Effect of application rate of a nitrification inhibitor,dicyandiamide (DCD), on nitrification rate,and ammonia-oxidizing bacteria and archaea growth in a grazed pasture soil: An incubation study

Purpose

Dicyandiamide (DCD) has been used commercially in New Zealand to reduce nitrate leaching and N₂O emissions in grazed pastures. However, there is a lack of information in the literature on the optimum rate of DCD to achieve the environmental benefits while at the same time reducing the cost of the technology. The objective of this study was to determine the effect of DCD application rate on its effectiveness to inhibit ammonia oxidizer growth and nitrification rate in a grazed pasture soil.

Materials and methods

The soil was a Templeton silt loam (Immature Pallic Soil; Udic Haplustepts) collected from Lincoln University Research Dairy Farm with a mixed pasture consisting of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) and was incubated alone (control) or with cow urine at 700 kg N/ha with 6 rates of DCD [0, 2.5, 5, 7.5, 10 (applied twice), 15 and 20 kg/ha] in incubation vessels. The incubation vessels were placed randomly in an incubator with a constant temperature of 12 °C. During 112 days of incubation, soil subsamples were taken at different time intervals to measure the concentrations of NO₃ ^?-N and NH₄ ⁺-N and the amoA gene copy numbers of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA).

Results and discussion

DCD applied at all the different rates inhibited nitrification in urine-treated soils, but the effectiveness increased with DCD application rate. In addition, AOB growth and the amounts of nitrate-N in the soil were significantly related to the application rate of DCD. However, AOA population abundance showed no relationship to the application rate of DCD. The DCD rate at which the AOB growth rate and nitrate-N concentration were halved (effective dosage that causes 50 % reduction in nitrification rate, or ED₅₀) was about 10 kg DCD/ha.

Conclusions

These results suggest that DCD applied at relatively low rates still slowed down the nitrification rate, and the current recommended rate of 10 kg DCD/ha for DCD use in New Zealand grazed pastures would result in a 50 % reduction in nitrification rate in this soil. The actual rate of DCD application used would depend on the cost of the product and the environmental and agronomic benefits that would result from its use.     

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.     

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 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.     

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-...     

Nitrate enhances N2O emission more than ammonium in a highly acidic soil

Purpose

Nitrous oxide (N₂O) is produced naturally in soils through microbial processes of nitrification and denitrification. In recent years, the long-term application of nitrogen-heavy fertilizers has led to the acidification of tea orchard soils with high N₂O emission. The present research aimed at finding out which process (nitrification or denitrification) dominates in N₂O production, whether certain fertilizer managements could reduce N₂O emission, and the effects of fertilizer management on the abundance of functional genes.

Materials and methods

Two nitrification inhibitors, 3, 4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD), combined with different N fertilizers (ammonium sulfate and potassium nitrate) were applied to highly acidic tea orchard soil in an aerobic incubation experiment. Both amoA and nosZ gene abundances from different treatments were determined by quantitative PCR. An anaerobic nitrate effect test was carried out using C₂H₂ inhibition method.

Results and discussion

The application of nitrate fertilizers significantly (P?<?0.05) enhanced total N₂O emission. A linear regression analysis between total N₂O emission and average nitrate contents indicated that denitrification is the dominant source of N₂O in this tea orchard soil. In the anaerobic incubation, no significant difference of N₂O emission was observed between KNO₃ and no KNO₃ treatments before 96 h. Quantitative PCR revealed lower copy numbers of nosZ in nitrate-associated fertilizer-treated soils than the soils from other treatments. Compared with the control, ammonium fertilizers with DCD or DMPP significantly (P?<?0.05) inhibited nitrate production as well as N₂O.

Conclusions

These results showed that denitrification is the dominant source of N₂O in this highly acidic soil. Nitrate addition could significantly inhibit the abundance of nitrous oxide reductase, therefore causing high N₂O emission. The application of ammonium fertilizers with DCD or DMPP could significantly reduce N₂O emission, possibly due to the effective inhibition of nitrate production.     

