Impact of plant functional group, plant species, and sampling time on the composition of nirK-type denitrifier communities in soil

We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.     

设施菜田不同碳氮管理对反硝化菌结构和功能的影响

【目的】通过6年长期定位试验,比较设施菜田不同碳氮管理下反硝化菌结构和功能的差异。【方法】采用末端限制性片段多态性(T-RFLP)和变性梯度凝胶电泳(DGGE)方法分别分析nir K/nir S和nos Z型反硝化菌群结构特征,利用自动连续在线培养监测体系(Robot系统)测定分析NO/(NO3-+NO2-)和N2O/(N2O+N2)产物比,并通过乙炔抑制法测定反硝化酶活性。【结果】传统施肥处理(CN)显著改变了nir K和nos Z型反硝化菌的结构,增加了NO/(NO3-+NO2-)和N2O/(N2O+N2)产物比。nir S型菌受碳氮管理影响较小。减氮(RN)和添加秸秆处理(RN+S)的nir K和nos Z型反硝化菌结构与CN处理的差异性显著,且会显著降低NO/(NO3-+NO2-)和N2O/(N2O+N2)产物比;与CN和RN相比,RN+S显著增加反硝化酶活性。【结论】设施菜田长期传统施肥措施改变了反硝化菌的结构和功能,增加土壤自身的NO产生能力并减弱了N2O还原N2的能力。减氮和添加秸秆管理能形成自身的反硝化菌群结构,并降低NO和N2O排放风险;秸秆的添加会促进反硝化潜在速率,降低菜田NO3-淋洗风险。     

Impact of Plant Functional Group, Plant Species, and Sampling Time on the Composition of nirK-Type Denitrifier Communities in Soil

We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.     

Abundance, diversity and functional gene expression of denitrifier communities in adjacent riparian and agricultural zones

Lands under riparian and agricultural management differ in soil properties, water content, plant species and nutrient content and are therefore expected to influence denitrifier communities, denitrification and nitrous oxide (N(2) O) emissions. Denitrifier community abundance, denitrifier community structure, denitrification gene expression and activity were quantified on three dates in a maize field and adjacent riparian zone. N(2) O emissions were greater in the agricultural zone, whereas complete denitrification to N(2) was greater in the riparian zone. In general, the targeted denitrifier community abundance did not change between agricultural and riparian zones. However, nosZ gene expression was greater in the riparian zone than the agricultural zone. The community structure of nirS-gene-bearing denitrifiers differed in June only, whereas the nirK-gene-bearing community structure differed significantly between the riparian and the agricultural zones at all dates. The nirK-gene-bearing community structure was correlated with soil pH, while no significant correlations were found between nirS-gene-bearing community structure and soil environmental variables or N(2) O emissions, denitrification or denitrifier enzyme activity. The results suggested for the nirK and nirS-gene-bearing communities different factors control abundance vs. community structure. The nirK-gene-bearing community structure was also more responsive than the nirS-gene-bearing community structure to change between the two ecosystems.     

Response of total and nitrate-dissimilating bacteria to reduced N deposition in a spruce forest soil profile

A field-scale manipulation experiment conducted for 16 years in a Norway spruce forest at Solling, Central Germany, was used to follow the long-term response of total soil bacteria, nitrate reducers and denitrifiers under conditions of reduced N deposition. N was experimentally removed from throughfall by a roof construction ('clean rain plot'). We used substrate-induced respiration (SIR) to characterize the active fraction of soil microbial biomass and potential nitrate reduction to quantify the activity of nitrate reducers. The abundance of total bacteria, nitrate reducers and denitrifiers in different soil layers was analysed by quantitative PCR of 16S rRNA gene, nitrate reduction and denitrification genes. Reduced N deposition temporarily affected the active fraction of the total microbial community (SIR) as well as nitrate reductase activity. However, the size of the total, nitrate reducer and denitrifier communities did not respond to reduced N deposition. Soil depth and sampling date had a greater influence on the density and activity of soil microorganisms than reduced deposition. An increase in the nosZ /16S rRNA gene and nosZ/nirK ratios with soil depth suggests that the proportion of denitrifiers capable of reducing N₂O into N₂ is larger in the mineral soil layer than in the organic layer.     

