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%.     

趋磁微生物处理污水研究

指出了趋磁性微生物能够在缺氧和厌氧条件下,以硝酸盐作为最终电子受体,产生反硝化反应,进行脱氮处理。趋磁微生物可以利用硫酸盐、硫代硫酸盐为电子受体,进行硫化反应,去除污水中的硫。趋磁微生物在磁场的作用下,可以定向移动。增加污泥的沉淀速度,减少沉淀池的容积及沉淀时间。趋磁微生物处理重金属污水,能够很好地解分选,回收有用的重金属。对趋磁微生物处理污水进行了研究。     

土壤反硝化酶活性测定方法及影响因素研究

借助土壤酶学的基本原理,初步建立了一种底物(KNO3)最佳浓度为10 g/L,土样不需二次离心的土壤反硝化酶活性测定方法——硝态氮剩余量法,并对其影响因素进行了研究。结果表明,该测定方法简便、快速、灵敏;有机肥的加入可显著提高土壤反硝化酶的活性,有机肥和化肥的共施是最佳的培肥措施;种植作物也可提高土壤反硝化酶活性;甲苯对土壤反硝化酶活性有明显的抑制作用,且土壤肥力水平越高,抑制幅度越大。     

五川流域农业土壤反硝化作用测定及其调控措施

反硝化作用是土壤氮素转化的一个重要过程，为探明五川流域内的农业土壤的反硝化作用强度及其影响因素，利用乙炔抑制-原状土柱培养法对其进行测定。通过3次试验测定，发现五川流域农业土壤具有较强的反硝化作用强度，在种植季节，土壤平均反硝化作用强度为0.1kgN·hm^-2·d^-1，最高达到0.6kgN·hm^-2·d^-1，其中蔬菜地反硝化作用强于其他土地利用类型。反硝化作用同土壤的NO3^-含量、含水量、温度以及pH都存在正相关关系，它们是流域土壤反硝化作用的主要影响因子。五川流域农业土壤经由反硝化作用氮损失量占流域平均施肥量的16%，高于国内其他地区。针对五川流域的环境和农业经济特点，提出了控制反硝化作用的措施：在温度较低的夜间进行施肥灌溉宜以防止氮肥损失，用农村富余的厩肥代替化肥以减轻反硝化作用的发生，同时加大节水灌溉力度。     

SBR生物脱氮机理及其影响因素

开发高效经济的生物脱氮技术一直是国内外科研工作者所关注的热点。本文介绍了SBR系统的微生物相、硝化－反硝化、亚硝化－反亚硝化生物脱氮技术及其影响因素的研究进展，并指出了当前SBR法在脱氮方面存在的问题以及发展趋势。     

土壤氮循环微生物过程的分子生态学研究进展

固氮作用、硝化作用和反硝化作用是土壤微生物参与氮素循环的三个重要方面。自分子生态学方法应用于土壤学后,土壤微生物作用于氮素循环过程的机理研究取得了若干重要进展。包括:1)利用固氮菌的nifH基因作为分子标记研究有机质、氮素与固氮微生物之间的关系,发现固氮微生物的丰度和群落结构与土壤有机质含量呈正相关。然而,固氮微生物的丰度和群落结构与土壤速效N含量呈负相关,施用氮肥会抑制固氮微生物的生长,施氮土壤固氮微生物数量减少,多样性降低。2)以氨氧化微生物功能基因为探针,揭示了土壤pH与氨氧化微生物分布关系密切,碱性土壤中氨氧化细菌是硝化作用主要参与者,而酸性土壤中氨氧化古菌是硝化作用的主导者。土壤中N素的含量也影响氨氧化微生物的数量和群落结构,施入氮肥后氨氧化微生物的数量和活性增加。3)利用反硝化功能基因为分子标记研究土壤因子对反硝化细菌群落的影响,阐明了土壤可溶性有机碳、pH和土壤水分是影响反硝化细菌数量和群落结构的重要因子,并且发现土壤反硝化细菌与土壤反硝化能力和氧化亚氮释放之间存在着一定的联系,但这种联系在不同研究对象中存在差异,需要进一步确认。目前,利用分子生物学技术解析土壤氮素循环微生物生态功能取得了重要的进展,但还需进一步深入。今后,将采用包括同位素在内的示踪技术与分子生物学方法相结合共同分析氮循环不同代谢过程微生物种群间的相互关系以及氧化亚氮产生与硝化和反硝化微生物之间的关系。     

Separating N2O production and consumption in intact agricultural soil cores at different moisture contents and depths

Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, N₂O. To implement management practices that minimize microbial N₂O production and maximize its consumption (i.e., complete denitrification), we must understand the interplay between simultaneously occurring biological and physical processes, especially how this changes with soil depth. Meaningfully disentangling of these processes is challenging and typical N₂O flux measurement techniques provide little insight into subsurface mechanisms. In addition, denitrification studies are often conducted on sieved soil in altered O₂ environments which relate poorly to in situ field conditions. Here, we developed a novel incubation system with headspaces both above and below the soil cores and field-relevant O₂ concentrations to better represent in situ conditions. We incubated intact sandy clay loam textured agricultural topsoil (0–10 cm) and subsoil (50–60 cm) cores for 3–4 days at 50% and 70% water-filled pore space, respectively. ¹⁵N-N₂O pool dilution and an SF₆ tracer were injected below the cores to determine the relative diffusivity and the net N₂O emission and gross N₂O emission and consumption fluxes. The relationship between calculated fluxes from the below and above soil core headspaces confirmed that the system performed well. Relative diffusivity did not vary with depth, likely due to the preservation of preferential flow pathways in the intact cores. Gross N₂O emission and uptake also did not differ with depth but were higher in the drier cores, contrary to expectation. We speculate this was due to aerobic denitrification being the primary N₂O consuming process and simultaneously occurring denitrification and nitrification both producing N₂O in the drier cores. We provide further evidence of substantial N₂O consumption in drier soil but without net negative N₂O emissions. The results from this study are important for the future application of the ¹⁵N-N₂O pool dilution method and N budgeting and modelling, as required for improving management to minimize N₂O losses.     

