Tillage system affects fertilizer-induced nitrous oxide emissions

Since the development of effective N₂O mitigation options is a key challenge for future agricultural practice, we studied the interactive effect of tillage systems on fertilizer-derived N₂O emissions and the abundance of microbial communities involved in N₂O production and reduction. Soil samples from 0–10 cm and 10–20 cm depth of reduced tillage and ploughed plots were incubated with dairy slurry (SL) and manure compost (MC) in comparison with calcium ammonium nitrate (CAN) and an unfertilized control (ZERO) for 42 days. N₂O and CO₂ fluxes, ammonium, nitrate, dissolved organic C, and functional gene abundances (16S rRNA gene, nirK, nirS, nosZ, bacterial and archaeal amoA) were regularly monitored. Averaged across all soil samples, N₂O emissions decreased in the order CAN and SL (CAN?=?748.8?±?206.3, SL?=?489.4?±?107.2 μg kg^?1) followed by MC (284.2?±?67.3 μg kg^?1) and ZERO (29.1?±?5.9 μg kg^?1). Highest cumulative N₂O emissions were found in 10–20 cm of the reduced tilled soil in CAN and SL. N₂O fluxes were assigned to ammonium as source in CAN and SL and correlated positively to bacterial amoA abundances. Additionally, nosZ abundances correlated negatively to N₂O fluxes in the organic fertilizer treatments. Soils showed a gradient in soil organic C, 16S rRNA, nirK, and nosZ with greater amounts in the 0–10 than 10–20 cm layer. Abundances of bacterial and archaeal amoA were higher in reduced tilled soil compared to ploughed soils. The study highlights that tillage system induced biophysicochemical stratification impacts net N₂O emissions within the soil profile according to N and C species added during fertilization.     

Abundance of transcripts of functional gene reflects the inverse relationship between CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions during mid-season drainage in acidic paddy soil

Agricultural management significantly affects methane (CH₄) and nitrous oxide (N₂O) emissions from paddy fields. However, little is known about the underlying microbiological mechanism. Field experiment was conducted to investigate the effect of the water regime and straw incorporation on CH₄ and N₂O emissions and soil properties. Quantitative PCR was applied to measure the abundance of soil methanogens, methane-oxidising bacteria, nitrifiers, and denitrifiers according to DNA and mRNA expression levels of microbial genes, including mcrA, pmoA, amoA, and nirK/nirS/nosZ. Field trials showed that the CH₄ and N₂O flux rates were negatively correlated with each other, and N₂O emissions were far lower than CH₄ emissions. Drainage and straw incorporation affected functional gene abundance through altered soil environment. The present (DNA-level) gene abundances of amoA, nosZ, and mcrA were higher with straw incorporation than those without straw incorporation, and they were positively correlated with high concentrations of soil exchangeable NH₄⁺ and dissolved organic carbon. The active (mRNA-level) gene abundance of mcrA was lower in the drainage treatment than in continuous flooding, which was negatively correlated with soil redox potential (Eh). The CH₄ flux rate was significantly and positively correlated with active mcrA abundance but negatively correlated with Eh. The N₂O flux rate was significantly and positively correlated with present and active nirS abundance and positively correlated with soil Eh. Thus, we demonstrated that active gene abundance, such as of mcrA for CH₄ and nirS for N₂O, reflects the contradictory relationship between CH₄ and N₂O emissions regulated by soil Eh in acidic paddy soils.     

Contrasting response of two grassland soils to N addition and moisture levels: N<Subscript>2</Subscript>O emission and functional gene abundance

Purpose

Nitrification and denitrification processes dominate nitrous oxide (N₂O) emission in grassland ecosystems, but their relative contribution as well as the abiotic factors are still not well understood.

Materials and methods

Two grassland soils from Duolun in Inner Mongolia, China, and Canterbury in New Zealand were used to quantitatively compare N₂O production and the abundance of bacterial and archaeal amoA, denitrifying nirK and nirS genes in response to N additions (0 and 100 μg NH₄ ⁺–N g^?1 dry soil) and two soil moisture levels (40 and 80 % water holding capacity) using microcosms.

