The effect of temperature and moisture on the source of N<Subscript>2</Subscript>O and contributions from ammonia oxidizers in an agricultural soil

In recent years, identification of the microbial sources responsible for soil N₂O production has substantially advanced with the development of isotope enrichment techniques, selective inhibitors, mathematical models and the discoveries of specific N-cycling functional genes. However, little information is available to effectively quantify the N₂O produced from different microbial pathways (e.g. nitrification and denitrification). Here, a ¹⁵N-tracing incubation experiment was conducted under controlled laboratory conditions (50, 70 and 85% water-filled pore space (WFPS) at 25 and 35 °C). Nitrification was the main contributor to N₂O production. At 50, 70 and 85% WFPS, nitrification contributed 87, 80 and 53% of total N₂O production, respectively, at 25 °C, and 86, 74 and 33% at 35 °C. The proportion of nitrified N as N₂O (P _N2O) increased with temperature and moisture, except for 85% WFPS, when P _N2O was lower at 35 °C than at 25 °C. Ammonia-oxidizing archaea (AOA) were the dominant ammonia oxidizers, but both AOA and ammonia-oxidizing bacteria (AOB) were related to N₂O emitted from nitrification. AOA and AOB abundance was significantly influenced by soil moisture, more so than temperature, and decreased with increasing moisture content. These findings can be used to develop better models for simulating N₂O from nitrification to inform soil management practises for improving N use efficiency.     

Nitrification, ammonia-oxidizing communities, and N2O and CH4 fluxes in an imperfectly drained agricultural field fertilized with coated urea with and without dicyandiamide

Agricultural soil is a major source of nitrous oxide (N₂O), and the application of nitrogen and soil drainage are important factors affecting N₂O emissions. This study tested the use of polymer-coated urea (PCU) and polymer-coated urea with the nitrification inhibitor dicyandiamide (PCUD) as potential mitigation options for N₂O emissions in an imperfectly drained, upland converted paddy field. Fluxes of N₂O and methane (CH₄), ammonia oxidation potential, and ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) abundances were monitored after the application of PCU, PCUD, and urea to upland soil. The results showed that urea application increased the ammonia oxidation potential and AOB and AOA abundances; however, the increase rate of AOB (4.6 times) was much greater than that of AOA (1.8 times). These results suggested that both AOB and AOA contributed to ammonia oxidation after fertilizer application, but the response of AOB was greater than AOA. Although PCU and PCUD had lower ammonia oxidation potential compared to urea treatment, they were not effective in reducing N₂O emissions. Large episodic N₂O emissions (up to 1.59 kg N ha^?1 day^?1) were observed following heavy rainfall 2 months after basal fertilizer application. The episodic N₂O emissions accounted for 55–80 % of total N₂O emissions over the entire monitoring period. The episodic N₂O emissions following heavy rainfall would be a major source of N₂O in poorly drained agricultural fields. Cumulative CH₄ emissions ranged from ?0.017 to ?0.07 kg CH₄ ha^?1, and fertilizer and nitrification inhibitor application did not affect CH₄ oxidation.     

Nitrous oxide emissions from grazed grassland as affected by a nitrification inhibitor,dicyandiamide, and relationships with ammonia-oxidizing bacteria and archaea

Purpose  

Nitrous oxide (N₂O) is a potent greenhouse gas and, in grazed grassland systems where animals graze outdoor pastures, most of the N₂O is emitted from animal urine nitrogen (N) deposited during grazing. Recently, ammonia-oxidizing archaea (AOA) were found to be present in large numbers in soils as well in the ocean, suggesting a potentially important role for AOA, in addition to ammonia-oxidizing bacteria (AOB), in the nitrogen cycle. The relationship between N₂O emissions and AOB and AOA populations is unknown. The objective of this study was to determine the quantitative relationship between N₂O emissions and AOB and AOA populations in nitrogen-rich grassland soils.     

