Preliminary study on the distribution of ammonia oxidizers and their contribution to potential ammonia oxidation in the plant-bed/ditch system of a constructed wetland

Purpose

Ammonia oxidation—as the rate-limiting step of nitrification—has been found to be performed by both ammonia-oxidizing archaea (AOA) and bacteria (AOB). However, how ammonium content and oxidation–reduction status regulate the distribution of ammonia oxidizers in constructed wetlands and their contribution to potential ammonia oxidation rate are still in dispute. This study aimed to explore the effects of ammonium content and oxidation–reduction status on the abundances of AOA/AOB and examine the contributions of AOA and AOB populations to ammonia oxidation rates in the plant-bed/ditch system of a constructed wetland.

Materials and methods

Sampling was carried out in the plant-bed/ditch system of the Shijiuyang Constructed Wetland, China. Three plant-bed soil cores were collected using a soil auger and sampled at depths of 0, 20, and 50 cm in 5-cm increments. Five ditch surface sediments (0–5 cm) were collected along the water flow direction. The abundances of AOA and AOB were investigated by quantitative polymerase chain reaction based on amoA genes. The potential ammonia oxidation rate was determined using the chlorate inhibition method.

Results and discussion

The results showed that AOA outnumbered AOB in the plant-bed surface soil which had lower ammonium content (4.67–7.63 mg kg^?1), but that AOB outnumbered AOA in the ditch surface sediment which had higher ammonium content (14.0–22.9 mg kg^?1). Ammonium content was found to be the crucial factor influencing the relative abundances of AOA and AOB in the surface samples of the plant-bed/ditch system. In the deep layers of the plant bed, AOA abundance outnumbered AOB, though much lower oxidation–reduction potential occurred along the water flow direction. Thus, the oxidation–reduction potential may be another factor influencing the distributions of AOA and AOB in the deep layers of the plant bed without significant difference in ammonium content (p?<?0.05). Moreover, the potential ammonia oxidation rate was significantly dominated by AOB rather than AOA in the plant-bed/ditch system.

Conclusions

The high ammonium content in the ditch sediment likely favored AOB. AOA seemed to persist more readily even under low oxidation–reduction potential in the deep layers of the plant bed. Ammonium content and the oxidation–reduction potential were important parameters influencing the distribution of AOA and AOB in the plant-bed/ditch system of Shijiuyang Constructed Wetland. AOB contributed more to ammonia oxidation than AOA, both in the plant-bed soils (r?=?0.592, p?=?0.0096) and in the ditch sediments (r?=?0.873, p?=?0.0002).     

Ammonia-oxidizing archaea are more important than ammonia-oxidizing bacteria in nitrification and NO3 −-N loss in acidic soil of sloped land

As part of a long-term sloped land use experiment established in 1995 at Taoyuan Agro-ecosystem Research Station (111°26′ E, 28°55′ N) in China, soil samples were collected from three land use types, including cropland (CL), natural forest, and tea plantation. Quantitative polymerase chain reaction and terminal restriction fragment length polymorphism were used to determine the abundance and community composition of amoA-containing bacteria (AOB) and archaea (AOA). The results indicate that land use type induced significant changes in soil potential nitrification rate and community composition, diversity, and abundance of AOB and AOA. Both AOB and AOA community compositions were generally similar between upper and lower slope positions (UP and LP), except within CL. The LP soils had significantly (p?<?0.05) higher diversity and abundance of both AOB and AOA than in the UP. Potential nitrification rate was significantly correlated (p?<?0.05) with diversity and abundance of AOA, but not with AOB. Among land use types, the NO₃ ^? and amoA-containing AOA runoff loss was greatest in CL. Nitrate-N runoff loss was significantly correlated (p?<?0.05) with the loss of AOA amoA copies in the runoff water. Furthermore, relationships between NO₃ ^?-N runoff loss and abundance of AOA but not of AOB at both slope positions were significantly correlated (p?<?0.05). These findings suggest that AOA are more important than AOB in nitrification and NO₃ ^?-N runoff loss in acidic soils across sloped land use types.     