Results and discussion

Soil moisture rather than N availability significantly increased the nitrification rate in the Duolun soil but not in the Canterbury soil. Moreover, N addition promoted denitrification enzyme activities in the Canterbury soil but not in the Duolun soil. The abundance of bacterial and archaeal amoA genes significantly increased as soil moisture increased in the Duolun soil, whereas in the Canterbury soil, only the abundance of bacterial amoA gene increased. The increase in N₂O flux induced by N addition was significantly greater in the Duolun soil than in the Canterbury soil, suggesting that nitrification may have a dominant role in N₂O emission for the Duolun soil, while denitrification for the Canterbury soil.

Conclusions

Microbial processes controlling N₂O emission differed in grassland soils, thus providing important baseline data in terms of global change.

     

Spatial patterns of microbial denitrification genes change in response to poultry litter placement and cover crop species in an agricultural soil

Subsurface-banding manure and winter cover cropping are farming techniques designed to reduce N loss. Little is known, however, about the effects of these management tools on denitrifying microbial communities and the greenhouse gases they produce. Abundances of bacterial (16S), fungal (ITS), and denitrification genes (nirK, nirS, nosZ-I, and nosZ-II) were measured in soil samples collected from a field experiment testing the combination of cereal rye and hairy vetch cover cropping with either surface-broadcasted or subsurface-banded poultry litter. The spatial distribution of genes was mapped to identify potential denitrifier hotspots. Spatial distribution maps showed increased 16S rRNA genes around the manure band, but no denitrifier hotspots. Soil depth and nitrate concentration were the strongest drivers of gene abundance, but bacterial gene abundance also differed by gene, soil characteristics, and management methods. Gene copy number of nirK was higher under cereal rye than hairy vetch and positively associated with soil moisture, while nirS gene copies did not differ between cover crop species. The nirS gene copies increased when manure was surface broadcasted compared to subsurface banded and was positively associated with pH. Soil moisture and pH were positively correlated to nosZ-II but not to nosZ-I gene copy numbers. We observed stronger correlations between nosZ-I and nirS, and nosZ-II and nirK gene copies compared to the reverse pairings. Agricultural management practices differentially affect spatial distributions of genes coding for denitrification enzymes, leading to changes in the composition of the denitrifying community.     

Microbial pathways for nitrous oxide emissions from sheep urine and dung in a typical steppe grassland

The aim of this study was to determine the responses of nitrifiers and denitrifiers to understand microbial pathways of nitrous oxide (N₂O) emissions in grassland soils that received inputs of sheep excreta. Sheep dung and synthetic sheep urine were applied at three different rates, simulating a single, double, or triple overlapping of urine or dung depositions in the field. Quantitative PCR and high-throughput sequencing were combined with process-based modeling to understand effects of sheep excreta on microbial populations and on pathways for N₂O production. Results showed that emissions of N₂O from urine were significantly higher than from dung, ranging from 0.12 to 0.78 kg N₂O-N ha^?1 during the 3 months. The N₂O emissions were significantly related to the bacterial amoA (r?=?0.373, P?<?0.001) and nirK (r?=?0.614, P?<?0.001) gene abundances. It was autotrophic nitrification that dominated N₂O production in the low urine-N rate soils, whereas it was denitrification (including nitrifier denitrification and heterotrophic denitrification) that dominated N₂O production in the high urine-N rate soils. Nitrifier denitrification was responsible for most of the N₂O emissions in the dung-treated soils. This study suggests that nitrifier denitrification is indeed an important pathway for N₂O emissions in these low fertility and dry grazed grassland ecosystems.     

Responses of soil nitrous oxide production and abundances and composition of associated microbial communities to nitrogen and water amendment

Soil moisture and nitrogen (N) are two important factors influencing N₂O emissions and the growth of microorganisms. Here, we carried out a microcosm experiment to evaluate effects of soil moisture level and N fertilizer type on N₂O emissions and abundances and composition of associated microbial communities in the two typical arable soils. The abundances and community composition of functional microbes involved in nitrification and denitrification were determined via quantitative PCR (qPCR) and terminal restriction length fragment polymorphism (T-RFLP), respectively. Results showed that N₂O production was higher at 90% water-filled pore (WFPS) than at 50% WFPS. The N₂O emissions in the two soils amended with ammonium were higher than those amended with nitrate, especially at relatively high moisture level. In both soils, increased soil moisture stimulated the growth of ammonia-oxidizing bacteria (AOB) and nitrite reducer (nirK). Ammonium fertilizer treatment increased the population size of AOB and nirK genes in the alluvial soil, while reduced the abundances of ammonia-oxidizing archaea (AOA) and denitrifiers (nirK and nosZ) in the red soil. Nitrate addition had a negative effect on AOA abundance in the red soil. Total N₂O emissions were positively correlated to AOB abundance, but not to other functional genes in the two soils. Changed soil moisture significantly affected AOA rather than AOB community composition in both soils. The way and extent of N fertilizers impacted on nitrifier and denitrifier community composition varied with N form and soil type. These results indicate that N₂O emissions and the succession of nitrifying and denitrifying communities are selectively affected by soil moisture and N fertilizer form in the two contrasting types of soil.     