Effects of 3,4-dimethylpyrazole phosphate (DMPP) on the abundance of ammonia oxidizers and denitrifiers in two different intensive vegetable cultivation soils

Purpose

Nitrification and denitrification in the N cycle are affected by various ammonia oxidizers and denitrifying microbes in intensive vegetable cultivation soils, but our current understanding of the effect these microbes have on N₂O emissions is limited. The nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP), acts by slowing nitrification and is used to improve fertilizer use efficiency and reduce N losses from agricultural systems; however, its effects on nitrifier and denitrifier activities in intensive vegetable cultivation soils are unknown.

Materials and methods

In this study, we measured the impacts of DMPP on N₂O emissions, ammonia oxidizers, and denitrifying microbes in two intensive vegetable cultivation soils: one that had been cultivated for a short term (1 year) and one that had been cultivated over a longer term (29 years). The quantitative PCR technique was used in this study. Three treatments, including control (no fertilizer), urea alone, and urea with DMPP, were included for each soil. The application rates of urea and DMPP were 1800 kg ha^?1 and 0.5% of the urea-N application rate.

Results and discussion

The application of N significantly increased N₂O emissions in both soils. The abundance of ammonia-oxidizing bacteria (AOB) increased significantly with high rate of N fertilizer application in both soils. Conversely, there was no change in the growth rate of ammonia-oxidizing archaea (AOA) in response to the applied urea despite the presence of larger numbers of AOA in these soils. This suggests AOB may play a greater role than AOA in the nitrification process, and N₂O emission in intensive vegetable cultivation soils. The application of DMPP significantly reduced soil NO₃^?-N content and N₂O emission, and delayed ammonia oxidation. It greatly reduced AOB abundance, but not AOA abundance. Moreover, the presence of DMPP was correlated with a significant decrease in the abundance of nitrite reductase (nirS and nirK) genes.

Conclusions

Long-term intensive vegetable cultivation with heavy N fertilization altered AOB and nirS abundance. In vegetable cultivation soils with high N levels, DMPP can be effective in mitigating N₂O emissions by directly inhibiting both ammonia oxidizing and denitrifying microbes.

     

The effects of silicon fertilizer on denitrification potential and associated genes abundance in paddy soil

This study evaluated the effect of silicate fertilizer on denitrification and associated gene abundance in a paddy soil. A consecutive trial from 2013 to 2015 was conducted including the following treatments: control (CK), mineral fertilizer (NPK), NPK plus sodium metasilicate (NPK + MSF), and NPK plus slag-based silicate fertilizer (NPK + SSF). Real-time quantitative PCR (qPCR) was used to analyze the abundances of nirS, nirK, and nosZ genes. Potential N₂O emissions and ammonium and nitrate concentrations were related to the nirS and nirK gene abundance. Compared with the NPK treatments, the addition of a Si fertilizer decreased N₂O emission rates and denitrification potential by 32.4–66.6 and 22.0–59.2%, respectively, which were probably related to increased rice productivity, soil Fe availability, and soil N depletion. The abundances of nirS and nirK genes were decreased by 17.7–35.8% and 21.1–43.5% with addition of silicate fertilizers, respectively. Rates of total N₂O and N₂O from denitrification (DeN₂O) emission were positively correlated with the nirS and nirK gene abundance. Nitrate, exchangeable NH₄ ⁺, and Fe concentrations were the main factors regulating the nirS and nirK gene abundance. Silicate fertilization during rice growth may serve as an effective approach to decreasing N₂O emissions.     

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.     

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.

     

Responses of bacterial and archaeal ammonia oxidisers of an acidic luvisols soil to different nitrogen fertilization rates after 9?years

It is still not clear which group of ammonia-oxidizing microorganisms plays the most important roles in nitrification in soils. Change in abundances and community compositions of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) under long-term different nitrogen (N) fertilization rates were investigated in an acidic luvisols soil using real-time polymerase chain reaction and denaturing gradient gel electrophoresis, respectively, based on the ammonia monooxygenase a-subunit gene. The experimental plan included the following treatments: control without N fertilization (N_CK), low N fertilization rate, middle N fertilization rate, and high N fertilization rate as 0, 100, 150, and 250?kg urea-N?ha^?1, respectively. Long-term different N fertilization rates did not significantly alter the total C and N contents of soil while it significantly decreased soil pH, which ranged from 5.60 to 5.20. The AOB abundance was more abundant in the N fertilization treatments than the N_CK treatment; the AOA abundance decreased by the increasing N fertilization rates, as did the ratios of AOA/AOB. The large differences in the potential nitrification rates among four treatments depended on the changes in AOA abundance but not to changes in AOB abundance. Phylogenetic analysis showed that the AOB communities were dominated by Nitrosospira clusters 1, 3, and 9 while all AOA sequences were grouped into soil/sediment cluster except for one sequence. Taken together, these results indicated that AOB and AOA preferred different soil N conditions and AOA were functionally more important in the nitrification than AOB in the acidic luvisols soil.     