Can phosphorus and nitrogen addition affect ammonia oxidizers in a high-phosphorus agricultural soil?

Ammonia-oxidizing archaea (AOA) and bacteria (AOB), which convert NH₃ to NO₂^? in soils, are important for agricultural production. It is well known that N addition can strongly affect soil ammonia oxidizers, but little is known about P addition. Based on microcosm experiments, this study assessed the responses of ammonia oxidizers to chemical P addition in a typically high P agricultural soil with or without N supply. Six treatments examined were neither N nor P, P alone (0.15, 0.45, and 0.75 g P₂O₅ kg^?1 soil, respectively), N alone (0.25 g N kg^?1 soil), and N plus P (0.25 g N and 0.15 g P₂O₅ kg^?1 soil). Quantitative real-time PCR for the abundance and high-throughput sequencing for community structure were applied. The results revealed that P addition did not affect the abundances and community structures of AOA and AOB, but N addition significantly increased AOB abundance and alter its community structure. Without N supply, continuously increasing soil P availability did not affect these two groups of ammonia oxidizers. This study highlights the relationship between soil P availability and ammonia oxidizers and suggests that soil P availability could be as a potential indicator for predicting N-related ecosystem functions in agricultural production.     

Fluctuations in the abundance of ammonia oxidizers and the contents of inorganic nitrogen in solarized soil

Solarization makes a great impact on the abundance of ammonia oxidizers and nitrifying activity in soil. To elucidate fluctuations in the abundance of ammonia oxidizers and nitrification in solarized soil, copy numbers of amoA gene of ammonia-oxidizing bacteria (AOB) and archaea (AOA), viable number of ammonia oxidizers and inorganic nitrogen contents were investigated in greenhouse experiments. The copy number of amoA gene and the viable number of ammonia oxidizers were determined by the quantitative polymerase chain reaction and most probable number methods, respectively. Abundance of AOB based on the estimation of amoA gene copy numbers and viable counts of ammonia oxidizers was decreased by the solarization treatment and increased during the tomato (Solanum lycopersicum L.) cultivation period following the solarization. Effect of solarization on the copy number of amoA gene of AOA was less evident than that on AOB. The proportion of nitrate in inorganic nitrogen contents was declined by the solarization and increased during the tomato cultivation period following the solarization. Positive correlations were found between the proportion of nitrate in inorganic nitrogen content and the copy number of bacterial or archaeal amoA gene or the viable number of ammonia oxidizers; the copy number of bacterial amoA gene showed a strong correlation with the viable number of ammonia oxidizers. The present study revealed influences of solarization on the fluctuation in the abundance of ammonia oxidizers and dynamics of inorganic nitrogen contents in soil and the results indicate that the determination of amoA gene of AOB is possibly a quick and useful diagnostic technique for evaluating suppression and restoration of nitrification following solarization.     

Direct and indirect influence of arbuscular mycorrhizal fungi on abundance and community structure of ammonia oxidizing bacteria and archaea in soil microcosms

Both arbuscular mycorrhizal (AM) fungi and ammonia oxidizers are important soil microbial groups in regulating soil N cycling. However, knowledge of their interactions, especially the direct influences of AM fungi on ammonia oxidizers is very limited to date. In the present study, a controlled microcosm experiment was established to examine the effects of AM fungi and N supply level on the abundance and community structure of ammonia oxidizing bacteria (AOB) and archaea (AOA) in the rhizosphere of alfalfa plants (Medicago sativa L.) inoculated with AM fungus Glomus intraradices. Effects were studied using combined approaches of quantitative polymerase chain reaction (qPCR) and terminal-restriction fragment length polymorphism (T-RFLP). The results showed that inoculation with AM fungi significantly increased the plant dry weights, total N and P uptake. Concomitantly, AM fungi significantly decreased the amoA gene copy numbers of AOA and AOB in the root compartment (RC) but not in the hyphal compartment (HC). Moreover, AM fungi induced some changes in AOA community structure in HC and RC, while only marginal variations in AOA composition were observed to respond to N supply level in HC. Neither RC nor HC showed significant differences in AOB composition irrespective of experimental treatments. The experimental results suggested that AM fungi could directly shape AOA composition, but more likely exerted indirect influences on AOA and AOB abundance via the plant pathway. In general, AM fungi may play an important role in mediating ammonia oxidizers, but the AOA community appeared to be more sensitive than the AOB community to AM fungi.     