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.     

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.     

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.     

Effects of trampling of a wet dairy pasture soil on soil porosity and on mitigation of nitrous oxide emissions by a nitrification inhibitor,dicyandiamide

Urine patches in dairy pastures are major sources of nitrous oxide (N₂O). Wet winters result in compaction damage to pastures because of animal trampling. The nitrification inhibitor, dicyandiamide (DCD), is effective at reducing N₂O emissions from urine patches. Here, we assessed the extent of damage to the physical quality of the soil by trampling and whether this influenced the ability of DCD to mitigate N₂O emissions. A field experiment was conducted where a sandy loam soil was trampled by a mechanical hoof just before urine and DCD application. Trampling reduced air permeability and pore continuity, but this had no effect on bulk density. Urine appeared to have contributed to pore collapse and blockage. Trampling increased average cumulative N₂O emissions from 1.74 to 4.66% of urine‐N applied. This effect was attributed to increased water‐filled pore space, aggregate destruction and suppression of grass growth. DCD was highly effective in reducing N₂O emissions, with the N₂O emission factor of the urine‐N being decreased by 58–63%. Trampling did not significantly affect the effectiveness of DCD in reducing N₂O emissions.     

Impact of urine and the application of the nitrification inhibitor DCD on microbial communities in dairy-grazed pasture soils

Tools to manage the emission of the greenhouse gas nitrous oxide (N₂O), an intermediate of both nitrification and denitrification, from soils are limited. To date, the nitrification inhibitor dicyandiamide (DCD) is one of the most effective tools available to livestock farmers for reducing N₂O emissions and minimizing leaching of nitrogen in response to increased urine deposition in grazed pasture systems. Despite its effectiveness in decreasing N losses from animal urine by inhibiting N processes in soils, the effect of DCD on the population structure of denitrifiers and overall bacterial community composition is still uncertain. Here we use three New Zealand dairy-grazed pasture soils to determine the effects of DCD application on microbial community richness and composition at both functional (genes involved in the denitrification process) and phylogenetic (overall bacterial community composition based on 16S rRNA profiling) levels. Results further confirm that the effects on microbial populations are minimal and transient in nature. The impact of DCD on microbial community structure was soil dependent, and a greater effect was attributed to intrinsic soil properties like soil texture, with community response to DCD in combination with urine being comparable to that under urine alone. Addition of DCD to cattle urine also reduced N₂O emission between 23 and 67%.     

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.     

pH regulates key players of nitrification in paddy soils

Increasing lines of evidence have suggested the functional importance of ammonia-oxidizing archaea (AOA) rather than bacteria (AOB) for nitrification in upland soils with low pH. However, it remains unclear whether niche specialization of AOA and AOB occurs in rice paddy wetlands constrained by oxygen availability. Using DNA-based stable isotope probing, we conclude that AOA dominated nitrification activity in acidic paddy soils (pH 5.6) while AOB dominated in alkaline soils (pH 8.2). Nitrification activity was stimulated by urea fertilization and accompanied by a significant increase of AOA in acid soils and AOB in alkaline soils. DNA-based stable isotope probing indicated significant assimilation of ¹³CO₂ for AOA only in acidic paddy soil, while AOB was the solely responsible for ammonia oxidation in the alkaline paddy soil. Phylogenetic analysis further indicated that AOA members within the soil group 1.1b lineage dominated nitrification in acid soils. Ammonia oxidation in the alkaline soil was catalyzed by Nitrosospira cluster 3-like AOB, suggesting that the physiological diversity of AOA is more complicated than previously thought, and soil pH plays important roles in shaping the community structures of ammonia oxidizers in paddy field.     

