1,721,030 research outputs found
Hot experience for cold-adapted microorganisms: Temperature sensitivity of soil enzymes
No full textHigh latitude and cold ecosystems, which constitute the major environment on Earth, are particularly threatened by global warming. Consequently, huge amounts of SOC stored in these ecosystems may be released to the atmosphere by accelerated enzymatic decomposition. Effects of intensive warming on temperature sensitivity and catalytic properties of soil enzymes were tested in cold-adapted alpine grassland of the Tibetan Plateau. We hypothesized that 1) maximal reaction rate will be insensitive to intensive warming at high temperature range (V-max - Q(10) = 1); 2) substrate affinity (K-m) remains constant at elevated temperatures due to expression of enzymes with less flexibility. These hypotheses were tested by examining the kinetics of six enzymes involved in carbon (cellobiohydrolase, beta-glucosidase, xylanase), nitrogen (tyrosine-aminopeptidase, leucine-aminopeptidase) and phosphorus (acid phosphomonoesterase) cycles after soil incubation at temperatures from 0 to 40 degrees C. Q(10) and E-a decreased at high temperature (25-40 degrees C). However, enzymes that degrade low quality polymers remained temperature-sensitive even above 25 degrees C (V-max - Q(10) = 2), which explains the faster decomposition of recalcitrant C compounds under warming. Substrate affinity of all enzymes gradually increased up to 20 degrees C. At 25 degrees C, however, K-m increased rapidly, leading to an extreme decrease in catalytic efficiency. Above 25 degrees C, K-m of C and N cycles remained nearly constant, while V-max gradually increased from 0 to 40 degrees C. These results reveal two important implications of warming: 1) there are some temperature thresholds (here 20-25 degrees C) that lead to sudden reductions in substrate affinity, decreasing temperature sensitivity and catalytic efficiency, 2) decoupled temperature sensitivity of V-max and K-m and the resulting maintenance of stable enzyme systems at high temperatures ensured efficient enzymatic functioning and persistent decomposition of SOM at temperatures much higher than the common adaptation range of the ecosystem. Thus, the temperature thresholds of strong changes in enzyme-based processes should be considered and included in the next generation of models in order to improve the prediction of SOM feedbacks to warming. (C) 2016 Elsevier Ltd. All rights reserved
Temperature selects for static soil enzyme systems to maintain high catalytic efficiency
No full textKnowledge on enzymatic mechanisms of acclimation to temperature is required to predict the effects of warming on decomposition of soil organic matter - the largest C stock in terrestrial ecosystems. Based on Michaelis-Menten kinetics we tested the hypothesis that enzyme affinity to substrate (K-m) is more sensitive to warming at cold than at warm temperatures. We also predicted a gradual increase in K-m values with increasing temperature. The kinetic parameters of six enzymes involved in cycles of C, (cellobiohydrolase, beta-glucosidase and xylanase), P (phosphatase), and N (leucine-aminopeptidase, tyrosine-aminopeptidase) were determined after one month of soil incubation at a temperature range 0-40 degrees C (with 5 increment). Contrary to our hypothesis, the increase in K-m with temperature was not gradual for most tested enzymes. Within large range of temperatures from 0 to 15 degrees C (phosphatase), 0-20 degrees C (enzymes involved in C cycle) and 0-40 degrees C (proteases) the hydrolytic activity was governed by enzymes with nearly constant substrate affinity. Temperature, therefore, mainly selected for soil enzyme systems maintaining static Km. The catalytic efficiency of the enzymes (V-max/K-m) increased from low to intermediate temperatures (0-20 degrees C) as a result of linear increase of V-max at constant K-m. Static K-m values were explained either by low flexibility (high structural stability) of a single enzyme type, which catalyzed the reaction over a broad temperature range, or by production of multiple isoenzymes each with different temperature optima but with similar affinity to substrate. Thus, maintaining static Km with temperature increase ensured high enzyme efficiency within a low and intermediate soil temperature range. (C) 2016 Elsevier Ltd. All rights reserved
Microbial and enzymes response to nutrient additions in soils of Mt. Kilimanjaro region depending on land use