Long-term fertilization effects on active ammonia oxidizers in an acidic upland soil in China

The effects of long-term fertilization of acidic soils on ammonia-oxidizing archaea (AOA) and bacteria (AOB) communities and its ecological implications remain poorly understood. We chose an acidic upland soil site under long-term (27-year) fertilization to investigate ammonia oxidizer communities under four different regimes: mineral N fertilizer (N), mineral NPK fertilizer (NPK), organic manure (OM) and an unfertilized control (CK). Soil net nitrification rates were significantly higher in OM soils than in CK, N or NPK soils. Quantitative analysis of the distribution of amoA genes by DNA-based stable isotope probing revealed that AOA dominate in CK, N and NPK soils, while AOB dominate in OM soils. Denaturing gradient gel electrophoresis and clone library analyses of amoA genes revealed that Group 1.1a-associated AOA (also referred to as Nitrosotalea) were the most dominant active AOA population (>92%), while Nitrosospira Cluster 3 and Cluster 9 were predominant among active AOB communities. The functional diversity of active ammonia oxidizers in acidic soils is affected by long-term fertilization practices, and the responses of active ammonia oxidizers to mineral fertilizer and organic manure are clearly different. Our results provide strong evidence that AOA are more highly adapted to growth at low pH and low substrate availability than AOB, and they suggest that the niche differentiation and metabolic diversity of ammonia oxidizers in acidic soils are more complex than previously thought.     

Effect of biochar on aerobic processes,enzyme activity,and crop yields in two sandy loam soils

Biochar added to agricultural soils may sequester carbon and improve physico-chemical conditions for crop growth, due to effects such as increased water and nutrient retention in the root zone. The effects of biochar on soil microbiological properties are less certain. We addressed the effects of wood-based biochar on soil respiration, water contents, potential ammonia oxidation (PAO), arylsulfatase activity (ASA), and crop yields at two temperate sandy loam soils under realistic field conditions. In situ soil respiration, PAO, and ASA were not significantly different in quadruplicate field plots with or without biochar (20 Mg ha^?1); however, in the same plots, volumetric water contents increased by 7.5 % due to biochar (P?=?0.007). Crop yields (oat) were not significantly different in the first year after biochar application, but in the second year, total yields of spring barley increased by 11 % (P??1, applied during two consecutive years, substantiated that biochar was not inhibitory to PAO and ASA as reference plots consistently showed lowest activities. For PAO, it was found that soil pH, rather than biochar rates, was a driving environmental variable. For ASA, the methodological approach was challenged by product sorption, but results did not suggest that biochar significantly stimulated the enzyme activity. Crop yields of maize in field experiments with 10–100 Mg biochar ha^?1 were unaffected by biochar except for a negative effect of the highest annual rates of 50 Mg ha^?1 in the first year after application. In conclusion, the present wood-based biochar poorly affected the measured microbial processes and generally resulted in similar crop yields in reference and biochar-amended soil plots.     

Contrasting response of nitrification capacity in three agricultural soils to N addition during short-term incubation

Purpose

Human disturbance is a major culprit driving imbalances in the biological transformation of nitrogen from the nonreactive to the reactive pool and is therefore one of the greatest concerns for nitrogen (N) cycling. The objective of this study was to compare potential nitrification rates and the abundance of ammonia oxidizers responsible for nitrification, with the amendment of external N in different agricultural soils.

