Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest

Boreal forests contain significant quantities of soil carbon that may be oxidized to CO₂ given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short‐term effects of wildfire to the long‐term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both ¹³C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.     

Salinity affects microbial activity and soil organic matter content in tidal wetlands

Climate change‐associated sea level rise is expected to cause saltwater intrusion into many historically freshwater ecosystems. Of particular concern are tidal freshwater wetlands, which perform several important ecological functions including carbon sequestration. To predict the impact of saltwater intrusion in these environments, we must first gain a better understanding of how salinity regulates decomposition in natural systems. This study sampled eight tidal wetlands ranging from freshwater to oligohaline (0–2 ppt) in four rivers near the Chesapeake Bay (Virginia). To help isolate salinity effects, sites were selected to be highly similar in terms of plant community composition and tidal influence. Overall, salinity was found to be strongly negatively correlated with soil organic matter content (OM%) and C : N, but unrelated to the other studied environmental parameters (pH, redox, and above‐ and below‐ground plant biomass). Partial correlation analysis, controlling for these environmental covariates, supported direct effects of salinity on the activity of carbon‐degrading extracellular enzymes (β‐1, 4‐glucosidase, 1, 4‐β‐cellobiosidase, β‐D‐xylosidase, and phenol oxidase) as well as alkaline phosphatase, using a per unit OM basis. As enzyme activity is the putative rate‐limiting step in decomposition, enhanced activity due to salinity increases could dramatically affect soil OM accumulation. Salinity was also found to be positively related to bacterial abundance (qPCR of the 16S rRNA gene) and tightly linked with community composition (T‐RFLP). Furthermore, strong relationships were found between bacterial abundance and/or composition with the activity of specific enzymes (1, 4‐β‐cellobiosidase, arylsulfatase, alkaline phosphatase, and phenol oxidase) suggesting salinity's impact on decomposition could be due, at least in part, to its effect on the bacterial community. Together, these results indicate that salinity increases microbial decomposition rates in low salinity wetlands, and suggests that these ecosystems may experience decreased soil OM accumulation, accretion, and carbon sequestration rates even with modest levels of saltwater intrusion.     

Role of a Ferric Ion-Reducing System in Sulfur Oxidation of Thiobacillus ferrooxidans

The properties of a ferric ion-reducing system which catalyzes the reduction of ferric ion with elemental sulfur was investigated with a pure strain of Thiobacillus ferrooxidans. In anaerobic conditions, washed intact cells of the strain reduced 6 mol of Fe³⁺ with 1 mol of elemental sulfur to give 6 mol of Fe²⁺, 1 mol of sulfate, and a small amount of sulfite. In aerobic conditions, the 6 mol of Fe²⁺ produced was immediately reoxidized by the iron oxidase of the cell, with a consumption of 1.5 mol of oxygen. As a result, Fe²⁺ production was never observed under aerobic conditions. However, in the presence of 5 mM cyanide, which completely inhibits the iron oxidase of the cell, an amount of Fe²⁺ production comparable to that formed under anaerobic conditions was observed under aerobic conditions. The ferric ion-reducing system had a pH optimum between 2.0 and 3.8, and the activity was completely destroyed by 10 min of incubation at 60°C. A short treatment of the strain with 0.5% phenol completely destroyed the ferric ion-reducing system of the cell. However, this treatment did not affect the iron oxidase of the cell. Since a concomitant complete loss of the activity of sulfur oxidation by molecular oxygen was observed in 0.5% phenol-treated cells, it was concluded that the ferric ion-reducing system plays an important role in the sulfur oxidation activity of this strain, and a new sulfur-oxidizing route is proposed for T. ferrooxidans.     

Nitrite: a key compound in N loss processes under acid conditions?

Summary Nitrite is very important in N transformation processes because it is an intermediate product in the aerobic nitrification as well as in the anaerobic denitrification process. Under soil conditions whereby aerobic and anaerobic zones are close to each other, the mobile nitrite can be a link between both N transformation processes. Because of its low stability in acid conditions, nitrite can be a key compound in N loss processes.The results are presented in three sets of incubation experiments using soil+added nitrite before and after oxidation of organic matter; soil+added nitrite and various iron oxide minerals; nitrite solutions without soil but with added ferrous iron.It was found that under acid conditions, soil organic matter as well as the soil mineral phase have a stimulating effect on the nitrite decomposition. Conditions favouring the solubility of Fe(III)-compounds and promoting the formation of Fe²⁺ increase the nitrite decomposition, even under slightly acid conditions. Of the gaseous decomposition products, only trace amounts of NO₂ occur while NO is the major component. Conditions whereby NO and NO₂ cannot escape from the medium promote production of some nitrite.     