Effect of P stoichiometry on the abundance of nitrogen-cycle genes in phosphorus-limited paddy soil

Previous studies have shown that phosphorus addition to P-limited soils increases gaseous N loss. A possible explanation for this phenomenon is element stoichiometry (specifically of C:N:P) modifying linked nutrient cycling, leading to enhanced nitrification and denitrification. In this study, we investigated how P stoichiometry influenced the dynamics of soil N-cycle functional genes. Rice seedlings were planted in P-poor soils and incubated with or without P application. Quantitative PCR was then applied to analyze the abundance of ammonia-oxidizing (amoA) and denitrifying (narG nirK, nirS, nosZ) genes in soil. P addition reduced bacterial amoA abundance but increased denitrifying gene abundance. We suggest this outcome is due to P-induced shifts in soil C:P and N:P ratios that limited ammonia oxidization while enhancing P availability for denitrification. Under P application, the rhizosphere effect raised ammonia-oxidizing bacterial abundance (amoA gene) and reduced nirK, nirS, and nosZ in rhizosphere soils. The change likely occurred through greater C input and O₂ release from roots, thus altering C availability and redox conditions for microbes. Our results show that P application enhances gaseous N loss potential in paddy fields mainly through stimulating denitrifier growth. We conclude that nutrient availability and elemental stoichiometry are important in regulating microbial gene responses, thereby influencing key ecosystem processes such as denitrification.

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Graphical abstract ?

     

Straw amendment to paddy soil stimulates denitrification but biochar amendment promotes anaerobic ammonia oxidation

Purpose

Organic matter amendment is usually used to improve soil physicochemical properties and to sequester carbon for counteracting climate change. There is no doubt that such amendment will change microbial activity and soil nitrogen transformation processes. However, the effects of straw and biochar amendment on anammox and denitrification activity and on community structure in paddy soil are unclear.

Materials and methods

We conducted a 30-day pot experiment using rice straw and rice straw biochar to deepen our understanding about the activity, microbial abundance, and community structure associated with soil nitrogen cycling during rice growth.

Results and discussion

Regarding activity, anammox contributed 3.1–8.1% of N₂ production and denitrification contributed 91.9–96.9% of N₂ production; straw amendment resulted in the highest denitrification rate (38.9 nmol N g^?1 h^?1), while biochar amendment resulted in the highest anammox rate (1.60 nmol N g^?1 h^?1). Both straw and biochar amendments significantly increased the hzsB and nosZ gene abundance (p < 0.05). Straw amendment showed the highest nosZ gene abundance, while biochar amendment showed the highest hzsB gene abundance. Phylogenetic analysis of the anammox bacteria 16S rRNA genes indicated that Candidatus Brocadia and Kuenenia were the dominant genera detected in all treatments.

Conclusions

Straw and biochar amendments have different influences on anaerobic ammonia oxidation and denitrification within paddy soil. Our results suggested that the changes in denitrification and anammox rates in the biochar and straw treatments were mainly linked to functional gene abundance rather than microbial community structure and that denitrification played the more major role in N₂ production in paddy soil.