Materials and methods

Three typical Chinese agricultural soils, QiYang (QY) acid soil, ShenYang (SY) neutral soil, and FengQiu (FQ) alkaline soil, were amended with 0, 20, 150, and 300 μg NH₄ ⁺-N g^?1 soil and incubated for 40 days. The abundance of ammonia oxidizing bacteria (AOB) and archaea (AOA) at the end of incubation in the soil microcosms was determined using the real-time PCR.

Results and discussion

There was a significant decrease in ammonium concentration in the QY soil from the highest to the lowest N-loading treatments, while no significant difference in ammonium concentrations was detected among the different N-loading treatments for the SY and FQ soils. A significantly higher potential nitrification rate (PNR) was observed in the FQ soil while lowest PNR was found in the QY soil. Quantitative PCR analysis of AOB amoA genes demonstrated that AOB abundance was significantly higher in the high N-loading treatments than in the control for the QY soil only, while no significant difference among treatments in the SY and FQ soils. A significant positive correlation between PNR and AOB amoA abundance, however, was found for the SY and FQ soils, but not for the QY soil. Little difference in AOA amoA abundance between different N-loading treatments was observed for all the soils.

Conclusions

This study suggested that ammonia oxidation capacity in the FQ and SY soils was higher than those in the QY soil with the addition of ammonium fertilizer for a short-term. These findings indicated that understanding the differential responses of biological nitrification to varying input levels of ammonium fertilizer is important for maximizing N use efficiency and thereby improving agricultural fertilization management.     

Responses of ammonia-oxidizing bacteria and archaea to nitrogen fertilization and precipitation increment in a typical temperate steppe in Inner Mongolia

As the first and rate-limiting step of nitrification, ammonia oxidation can be realized either by ammonia-oxidizing bacteria (AOB) or archaea (AOA). However, the key factors driving the abundance, community structure and activity of ammonia oxidizers are still unclear, and the relative importance of AOA and AOB in ammonia oxidation is unresolved. In the present study, we examined the effects of long-term (6 years) nitrogen (N) addition and simulated precipitation increment on the abundance and community composition of AOA and AOB based on a field trial in a typical temperate steppe of northern China. We used combined approaches of quantitative PCR, terminal-restriction fragment length polymorphism (T-RFLP) and clone library analyses of amoA genes. The study objective was to determine (1) AOA and AOB diversity and activity in response to N addition and increased precipitation and (2) the relative contributions of AOA and AOB to soil ammonia oxidation in the typical temperate steppe. The results showed that the potential nitrification rate (PNR) increased with N addition, but decreased with increased precipitation. Both N addition and increased precipitation significantly increased AOB but not AOA abundance, and a significant correlation was only observed between PNR and AOB amoA gene copies. The T-RFLP analysis showed that both N and precipitation were key factors in shaping the composition of AOB, while AOA were only marginally influenced. Phylogenetic analysis indicated that all AOA clones fell within the soil and sediment lineage while all AOB clones fell within the Nitrosospira. The study suggested that AOA and AOB had distinct physiological characteristics and ecological niches. AOB were shown to be more sensitive to N and precipitation than AOA, and the ammonia oxidation process was therefore supposed to be mainly driven by AOB in this temperate steppe.     

Responses of bacterial and archaeal ammonia oxidizers to soil organic and fertilizer amendments under long-term management

Ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) co-exist in soil, but their relative distribution may vary depending on the environmental conditions. Effects of changes in soil organic matter and nutrient content on the AOB and AOA are poorly understood. Our aim was to compare effects of long-term soil organic matter depletion and amendments with labile (straw) and more recalcitrant (peat) organic matter, with and without easily plant-available nitrogen, on the activities, abundances and community structures of AOB and AOA. Soil was sampled from a long-term field site in Sweden that was established in 1956. The potential ammonia oxidation rates, the AOB and AOA amoA gene abundances and the community structures of both groups based on T-RFLP of amoA genes were determined. Straw amendment during 50 years had not altered any of the measured soil parameters, while the addition of peat resulted in a significant increase of soil organic carbon as well as a decrease in pH. Nitrogen fertilization alone resulted in a small decrease in soil pH, organic carbon and total nitrogen, but an increase in primary production. Type and amount of organic matter had an impact on the AOB and AOA community structures and the AOA abundance. Our findings confirmed that AOA are abundant in soil, but showed that under certain conditions the AOB dominate, suggesting niche differentiation between the two groups at the field site. The large differences in potential rates between treatments correlated to the AOA community size, indicating that they were functionally more important in the nitrification process than the AOB. The AOA abundance was positively related to addition of labile organic carbon, which supports the idea that AOA could have alternative growth strategies using organic carbon. The AOB community size varied little in contrast to that of the AOA. This indicates that the bacterial ammonia oxidizers as a group have a greater ecophysiological diversity and potentially cover a broader range of habitats.     

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.     

Nitrapyrin affects the abundance of ammonia oxidizers rather than community structure in a yellow clay paddy soil

Purpose

Yellow clay paddy soil (Oxisols) is a low-yield soil with low nitrogen use efficiency (NUE) in southern China. The nitrification inhibitor nitrapyrin (2-chloro-6- (tricholoromethyl)-pyridine, CP) has been applied to improve NUE and reduce environmental pollution in paddy soil. However, the effects of nitrapyrin combined with nitrogen fertilizers on ammonia oxidizers in yellow clay paddy soil have not been examined.

Materials and methods

A randomized complete block design was set with three treatments: (1) without nitrogen fertilizer (CK), (2) common prilled urea (PU), and (3) prilled urea with nitrapyrin (NPU). Soil samples were collected from three treatments where CK, PU, and NPU had been repeatedly applied over 5 years. Soil samples were analyzed by quantitative PCR and 454 high-throughput pyrosequencing of the amoA gene to investigate the influence of nitrapyrin combined with nitrogen on the abundance and community structure of ammonia oxidizers in yellow clay paddy soil.

Results and discussion

The potential nitrification rate (PNR) of the soil was significantly correlated with the abundances of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Application of urea significantly stimulated AOA and AOB growth, whereas nitrapyrin exhibited inhibitory effects on AOA. Phylogenetic analysis showed that the most dominant operational taxonomic units (OTUs) of AOA and AOB were affiliated with the Nitrosotalea cluster and Nitrosospira cluster 12, respectively. AOA and AOB community structures were not altered by urea and nitrapyrin application.

Conclusions

Nitrogen fertilization stimulated nitrification and increased the population sizes of AOA and AOB. Nitrapyrin affected the abundance, but not community structure of ammonia oxidizers in yellow clay soil. Our results suggested that nitrapyrin improving NUE and inhibiting PNR was attributable to the inhibition of AOA growth.

     

Comparative diversity and abundance of ammonia monooxygenase genes in mulched and vegetated soils

Soil microbial habitats are altered by mulching, a common practice in urban areas during which vegetation is removed and soils covered to suppress weeds and retain moisture. Soil microorganisms drive nitrogen-cycling processes in mulched soils, because living plants no longer take up ammonium-N released during decomposition of residual organic matter. Because ammonia oxidizers carry out the first, rate-limiting step of nitrification, we compared ammonia oxidizers in experimental, unfertilized plots of mulched and vegetated soils. We hypothesized that mulched and vegetated soils would support contrasting communities of bacterial and archaeal ammonia oxidizers, as determined by quantitative PCR and primers specific for genes encoding ammonia monooxygenase subunit A (amoA). Clone libraries of archaeal amoA also were constructed to compare diversity in soil cores, duplicate blocked plots, and treatments (bark-mulched, gravel-mulched, and unmanaged old field vegetation). Gene copies from ammonia-oxidizing bacteria (AOB) ranged from 2.2 × 10⁶ to 2.7 × 10⁷ gene copies per gram dry soil and did not differ across treatments. In contrast, gene copies from ammonia-oxidizing archaea (AOA) ranged from 9.1 × 10⁵ to 1.0 × 10⁸ copies per gram dry soil, with bark-mulched soils having significantly lower abundance. Community structure of AOA in gravel-mulched soils was distinct from the other two treatments. At 97% amino acid similarity, 22 operational taxonomic units, or OTUs, were identified, with only one OTU found in all 18 clone libraries. This ubiquitous OTU-1, which was highly similar to published amoA sequences recovered from soils, comprised 55% of all 482 translated sequences. Greater variability in OTU richness was observed among cores from mulched soils than from vegetated soils. Our observations supported our hypothesis that AOA communities differ in mulched and vegetated soils, with mulched soils providing altered and variable microniches for these N cycling microorganisms.     