Actinobacterial nitrate reducers and proteobacterial denitrifiers are abundant in N2O-metabolizing palsa peat

Palsa peats are characterized by elevated, circular frost heaves (peat soil on top of a permanently frozen ice lens) and are strong to moderate sources or even temporary sinks for the greenhouse gas nitrous oxide (N(2)O). Palsa peats are predicted to react sensitively to global warming. The acidic palsa peat Skalluvaara (approximate pH 4.4) is located in the discontinuous permafrost zone in northwestern Finnish Lapland. In situ N(2)O fluxes were spatially variable, ranging from 0.01 to -0.02 μmol of N(2)O m(-2) h(-1). Fertilization with nitrate stimulated in situ N(2)O emissions and N(2)O production in anoxic microcosms without apparent delay. N(2)O was subsequently consumed in microcosms. Maximal reaction velocities (v(max)) of nitrate-dependent denitrification approximated 3 and 1 nmol of N(2)O per h per gram (dry weight [g(DW)]) in soil from 0 to 20 cm and below 20 cm of depth, respectively. v(max) values of nitrite-dependent denitrification were 2- to 5-fold higher than the v(max) nitrate-dependent denitrification, and v(max) of N(2)O consumption was 1- to 6-fold higher than that of nitrite-dependent denitrification, highlighting a high N(2)O consumption potential. Up to 12 species-level operational taxonomic units (OTUs) of narG, nirK and nirS, and nosZ were retrieved. Detected OTUs suggested the presence of diverse uncultured soil denitrifiers and dissimilatory nitrate reducers, hitherto undetected species, as well as Actino-, Alpha-, and Betaproteobacteria. Copy numbers of nirS always outnumbered those of nirK by 2 orders of magnitude. Copy numbers of nirS tended to be higher, while copy numbers of narG and nosZ tended to be lower in 0- to 20-cm soil than in soil below 20 cm. The collective data suggest that (i) the source and sink functions of palsa peat soils for N(2)O are associated with denitrification, (ii) actinobacterial nitrate reducers and nirS-type and nosZ-harboring proteobacterial denitrifiers are important players, and (iii) acidic soils like palsa peats represent reservoirs of diverse acid-tolerant denitrifiers associated with N(2)O fluxes.     

Association of earthworm-denitrifier interactions with increased emission of nitrous oxide from soil mesocosms amended with crop residue

Earthworm activity is known to increase emissions of nitrous oxide (N(2)O) from arable soils. Earthworm gut, casts, and burrows have exhibited higher denitrification activities than the bulk soil, implicating priming of denitrifying organisms as a possible mechanism for this effect. Furthermore, the earthworm feeding strategy may drive N(2)O emissions, as it determines access to fresh organic matter for denitrification. Here, we determined whether interactions between earthworm feeding strategy and the soil denitrifier community can predict N(2)O emissions from the soil. We set up a 90-day mesocosm experiment in which (15)N-labeled maize (Zea mays L.) was either mixed in or applied on top of the soil in the presence or absence of the epigeic earthworm Lumbricus rubellus and/or the endogeic earthworm Aporrectodea caliginosa. We measured N(2)O fluxes and tested the bulk soil for denitrification enzyme activity and the abundance of 16S rRNA and denitrifier genes nirS and nosZ through real-time quantitative PCR. Compared to the control, L. rubellus increased denitrification enzyme activity and N(2)O emissions on days 21 and 90 (day 21, P = 0.034 and P = 0.002, respectively; day 90, P = 0.001 and P = 0.007, respectively), as well as cumulative N(2)O emissions (76%; P = 0.014). A. caliginosa activity led to a transient increase of N(2)O emissions on days 8 to 18 of the experiment. Abundance of nosZ was significantly increased (100%) on day 90 in the treatment mixture containing L. rubellus alone. We conclude that L. rubellus increased cumulative N(2)O emissions by affecting denitrifier community activity via incorporation of fresh residue into the soil and supplying a steady, labile carbon source.     