No full textMicrobial and enzyme activities can be used to identify and assess the impacts of changes in land use management on soil quality. However, only few studies have investigated the effects of land use and nutrient additions on enzyme activities and microbial processes in tropical African soils. Glucose and nutrients (N and P) were added to soils (0-20 cm) from natural and agricultural ecosystems: (1) savannah, (2) maize fields, (3) lower montane forest, (4) coffee plantation, (5) grasslands (6) Chagga homegardens common at Mt. Kilimanjaro region and East Africa. Microbial biomass and activities of beta-glucosidase, cellobiohydrolase, chitinase and phosphatase were monitored over 60 days incubation period. Microbial biomass content and enzyme activities were generally higher in soils under natural vegetation compared to corresponding agricultural soils. Decline in microbial biomass C content over time was higher in natural ecosystems compared to agricultural soils. However, the microbial biomass C content in Chagga homegarden soils was relatively stable. Land use was negatively correlated to beta-glucosidase, cellobiohydrolase and chitinase activity, but positively correlated to phosphatase activity. beta-glucosidase and cellobiohydrolase, involved in the C-cycle, were the most sensitive to landuse change. Chitinase activity was 2-6 times higher in soils under natural vegetation compared to corresponding arable soils. Phosphatase displayed very high activities in all land use types. This is attributed to the high P retention capacity common for andic soils similar to those occurring at Mt. Kilimanjaro region. Increased P availability stimulated enzyme activities in lower montane forest and Chagga homegarden soils. Overall, microbial biomass and enzyme activities showed a strong decrease with increased land use intensity and should therefore be taken into consideration in monitoring and assessing the impact of land use change at Mt. Kilimanjaro region. (C) 2015 Elsevier Masson SAS. All rights reserved
Corrigendum to Razavi et al. (2016) “Rhizosphere shape of lentil and maize: Spatial distribution of enzyme activities” [Soil Biol. Biochem. 96, 229–237]
No full textLand use affects soil biochemical properties in Mt. Kilimanjaro region
No full textMicrobial parameters have been used to monitor changes in soil quality. Soils from four land use systems common in East Africa and present in the Mt. Kilimanjaro region: (1) montane forest, (2) savannah (3) maize fields and (4) Chagga homegardens were used in laboratory incubations to assess the effects of landuse changes on soil quality. Soil organic matter mineralization and the following microbial parameters: microbial biomass C, mineralization quotient, metabolic quotient and activities of four enzymes: beta-glucosidase, cellobiohydrolase, phosphatase and chitinase were determined. Microbial biomass C content, beta-glucosidase, cellobiohydrolase and chitinase activities were higher in natural systems compared to agricultural soils. High phosphatase activity observed in all land use types reflected strong phosphorus limitation in andic soils of the Mt. Kilimanjaro region. Chitinase activity in montane forest soils was 3 times higher than in Chagga homegardens. Mineralization quotient and cellobiohydrolase activity best exhibited the effect of land-use changes on soil quality in the Mt. Kilimanjaro region. Cellobiohydrolase activity was up to 3 times higher under natural ecosystems compared to agroecosystems. A high percentage of microbial biomass C content in total organic C and low metabolic quotient were observed in Chagga homegarden soils. Soil enzymes (especially cellobiohydrolase) best distinguished between natural and agricultural ecosystems, and are therefore useful for monitoring changes in soil quality. In conclusion, the measured microbial parameters clearly show that the microbial organisms in traditional Chagga homegardens system have high substrate use efficiency. This demonstrates that traditional agroforestry systems promotes soil fertility and are more suitable for agricultural production in the tropics compared to monocropping systems like maize plantations. (C) 2016 Elsevier B.V. All rights reserved
Effect of land use and management practices on microbial biomass and enzyme activities in subtropical top-and sub-soils
No full textLand-use change, especially from forest to intensive agriculture, is negatively impacting soil quality and sustainability. Soil biological activities are sensitive indicators of such land-use impacts. We tested two hypotheses: i) land use and management practices affect microbial properties (microbial biomass and enzyme activities) in topsoil (0-20 cm), but have no effects in subsoil (20-100 cm); and ii) microbial properties in topsoil are highest in forest, followed by organic farming and then conventional farming. Total organic C and N contents as well as microbial biomass were significantly higher in the organic farming topsoil compared with conventional farming and forest. Except xylanase and acid phosphatase, enzyme activities (beta-glucosidase, cellobiohydrolas, chitinase, sulfatase, leucine