长期施肥对棕壤氨氧化细菌和古菌丰度的影响

【目的】氨氧化是氮转化过程的限速步骤,其由氨氧化微生物所驱动。本研究旨在探明 37 年玉米–大豆轮作施肥条件下影响棕壤氨氧化微生物丰度的主要影响因子及变化规律。【方法】以沈阳农业大学棕壤肥料长期定位试验耕层土壤 (0—20 cm) 为材料,选取其中 9 个施肥处理进行取样分析:不施肥 (CK)、低量氮肥 (N₁)、高量氮肥 (N₂)、氮磷肥 (N₁P)、氮磷钾肥 (N₁PK)、高量有机肥 (M₂)、高量有机肥 + 低量氮肥 (M₂N₁)、高量有机肥 + 氮磷肥 (M₂N₁P)、高量有机肥 + 氮磷钾肥 (M₂N₁PK)。采用实时荧光定量 PCR 技术测定其氨氧化微生物丰度,通过对土壤基本化学性质和氨氧化微生物丰度的冗余分析找出影响氨氧化微生物丰度的主要因素。【结果】施用有机肥处理的土壤 pH、有机质、全氮、碱解氮、速效钾、速效磷、铵态氮、硝态氮含量明显高于不施肥和单施化肥处理。各施肥处理土壤有机质、全氮、碱解氮、速效钾、速效磷的含量总体呈现有机肥处理 > 化肥处理 > CK;与不施肥处理 (CK) 相比,单施化肥处理显著降低了土壤 pH 值,施用有机肥处理显著提高了土壤 pH 值,其中 N₂ 处理的土壤 pH 最低,M₂ 处理的土壤 pH 最高。不同施肥处理氨氧化细菌 (AOB) 的丰度为 0.94 × 106～5.77 × 106 copies/g 干土,氨氧化古菌 (AOA) 的丰度为 3.56 × 106～1.22 × 107 copies/g 干土;施用有机肥处理 AOB 和 AOA 丰度显著高于不施肥和单施化肥处理,其中 M₂ 处理的 AOB 和 AOA 丰度最高,单施氮肥处理的 AOB 和 AOA 丰度最低。冗余分析 (RDA) 表明,影响棕壤 AOB 和 AOA 丰度的主要环境因子有土壤 pH、有机质、全氮、碱解氮、速效磷、速效钾,且与 AOB 和 AOA 丰度呈正相关关系。【结论】长期轮作施肥显著改变了棕壤的化学性质,从而对氨氧化微生物的丰度产生了显著影响。长期施用有机肥显著提高了土壤养分含量及 AOB 和 AOA 的丰度,对维持土壤氨氧化微生物的数量起到十分重要的作用;同时试验结果也为今后通过改变土壤 pH、有机质、全氮、碱解氮、速效磷、速效钾等性质对 AOB 和 AOA 进行调节提供了依据。     

Temporal changes in abundance and composition of ammonia-oxidizing bacterial and archaeal communities in a drained peat soil in relation to N<Subscript>2</Subscript>O emissions

Purpose  

Boreal peat soils comprise about 3% of the terrestrial environments, and when drained, they become sources of the greenhouse gas nitrous oxide (N₂O). Ammonia oxidation can result in N₂O emissions, either directly or by fuelling denitrification, but we know little about the ecology of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in peat soils. Our aim was to determine temporal alterations in abundance and composition of these communities in a drained and forested peat soil in relation to N₂O emissions and ammonia oxidation activity.     