     

Distinct effect of nitrogen fertilisation and soil depth on nitrous oxide emissions and nitrifiers and denitrifiers abundance

The nitrous oxide and molecular N emissions from 5-cm length subsamples taken from 20-cm length sample corers containing eutric Cambisol soil fertilised either with urea, ammonium or nitrate for 1 year have been examined using gas chromatography. At the beginning of the incubation, the same N rate (260 kg N/ha) was added to the soil and kept constant during the experiment. The total abundance of the soil Bacteria and Archaea and that of nitrifiers and denitrifiers was estimated by quantitative PCR of the corresponding biotic variables 16S rRNA, amoA and napA, narG, nirK, nirS, norB, nosZI and nosZII genes. The abiotic variables dissolved oxygen, pH, exchangeable NH₄⁺-N and NO₃^?-N contents and total C and total N were also analysed. None of the three fertilisers affected the total abundance of Bacteria and Archaea and nitrification was the main driver of nitrous oxide production in the 0- to 5-cm and 5- to 10-cm soil layers while denitrification was in the 10- to 15-cm and 15- to 20-cm soil horizons. Parallel to the reduction in the content of dissolved oxygen along the soil profile, there was a decrease in the total and relative abundance of the bacterial and archaeal amoA gene and an increase in the abundances of the denitrification genes, mainly in the 10- to 15-cm and 15- to 20-cm soil layers. A non-metric multidimensional scaling plot comparing the biotic and abiotic variables examined in each of the four 5-cm soil subsamples and the whole 20-cm sample showed a disparate effect of N fertilisation on N gas emissions and abundance of nitrifiers and denitrifiers bacterial and archaeal communities.     

Management practices have a major impact on nitrifier and denitrifier communities in a semiarid grassland ecosystem

Purpose

Nitrification and denitrification, two of the key nitrogen (N) transformation processes in the soil, are carried out by a diverse range of microorganisms and catalyzed by a series of enzymes. Different management practices, such as continuous grazing, mowing, and periodic fencing off from grazing, dramatically influenced grassland ecosystems. This study aimed to examine the effects of management practices on the abundance and community structure of nitrifier and denitrifier communities in grassland ecosystems.

Materials and methods

Soil samples were collected from a semiarid grassland ecosystem in Xilingol region, Inner Mongolia, where long-term management practices including free-grazing, different periods of enclosure from grazing, and different frequencies of mowing were conducted. Real-time quantitative polymerase chain reaction (Q-PCR), denaturing gradient gel electrophoresis (DGGE), sequencing, and phylogenetic analysis were applied to estimate the abundance and composition of amoA, nirS, nirK, and nosZ genes.

Results and discussion

The ammonia-oxidizing archaea (AOA) amoA copies were in the range 5.99?×?10⁸ to 8.60?×?10⁸, while those of ammonia-oxidizing bacteria (AOB) varied from 3.02?×?10⁷ to 4.61?×?10⁷. The abundance of AOA was substantially higher in the light grazing treatment (LG) than in the mowing treatments. The quantity and intensity of DGGE bands of AOA varied with pasture management. In stark contrast, AOB population abundance and community structure remained largely unchanged in all the soils irrespective of the management practices. All these results suggested that ammonia oxidizers were dominated by AOA. The higher gene abundance and greater intensity of DGGE bands of nirS and nosZ under the enclosure treatments would suggest greater stimulated denitrification. The ratio of nosZ/(nirS?+?nirK) was higher in mowing treatments than in the free-grazing and enclosure treatments, possibly leading to more complete denitrification. Correlation analysis indicated that soil moisture and inorganic nitrogen content were the two main soil environmental variables that influence the community structure of nitrifiers and denitrifiers.

Conclusions

In this semiarid neutral to alkaline grassland ecosystem under low temperature conditions, AOA mainly affiliated with Nitrososphaera dominated nitrification. These results clearly demonstrate that grassland management practices can have a major impact on nitrifier and denitrifier communities in this semiarid grassland ecosystem, under low temperature conditions.

     

Nitrous oxide (N<Subscript>2</Subscript>O)-reducing denitrifier-inoculated organic fertilizer mitigates N<Subscript>2</Subscript>O emissions from agricultural soils