Shift in abundance and structure of soil ammonia-oxidizing bacteria and archaea communities associated with four typical forest vegetations in subtropical region

Purpose

Nitrogen (N) is one of the most important elements that can limit plant growth in forest ecosystems. Ammonia-oxidizing bacteria (AOB) and archaea (AOA) are considered as the key drivers of global N biogeochemical cycling. Soil ammonia-oxidizing microbial communities associated with subtropical vegetation remain poorly characterized. The aim of this study was to determine how AOA and AOB abundance and community structure shift in response to four typical forest vegetations in subtropical region.

Materials and methods

Broad-leaved forest (BF), Chinese fir forest (CF), Pinus massoniana forest (PF), and moso bamboo forest (MB) were widely distributed in the subtropical area of southern China and represented typical vegetation types. Four types of forest stands of more than 30 years grew adjacent to each other on the same soil type, slope, and elevation, were chosen for this experiment. The abundance and community structure of AOA and AOB were characterized by using real-time PCR and denaturing gradient gel electrophoresis (DGGE). The impact of soil properties on communities of AOA and AOB was tested by canonical correspondence analysis (CCA).

Results and discussion

The results indicated that AOB dominated in numbers over AOA in both BF and MB soils, while the AOA/AOB ratio shifted with different forest stands. The highest archaeal and bacterial amoA gene copy numbers were detected in CF and BF soils, respectively. The AOA abundance showed a negative correlation with soil pH and organic C but a positive correlation with NO₃ ^??N concentration. The structures of AOA communities changed with vegetation types, but vegetation types alone would not suffice for shaping AOB community structure among four forest soils. CCA results revealed that NO₃ ^??N concentration and soil pH were the most important environmental gradients on the distribution of AOA community except vegetation type, while NO₃ ^??N concentration, soil pH, and organic C significantly affected the distribution of the AOB communities.

Conclusions

These results revealed the differences in the abundance and structure of AOA and AOB community associated with different tree species, and AOA was more sensitive to vegetation and soil chemical properties than AOB. N bioavailability could be directly linked to AOA and AOB community, and these results are useful for management activities, including forest tree species selection in areas managed to minimize N export to aquatic systems.     

Laboratory study of the effects of two nitrification inhibitors on greenhouse gas emissions from a slurry-treated arable soil: impact of diurnal temperature cycle

An automated laboratory soil incubation system enabled the effects on gaseous emissions from a soil to be quantified accurately, when amended with slurry plus a nitrification inhibitor: dicyandiamide (DCD), or 3,4-dimethylpyrazole phosphate (DMPP). Nitrification inhibitors applied with slurry under simulated Portuguese conditions were very efficient in reducing N₂O emission, and did not increase CH₄ emissions significantly, when the soil was predominantly aerobic. The inhibitors were also indirectly effective in reducing N₂O emissions due to denitrification during a subsequent anaerobic phase. All gaseous emissions followed strong diurnal patterns that were positively correlated with soil temperature and obeyed a Q₁₀=2 relationship. The widespread use of DCD and DMPP inhibitors with slurry applied to Portuguese soils could have the potential to reduce N₂O emissions from this source by ten- to 20-fold.     

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.

     

石灰和双氰胺对红壤酸化和硝化作用的影响及其机制

施用石灰是改良酸性土壤的重要措施,但其对土壤硝化作用的增强不仅加速土壤酸化,也增加硝态氮流失风险。传统的硝化抑制剂双氰胺(Dicyandiamide,DCD)能否在石灰改变pH的条件下始终有效抑制硝化是当前红壤区生产中亟需解决的问题。采用短期土壤培养试验,探讨了不同用量石灰与DCD配合施用对土壤酸化和硝化作用的影响及其机制。结果表明:施用一定量的石灰(≤4 g·kg^–1)显著提高土壤pH,通过促进氨氧化细菌的生长以促进硝化作用。在不同pH条件下,DCD对红壤硝化过程均有显著抑制效果。在较高pH(pH 7.0~7.8)条件下,DCD主要通过降低氨氧化细菌的丰度以抑制硝化,而在低pH(pH<6.0)条件下,DCD对氨氧化古菌和氨氧化细菌的丰度均有抑制作用。此外,DCD通过抑制土壤硝化,显著提高了土壤pH。上述结果表明,适宜量(2~4 g·kg^–1)的石灰和DCD结合施用不仅能够减缓红壤酸化,而且能够抑制硝化作用,降低硝态氮的潜在环境风险。     