aminopeptidase and tyrosine aminopeptidase) were also higher in organic farming soil. Crop residues and rhizodeposits support higher microbial biomass, leading to enhanced enzyme activities in organic farming soil. Incorporation of rice stubble and limitation of available phosphorus explain the higher xylanase and acid phosphatase activities, respectively, in conventional farming soil. Litter removal leads to a deficiency of labile C and N, resulting in lower enzyme activities in forest soil. Total C and N contents were higher in subsoil under organic farming. Although there was no effect of land use on microbial biomass in subsoil, activities of most enzymes were higher under organic farming. Overall, our results indicate that land-use change significantly alters microbial properties in topsoil, with modest effects in subsoil. Microbial properties should be considered in environmental risk assessments and models as indicators of ecosystem disturbance caused by land-use and management practices. (C) 2017 Elsevier B.V. All rights reserved.Erasmus mundus (Experst4Asia
Rhizosphere shape of lentil and maize: Spatial distribution of enzyme activities
No full textThe rhizosphere, the small soil volume that surrounds and is influenced by plant roots, is one of the most dynamic biological interfaces on Earth. Enzymes, produced by both roots and microorganisms, are the main biological drivers of SOM decomposition. In situ soil zymography was applied to test hypotheses that 1) the spatial pattern of rhizosphere activity is enzyme-specific and 2) the distribution of enzyme activity along the roots is dependent on root system and plant species. Lentil (Lens culinaris) and maize (Zea mays L.), two species with contrasting root physiology, were chosen to test their effects on spatial distribution of activities of p-glucosidase, cellobiohydrolase, leucine-aminopeptidase and phosphatase. The extent of the rhizosphere for each enzyme and plant species was estimated as a function of distance from the root. For the first time, we demonstrated plant-specific patterns of exoenzyme distribution: these were uniform along the lentil roots, whereas in the rhizosphere of maize, the enzyme activities were higher at the apical or proximal root parts. We conclude that the shape and extent of the rhizosphere for enzyme activities is plant species specific and varies due to different rhizosphere processes (e.g. root exudation) and functions (e.g. nutrient mobilization abilities). The extension of enzyme activity into the rhizosphere soil was minimal (1 mm) for enzymes responsible for the C cycle and maximal (3.5 mm) for enzymes of the phosphorus cycle. This should be considered in assessments and modeling of rhizosphere extension and the corresponding effects on soil properties and functions. (C) 2016 Elsevier Ltd. All rights reserved
Spatial distribution and catalytic mechanisms of beta-glucosidase activity at the root-soil interface
No full textWe compared modifications of soil zymography, a new in situ technique to visualize enzyme activities, based on contact of fluorgenic substrate-saturated membranes with soil either through the gel layer (gel zymography) or without gel application (direct zymography). We coupled zymography with quantitative measurements of enzyme kinetics to characterize catalytic mechanisms of beta-glucosidase activity at the plant-soil interface including root surface (rhizoplane), rhizosphere, and bulk soil. Direct zymography refined and focused image resolution. The area of hotspots (i.e., spots with most intensive enzyme activity) as well as color intensity ratios estimated using direct zymography exceeded by a factor of 2 the corresponding values obtained with gel zymography. As determined by direct zymography, the percentage of hotspots associated to root surfaces was 58-68 % of total hotspot area. Hotspot area comprised only 6.8 +/- 0.1 % of the total area of an image and 9.0 +/- 3 % of the root surface area. The intensity of beta-glucosidase activity, however, was up to 20 times higher in the hotspots versus bulk soil. The contribution of rhizosphere to beta-glucosidase activity of the whole image (77-82 %) was four times higher than the contribution of the root surface. Enzyme kinetic parameters indicated different enzyme systems in bulk and rhizosphere soil. Higher substrate affinity and catalytic efficiency in bulk than in rhizosphere soil suggested relative domination of microorganisms with more efficient enzyme systems in the former. Coupling direct zymography and kinetic assays enabled mapping the two-dimensional (2D) distribution of enzyme activity at the root-soil interface and estimating the catalytic properties of root-associated and soil-associated enzymes
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