Chronic Nitrogen Fertilization Modulates Competitive Interactions Among Microbial Ammonia Oxidizers in a Loess Soil

Nitrogen(N) application may lead to niche segregation of soil ammonia-oxidizing archaea(AOA) and bacteria(AOB), thereby reducing the competitive interactions between AOA and AOB due to higher ammonium substrate availability. However, the adaptive mechanisms of AOA and AOB under N enrichment remain poorly understood. Stable isotope probing(SIP) microcosm incubation was employed to reveal community changes of active AOA and AOB in a loess soil from a field experiment growing potatoes that received no N(control, CK), low N(LN, 75 kg N ha~(-1)), and high N(HN, 375 kg N ha~(-1)). The results showed that the soil potential nitrification rate(PNR) was measured by culturing of the soil samples from the field experiment. Soil PNR was significantly increased in HN by87.5% and 67.5% compared with CK and LN, respectively. Compared with CK, the~(13)C-amoA genes of soil AOA and AOB in HN had 2.58 × 10~4 and 1.55 × 10~6 copies, representing 1.6-and 16.2-fold increase respectively. It was indicated that AOB dominated soil ammonia oxidation. A phylogenetic analysis of the~(13)C-amoA gene showed that N application significantly increased the proportion of54 d9-like AOA up to 90% in HN, while the Nitrososphaera gargensis-like and Nitrososphaera viennensis-like AOA were inhibited and completely disappeared. Nitrogen application also resulted in the community shift of active AOB-dominant group from Nitrosospira briensis-like to Nitrosospira sp. TCH711-like. Our study provides compelling evidence for the emergence and maintenance of active nitrifying communities under the intensified N input to an agricultural ecosystem.     

Soil pH and plant diversity drive co-occurrence patterns of ammonia and nitrite oxidizer in soils from forest ecosystems

In this study, we investigated how co-occurrence patters of ammonia and nitrite oxidizers, which drive autotrophic nitrification, are influenced by tree species composition as well as soil pH in different forest soils. We expected that a decline of ammonia oxidizers in coniferous forests, as a result of excreted nitrification inhibitors and at acidic sites with low availability of ammonia, would reduce the abundance of nitrite-oxidizing bacteria (NOB). To detect shifts in co-occurrence patterns, the abundance of key players was measured at 50 forest plots with coniferous respectively deciduous vegetation and different soil pH levels in the region Schwäbische Alb (Germany). We found ammonia-oxidizing archaea (AOA) and Nitrospira-like NOB (NS) to be dominating in numbers over their counterparts across all forest types. AOA co-occurred mostly with NS, while bacterial ammonia oxidizers (AOB) were correlated with Nitrobacter-like NOB (NB). Co-occurrence patterns changed from tight significant relationships of all ammonia and nitrite oxidizers in deciduous forests to a significant relationship of AOB and NB in coniferous forests, where AOA abundance was reduced. Surprisingly, no co-occurrence structures between ammonia and nitrite oxidizers could be determined at acidic sites, although abundances were correlated to the respective nitrogen pools. This raises the question whether interactions with heterotrophic nitrifiers may occur, which needs to be addressed in future studies.     