Short-term influence of recolonisation by the polycheate worm [2pt] Nereis succinea on oxygen and nitrogen fluxes and [2pt] denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sediment-water column fluxes of oxygen and dissolved inorganic nitrogen, and rates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of stimulation was stable with time. Similarly, ammonium fluxes to the water column were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally greater in the faunated microcosms, but highly variable with time. Denitrification rates were positively stimulated by N. succinea populations, denitrification of water column nitrate was stimulated 10-fold in comparison to denitrification coupled to nitrification in the sediments. Rates of denitrification of water column nitrate were not significantly different from rates in undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification–denitrification were 3-fold lower in the experimental set-up. These results may reflect the relative growth rates of nitrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly reduced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dystrophic crisis, may play a significant role in the reoxidation and detoxification of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low capacity of nitrifying bacteria to colonise the new burrow structures, these losses would be highly dependent upon water column nitrate concentrations.     

Community-specific pH response of denitrification: experiments with cells extracted from organic soils

Denitrifying prokaryotes are phylogenetically and functionally diverse. Little is known about the relationship between soil denitrifier community composition and functional traits. We extracted bacterial cells from three cultivated peat soils with contrasting native pH by density gradient centrifugation and investigated their kinetics of oxygen depletion and NO2 -, NO, N(2) O and N(2) accumulation during initially hypoxic batch incubations (0.5-1 μM O(2)) in minimal medium buffered at either pH 5.4 or 7.1 (2 mM glutamate, 2 mM NO3 -). The three communities differed strikingly in NO2 - accumulation and transient N(2) O accumulation at the two pH levels, whereas NO peak concentrations (24-53 nM) were similar across all communities and pH treatments. The results confirm that the communities represent different denitrification regulatory phenotypes, as indicated by previous denitrification bioassays with nonbuffered slurries of the same three soils. The composition of the extracted cells resembled that of the parent soils (PCR-TRFLP analyses of 16S rRNA genes, nirK, nirS and nosZ), which were found to differ profoundly in their genetic composition (Braker et al., ). Together, this suggests that direct pH response of denitrification depends on denitrifier community composition, with implications for the propensity of soils to emit N(2) O to the atmosphere.     

Spatially tripartite interactions of denitrifiers in arctic ecosystems: activities, functional groups and soil resources

Soil denitrification is one of the most significant contributors to global nitrous oxide (N(2) O) emissions, and spatial patterns of denitrifying communities and their functions may reveal the factors that drive denitrification potential and functional consortia. Although denitrifier spatial patterns have been studied extensively in most soil ecosystems, little is known about these processes in arctic soils. This study aimed to unravel the spatial relationships among denitrifier abundance, denitrification potential and soil resources in 279 soil samples collected from three Canadian arctic ecosystems encompassing 7° in latitude and 27° in longitude. The abundance of nirS (10(6) -10(8) copies?g(-1) dry soil), nirK (10(3) -10(7) copies?g(-1) dry soil) and nosZ (10(6) -10(7) copies?g(-1) dry soil) genes in these soils is in the similar range as non-arctic soil ecosystems. Potential denitrification in Organic Cryosols (1034?ng?N(2) O-N?g(-1) soil) was 5-11 times higher than Static/Turbic Cryosols and the overall denitrification potential in Cryosols was also comparable to other ecosystems. We found denitrifier functional groups and potential denitrification were highly spatially dependent within a scale of 5?m. Functional groups and soil resources were significantly (P?相似文献   

Contrasting denitrifier communities relate to contrasting N2O emission patterns from acidic peat soils in arctic tundra

Cryoturbated peat circles (that is, bare surface soil mixed by frost action; pH 3–4) in the Russian discontinuous permafrost tundra are nitrate-rich ‘hotspots'' of nitrous oxide (N₂O) emissions in arctic ecosystems, whereas adjacent unturbated peat areas are not. N₂O was produced and subsequently consumed at pH 4 in unsupplemented anoxic microcosms with cryoturbated but not in those with unturbated peat soil. Nitrate, nitrite and acetylene stimulated net N₂O production of both soils in anoxic microcosms, indicating denitrification as the source of N₂O. Up to 500 and 10 μ nitrate stimulated denitrification in cryoturbated and unturbated peat soils, respectively. Apparent maximal reaction velocities of nitrite-dependent denitrification were 28 and 18 nmol N₂O g_DW⁻¹ h⁻¹, for cryoturbated and unturbated peat soils, respectively. Barcoded amplicon pyrosequencing of narG, nirK/nirS and nosZ (encoding nitrate, nitrite and N₂O reductases, respectively) yielded ≈49 000 quality-filtered sequences with an average sequence length of 444 bp. Up to 19 species-level operational taxonomic units were detected per soil and gene, many of which were distantly related to cultured denitrifiers or environmental sequences. Denitrification-associated gene diversity in cryoturbated and in unturbated peat soils differed. Quantitative PCR (inhibition-corrected per DNA extract) revealed higher copy numbers of narG in cryoturbated than in unturbated peat soil. Copy numbers of nirS were up to 1000 × higher than those of nirK in both soils, and nirS nirK⁻¹ copy number ratios in cryoturbated and unturbated peat soils differed. The collective data indicate that the contrasting N₂O emission patterns of cryoturbated and unturbated peat soils are associated with contrasting denitrifier communities.     