The only known sink for nitrous oxide (N₂O) is biochemical reduction to dinitrogen (N₂) by N₂O reductase (N₂OR). We hypothesized that the application of N₂O-reducing denitrifier-inoculated organic fertilizer could enhance soil N₂O consumption while the disruption of nosZ genes could result in inactivation of N₂O consumption. To test such hypotheses, a denitrifier-inoculated granular organic fertilizer was applied to both soil microcosms and fields. Of 41 denitrifier strains, 38 generated ³⁰N₂ in the end products of denitrification (³⁰N₂ and ⁴⁶N₂O) after the addition of Na¹⁵NO₃ in culture condition, indicating their high N₂O reductase activities. Of these 41 strains, 18 were screened in soil microcosms after their inoculation into the organic fertilizer, most of which were affiliated with Azospirillum and Herbaspirillum. These 18 strains were nutritionally starved to improve their survival in soil, and 14 starved and/or non-starved strains significantly decreased N₂O emissions in soil microcosms. However, the N₂O emission had not been decreased in soil microcosms after inoculating with a nosZ gene-disruptive strain, suggesting that N₂O reductase activity might be essential for N₂O consumption. Although the decrease of N₂O was not significant at field scales, the application of organic fertilizer inoculated with Azospirillum sp. TSH100 and Herbaspirillum sp. UKPF54 had decreased the N₂O emissions by 36.7% in Fluvisol and 23.4% in Andosol in 2014, but by 21.6% in Andosol in 2015 (H. sp. UKPF54 only). These results suggest that the application of N₂O-reducing denitrifier-inoculated organic fertilizer may enhance N₂O consumption or decrease N₂O emissions in agricultural soils.     

Chemical nature of soil organic carbon under different long-term fertilization regimes is coupled with changes in the bacterial community composition in a Calcaric Fluvisol

Fertilization is an important factor influencing the chemical structure of soil organic carbon (SOC) and soil microbial communities; however, whether any connection exists between the two under different fertilization regimes remains unclear. Soils from a 27-year field experiment were used to explore potential associations between SOC functional groups and specific bacterial taxa, using quantitative multiple cross-polarization magic-angle spinning ¹³C nuclear magnetic resonance and 16S rRNA gene sequencing. Treatments included balanced fertilization with organic materials (OM) and with nitrogen (N), phosphorus (P), and potassium (K) mineral fertilizers (NPK); unbalanced fertilization without one of the major elements (NP, PK, or NK); and an unamended control. These treatments were divided into four distinct groups, namely OM, NPK, NP plus PK, and NK plus control, according to their bacterial community composition and SOC chemical structure. Soil total P, available P, and SOC contents were the major determinants of bacterial community composition after long-term fertilization. Compared to NPK, the OM treatment generated a higher aromatic C–O and OCH₃ and lower alkyl C and OCH abundance, which were associated with the enhanced abundance of members of the Acidobacteria subgroups 6 and 5, Cytophagaceae, Chitinophagaceae, and Bacillus sp.; NP plus PK treatments resulted in a higher OCH and lower aromatic C–C abundance, which showed a close association with the enrichment of unclassified Chloracidobacteria, Syntrophobacteraceae, and Anaerolineae and depletion of Bacillales; and NK plus control treatments resulted in a higher abundance of aromatic C–C, which was associated with the enhanced abundance of Bacillales. Our results indicate that different fertilization regimes changed the SOC chemical structure and bacterial community composition in different patterns. The results also suggest that fertilization-induced variations in SOC chemical structure were strongly associated with shifts in specific microbial taxa which, in turn, may be affected by changes in soil properties.     

Response of ammonia oxidizers and denitrifiers to repeated applications of a nitrification inhibitor and a urease inhibitor in two pasture soils

Purpose

The nitrification inhibitor 3,4-dimethylpyrazol-phosphate (DMPP) and the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) can mitigate N losses through reducing nitrification and ammonia volatilization, respectively. However, the impact of repeated applications of these inhibitors on nitrogen cycling microorganisms is not well documented. This study aimed to investigate the changes in the abundance and community structure of the functional microorganisms involved in nitrification and denitrification in Australian pasture soils after repeated applications of DMPP and nBTPT.

Materials and methods

Soil was collected in autumn and spring, 2014 from two pasture sites where control, urea, urea ammonium nitrate, and urea-coated inhibitors had been repeatedly applied over 2 year. Soil samples were analyzed to determine the potential nitrification rates (PNRs), the abundances of amoA, narG, nirK and bacterial 16S rRNA genes, and the community structure of ammonia oxidizers.

Results and discussion

Two years of urea application resulted in a significantly lower soil pH at Terang and a significant decrease in total bacterial 16S rRNA gene abundance at Glenormiston and led to significantly higher PNRs and abundances of ammonia oxidizers compared to the control. Amendment with either DMPP or nBTPT significantly decreased PNRs and the abundance of amoA and narG genes. However, there was no fertilizer- or inhibitor-induced change in the community structure of ammonia oxidizers.