Does Atmospheric NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition Alter the Abundance and Activity of Ligninolytic Fungi in Forest Soils?

Although field studies have demonstrated an ecosystem-specific effect of experimental atmospheric nitrogen (N) deposition on litter decomposition, a mechanistic understanding of how ligninolytic microbial communities respond to atmospheric deposition is lacking. Because high levels of inorganic N suppress lignin decomposition by some basidiomycetes, it is plausible that the abundance and activity of these key microorganisms underlies differential ecosystem responses of decomposition to atmospheric N deposition. We hypothesize that: (a) atmospheric N deposition will cause an ecosystem-specific reduction in basidiomycete activity and abundance with greatest decreases in ecosystems with lignin-rich forest litter and (b) the abundance of lignin degrading basidiomycetes will be positively correlated with ligninolytic enzyme activity. To test these hypotheses, we measured the effects of experimental N deposition on the potential activity of phenol oxidase enzymes, and the abundance of basidiomycete genes encoding laccase, a primary phenol oxidase enzyme, in three hardwood forests spanning a range of leaf litter lignin content. The black oak-white oak (BOWO) contains high lignin litter, the sugar maple-basswood (SMBW) has low lignin litter, and the sugar maple-red oak (SMRO) is intermediate. An ecosystem by N deposition interaction significantly influenced phenol oxidase activity in the surface soil (P = 0.05), where phenol oxidase activity decreased with increasing experimental N deposition in the BOWO ecosystem. No consistent response to N deposition was evident for surface soil phenol oxidase activity within either the SMRO or SMBW ecosystem. This interaction did not influence laccase gene abundance. Instead, basidiomycete laccase gene abundance was reduced by experimental N deposition (main effect) in surface soil. There was only a weak correlation between basidiomycete laccase gene abundance and potential phenol oxidase enzyme activity, suggesting that the abundance of organisms possessing laccase genes may not control phenol oxidase activity in soil. Our results suggest that the regulation of laccase gene expression may mediate the decomposition response to atmospheric N deposition.     

Turnover of labile and recalcitrant soil carbon differ in response to nitrate and ammonium deposition in an ombrotrophic peatland

The effects of 4 years of simulated nitrogen deposition, as nitrate (NO₃^?) and ammonium (NH₄⁺), on microbial carbon turnover were studied in an ombrotrophic peatland. We investigated the mineralization of simple forms of carbon using MicroResp^? measurements (a multiple substrate induced respiration technique) and the activities of four soil enzymes involved in the decomposition of more complex forms of carbon or in nutrient acquisition: N‐acetyl‐glucosaminidase (NAG), cellobiohydrolase (CBH), acid phosphatase (AP), and phenol oxidase (PO). The potential mineralization of labile forms of carbon was significantly enhanced at the higher N additions, especially with NH₄⁺ amendments, while potential enzyme activities involved in breakdown of more complex forms of carbon or nutrient acquisition decreased slightly (NAG and CBH) or remained unchanged (AP and PO) with N amendments. This study also showed the importance of distinguishing between NO₃^? and NH₄⁺ amendments, as their impact often differed. It is possible that the limited response on potential extracellular enzyme activity is due to other factors, such as limited exposure to the added N in the deeper soil or continued suboptimal functioning of the enzymes due to the low pH, possibly via the inhibitory effect of low phenol oxidase activity.     

Anin vitro effect of soil organic matter fractions and synthetic humic acids on the generation of superoxide radicals

Summary Humic acid, and the acid-extracted residue obtained from it, stimulated the production of superoxide radicals (O₂ ^.–) generated in the xanthine-xanthine oxidase system. Several synthetic humic acids, prepared by the oxidation of simple phenolic substances, also stimulated the production of O₂ ^.– but the degree of stimulation depended on the initial phenol. Fulvic acid and water-extractable soil organic matter were less effective in stimulating O₂ ^.– production than was humic acid. The activity of superoxide dismutase, an enzyme which destroys O₂ ^.–, was also enhanced by HA. In contrast, fulvic acid and water-extractable soil organic matter had little effect on the activity of the dismutase.     