Genetic characterization of denitrifier communities with contrasting intrinsic functional traits

Microorganisms capable of denitrification are polyphyletic and exhibit distinct denitrification regulatory phenotypes (DRP), and thus, denitrification in soils could be controlled by community composition. In a companion study (D?rsch et al., 2012) and preceding work, ex situ denitrification assays of three organic soils demonstrated profoundly different functional traits including N(2) O/N(2) ratios. Here, we explored the composition of the underlying denitrifier communities by analyzing the abundance and structure of denitrification genes (nirK, nirS, and nosZ). The relative abundance of nosZ (vs. nirK + nirS) was similar for all communities, and hence, the low N(2) O reductase activity in one of the soils was not because of the lack of organisms with this gene. Similarity in community composition between the soils was generally low for nirK and nirS, but not for nosZ. The community with the most robust denitrification (consistently low N(2) O/N(2) ) had the highest diversity/richness of nosZ and nirK, but not of nirS. Contrary results found for a second soil agreed with impaired denitrification (low overall denitrification activity, high N(2) O/N(2) ). In conclusion, differences in community composition and in the absolute abundance of denitrification genes clearly reflected the functional differences observed in laboratory studies and may shed light on differences in in situ N(2) O emission of the soils.     

农田和森林土壤中氧化亚氮的产生与还原

采用土壤淤浆方法对丹麦农田和山毛榉森林土壤反硝化过程中Ｎ２Ｏ的产生与还原进行了研究。同时考察了硝酸根和铵离子对反硝化作用的影响。结果表明,森林土壤反硝化活性大于农田土壤,但农田土壤中Ｎ２Ｏ还原活性大于森林土壤,表现在农田和森林土壤中Ｎ２Ｏ／Ｎ２的产生比率分别为０．１１和３．６５。硝酸根和铵离子能促进两种土壤中的Ｎ２Ｏ产生,但可降低农田土壤中的Ｎ２Ｏ还原速率,与农田土壤相比,硝酸根可降低森林土壤Ｎ２     

Influence of temperature and soil water content on bacterial, archaeal and denitrifying microbial communities in drained fen grassland soil microcosms

In this study, microcosms were used to investigate the influence of temperature (4 and 28 degrees C) and water content (45% and 90% WHC) on microbial communities and activities in carbon-rich fen soil. Bacterial, archaeal and denitrifier community composition was assessed during incubation of microcosms for 12 weeks using terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA and nitrous oxide reductase (nosZ) genes. In addition, microbial and denitrifier abundance, potential denitrification activity and production of greenhouse gases were measured. No detectable changes were observed in prokaryote or denitrifier abundance. In general, cumulatively after 12 weeks more carbon was respired at the higher temperature (3.7 mg CO(2) g(-1) soil), irrespective of the water content, whereas nitrous oxide production was greater under wet conditions (98-336 microg N(2)O g(-1) soil). After an initial lag phase, methane emissions (963 microg CH(4) g(-1) soil) were observed only under warm and wet conditions. T-RFLP analyses of bacterial 16S rRNA and nosZ genes revealed small or undetectable community changes in response to temperature and water content, suggesting that bacterial and denitrifying microbial communities are stable and do not respond significantly to seasonal changes in soil conditions. In contrast, archaeal microbial community structure was more dynamic and was strongly influenced by temperature.     

