Characterization of Gaseous Ozone Decomposition in Soil

Laboratory scale batch experiments were performed to investigate the decomposition characteristics of gaseous ozone in porous media. The decomposition rates of gaseous ozone in several solid media were determined, and the relationship of moisture content with sorbed ozone molecules was evaluated. Ozone decomposition in control and glass beads packed columns followed second-order reaction kinetics, while ozone consumption in a sand-packed column demonstrated first-order kinetics with a rate constant of 0.0109 min^?1 and half-life of 1.0 h. The presence of typical metal oxides in the soil resulted in ozone consumption rates in the following order: hematite (Fe₂O₃) > silica-alumina (SiO₂Al₂O₃) > alumina (Al₂O₃) > silica (SiO₂). Ozone decomposition was highly dependent upon soil moisture content. Over 90% of the total ozone mass decomposed in the field soil with moisture content at less than 1 wt%, whereas as low as 5–15% of the total ozone mass degraded with moisture content at more than 2 wt%. In conclusion, ozone decomposition in soils was primarily controlled not only by soil organic matter but also by reactive metal oxides on the soil surface. These two factors were shown to be highly dependent upon soil moisture content.     

Conversion of hardwood forests to spruce and pine plantations strongly reduced soil methane sink in Germany

Well‐drained forest soils are thought to be a significant sink for atmospheric methane. Recent research suggests that land use change reduces the soil methane sink by diminishing populations of methane oxidizing bacteria. Here we report soil CH₄ uptake from ‘natural’ mature beech forests and from mature pine and spruce plantations in two study areas of Germany with distinct climate and soils. The CH₄ uptake rates of both beech forests at Solling and Unterlüß were about two–three times the CH₄ uptake rates of the adjacent pine and spruce plantations, indicating a strong impact of forest type on the soil CH₄ sink. The CH₄ uptake rates of sieved mineral soils from our study sites confirmed the tree species effect and indicate that methanotrophs were mainly reduced in the 0–5 cm mineral soil depth. The reasons for the reduction are still unknown. We found no site effect between Solling and Unterlüß, however, CH₄ uptake rates from Solling were significantly higher at the same effective CH₄ diffusivity. This potential site effect was masked by higher soil water contents at Solling. Soil pH (H₂O) explained 71% of the variation in CH₄ uptake rates of sieved mineral soils from the 0–5 cm depth, while cation exchange capacity, soil organic carbon, soil nitrogen and total phosphorous content were not correlated with CH₄ uptake rates. Comparing 1998–99, annual CH₄ uptake rates increased by 69–111% in the beech and spruce stands and by 5–25% in the pine stands, due primarily to differences in growing season soil moisture. Cumulative CH₄ uptake rates from November throughout April were rather constant in both years. The CH₄ uptake rates of each stand were separately predicted using daily average soil matric potential and a previously developed empirical model. The model results revealed that soil matric potential explains 53–87% of the temporal variation in CH₄ uptake. The differences between measured and predicted annual CH₄ uptake rates were less than 10%, except for the spruce stand at Solling in 1998 (17%). Based on data from this study and from the literature, we calculated a total reduction in the soil CH₄ sink of 31% for German forests due in part to conversion of deciduous to coniferous forests.     

Persistence of Viruses in Desert Soils Amended with Anaerobically Digested Sewage Sludge