Combined effects of waterlogging and salinity on electrochemistry, water-soluble cations and water dispersible clay in soils with various salinity levels

The combination effects of waterlogging and salinity on redox potential (Eh), pH, electric conductivity (EC), water-soluble cations (NH₄ ⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺) and water-dispersible clay (WDC) were studied in six soils collected near salt lakes in western Australia. The soils with various salinity levels were incubated under a waterlogged condition at 30 °C for 12 weeks. The Eh, pH, EC, and cations of soil solutions were monitored over the waterlogged period. The Eh values generally dropped to the lowest point within 12 days of waterlogging, then increased slightly, and reached equilibrium after 4 weeks of waterlogging. Increasing salinity levels increased soil Eh. While waterlogging increased soil pH in the first 3–4 weeks, increasing salinity level decreased soil pH during the entire waterlogging period. Waterlogging increased the EC values in the first 2 weeks, partly due to dissolution of insoluble salts. The concentrations of water-soluble NH₄ ⁺ were significantly increased with salinity level and waterlogging, and reached maximum values at week 2, and then declined to the initial level. Waterlogging and salinity increased the concentrations of water-soluble K⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺ ions, but the magnitudes of changes were greatly affected by soil properties. Increases in water-soluble K⁺, Ca²⁺ and Mg²⁺ were attributed to increased solubility of insoluble salts, and increased competition for the adsorption sites of the soil exchange complex due to elevated concentrations of Na⁺, Fe²⁺ and Mn²⁺. Increases in water-soluble Fe²⁺ and Mn²⁺ induced by waterlogging were attributed to the dissolution of Fe and Mn oxides under reduced conditions. Waterlogging increased, but salinity decreased, the amounts of water-dispersible clay in the soils of low EC value. The higher salinity level can counteract the adverse effect of waterlogging on clay flocculation.     

模拟氮沉降对太岳山油松林土壤酶活性的影响

为研究土壤酶活性对氮沉降增加的响应,以山西太岳山油松人工林和天然林为研究对象,于2009年8月开始实施模拟氮沉实验,试验设置对照(CK,0 kg N hm-2a-1);低氮(LN,50 kg N hm-2a-1);中氮(MN,100 kg N hm-2a-1);高氮(HN,150 kg N hm-2a-1)4种氮处理,自2012年起每年5、7、9月在各处理样方采集表层0—20 cm土壤,测定土壤酶活性(过氧化物酶、多酚氧化酶、纤维素酶、蔗糖酶、脲酶、中性磷酸酶)。研究结果表明:施氮处理下的脲酶与中性磷酸酶活性均有所提高,而低氮处理下天然林中的多酚氧化酶与人工林中的蔗糖酶显著低于对照,中氮、高氮处理下过氧化物酶、多酚氧化酶、天然林中的纤维素酶以及人工林中的蔗糖酶显著降低。总的来说,人工模拟氮沉降促进了土壤中脲酶和中性磷酸酶的活性,抑制了过氧化物酶和多酚氧化酶的活性,并降低了天然林土壤中的纤维素酶活性和人工林中的蔗糖酶活性,但对天然林中蔗糖酶和人工林中的纤维素酶无影响。主导木质素降解的多酚氧化酶活性与纤维素酶、蔗糖酶活性显著相关,纤维素酶与蔗糖酶活性的下降可能是由木质素降解受到抑制,土壤微生物可利用碳源减少所引起。另外,受到天然林土壤含氮量较高的影响,与人工林相比,天然林的多酚氧化酶活性对模拟氮沉降更敏感。由于被抑制的酶均与土壤有机质降解密切相关,氮沉降增加将减缓山西油松林土壤有机质的降解,有利于有机质在土壤中的积累。     

The multiple effects of ethylenediaminetetraacetate in several model lipid peroxidation systems