Nitrate reduction in a groundwater microcosm determined by N gas chromatography-mass spectrometry

Aerobic and anaerobic groundwater continuous-flow microcosms were designed to study nitrate reduction by the indigenous bacteria in intact saturated soil cores from a sandy aquifer with a concentration of 3.8 mg of NO(3)-N liter. Traces of NO(3) were added to filter-sterilized groundwater by using a Darcy flux of 4 cm day. Both assimilatory and dissimilatory reduction rates were estimated from analyses of N(2), N(2)O, NH(4), and N-labeled protein amino acids by capillary gas chromatography-mass spectrometry. N(2) and N(2)O were separated on a megabore fused-silica column and quantified by electron impact-selected ion monitoring. NO(3) and NH(4) were analyzed as pentafluorobenzoyl amides by multiple-ion monitoring and protein amino acids as their N-heptafluorobutyryl isobutyl ester derivatives by negative ion-chemical ionization. The numbers of bacteria and their [methyl-H]thymidine incorporation rates were simultaneously measured. Nitrate was completely reduced in the microcosms at a rate of about 250 ng g day. Of this nitrate, 80 to 90% was converted by aerobic denitrification to N(2), whereas only 35% was denitrified in the anaerobic microcosm, where more than 50% of NO(3) was reduced to NH(4). Assimilatory reduction was recorded only in the aerobic microcosm, where N appeared in alanine in the cells. The nitrate reduction rates estimated for the aquifer material were low in comparison with rates in eutrophic lakes and coastal sediments but sufficiently high to remove nitrate from an uncontaminated aquifer of the kind examined in less than 1 month.     

Denitrification Activity of a Remarkably Diverse Fen Denitrifier Community in Finnish Lapland Is N-Oxide Limited

Peatlands cover more than 30% of the Finnish land area and impact N₂O fluxes. Denitrifiers release N₂O as an intermediate or end product. In situ N₂O emissions of a near pH neutral pristine fen soil in Finnish Lapland were marginal during gas chamber measurements. However, nitrate and ammonium fertilization significantly stimulated in situ N₂O emissions. Stimulation with nitrate was stronger than with ammonium. N₂O was produced and subsequently consumed in gas chambers. In unsupplemented anoxic microcosms, fen soil produced N₂O only when acetylene was added to block nitrous oxide reductase, suggesting complete denitrification. Nitrate and nitrite stimulated denitrification in fen soil, and maximal reaction velocities (v_max) of nitrate or nitrite dependent denitrification where 18 and 52 nmol N₂O h^-1 g_DW ^-1, respectively. N₂O was below 30% of total produced N gases in fen soil when concentrations of nitrate and nitrite were <500 μM. v_max for N₂O consumption was up to 36 nmol N₂O h^-1 g_DW ^-1. Denitrifier diversity was assessed by analyses of narG, nirK/nirS, and nosZ (encoding nitrate-, nitrite-, and nitrous oxide reductases, respectively) by barcoded amplicon pyrosequencing. Analyses of ~14,000 quality filtered sequences indicated up to 25 species-level operational taxonomic units (OTUs), and up to 359 OTUs at 97% sequence similarity, suggesting diverse denitrifiers. Phylogenetic analyses revealed clusters distantly related to publicly available sequences, suggesting hitherto unknown denitrifiers. Representatives of species-level OTUs were affiliated with sequences of unknown soil bacteria and Actinobacterial, Alpha-, Beta-, Gamma-, and Delta-Proteobacterial sequences. Comparison of the 4 gene markers at 97% similarity indicated a higher diversity of narG than for the other gene markers based on Shannon indices and observed number of OTUs. The collective data indicate (i) a high denitrification and N₂O consumption potential, and (ii) a highly diverse, nitrate limited denitrifier community associated with potential N₂O fluxes in a pH-neutral fen soil.     

Manipulation of the Dissolved Organic Carbon Pool in an Agricultural Stream: Responses in Microbial Community Structure, Denitrification, and Assimilatory Nitrogen Uptake