Conclusions

These results suggest that there were inhibitory effects of DMPP and nBTPT on the functional groups mediating nitrification and denitrification, while no significant impact on the community structure of ammonia oxidizers was observed. The application of nitrification or urease inhibitor appears to be an effective approach targeting specific microbial groups with minimal effects on soil pH and the total bacterial abundance.

     

Nitrogen transformation rates and N<Subscript>2</Subscript>O producing pathways in two pasture soils

Purpose

Better understanding of N transformations and the regulation of N₂O-related N transformation processes in pasture soil contributes significantly to N fertilizer management and development of targeted mitigation strategies.

Materials and methods

¹⁵N tracer technique combined with acetylene (C₂H₂) method was used to measure gross N transformation rates and to distinguish pathways of N₂O production in two Australian pasture soils. The soils were collected from Glenormiston (GN) and Terang (TR), Victoria, Australia, and incubated at a soil moisture content of 60% water-filled pore space (WFPS) and at temperature of 20 °C.

Results and discussion

Two tested pasture soils were characterized by high mineralization and immobilization turnover. The average gross N nitrification rate (n_tot) was 7.28 mg N kg^?1 day^?1 in TR soil () and 5.79 mg N kg^?1 day^?1 in GN soil. Heterotrophic nitrification rates (n_h), which accounting for 50.8 and 41.9% of n_tot, and 23.4 and 30.1% of N₂O emissions in GN and TR soils, respectively, played a role similar with autotrophic nitrification in total nitrification and N₂O emission. Denitrification rates in two pasture soils were as low as 0.003–0.004 mg N kg^?1 day^?1 under selected conditions but contributed more than 30% of N₂O emissions.

Conclusions

Results demonstrated that two tested pasture soils were characterized by fast N transformation rates of mineralization, immobilization, and nitrification. Heterotrophic nitrification could be an important NO₃^?–N production transformation process in studied pasture soils. Except for autotrophic nitrification, roles of heterotrophic nitrification and denitrification in N₂O emission in two pasture soils should be considered when developing mitigation strategies.

     

Application of compost and clay under water-stressed conditions influences functional diversity of rhizosphere bacteria

Applications of compost and clay to ameliorate soil constraints such as water stress are potential management strategies for sandy agricultural soils. Water repellent sandy soils in rain-fed agricultural systems limit production and have negative environmental effects associated with leaching and soil erosion. The aim was to determine whether compost and clay amendments in a sandy agricultural soil influenced the rhizosphere microbiome of Trifolium subterraneum under differing water regimes. Soil was amended with compost (2% w/w), clay (5% w/w) and a combination of both, in a glasshouse experiment with well-watered and water-stressed (70 and 35% field capacity) treatments. Ion Torrent 16S rRNA sequencing and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis of functional gene prediction were used to characterise the rhizosphere bacterial community and its functional component involved in nitrogen (N) cycling and soil carbon (C) degradation. Compost soil treatments increased the relative abundance of copiotrophic bacteria, decreased labile C and increased the abundance of recalcitrant C degrading genes. Predicted N cycling genes increased with the addition of clay (N₂ fixation, nitrification, denitrification) and compost + clay (N₂ fixation, denitrification) and decreased with compost (for denitrification) amendment. Water stress did not alter the relative abundance of phylum level taxa in the presence of compost, although copiotrophic Actinobacteria increased in relative abundance with addition of clay and with compost + clay. A significant role of compost and clay under water stress in influencing the composition of rhizosphere bacteria and their implications for N cycling and C degradation was demonstrated.     

Long-term fertilisation regimes affect the composition of the alkaline phosphomonoesterase encoding microbial community of a vertisol and its derivative soil fractions