The effect of soil pH and dicyandiamide (DCD) on N2O emissions and ammonia oxidiser abundance in a stimulated grazed pasture soil

Purpose

Nitrous oxide (N₂O) is a potent greenhouse gas which is mainly produced from agricultural soils through the processes of nitrification and denitrification. Although denitrification is usually the major process responsible for N₂O emissions, N₂O production from nitrification can increase under some soil conditions. Soil pH can affect N₂O emissions by altering N transformations and microbial communities. Bacterial (AOB) and archaeal (AOA) ammonia oxidisers are important for N₂O production as they carry out the rate-limiting step of the nitrification process.

Material and methods

A field study was conducted to investigate the effect of soil pH changes on N₂O emissions, AOB and AOA community abundance, and the efficacy of a nitrification inhibitor, dicyandiamide (DCD), at reducing N₂O emissions from animal urine applied to soil. The effect of three pH treatments, namely alkaline treatment (CaO/NaOH), acid treatment (HCl) and native (water) and four urine and DCD treatments as control (no urine or DCD), urine-only, DCD-only and urine + DCD were assessed in terms of their effect on N₂O emissions and ammonia oxidiser community growth.

Results and discussion

Results showed that total N₂O emissions were increased when the soil was acidified by the acid treatment. This was probably due to incomplete denitrification caused by the inhibition of the assembly of the N₂O reductase enzyme under acidic conditions. AOB population abundance increased when the pH was increased in the alkaline treatment, particularly when animal urine was applied. In contrast, AOA grew in the acid treatment, once the initial inhibitory effect of the urine had subsided. The addition of DCD decreased total N₂O emissions significantly in the acid treatment and decreased peak N₂O emissions in all pH treatments. DCD also inhibited AOB growth in both the alkaline and native pH treatments and inhibited AOA growth in the acid treatment.

Conclusions

These results show that N₂O emissions increase when soil pH decreases. AOB and AOA prefer different soil pH environments to grow: AOB growth is favoured in an alkaline pH and AOA growth favoured in more acidic soils. DCD was effective in inhibiting AOB and AOA when they were actively growing under the different soil pH conditions.     

Identifying response groups of soil nitrifiers and denitrifiers to grazing and associated soil environmental drivers in Tibetan alpine meadows

Defining response groups within N-related microbial communities is needed to predict land management effect on soil N dynamics, but information on such response groups and associated environmental drivers is scarce. We investigated the abundance and major populations of ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nirS- and nirK-harboring denitrifiers under different grazing managements in Tibetan alpine meadow soils. Grazing increased AOB and AOA abundances up to 42 fold and 3.7 fold, respectively, and increased the percentage of AOB within total ammonia oxidizers from 3.1% to 10.8%. The abundance of nirK-like denitrifiers increased with grazing intensity, while the abundance of nirS-like denitrifiers tended to decrease. However, sub-groups within each of these broad groups of (de)nitrifiers responded differently to grazing. Soil nitrate was the main driver of the abundance of denitrifier sub-groups (nirK or nirS) positively responding to grazing, while soil moisture and carbon concentration were the main drivers of the abundance of denitrifier sub-groups negatively responding to grazing. AOB and nirK-harboring denitrifiers thus generally responded more positively to grazing than AOA and nirS-harboring denitrifiers, but significant functional diversity existed within each group. Our approach demonstrates the usefulness of the concept of response groups to better characterize and understand (de)nitrifier response to grazing.     

Decreased nitrous oxide emissions associated with functional microbial genes under bio-organic fertilizer application in vegetable fields