Pima County, Ariz., is currently investigating the potential benefits of land application of sewage sludge. To assess risks associated with the presence of pathogenic enteric viruses present in the sludge, laboratory studies were conducted to measure the inactivation rate (k = log₁₀ reduction per day) of poliovirus type 1 and bacteriophages MS2 and PRD-1 in two sludge-amended desert agricultural soils (Brazito Sandy Loam and Pima Clay Loam). Under constant moisture (approximately -0.05 × 10⁵ Pa for both soils) and temperatures of 15, 27, and 40°C, the main factors controlling the inactivation of these viruses were soil temperature and texture. As the temperature increased from 15 to 40°C, the inactivation rate increased significantly for poliovirus and MS2, whereas, for PRD-1, a significant increase in the inactivation rate was observed only at 40°C. Clay loam soils afforded more protection to all three viruses than sandy soils. At 15°C, the inactivation rate for MS2 ranged from 0.366 to 0.394 log₁₀ reduction per day in clay loam and sandy loam soils, respectively. At 27°C, this rate increased to 0.629 log₁₀ reduction per day in clay loam soil and to 0.652 in sandy loam soil. A similar trend was observed for poliovirus at 15°C (k = 0.064 log₁₀ reduction per day, clay loam; k = 0.095 log₁₀ reduction per day, sandy loam) and 27°C (k = 0.133 log₁₀ reduction per day, clay loam; k = 0.154 log₁₀ reduction per day, sandy loam). Neither MS2 nor poliovirus was recovered after 24 h at 40°C. No reduction of PRD-1 was observed after 28 days at 15°C and after 16 days at 27°C. At 40°C, the inactivation rates were 0.208 log₁₀ reduction per day in amended clay loam soil and 0.282 log₁₀ reduction per day in sandy loam soil. Evaporation to less than 5% soil moisture completely inactivated all three viruses within 7 days at 15°C, within 3 days at 27°C, and within 2 days at 40°C regardless of soil type. This suggests that a combination of high soil temperature and rapid loss of soil moisture will significantly reduce risks caused by viruses in sludge.     

Rapid laboratory measurement of the temperature dependence of soil respiration and application to changes in three diverse soils through the year

Determining the temperature dependence of soil respiration is needed to test predictive models such as Arrhenius-like functions and macro-molecular rate theory (MMRT). We tested a method for rapid measurement of respiration using a temperature gradient block, cooled at one end (~2 °C) and heated at the other (~50 °C) that accommodated 44 tubes containing soil incubated at roughly 1 °C increments. Gas samples were taken after 5 h incubation and analysed for CO₂. The temperature gradient block allowed rapid assessment of temperature dependence of soil respiration with the precision needed to test models and explore existing theories of how temperature and moisture interact to control biochemical processes. Temperature response curves were well fitted by MMRT and allowed calculation of the temperature at which absolute temperature sensitivity was maximal (T_inf). We measured temperature response of three soils at seven moisture contents and showed that the absolute rate and sensitivity of respiration was partly dependent on adjusted moisture content. This result implied that comparisons between soils need to be made at a common moisture content. We also measured potential changes in the temperature dependence (and sensitivity) of respiration for three different soils collected at one site throughout a year. T_inf ranged from 43 to 51 °C for the three soils. T_inf and temperature sensitivity were not dependent on soil type collected but was partly dependent on time of year of collection. Temporal changes in temperature response suggested that the microbial communities may tune their metabolisms in response to changes in soil temperatures.     

Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.     

Microbial and Environmental Controls of Methane Fluxes Along a Soil Moisture Gradient in a Pacific Coastal Temperate Rainforest

Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO₂), whereas methane (CH₄) has not received the same attention. Soil hydrology is a key driver of CH₄ dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH₄ cycling and the resultant net CH₄ exchange is not well understood at this scale. We studied the growing season variations of in situ CH₄ fluxes, microbial gene abundances of methanotrophs (CH₄ oxidizers) and methanogens (CH₄ producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH₄ displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH₄ m^?2 h^?1) to a net emission in the wetter soils (0–90 μmol CH₄ m^?2 h^?1). Seasonal variations of CH₄ fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH₄ emission was inversely related to the water table depth in the wet soils. Spatial variability of CH₄ exchange was related to the abundance of genes involved in CH₄ oxidation and production, but there was no indication of a temporal link between microbial groups and CH₄ exchange. Our data show that the abundances of genes involved in CH₄ oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type.     

Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ^?) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it challenging to determine the direct causes of increased soil NO₃ ^?. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ^?. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ^? uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ^? caused slightly elevated NO₃ ^? concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ^? uptake rates. Despite reduced microbial NO₃ ^? uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ^? concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.     