Carbon (C) and nitrogen (N) are strongly coupled across ecosystems due to stoichiometrically balanced assimilatory demand as well as dissimilatory processes such as denitrification. Microorganisms mediate these biogeochemical cycles, but how microbial communities respond to environmental changes, such as dissolved organic carbon (DOC) availability, and how those responses impact coupled biogeochemical cycles in streams is not clear. We enriched a stream in central Indiana with labile DOC for 5?days to investigate coupled C and N cycling. Before, and on day 5 of the enrichment, we examined assimilatory uptake and denitrification using whole-stream ¹⁵N-nitrate tracer additions and short-term nitrate releases. Concurrently, we measured bacterial and denitrifier abundance and community structure. We predicted N assimilation and denitrification would be stimulated by the addition of labile C and would be mediated by increases in bacterial activity, abundance, and a shift in community structure. In response to the twofold increase in DOC concentrations in the water column, N assimilation increased throughout the enrichment. Community respiration doubled during the enrichment and was associated with a change in bacterial community structure (based on terminal restriction fragment length polymorphisms of the 16S rRNA gene). In contrast, there was little response in denitrification or denitrifier community structure, likely because labile C was assimilated by heterotrophic communities on the stream bed prior to reaching denitrifiers within the sediments. Our results suggest that coupling between C and N in streams involves potentially complex interactions with sediment texture and organic matter, microbial community structure, and possibly indirect biogeochemical pathways.     

Effects of Shading Soybean Plants on N2O Emission from Soil

The effect of photosynthesis on N₂O emission from soil was investigated by shading soybean (Gycline max. L) plant at flowering, pod-setting and grain-filling stages. The results showed that by stopping photosynthesis through shading the plants stimulated N₂O emission significantly at flowering stage and pod-setting stage, and suppressed N₂O emission dramatically at grain-filling stage. At flowering stage, soybean species seem to rely mainly on fertilizer N and shaded plants decreased the N uptake. Interaction between the relative increase in available N for N₂O production by shading and the presence of root exudates promoted N transformation (nitrification/denitrification) and N₂O emission. At pod-setting stage, the available soil nitrogen seems to be a critical limiting factor and without substantial release of symbiotically fixed N through plant roots, resulted in a weak effect of shading on N₂O emission. At grain-filling stage, available N for N₂O production was derived from symbiotically fixed N and was greatly affected by photosynthesis. These results indicated that the effect of soybean growth on N₂O emission from soil varies with plant growth stages as available N for N₂O production is mainly from fertilizer N and organic mineralization during the early growth of soybean plants, while N₂O emission is controlled by the quantity and perhaps also the quality of root exudates, which is closely related with plant photosynthesis in the late season of soybean growth.     

Effects of selected root exudate components on soil bacterial communities

Low-molecular-weight organic compounds in root exudates play a key role in plant-microorganism interactions by influencing the structure and function of soil microbial communities. Model exudate solutions, based on organic acids (OAs) (quinic, lactic, maleic acids) and sugars (glucose, sucrose, fructose), previously identified in the rhizosphere of Pinus radiata, were applied to soil microcosms. Root exudate compound solutions stimulated soil dehydrogenase activity and the addition of OAs increased soil pH. The structure of active bacterial communities, based on reverse-transcribed 16S rRNA gene PCR, was assessed by denaturing gradient gel electrophoresis and PhyloChip microarrays. Bacterial taxon richness was greater in all treatments than that in control soil, with a wide range of taxa (88-1043) responding positively to exudate solutions and fewer (<24) responding negatively. OAs caused significantly greater increases than sugars in the detectable richness of the soil bacterial community and larger shifts of dominant taxa. The greater response of bacteria to OAs may be due to the higher amounts of added carbon, solubilization of soil organic matter or shifts in soil pH. Our results indicate that OAs play a significant role in shaping soil bacterial communities and this may therefore have a significant impact on plant growth.     

Soil Formate Regulates the Fungal Nitrous Oxide Emission Pathway

Fungal activity is a major driver in the global nitrogen cycle, and mounting evidence suggests that fungal denitrification activity contributes significantly to soil emissions of the greenhouse gas nitrous oxide (N₂O). The metabolic pathway and oxygen requirement for fungal denitrification are different from those for bacterial denitrification. We hypothesized that the soil N₂O emission from fungi is formate and O₂ dependent and that land use and landforms could influence the proportion of N₂O coming from fungi. Using substrate-induced respiration inhibition under anaerobic and aerobic conditions in combination with ¹⁵N gas analysis, we found that formate and hypoxia (versus anaerobiosis) were essential for the fungal reduction of ¹⁵N-labeled nitrate to ¹⁵N₂O. As much as 65% of soil-emitted N₂O was attributable to fungi; however, this was found only in soils from water-accumulating landforms. From these results, we hypothesize that plant root exudates could affect N₂O production from fungi via the proposed formate-dependent pathway.