Nitrous oxide production of heavy metal contaminated soil

Arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills of central and northern Mexico. Interest in these heavy metals has increased recently as they contaminate drinking water and aquifers in large parts of the world and severely affect human health, but little is known about how they affect biological functioning of soil. Soils were sampled in seven locations along a gradient of heavy metal contamination with distance from a mine in San Luis Potosí (Mexico), active since about 1800 AD. C mineralization and N₂O production were monitored in an aerobic incubation experiment. Concentrations of As in the top 0-10 cm soil layer ranged from 8 to 22,992 mg kg⁻¹, from 31 to 1845 mg kg⁻¹ for Pb, from 27 to 1620 mg kg⁻¹ for Cu and from 81 to 4218 mg kg⁻¹ for Zn. There was a significant negative correlation between production rates of CO₂ and concentrations of As, Pb, Cu and Zn, and there was a significant positive correlation with pH, water holding capacity (WHC), total N and soil organic C. There was a significant negative correlation (P<0.05) between production rate of nitrous oxide (N₂O) attributed to nitrification by the inhibition method in soil incubated at 50% WHC and total concentrations of Pb and Zn, and there was a significant positive correlation (P<0.05) with pH and total N content. There was a significant negative correlation (P<0.05) between the production rate of N₂O attributed to denitrification by the inhibition method in soil incubated at 100% WHC and total concentrations of Pb, Cu and Zn, and a significant positive correlation (P<0.01) with pH; there was a significant positive correlation (P<0.05) between the production of N₂O attributed to other processes by the inhibition method and WHC, inorganic C and clay content. A negative value for production rate of N₂O attributed to nitrifier denitrification by the inhibition method was obtained at 100% WHC. The large concentrations of heavy metals in soil inhibited microbial activity and the production rate of N₂O attributed to nitrification by the inhibition method when soil was incubated at 50% WHC and denitrification when soil was incubated at 100% WHC. The inhibitor/suppression technique used appeared to be flawed, as negative values for nitrifier denitrification were obtained and as the production rate of N₂O through denitrification increased when soil was incubated with C₂H₂.     

Effect of sodium chloride on denitrification in glucose amended soil treated with ammonium and nitrate nitrogen

Laboratory incubations were conducted to study the effect of sodium chloride (NaCl) on denitrification and respiratory gases (CO₂, O₂) from soil treated with ammonium or nitrate and incubated at 20 % moisture. The same samples were assayed for denitrifying enzyme activity (DEA) after incubation at 40 % moisture with glucose and NO₃^–. Under aerobic conditions (20 % water content), a flush of activity was observed at 6 hours after start of incubation and subsided to negligible levels at 12 hours. Sodium chloride significantly depressed N₂O and CO₂ emissions and O₂ consumption. Significantly more loss of N₂O occurred from NH₄⁺‐ than NO₃^–‐treated soil at all NaCl levels and was attributed to higher microbial activity. A highly significant positive correlation was obtained between N₂O emission and respiratory gases. The respiratory quotient (CO₂ evolved/O₂) was higher for NH₄⁺‐treated soil and decreased with the amount of NaCl. At 40 % moisture, N₂O emissions were higher than at 20 % and peaked at 37 hours followed by a sharp decrease. Short‐term incubations of soil with NH₄⁺ or NO₃^– did not have an effect on denitrifying enzyme activity (DEA) while NaCl had a positive effect, particularly in previously NO₃^–‐treated soil.     

Effect of increasing oxygen concentration on total denitrification and nitrous oxide release from soil by different bacteria

Summary The effect of increasing oxygen concentrations (0, 5, 10 and 20 Vol% O₂) on total denitrification and N₂0 release was studied in model experiments using a neutral pH loamy soil relatively rich in easily decomposable organic matter and supplied with nitrate (300 g nitrate N/g dry soil). The sterilized soil was inoculated with three different denitrifying bacteria (Bacillus licheniformis,Aeromonas denitrificans andAzospirillum lipoferum) and incubated (80% WHC, 30°C). The gas volume was analysed for O₂, CO₂, N₂O, NO and N₂ by gas chromatography and the soil investigated for changes in ammonium, nitrite, nitrate, pH, total N and C as well as water-extractable C. WithB. licheniformis andAeromonas denitrificans total denitrification increased remarkably with increasing pO₂ as the result of intensified mineralization.Azospirillum lipoferum, however, showed the highest activity at 5 vol% O₂. WithB. licheniformis N₂O was released only in anaerobic conditions and at 5 Vol% O₂ (maximum) or 10 Vol% 02, but not at 20 Vol%, whereasAeromonas denitrificans produced N₂O only in the presence of He gas (maximum) or at 5 Vol% O₂. In contrast to these bacteria, N₂O production withAzospirillum lipoferum was restricted to 10 Vol% O₂ (maximum) and to 20 Vol% 02, with some traces at 5 vol% O₂. With a certain set of conditions, total denitrification and N₂O formation seem to be governed by the mineralization rate of the organisms in question. The increased demand for electron acceptors by a high turnover rate rather than the presence of anaerobic conditions seems to have determined the rate of denitrification.