On the assessment of root and soil respiration for soils of different textures: interactions with soil moisture contents and soil CO2 concentrations

Estimates of root and soil respiration are becoming increasingly important in agricultural and ecological research, but there is little understanding how soil texture and water content may affect these estimates. We examined the effects of soil texture on (i) estimated rates of root and soil respiration and (ii) soil CO₂ concentrations, during cycles of soil wetting and drying in the citrus rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.). Plants were grown in soil columns filled with three different soil mixtures varying in their sand, silt and clay content. Root and soil respiration rates, soil water content, plant water uptake and soil CO₂ concentrations were measured and dynamic relationships among these variables were developed for each soil texture treatment. We found that although the different soil textures differed in their plant-soil water relations characteristics, plant growth was only slightly affected. Root and soil respiration rates were similar under most soil moisture conditions for soils varying widely in percentages of sand, silt and clay. Only following irrigation did CO₂ efflux from the soil surface vary among soils. That is, efflux of CO₂ from the soil surface was much more restricted after watering (therefore rendering any respiration measurements inaccurate) in finer textured soils than in sandy soils because of reduced porosity in the finer textured soils. Accordingly, CO₂ reached and maintained the highest concentrations in finer textured soils (> 40 mmol CO₂ mol⁻¹). This study revealed that changes in soil moisture can affect interpretations of root and soil measurements based on CO₂ efflux, particularly in fine textured soils. The implications of the present findings for field soil CO₂ flux measurements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

放牧对滇西北高原纳帕海沼泽化草甸湿地土壤氮转化的影响

选择位于滇西北高原纳帕海国际重要湿地内的典型沼泽化草甸湿地为研究对象,采用原位土柱室内控制实验法研究了放牧干扰(猪翻拱扰动和牲畜践踏)对沼泽化草甸湿地土壤氮转化的影响。研究结果表明,放牧活动显著提高了沼泽化草甸湿地表层土壤的容重和pH值,降低了土壤含水率、TOC、TN和NH_4~+-N含量,而对NO_3~--N含量影响不显著。放牧干扰下沼泽化草甸湿地土壤的矿化速率和硝化速率均表现为猪翻拱扰动样地(ZG)牲畜践踏样地(JT)对照样地(CK);表现为ZGJTCK。放牧干扰促进了沼泽化草甸湿地土壤的矿化和硝化作用,猪的翻拱活动比牲畜践踏活动对土壤氮矿化和硝化作用的促进作用更显著。放牧干扰下沼泽化草甸湿地土壤的反硝化速率表现为ZGCKJT,猪的翻拱活动促进了土壤N_2O气体的排放,而牲畜践踏活动抑制了土壤N_2O气体的排放。相关性分析表明,受放牧干扰的沼泽化草甸湿地土壤的矿化和硝化速率均与土壤容重、pH呈显著正相关,与土壤含水率、NH_4~+-N、TOC、TN含量呈显著负相关;反硝化速率与TOC含量呈显著负相关。     

Linking activity,composition and seasonal dynamics of atmospheric methane oxidizers in a meadow soil

Microbial oxidation is the only biological sink for atmospheric methane. We assessed seasonal changes in atmospheric methane oxidation and the underlying methanotrophic communities in grassland near Giessen (Germany), along a soil moisture gradient. Soil samples were taken from the surface layer (0–10 cm) of three sites in August 2007, November 2007, February 2008 and May 2008. The sites showed seasonal differences in hydrological parameters. Net uptake rates varied seasonally between 0 and 70 μg CH₄ m⁻² h⁻¹. Greatest uptake rates coincided with lowest soil moisture in spring and summer. Over all sites and seasons, the methanotrophic communities were dominated by uncultivated methanotrophs. These formed a monophyletic cluster defined by the RA14, MHP and JR1 clades, referred to as upland soil cluster alphaproteobacteria (USCα)-like group. The copy numbers of pmoA genes ranged between 3.8 × 10⁵–1.9 × 10⁶ copies g⁻¹ of soil. Temperature was positively correlated with CH₄ uptake rates (P<0.001), but had no effect on methanotrophic population dynamics. The soil moisture was negatively correlated with CH₄ uptake rates (P<0.001), but showed a positive correlation with changes in USCα-like diversity (P<0.001) and pmoA gene abundance (P<0.05). These were greatest at low net CH₄ uptake rates during winter times and coincided with an overall increase in bacterial 16S rRNA gene abundances (P<0.05). Taken together, soil moisture had a significant but opposed effect on CH₄ uptake rates and methanotrophic population dynamics, the latter being increasingly stimulated by soil moisture contents >50 vol% and primarily related to members of the MHP clade.     