Bio-organic fertilizers enriched with plant growth-promoting microbes(PGPMs)have been widely used in crop fields to promote plant growth and maintain soil microbiome functions.However,their potential effects on N₂O emissions are of increasing concern.In this study,an in situ measurement experiment was conducted to investigate the effect of organic fertilizer containing Trichoderma guizhouense(a plant growth-promoting fungus)on soil N₂O emissions from a greenhouse vegetable field.The following four treatments were used:no fertilizer(control),chemical fertilizer(NPK),organic fertilizer derived from cattle manure(O),and organic fertilizer containing T.guizhouense(O+T,referring to bio-organic fertilizer).The abundances of soil N cycling-related functional genes(amoA)from ammonium-oxidizing bacteria(AOB)and archaea(AOA),as well as nirS,nirK,and nosZ,were simultaneously determined using quantitative PCR(qPCR).Compared to the NPK plot,seasonal total N₂O emissions decreased by 11.7%and 18.7%in the O and O+T plots,respectively,which was attributed to lower NH₄⁺-N content and AOB amoA abundance in the O and O+T plots.The nosZ abundance was significantly greater in the O+T plot,whilst the AOB amoA abundance was significantly lower in the O+T plot than in the O plot.Relative to the organic fertilizer,bio-organic fertilizer application tended to decrease N₂O emissions by 7.9%and enhanced vegetable yield,resulting in a significant decrease in yield-scaled N₂O emissions.Overall,the results of this study suggested that,compared to organic and chemical fertilizers,bio-organic fertilizers containing PGPMs could benefit crop yield and mitigate N₂O emissions in vegetable fields.     

Impacts of continuous excessive fertilization on soil potential nitrification activity and nitrifying microbial community dynamics in greenhouse system

Purpose

Sampling and analysis of greenhouse soils were conducted in Shouguang, China, to study continuous excessive fertilization effect on nitrifying microbial community dynamics in greenhouse environment.

Materials and methods

Potential nitrification activity (PNA), abundance, and structure of nitrifying microbial communities as well as the correlations with soil properties were investigated.

Results and discussion

Short-term excessive fertilization increased soil nutrient contents and the diversity of nitrifying microbial communities under greenhouse cultivation. However, the abundance and diversity of nitrifying communities decreased greatly due to the increase of soil acidity and salinity after 14 years of high fertilization in greenhouse. There was a significant positive correlation between soil PNA and the abundance of ammonia-oxidizing bacteria (AOB) but not that of ammonia-oxidizing archaea (AOA) in topsoil (0–20 cm) when pH ≥7. Soil PNA and AOB were strongly influenced by soil pH. The groups of Nitrososphaeraceae, Nitrosomonadaceae, and Nitrospiraceae were predominant in the AOA, AOB, and nitrite-oxidizing bacteria (NOB) communities, respectively. Nitrifying community structure was significantly correlated with soil electrical salinity (EC), organic carbon (OC), and nitrate nitrogen (NO₃ ^?–N) content by redundancy analysis (RDA).

Conclusions

Nitrification was predominated by AOB in greenhouse topsoil with high fertilizer loads. Soil salinity, OC, NO₃ ^?–N content, and pH affected by continuous excessive fertilization were the major edaphic factors in shaping nitrifying community structure in greenhouse soils.

     

Abundance and community structure of ammonia-oxidizing archaea and bacteria in an acid paddy soil

Nitrification is essential to the nitrogen cycle in paddy soils. However, it is still not clear which group of ammonia-oxidizing microorganisms plays more important roles in nitrification in the paddy soils. The changes in the abundance and composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were investigated by real-time PCR, terminal restriction fragment length polymorphism, and clone library approaches in an acid red paddy soil subjected to long-term fertilization treatments, including treatment without fertilizers (CT); chemical fertilizer nitrogen (N); N and potassium (NK); N and phosphorus (NP); N, P, and K (NPK); and NPK plus recycled crop residues (NPK+C). The AOA population size in NPK+C was higher than those in CT, while minor changes in AOB population sizes were detected among the treatments. There were also some changes in AOA community composition responding to different fertilization treatments. Still few differences were detected in AOB community composition among the treatments. Phylogenetic analysis showed that the AOA sequences fell into two main clusters: cluster A and cluster soil/sediment. The AOB composition in this paddy soil was dominated by Nitrosospira cluster 12. These results suggested that the AOA were more sensitive than AOB to different fertilization treatments in the acid red paddy soil.     