Methanotroph abundance not affected by applications of animal urine and a nitrification inhibitor,dicyandiamide, in six grazed grassland soils

Purpose  

Methanotrophs are an important group of methane (CH₄)-oxidizing bacteria in the soil, which act as a major sink for the greenhouse gas, CH₄. In grazed grassland, one of the ecologically most sensitive areas is the animal urine patch soil, which is a major source of both nitrate (NO₃ ⁻) leaching and nitrous oxide (N₂O) emissions. Nitrification inhibitors, such as dicyandiamide (DCD), have been used to mitigate NO₃ ⁻ leaching and N₂O emissions in grazed pastures. However, it is not clear if the high nitrogen loading rate in the animal urine patch soil and the use of nitrification inhibitors would have an impact on the abundance of methanotrophs in grazed grassland soils. The purpose of this study was to determine the effect of animal urine and DCD on methanotroph abundance in grazed grassland soils.     

The short-term effects of nitrification inhibitors on the abundance and expression of ammonia and nitrite oxidizers in a long-term field experiment comparing land management

Microcosms were set up to evaluate the effect of nitrification inhibitors (DCD, c-PTiO, and NaClO₃) on the abundance and expression of ammonia-oxidizing bacteria (AOB) and archaea (AOA), as well as the nitrite-oxidizing bacteria (NOB) Nitrospira and Nitrobacter. Both DCD and NaClO₃ inhibited the net nitrification rate, while c-PTiO had no significant effects, and NaClO₃ had a much greater inhibitory effect (> 60%) in all soils than DCD. No significant changes in total microbial abundance were observed with DCD and NaClO₃. DCD limited only the growth of AOB; however, NaClO₃ inhibited growth of both AOA and Nitrospira-NOB with no significant effects on AOB and Nitrobacter-NOB. Probably NaClO₃ inhibited both ammonia oxidation and nitrite oxidation. This is the first report to reveal the inhibitory effects of NaClO₃ on a specific nitrification process, helping to clarify the ecological niche of nitrifiers and the potential of nitrification inhibitors applied to soil.     

pH-dependent distribution of soil ammonia oxidizers across a large geographical scale as revealed by high-throughput pyrosequencing

Purpose

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) are ubiquitous and important for nitrogen transformations in terrestrial ecosystems. However, the distribution patterns of these microorganisms as affected by the terrestrial environments across a large geographical scale are not well understood. This study was designed to gain insights into the ecological characteristics of AOA and AOB in 65 soils, collected from a wide range of soil and ecosystem types.

Materials and methods

Barcoded pyrosequencing in combination with quantitative PCR was employed to characterize the relative abundance, diversity, and community composition of archaeal 16S rRNA gene, and AOA and AOB amoA genes in 65 soil samples.

Results and discussion

The operational taxonomic unit richness and Shannon diversity of Thaumarchaeota, AOA, and AOB were highly variable among different soils, but their variations were best explained by soil pH. Soil pH was strongly correlated with the overall community composition of ammonia oxidizers, as measured by the pairwise Bray–Curtis dissimilarity across all sites. These findings were further corroborated by the evident pH-dependent distribution patterns of four thaumarchaeal groups (I.1a-associated, I.1b, I.1c, and I.1c-associated) and four AOB clusters (2, 3a.1, 10, and 12). The ratios of AOA to AOB amoA gene copy numbers significantly decreased with increasing pH, suggesting a competitive advantage of AOA over AOB in acidic soils.

Conclusions

These results suggest that the distribution of ammonia oxidizers across large-scale biogeographical settings can be largely predicted along the soil pH gradient, thus providing important indications for the ecological characteristics of AOA and AOB in different soils.