Alkaline phosphomonoesterase (ALP) mainly originates from soil microbial secretion and plays a crucial role in the turnover of soil phosphorus (P). To examine the response of ALP-encoding microbial communities (analysed for the biomarker of the ALP gene, phoD) of soils and derivative soil fractions to different fertilisation regimes, soil samples were collected from a long-term experimental field (over 35 years). The different organic P (Po) pools of soil fractions and the ALP activity of soil were also determined. Compared with chemical-only fertilised soils, the ALP activity was 232–815% higher in organic-amended soils, and the highest enzyme activity was observed in the organic-only fertilised treatment. The abundance of the phoD gene harbouring in soil fractions, determined by quantitative PCR (qPCR), was affected by different fertilisations. The highest abundance of the phoD gene was generally detected in the 2–63-μm-sized fraction (silt), but most phoD-encoding microbial species were associated to the 0.1–2-μm-sized fraction (clay) in the chemical-only fertilised soil. The contents of labile Po (LPo), moderately labile Po (MLPo) and fulvic acid-associated Po (FAPo) were significantly correlated with the phoD gene abundance, whereas only LPo content was significantly correlated with the ALP activity. The dominant phoD-encoding phylas were Actinobacteria and Proteobacteria, according to a high-throughput sequencing. Bradyrhizobium, a N₂-fixer identified as a phoD-encoding genus, showed the highest abundance in fertilised soils. The abundance of Bradyrhizobium, Streptomyces, Modestobacter, Lysobacter, Frankia and Burkholderia increased with the organic-only amendment and was significantly correlated with the ALP activity. According to structure equation models (SEM), pH and LPo content significantly and directly affected the ALP activity; the soil organic C (C_org) content was related to composition and abundances of phoD-harbouring microbial communities; since both microbial properties were correlated to the ALP activity, the C_org content was indirectly related to the ALP activity. In conclusion, soil management practices can be used to optimise the contents of soil available P and the organic P with regulation of soil ALP activity and the community composition of corresponding microbes.     

Nitrous oxide production by fungi in soils under different moisture levels

The production of nitrous oxide (N₂O) by facultative anaerobic fungi from the Fusarium, Trichoderma, and Paecylomyces genera was detected. Representatives of the genus Mucorales did not produce N₂O. The formation of N₂O in sterile soddy-podzolic soil inoculated by Fusarium oxysporum and F. solani increased significantly with the rise of the soil water content from 16–20% (50–60% of the field water capacity) to 30% (the field water capacity) with maximum values reached at the water content of 50% (the total soil water capacity). The production of N₂O by fungi at the soil water content of 50% was often higher under microaerobic conditions than under anaerobic conditions created via substitution of argon for atmospheric air in the flasks. The activity of N₂O production by fungi in the soil increased by several times upon nitrite or nitrate amendments. The specific activity of N₂ O formation in the soil was 0.38 ± 0.15 nmol N₂O/(h per mg) of dry mycelium. It was significantly lower than the rate of N₂O formation by Fusarium oxysporum 11dn1 in the nitrite-containing media and close to the rate of N₂O formation by this fungus in the nitrate-containing media. A comparison of the rate of N₂O release by active strain Fusarium oxysporum 11dn1 inoculated into the sterile soil with the rate of denitrification processes in the nonsterile soil showed that the contribution of soil fungi to the total emission of gaseous nitrogen compounds from the soil may reach 8% under optimum conditions.     

The effects of different irrigation regimes on nitrous oxide emissions and influencing factors in greenhouse tomato fields

Purpose

Intensive agricultural practices have enhanced problems associated with the competing use of limited water resources. Nitrous oxide (N₂O) is a major contributor to global warming. It is important for researchers to ascertain the relationship between irrigation and soil N₂O emissions in order to identify mitigation strategies to reduce nitrous oxide emissions. Different irrigation amounts affect soil water dynamics and nitrogen turnover. The effect of three lower limits of irrigation on soil N₂O emissions, influencing factors, and abundance of genes involved in nitrification and denitrification were investigated in tomato irrigated in a greenhouse.

Materials and methods

Observations were performed between April and August 2015 in a long-term irrigated field subjected to different lower limits of irrigation: 20 kPa (D₂₀), 30 kPa (D₃₀), and 40 kPa (D₄₀) from greenhouse soil during the tomato crop season. Soil N₂O fluxes were monitored using the static chamber-gas chromatograph method. Copy numbers of genes were determined using the real-time quantitative polymerase chain reaction (real-time PCR) technique. Characteristics of soil N₂O emissions were analyzed, and differences between irrigation regimes were determined. The effects of influencing factors on soil N₂O emissions were analyzed, including soil temperature, soil moisture, soil pH, and soil mineral nitrogen, as well as changes in the abundance of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) based on amoA genes and denitrifier genes (nosZ, nirK, and cnorB).