Effects of biochar application on soil methane emission at different soil moisture levels

The aim of this study was to investigate the effects of biochar application on soil methane (CH₄) emission. Experiments were conducted over an 84-day incubation period with the following treatments: each of two soils (a paddy soil and a forest soil) was treated with or without biochar at three soil moisture levels (35, 60, and 100 % water-filled pore space (WFPS) for the paddy soil; 35, 60, and 85 % WFPS for the forest soil). Biochar application (P?<?0.05) significantly increased soil pH and stimulated C mineralization at the early incubation stage. The effects of biochar application on CH₄ emission were related to the soil moistures, with reduction of CH₄ emission at 35 and 60 % WFPS and stimulation at the highest soil moisture. While both soils changed from CH₄ sinks to sources by increasing soil moisture regardless of biochar addition, the effect was enhanced with biochar application. At lower soil moistures, the CH₄ oxidation activity in soils was higher with biochar than without biochar, while the trend became opposite at higher soil moistures. Therefore, the CH₄ production and consumption processes were influenced by different soil moisture levels and microbial communities of different soils.     

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.

     

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.

     

15N示踪法研究生物炭施用对水稻—土壤系统氮肥去向的影响

为探究施用水稻秸秆生物炭对水稻产量、氮肥利用率、氮肥残留及损失的影响,采用盆栽试验结合¹⁵N示踪技术,分析了施用水稻秸秆生物炭对水稻生物量、氮素积累量、肥料氮去向以及氨氧化微生物的影响。研究共设置5个处理:不施氮肥(N0)、单施化肥(CF)、施化肥配施0.5%生物炭(BC1)、施化肥配施1%生物炭(BC2)和施化肥配施2%生物炭(BC3)。结果表明:与CF处理相比,BC2和BC3处理均显著提高水稻产量,增产率分别为19.3%和22.0%。施用生物炭显著增加水稻氮素积累量和表观利用率。施用生物炭的水稻籽粒肥料氮积累和总肥料氮积累量较CF处理分别提高18.6%~23.4%和18.5%~26.5%。然而,施用生物炭处理与CF处理之间的籽粒土壤氮吸收量没有显著差异。BC1、BC2和BC3处理的氮肥利用率分别为30.4%,28.5%和29.3%,均显著高于CF处理(24.1%)。施用生物炭有利于肥料氮在土壤中的 残留,从而减少损失。因此,施用生物炭的肥料氮损失率(25.7%~27.5%)显著低于单施化肥处理(38.4%)。与CF处理相比,高量施用生物炭(BC3)显著降低氨氧化细菌的amoA基因拷贝数,但施用生物炭对氨氧化古菌丰度没有显著影响。综上表明,施用水稻秸秆生物炭是提高水稻产量和氮肥利用率,同时还是有效减少氮素损失的一种有效措施。     

Biosurfactant in Membrane Separation of Atrazine from Water

This study examines the effects of land use change on nitrate concentration and the abundances of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and narG-containing denitrifiers in shallow groundwater. The results reveal a general increase of nitrate in shallow groundwater following the change of land use from paddy fields to vegetable patches. Furthermore, a significant relationship between NO₃ ^?-N concentrations was observed both in groundwater and in soil at soil depths of 0–20, 20–40, 40–60, 60–80, and 80–100 cm. With regard to gene abundance in groundwater, the AOB amoA gene was most abundant and the AOA amoA gene copy numbers were lowest from the field with long-term paddy cultivation compared with the field under vegetable cultivation. The narG gene copy numbers were higher from the field under short-term vegetable cultivation compared with fields under long-term vegetable cultivation. The NO₃ ^?-N concentrations in groundwater correlated positively with AOA amoA gene copy numbers, negatively with the AOB amoA gene, but with no significant relationship with the narG gene. In conclusion, land use change from paddy fields to vegetable patches increases nitrate in groundwater, which is correlated significantly with nitrate in soil and the abundance of the amoA gene, but is not related to the narG gene in groundwater. This study also suggests that the removal of groundwater nitrate pollution is not feasible through biological denitrification without additional denitrifiers and that it might even become more aggravated because of the AOA.