Experiments were performed which illustrate the various ways EDTA can influence lipid peroxidation. Either detergent-dispersed linoleate, or liposomes made from extracted microsomal phospholipids were utilized as substrates for peroxidation. Peroxidation was accomplished using Fe²⁺ or Fe³⁺. In systems utilizing Fe²⁺, EDTA chelation facilitated Fe²⁺ autoxidation which in turn caused peroxidation of detergent-dispersed linoleate. Peroxidation was not initiated during EDTA-Fe²⁺ autoxidation when the substrate lipids were in a liposomal configuration. Systems utilizing Fe³⁺ required an enzyme (either xanthine oxidase or NADPH-cytochrome P₄₅₀ reductase) to reduce the iron for peroxidative activity. EDTA chelation of Fe³⁺ enhanced the xanthine oxidase and NADPH-cytochrome P₄₅₀ reductase-catalyzed peroxidation of detergent-dispersed linoleate, presumably by facilitating the reduction of Fe³⁺. Catalase and mannitol inhibited both EDTA-Fe²⁺- and EDTA-Fe³⁺-dependent lipid peroxidation. EDTA-Fe³⁺ was not capable of initiating peroxidation of phospholipid liposomes following enzymatic reduction by either enzyme, but ADP-chelated iron effectively initiated liposomal peroxidation in similar systems. With xanthine oxidase-catalyzed peroxidation of liposomes with ADP-Fe³⁺, the inclusion of EDTA-Fe³⁺ caused a modest enhancement of activity. EDTA-Fe³⁺ greatly stimulated NADPH-cytochrome P₄₅₀ reductase-catalyzed peroxidation of liposomes with ADP-Fe³⁺. In contrast, the addition of EDTA, rather than EDTA-Fe³⁺ inhibited the liposomal peroxidation catalyzed by either enzyme with ADP-Fe³⁺ when the EDTA concentration exceeded the concentration of Fe³⁺.     

Ethylene production in rice bronzing leaves induced by ferrous iron

Bronzing, a nutritional disorder of rice plants which is widely distributed in tropical lowlands, was induced by dipping the cut end of rice leaves into FeSO₄ solution (pH 3.5). Ethylene production; the activities of peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase; and the effects of Co²⁺, aminoethoxyvinylglycine, Ag⁺, cycloheximide, and 1-aminocyclopropane-1-carboxylate, were investigated in the course of bronzing development. It was found that ethylene production could be stimulated up to about 20 times that of the control by Fe²⁺, and a peak could be reached at about 24 h after incubation. The Fe²⁺-treated leaves also had 10-fold higher peroxidase activity than the control, whereas in vitro enzyme activity was inhibited by Fe²⁺. Cycloheximide retarded in vivo stimulation of peroxidase, indicating that in vivo stimulation resulted from inducing de novo synthesis of the enzyme. No changes in the activities of phenylalanine ammonia-lyase and polyphenol oxidase were observed. The results, obtained from the incubation of leaves with Co²⁺, aminoethoxyvinylglycine, Ag⁺, cycloheximide, or 1-aminocyclopropane-1-carboxylate, showed that ethylene production was the effect of Fe²⁺ stress and that it was not involved in the process of bronzing development, which is probably an acclimation process to enable plants to cope with stress. The accelerated peroxidase activity may be associated with bronzing development.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine - EFE ethylene forming enzyme - PAL phenylalanine ammonia-lyase - POD peroxidase - PPO polyphenol oxidase - SE standard error     

Response of the microbial community to water table variation and nutrient addition and its implications for in situ preservation of organic archaeological remains in wetland soils

Wetland environments can preserve organic archaeological remains because of their anaerobic nature. The ongoing discovery of archaeological sites in wetlands is associated with a lack of funds for excavation and preservation. This situation has led to the consideration of preservation in situ the preferred option for dealing with the majority of waterlogged archaeological remains in England. To expand our understanding of the burial environment, we studied changes in environmental variables along with counts of total bacteria and microbial ¹⁴C-leucine assimilation down the soil profile at two wetlands in the North of England. Soil cores were sampled at five depth intervals between 10 and 100 cm. To test whether the addition of nutrients induces bacterial activity in the soil, inorganic phosphate and combined nitrogen were added to soil samples and the rate of ¹⁴C-leucine assimilation was recorded. Redox potential readings were positive above the water table and negative below. The total number of bacteria and the ¹⁴C-leucine assimilation rates differed among sites, but always decreased with increasing soil depth. Nutrient availability was limiting for the microbial communities in the upper soil horizons, but did not appear to limit those in the lower soil. These results allow a better understanding of the physico-chemical and microbiological conditions that potentially favour or inhibit the decomposition of organic archaeological remains at the studied wetlands.     