Variation in Soil Methane Release or Uptake Responses to Biochar Amendment: A Separate Meta-analysis

Agricultural soils play an important role in the atmospheric methane (CH₄) budget, where paddy soils can contribute significant CH₄ to atmosphere whereas upland soils may act as a source or sink of atmospheric CH₄, dependent on soil water conditions. Biochar amendments have effects on soil CH₄ production or oxidation processes in individual experiments, but the causative mechanisms are yet to be fully elucidated. To synthesize the response of soil CH₄ release or uptake to biochar amendment, we performed a meta-analysis using data from 61 peer-reviewed papers with 222 updated paired measurements. When averaged across all studies, biochar amendment significantly decreased CH₄ release rates by 12% for paddy soils and 72% for upland soils, and CH₄ uptake rates by 84% for upland soils. Neither soil CH₄ release nor uptake responses to biochar amendment were significant in field soils. Nitrogen (N) fertilizer application would weaken the response of soil CH₄ release or uptake to biochar amendment. Biochar-incurred decreases in soil CH₄ release and uptake rates were the largest in medium-textured soils or neutral-pH soils. Soil CH₄ release or uptake responses to biochar were also significantly altered by biochar characteristics, such as feedstock source, C/N ratio, pH, and pyrolysis temperature. The results of this synthesis suggest that the role of biochar in soil CH₄ mitigation potential might have been exaggerated, particularly in fields when biochar is applied in combination with N fertilizer.     

土壤湿度对中国南部热带森林土壤甲烷吸收的影响

森林土壤是一个重要的大气甲烷的汇。然而,相较于寒带和温带,在热带尤其是东南亚地区,森林土壤甲烷通量的观测较少,这限制了目前对热带森林土壤甲烷通量与环境因子之间关系的认识,也给热带森林土壤甲烷汇的估算带来了一定的不确定性。在中国海南省吊罗山国家森林公园的热带森林土壤,采用激光光谱法测量了2016年9月至2018年9月逐月的土壤甲烷通量,并分析了其与周围环境因子的关系。结果表明：研究区土壤是甲烷的汇,山顶样地的年平均吸收量为0.95 kg CH₄-C hm^-2 a^-1,山脚样地的年平均吸收量为1.93 kg CH₄-C hm^-2 a^-1。干季(11月—次年4月)的甲烷吸收通量明显高于湿季(5—10月),占到全年甲烷吸收的68%。山顶样地年平均土壤湿度为19.2%,年内的波动较小(2.8%)。而山脚样地的年平均湿度相对较低,为12.7%,且年内波动大(5.4%)。土壤湿度是控制甲烷吸收最主要的环境因子,可以解释月际甲烷吸收变化的76%,甲烷吸收通量与土壤温度的相...     

Effect of soil nitrogen,carbon and moisture on methane uptake by dry tropical forest soils

Methane uptake was measured for two consecutive years for four forest and one savanna sites in a seasonally dry tropical region of India. The soils were nutrient-poor and well drained. These sites differed in vegetational cover and physico-chemical features of the soil. There were significant differences in CH₄ consumption rates during the two years (mean 0.43 and 0.49 mg m^-2 h^-1), and at different sites (mean 0.36 to 0.57 mg m^-2 h^-1). The mean uptake rate was higher (P < 0.05) in dry seasons than in the rainy season at all the sites. There was a significant season and site interaction, indicating that the effect of different seasons differed across the sites. There was a positive relation between soil moisture and CH₄ uptake rates during summer (the driest period) and a negative relation during the rest of the year. The results suggested that seasonally dry tropical forests are a strong sink for CH₄, and C and N status of soils regulates the strength of the sink in the long term.     