Results and discussion

Our results showed that peaks in N₂O emissions occurred 1–5 days after each irrigation. During the whole tomato growth period, soil N₂O fluxes were lowest under D₃₀ treatment compared with those under D₂₀ and D₄₀ treatments. Soil NO₃ ^?-N concentrations were significantly higher than NH₄ ⁺-N concentrations. Soil N₂O fluxes were significantly related to soil moisture, NH₄ ⁺-N concentrations (P < 0.01), soil pH, and AOA copy numbers (P < 0.05). There was no consistent correlation between soil N₂O emissions, soil temperature, and soil NO₃ ^?-N concentrations. Different irrigation regimes significantly affected AOA copy numbers but did not affect the expression of other genes. AOA copy numbers were higher than those of AOB. Soil N₂O fluxes significantly affected the AOA copy numbers and potential nitrification rates (P < 0.05).

Conclusions

Soil moisture, pH, and NH₄ ⁺-N concentration were important factors affecting soil N₂O emissions. Compared with other genes associated with nitrification and denitrification, AOA plays an important role in N₂O emissions from greenhouse soils. Selecting a lower limit of irrigation of 30 kPa could effectively reduce N₂O emissions from vegetable soils.

     

Nitrogen Removal and N<Subscript>2</Subscript>O Emission During Low Carbon Wastewater Treatment Using the Multiple A/O Process

With the organic carbon of acetate (SBR-A) and propionate (SBR-P), the effect of organic carbon sources on nitrogen removal and nitrous oxide (N₂O) emission in the multiple anoxic and aerobic process was investigated. The nitrogen removal percentages in SBR-A and SBR-P reactor were both 72%, and the phosphate removal percentages were 97 and 85.4%, respectively. During nitrification, both the NH₄ ⁺-N oxidation rate in the SBR-A and SBR-P had a small change without the influence of the addition of nitrite nitrogen (NO₂ ^?-N). With the addition of 10 mg/L NO₂ ^?-N, the nitrate nitrogen (NO₃ ^?-N) production rate, N₂O accumulation rate and emission factor had increased. At the same time, the N₂O emission factor of SBR-A and SBR-P reactors increased from 2.13 and 0.87% to 4.66 and 2.08%, respectively. During exogenous denitrification, when nitrite was used as electron acceptor, the N₂O emission factors were 34.1 and 8.6 times more than those of NO₃ ^?-N as electron acceptor in SBR-A and SBR-P. During endogenous denitrification with NO₂ ^?-N as electron acceptor, the accumulation rate and emission factor of N₂O were higher than those of NO₃ ^?-N as electron acceptor. High-throughput sequencing test showed that the dominant bacteria were Proteobacteria and Bacteroidetes in both reactors at the phylum level, while the main denitrification functional bacteria were Thauera sp., Zoogloea sp. and Dechloromonas sp. at the genus level.     

Linkage of N2O emissions to the abundance of soil ammonia oxidizers and denitrifiers in purple soil under long-term fertilization

Abstract

Microbial nitrification and denitrification are responsible for the majority of soil nitrous (N₂O) emissions. In this study, N₂O emissions were measured and the abundance of ammonium oxidizers and denitrifiers were quantified in purple soil in a long-term fertilization experiment to explore their relationships. The average N₂O fluxes and abundance of the amoAgene in ammonia-oxidizing bacteria during the observed dry season were highest when treated with mixed nitrogen, phosphorus and potassium fertilizer (NPK) and a single N treatment (N) using NH₄HCO₃as the sole N source; lower values were obtained using organic manure with pig slurry and added NPK at a ratio of 40%:60% (OMNPK),organic manure with pig slurry (OM) and returning crop straw residue plus synthetic NH₄HCO₃fertilizer at a ratio of 15%:85% (SRNPK). The lowest N₂O fluxes were observed in the treatment that used crop straw residue(SR) and in the control with no fertilizer (CK). Soil NH₄⁺provides the substrate for nitrification generating N₂O as a byproduct. The N₂O flux was significantly correlated with the abundance of the amoA gene in ammonia-oxidizing bacteria (r = 0.984, p < 0.001), which was the main driver of nitrification. During the wet season, soil nitrate (NO₃^?) and soil organic matter (SOC) were found positively correlated with N₂O emissions (r = 0.774, p = 0.041 and r = 0.827, p = 0.015, respectively). The nirS gene showed a similar trend with N₂O fluxes. These results show the relationship between the abundance of soil microbes and N₂O emissions and suggest that N₂O emissions during the dry season were due to nitrification, whereas in wet season, denitrification might dominate N₂O emission.