High-throughput Fluorometric Measurement of Potential Soil Extracellular Enzyme Activities

Microbes in soils and other environments produce extracellular enzymes to depolymerize and hydrolyze organic macromolecules so that they can be assimilated for energy and nutrients. Measuring soil microbial enzyme activity is crucial in understanding soil ecosystem functional dynamics. The general concept of the fluorescence enzyme assay is that synthetic C-, N-, or P-rich substrates bound with a fluorescent dye are added to soil samples. When intact, the labeled substrates do not fluoresce. Enzyme activity is measured as the increase in fluorescence as the fluorescent dyes are cleaved from their substrates, which allows them to fluoresce. Enzyme measurements can be expressed in units of molarity or activity. To perform this assay, soil slurries are prepared by combining soil with a pH buffer. The pH buffer (typically a 50 mM sodium acetate or 50 mM Tris buffer), is chosen for the buffer''s particular acid dissociation constant (pKa) to best match the soil sample pH. The soil slurries are inoculated with a nonlimiting amount of fluorescently labeled (i.e. C-, N-, or P-rich) substrate. Using soil slurries in the assay serves to minimize limitations on enzyme and substrate diffusion. Therefore, this assay controls for differences in substrate limitation, diffusion rates, and soil pH conditions; thus detecting potential enzyme activity rates as a function of the difference in enzyme concentrations (per sample).Fluorescence enzyme assays are typically more sensitive than spectrophotometric (i.e. colorimetric) assays, but can suffer from interference caused by impurities and the instability of many fluorescent compounds when exposed to light; so caution is required when handling fluorescent substrates. Likewise, this method only assesses potential enzyme activities under laboratory conditions when substrates are not limiting. Caution should be used when interpreting the data representing cross-site comparisons with differing temperatures or soil types, as in situ soil type and temperature can influence enzyme kinetics.     

Involutin Is an Fe3+ Reductant Secreted by the Ectomycorrhizal Fungus Paxillus involutus during Fenton-Based Decomposition of Organic Matter

Ectomycorrhizal fungi play a key role in mobilizing nutrients embedded in recalcitrant organic matter complexes, thereby increasing nutrient accessibility to the host plant. Recent studies have shown that during the assimilation of nutrients, the ectomycorrhizal fungus Paxillus involutus decomposes organic matter using an oxidative mechanism involving Fenton chemistry (Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + ˙OH + H₂O), similar to that of brown rot wood-decaying fungi. In such fungi, secreted metabolites are one of the components that drive one-electron reductions of Fe³⁺ and O₂, generating Fenton chemistry reagents. Here we investigated whether such a mechanism is also implemented by P. involutus during organic matter decomposition. Activity-guided purification was performed to isolate the Fe³⁺-reducing principle secreted by P. involutus during growth on a maize compost extract. The Fe³⁺-reducing activity correlated with the presence of one compound. Mass spectrometry and nuclear magnetic resonance (NMR) identified this compound as the diarylcyclopentenone involutin. A major part of the involutin produced by P. involutus during organic matter decomposition was secreted into the medium, and the metabolite was not detected when the fungus was grown on a mineral nutrient medium. We also demonstrated that in the presence of H₂O₂, involutin has the capacity to drive an in vitro Fenton reaction via Fe³⁺ reduction. Our results show that the mechanism for the reduction of Fe³⁺ and the generation of hydroxyl radicals via Fenton chemistry by ectomycorrhizal fungi during organic matter decomposition is similar to that employed by the evolutionarily related brown rot saprotrophs during wood decay.     

Callus Induction from Insect Galls by Dryocosmus kuriphilus on Chestnut Tree

Dehydrodicaffeic acid dilactone (DDACD) was found in a cultured mushroom by screening for catechol-O-methyltransferase inhibitors. The enzyme which converts two molecules of caffeic acid to DDCAD has been extracted from the mushroom and purified and the enzyme reaction has been studied. It was markedly inhibited by reducing agents, such as NADPH, NADH, glutathione and ascorbic acid but stimulated by Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cu⁺ and Zn²⁺ ions. Sodium diethyldithiocarbamate and sodium cyanide known to be copper chelating agents inactivated the enzyme, but activity was restored by addition of Cu²⁺ or Cu⁺. Although the enzymic reaction did not occur under anaerobic conditions, ¹⁸O-oxygen was not incorporated into DDCAD. o-Diphenol oxidase catalyzed DDCAD formation from caffeic acid and the DDCAD-forming enzyme catalyzed the formation of DOPAchrome from DOPA. Thus, the DDCAD-forming enzyme is a type of o-diphenol oxidase. Peroxidase and hydrogen peroxide produced DDCAD from caffeic acid.