内蒙古典型草原植物功能型对土壤甲烷吸收的影响

甲烷(CH₄)是仅次于CO₂的重要温室气体。内蒙古草原是欧亚温带草原的重要类型, 具有典型的生态地域代表性。该文以内蒙古温带典型草原为研究对象, 通过人工剔除植物种的方法来确定群落中的植物功能型, 并应用静态箱技术, 观测土壤CH₄的吸收, 以理解植物功能型对土壤CH₄吸收的影响。结果表明: 1)土壤CH₄的吸收受温度和水分变化的影响, 具有明显的季节差异, 且与温度显著相关。2)在2008年和2009年所测的大部分月份中, 植物功能型的土壤CH₄吸收量之间没有显著差异; 然而在植物生长旺季(8月), 不同植物功能型的土壤CH₄吸收量之间存在显著差异, 多年生丛生禾草的土壤CH₄吸收量最小。3)处理中一、二年生植物、多年生杂类草的存在能够增加土壤CH₄的吸收量, 而处理中多年生根茎类禾草、多年生丛生禾草的存在对土壤CH₄吸收的影响不大。这可能是因为, 植物功能型影响土壤的微生物代谢和环境因子, 进而影响土壤CH₄吸收量。该试验说明, 在痕量气体层面上, 植物功能型组成在生态系统功能中具有重要作用, 特别是群落中的亚优势种和伴生种(一、二年生植物、多年生杂类草), 通过调控土壤微生物和环境因子, 对地-气的CH₄交换产生重要影响。     

Denitrification in Low pH Spodosols and Peats Determined with the Acetylene Inhibition Method

Potential denitrification rates were determined for predominantly acid (pH ≥ 3.6) horizons of forestal, miry, and agricultural soils from 22 locations in southern Finland. The acetylene inhibition method was used with nitrate-amended water-logged soils incubated in an N₂ atmosphere containing 2.5 or 5% C₂H₂. Complete inhibition of the reduction of N₂O to N₂ was observed in 99.3% of the samples. The denitrification rates varied from 0.12 to 53.8 μg of N·cm^-3·day^-1. Correlation between denitrification rate and soil pH was highly significant: r = 0.84 on a volume basis, and r = 0.44 on a weight basis. Vegetation type and amount of soil organic matter had a minor or no effect, respectively. In spodosolized soils the rates were significantly higher for B horizons than for A horizons. These results show that denitrification can occur in acid soils.     

Coral uptake of inorganic phosphorus and nitrogen negatively affected by simultaneous changes in temperature and pH

The effects of ocean acidification and elevated seawater temperature on coral calcification and photosynthesis have been extensively investigated over the last two decades, whereas they are still unknown on nutrient uptake, despite their importance for coral energetics. We therefore studied the separate and combined impacts of increases in temperature and pCO₂ on phosphate, ammonium, and nitrate uptake rates by the scleractinian coral S. pistillata. Three experiments were performed, during 10 days i) at three pH_T conditions (8.1, 7.8, and 7.5) and normal temperature (26°C), ii) at three temperature conditions (26°, 29°C, and 33°C) and normal pH_T (8.1), and iii) at three pH_T conditions (8.1, 7.8, and 7.5) and elevated temperature (33°C). After 10 days of incubation, corals had not bleached, as protein, chlorophyll, and zooxanthellae contents were the same in all treatments. However, photosynthetic rates significantly decreased at 33°C, and were further reduced for the pH_T 7.5. The photosynthetic efficiency of PSII was only decreased by elevated temperature. Nutrient uptake rates were not affected by a change in pH alone. Conversely, elevated temperature (33°C) alone induced an increase in phosphate uptake but a severe decrease in nitrate and ammonium uptake rates, even leading to a release of nitrogen into seawater. Combination of high temperature (33°C) and low pH_T (7.5) resulted in a significant decrease in phosphate and nitrate uptake rates compared to control corals (26°C, pH_T = 8.1). These results indicate that both inorganic nitrogen and phosphorus metabolism may be negatively affected by the cumulative effects of ocean warming and acidification.     

Abrupt permafrost thaw drives spatially heterogeneous soil moisture and carbon dioxide fluxes in upland tundra