不同生物硝化抑制剂对红壤性水稻土N2O排放的影响及其机制

为揭示不同生物硝化抑制剂(BNIs)对红壤性水稻土N₂O排放的影响差异及作用机制,通过21 d的土柱淹水培养试验,比较了三种BNIs 1,9-癸二醇(1,9-D)、亚麻酸(LN)和3-(4-羟基苯基)丙酸甲酯(MHPP)与化学合成硝化抑制剂双氰胺(DCD)对土壤N₂O排放及相关硝化、反硝化功能基因的影响。结果表明:不同BNIs(1,9-D、LN、MHPP)可以显著平均降低土壤N₂O日排放峰值40.1%;1,9-D和MHPP可分别抑制N₂O排放总量44.5%和43.9%,而DCD和LN对N₂O排放总量没有显著影响。1,9-D和MHPP对AOA(氨氧化古菌)、AOB(氨氧化细菌)硝化菌和nirS、nirK型反硝化菌的调控均有所不同,1,9-D可以同时抑制AOA、AOB和nirS微生物的生长;MHPP仅可以抑制AOA的生长;其中,AOA-amoA和nirS基因丰度与土壤N₂O的排放呈显著正相关关系。同时,1,9-D和MHPP均增加了nosZ基因丰度及其与AOA-...     

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.

     

Effect of soil aggregate size and dicyandiamide on N2O emissions and ammonia oxidizer abundance in a grazed pasture soil

Nitrous oxide (N₂O) is a potent greenhouse gas, which is mainly produced from agricultural soils. Ammonia oxidation is the rate‐determining step in N₂O production, and the process is carried out by ammonia oxidizers, bacteria and archaea. Soil aggregate size has been shown to alter soil properties, which affect N₂O emissions and bacterial communities. However, the effect of aggregate size on temporal and total N₂O emissions and ammonia‐oxidizing bacteria (AOB) and archaea (AOA) is not fully understood. This incubation study investigated the effect of three different soil aggregate sizes on N₂O emissions and ammonia oxidizer abundance under high urine‐N concentrations and the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), at reducing N₂O emissions in different aggregate soils. It was found that temporal patterns of N₂O emissions were affected by aggregate size with higher peak emissions in the large and medium aggregates. However, the total emissions were the same due to a ‘switch’ in emissions at day 66, after which smaller aggregates produced higher N₂O emissions. It is suggested that the switch was caused by an increase in aggregate disruption in the small aggregates, following the urine application, due to their higher surface area to volume ratio. AOB and AOA abundances were not significantly affected by aggregate size. DCD was effective in reducing N₂O emissions in all aggregate sizes by an average of 79%. These results suggest that similar ammonia oxidizer abundance is found in soils of different aggregate sizes, and the efficacy of DCD in reducing N₂O emissions was not affected by aggregate size of the soil.     

Effects of biochar and 3,4-dimethylpyrazole phosphate (DMPP) on soil ammonia-oxidizing bacteria and nosZ-N2O reducers in the mitigation of N2O emissions from paddy soils

Purpose

Paddy fields are an important source of nitrous oxide (N₂O) emission. The application of biochar or the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) to paddy soils have been proposed as technologies to mitigate N₂O emissions, but their mechanisms remain poorly understood.

Methods

An experiment was undertaken to study the combined and individual effects of biochar and DMPP on N₂O emission from a paddy field. Changes in soil microbial community composition were investigated. Four fertilized treatments were established as follows: fertilizer only, biochar, DMPP, and biochar combined with DMPP; along with an unfertilized control.

Results

The application of biochar and/or DMPP decreased N₂O emission by 18.9–39.6% compared with fertilizer only. The combination of biochar and DMPP exhibited higher efficiency at suppressing N₂O emission than biochar alone but not as effective as DMPP alone. Biochar promoted the growth of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), while DMPP suppressed AOB and increased AOA. Applying biochar with DMPP reduced the impact of DMPP on AOB. The nirS-/nirK- denitrifiers were decreased and nosZ-N₂O reducers were increased by DMPP and the combination of DMPP and biochar. The abundance of the nirK gene was increased by biochar at the elongation and heading stages of rice development. Compared with fertilizer only, the application of biochar and/or DMPP promoted the abundance of nosZ genes.

Conclusion

These results suggest that applying biochar and/or DMPP to rice paddy fields is a promising strategy to reduce N₂O emissions by regulating the dynamics of ammonia oxidizers and N₂O reducers.

     

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.