were also explored. The results indicated that co-denitrification, significantly affected by dissolved oxygen, pH and potential denitrification activity, had to be taken into account in N2O formation. When the addition of nitrate nitrogen changed from one-time to intermittent, the contribution of co-denitrification increased from (1.87 ± 0.3) % to (34.2 ± 3.8) %. As a result, N2O emission and conversion rate increased from (5.17 ± 1.4) × 10−3 mg/m2·h to (6.77 ± 1.7) × 10−3 mg/m2·h and from (0.17 ± 0.02) % to (0.232 ± 0.01) %, respectively. The results also suggested that, given the increasing scale of SWIS applications, more attention should be paid to the nitrogen cycle in the system. To effectively mitigate the generation and conversion of N2O, measures to reduce nitrate accumulation need to be taken.