Permafrost thaw causes the seasonally thawed active layer to deepen, causing the Arctic to shift toward carbon release as soil organic matter becomes susceptible to decomposition. Ground subsidence initiated by ice loss can cause these soils to collapse abruptly, rapidly shifting soil moisture as microtopography changes and also accelerating carbon and nutrient mobilization. The uncertainty of soil moisture trajectories during thaw makes it difficult to predict the role of abrupt thaw in suppressing or exacerbating carbon losses. In this study, we investigated the role of shifting soil moisture conditions on carbon dioxide fluxes during a 13-year permafrost warming experiment that exhibited abrupt thaw. Warming deepened the active layer differentially across treatments, leading to variable rates of subsidence and formation of thermokarst depressions. In turn, differential subsidence caused a gradient of moisture conditions, with some plots becoming consistently inundated with water within thermokarst depressions and others exhibiting generally dry, but more variable soil moisture conditions outside of thermokarst depressions. Experimentally induced permafrost thaw initially drove increasing rates of growing season gross primary productivity (GPP), ecosystem respiration (R_eco), and net ecosystem exchange (NEE) (higher carbon uptake), but the formation of thermokarst depressions began to reverse this trend with a high level of spatial heterogeneity. Plots that subsided at the slowest rate stayed relatively dry and supported higher CO₂ fluxes throughout the 13-year experiment, while plots that subsided very rapidly into the center of a thermokarst feature became consistently wet and experienced a rapid decline in growing season GPP, R_eco, and NEE (lower carbon uptake or carbon release). These findings indicate that Earth system models, which do not simulate subsidence and often predict drier active layer conditions, likely overestimate net growing season carbon uptake in abruptly thawing landscapes.     

N2O emission rates in a California meadow soil are influenced by fertilizer level, soil moisture and the community structure of ammonia-oxidizing bacteria

The response of nitrous oxide (N₂O) emission rates and β‐proteobacterial ammonia‐oxidizing (AOB) communities to manipulations of temperature, soil moisture and nitrogenous fertilizer concentration were studied for 16–20 weeks in a multifactorial laboratory experiment using a California meadow soil. Interactions among these three environmental factors influenced the N₂O emission rates, and two patterns of N₂O emission rates due to nitrification (NitN₂O) were observed. First, in soils receiving low or moderate amounts of fertilizer, the rates decreased sharply in response to increasing soil moisture and temperature. Second, in soils receiving high amounts of fertilizer, the rates were influenced by an interaction between soil moisture and temperature, such that at 20 °C increasing soil moisture resulted in an increase in the rates, and at 30 °C the highest rate was observed at moderate soil moisture. We used path analysis to identify the interrelationships that best explain these two patterns. Path analysis revealed that in the high fertilizer (HF) treatment, the major path by which ammonia influenced NitN₂O rates was indirect through an influence on the abundance of one particular phylogenetic group (AOB ‘cluster 10’). In contrast, in the low and moderate fertilizer treatments soil moisture influenced the rates both directly (the major path) and indirectly through AOB community structure. Although terminal restriction fragment length polymorphism (T‐RFLP) analysis revealed shifts in the community structure of AOB in all treatments, the shifts at HF concentrations were particularly striking, with dominance by three different phylogenetic groups under different combinations of the three environmental factors. The high emission rates observed at the lowest soil moistures suggest that bacterial nitrifiers may use denitrification as a stress response.     

森林土壤甲烷吸收的主控因子及其对增氮的响应研究进展

森林土壤甲烷(CH4)吸收在生态系统碳、氮循环和碳平衡研究中具有重要作用。论述了森林土壤CH4的产生和消耗过程及其主控因子,有效氮不同的森林土壤CH4吸收对氮素输入的响应差异及其驱动机制,并且明确了现有研究的不足和未来研究的重点。研究表明:大气氮沉降输入倾向于抑制富氮森林土壤的CH4吸收,而对贫氮森林土壤CH4吸收具有显著的促进作用,其内在的氮素调控机制至今尚不明确。主要的原因是过去通过高剂量施氮试验所得出的理论难以准确地解释低水平氮沉降情景下森林土壤CH4吸收过程,有关森林土壤CH4吸收对大气氮沉降响应的微生物学机理也缺乏系统性研究。未来研究的重点是探讨森林土壤CH4物理扩散和净吸收过程对施氮类型、剂量的短期与长期响应,量化深层土壤CH4累积和消耗对表层土壤CH4吸收的贡献,揭示森林土壤CH4吸收对增氮响应的物理学与生物化学机制。另外,研究森林土壤甲烷氧化菌群落活性、结构对施氮类型和剂量的响应,阐明土壤CH4吸收与甲烷氧化菌群落组成的内在联系,有助于深入揭示森林土壤CH4吸收对增氮响应的微生物学机制。     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.