On the other hand, DDCAD was non-enzymatically synthesized from caffeic acid in the presence of CuCl₂ in 64% yield. In both enzymic and non-enzymic syntheses, both (+)- DDCAD and (?)-DDCAD were produced.     

Priming and microbial nutrient limitation in lowland tropical forest soils of contrasting fertility

Priming is an increase in soil organic carbon decomposition following input of labile organic carbon. In temperate soils where biological activity is limited commonly by nitrogen availability, priming is expected to occur through microbial acquisition of nitrogen from organic matter or stimulated activity of recalcitrant-carbon degrading microorganisms. However, these priming mechanisms have not yet been assessed in strongly weathered tropical forest soils where biological activity is often limited by the availability of phosphorus. We examined whether microbial nutrient limitation or community dynamics drive priming in three lowland tropical forest soils of contrasting fertility (‘low’, ‘mid’ and ‘high’) by applying C₄-sucrose (alone or in combination with nutrients; nitrogen, phosphorus and potassium) and measuring (1) the δ¹³C-signatures in respired CO₂ and in phospholipid fatty acid (PLFA) biomarkers, and (2) the activities of enzymes involved in nitrogen (N-acetyl β-glucosaminidase), phosphorus (phosphomonoesterase) and carbon (β-glucosidase, cellobiohydrolase, xylanase, phenol oxidase) acquisition from organic compounds. Priming was constrained in part by nutrient availability, because priming was greater when sucrose was added alone compared to when added with nutrients. However, the greatest priming with sucrose addition alone was detected in the medium fertility soil. Priming occurred in parallel with stimulated activity of phosphomonoesterase and phenol oxidase (but not N-acetyl β-glucosaminidase); when sucrose was added with nutrients there were lower activities of phosphomonoesterase and phenol oxidase. There was no evidence according to PLFA δ¹³C-incorporation that priming was caused by specific groups of recalcitrant-carbon degrading microorganisms. We conclude that priming occurred in the intermediate fertility soil following microbial mineralization of organic nutrients (phosphorus in particular) and suggest that priming was constrained in the high fertility soil by high nutrient availability and in the low fertility soil by the low concentration of soil organic matter amenable to priming. This first study of priming mechanisms in tropical forest soils indicates that input of labile carbon can result in priming by microbial mineralization of organic nutrients, which has important implications for understanding the fate of organic carbon in tropical forest soils.     

漓江水陆交错带不同植被类型的土壤酶活性

水陆交错带是内陆水生生态系统与陆地生态系统之间的功能界面区,其包含了高地到低地直到水体的区域,是土壤有机质源、汇和转换器。土壤中有机物的分解以及营养物质的转化不仅影响到植物的生长,也对水体质量产生间接影响。土壤酶几乎参与土壤中有机物质的分解与合成的全过程,直接或间接影响着土壤一系列的生物化学反应,对生态系统的物质循环产生重要影响。不少学者围绕农田土壤、林地土壤以及湿地土壤探讨了不同植被下酶活性的变异。水陆交错带植被种类丰富,周期性的淹水条件加剧了土壤性质变异的复杂性。但目前水陆交错带不同植被类型土壤酶活性差异的研究不多。以漓江水陆交错带土壤为研究对象,对苔藓、草本和灌丛3种植被类型下的土壤溶解性化学成分、4种土壤水解酶即糖苷酶、几丁质酶、亮氨酸氨基肽酶和磷酸酶以及2种氧化还原酶即酚氧化酶和过氧化物酶的活性,以及土壤性质与酶活性之间的关系进行了研究。结果表明,苔藓植被下土壤的糖苷酶和酚氧化酶活性显著高于草本和灌丛,草本植被下土壤的过氧化物酶活性显著高于苔藓和灌丛,灌丛植被下土壤几丁质酶活性显著高于苔藓和草本,但不同植被类型的土壤亮氨酸氨基肽酶活性无显著差异。相关分析表明,土壤水分含量与糖苷酶和酚氧化酶活性呈显著正相关,而与几丁质酶和碱性磷酸酶活性呈显著负相关。土壤有机碳和易氧化碳均与糖苷酶和酚氧化酶活性呈极显著负相关,与几丁质酶活性呈显著正相关。土壤溶解性有机碳与亮氨酸氨基肽酶和酚氧化酶呈显著正相关。综合认为,水陆交错带不同种类土壤酶在不同植被类型间的差异有别,土壤水分含量和土壤有机碳显著影响土壤酶活性的变化。不同植被类型土壤酶活性的差异不仅与植被类型有关,与水陆交错带微地形以及土壤性质的空间异质性也有密切关系,需运用长期控制试验手段开展研究。     

Preparation and some properties of cholesterol oxidase from Rhodococcus sp. R14-2

Rhodococcus sp. R_14-2, isolated from Chinese Jin-hua ham, produces a novel extracellular cholesterol oxidase (COX). The enzyme was extracted from fermentation broth and purified 53.1-fold based on specific activity. The purified enzyme shows a single polypeptide band on SDS-PAGE with an estimated molecular weight of about 60 kDa, and has a pI of 8.5. The first 10 amino acid residues of the NH₂-terminal sequence of the enzyme are A-P-P-V-A-S-C-R-Y-C, which differs from other known COXs. The enzyme is stable over a rather wide pH range of 4.0–10.0. The optimum pH and temperature of the COX are pH 7.0 and 50°C, respectively. The COX rapidly oxidizes 3β-hydroxysteroids such as cholesterol and phytosterols, but is inert toward 3α-hydroxysteroids. Thus, the presence of a 3β-hydroxyl group appears to be essential for substrate activity. The Michaelis constant (Km) for cholesterol is estimated at 55 μM; the COX activity was markedly inhibited by metal ions such as Hg²⁺ and Fe³⁺ and inhibitors such as p-chloromercuric benzoate, mercaptoethanol and fenpropimorph. Inhibition caused by p-chloromercuric benzoate, mercuric chloride, or silver nitrate was almost completely prevented by the addition of glutathione. These suggests that -SH groups may be involved in the catalytic activity of the present COX.     

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Wetlands have been widely applied for water quality amelioration. Enzymatic analysis was applied in a study of decomposition in constructed wetlands. We hypothesise that soil enzyme activities would be lower in wetland sediment than adjacent upland and that the lower soil enzyme activities are partly responsible for the water quality amelioration. Four soil enzyme activities (β-glucosidase, β-N-acetylglucosaminidase, phosphatase, and arylsulfatase) and microbial activity (electron transport system activity) were measured across a transect from a upland soil to a wetland sediment in two constructed wetland sites in the USA. Along with the activities, hydrochemistry was determined in inflow and outflow of the wetlands. In both wetlands, the enzyme activities in the sediments were significantly lower than the adjacent upland soils. For hydrochemistry, significant decreases were observed in phosphate and nitrate concentrations in outflow water compared to inflow water. However, there were no significant changes in other anions (F^-, Cl^-, SO ₄ ^2- . For dissolved organic carbon, it seems that the wetlands would be a source rather than a sink. The results suggest that the enzymatic approach represents a valuable method to assess decomposition processes in wetland sediments, and that characteristically low enzyme activities in the sediments may be important in the water quality amelioration function. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laccase Gene Composition and Relative Abundance in Oak Forest Soil is not Affected by Short-Term Nitrogen Fertilization

Anthropogenic nitrogen (N) deposition affects a wide range of soil processes including phenol oxidase (PO) activity and soil organic matter dynamics. Depression of phenol oxidase activity in response to N saturation is believed to be mediated by the activity of white-rot basidiomycetes, whose production of extracellular oxidative enzymes can be limited by high N availability. We examined the effect of short-term N deposition on basidiomycete laccase gene diversity and relative abundance in temperate oak forest soil in which significant decreases in phenol oxidase and increased SOM have been recorded in response to experimental N deposition. UniFrac was used to compare the composition of laccase genes between three control- and three nitrogen-fertilized (80 kg⁻¹ ha⁻¹ per year) oak forest soils. The relative abundance of laccase genes was determined from qPCR analysis of laccase and basidiomycete ITS gene abundances. Our results indicate that there was no significant shift in the composition of laccase genes between control- and N-fertilized soils, nor was there a significant change in the relative abundance of laccase genes. These data suggest that N deposition effects on mineral soil PO activity do not result from changes in laccase gene diversity of white-rot basidiomycetes but are likely the result of altered microbial abundance or expression in this ecosystem type. Furthermore, laccase gene composition may be tied to factors that structure microbial communities in general, as soil laccase gene communities are more similar to other forest soils than with the corresponding litter.