冻融环境下亚高山森林凋落叶添加对土壤腐殖质动态的影响

亚高山森林土壤形成过程中,胡敏酸、富里酸等腐殖物质的累积是维持土壤肥力及物质循环的重要途径,它受到土壤基质质量、凋落叶和环境因素的调控.本研究以川西亚高山典型的针叶林、针阔混交林和阔叶林土壤为对象,采用室内培养控制冻融环境和凋落叶添加的方法,研究冻融环境下凋落叶添加对土壤腐殖物质累积的影响.结果表明:冻融循环环境下针叶林土壤腐殖质含量升高,而针阔混交林和阔叶林土壤腐殖质含量降低,且凋落叶对土壤腐殖质含量无显著影响.培养前期冻融环境下3种林型土壤胡敏酸净累积,净累积量大小为针阔混交林>针叶林>阔叶林,富里酸含量下降,下降程度为阔叶林>针阔混交林>针叶林,且凋落叶对土壤胡敏酸和富里酸含量无显著影响.随培养时间的延长,3种林型土壤胡敏酸及富里酸含量均下降.这表明,凋落叶对土壤腐殖质含量的影响与土壤基质质量存在密切关系,且受到冬季土壤冻融时间长短的影响.     

西双版纳不同热带生态系统土壤有机质的光谱学特性

 为了探讨西双版纳地区土地利用变化对土壤有机质含量及其化学组成的影响,选取了相邻的次生林、耕种6年的农田和定植3年的橡胶园样地,对其0～5 cm和5～20 cm表层土壤中有机质含量、胡敏酸的光谱学特性进行了分析。研究结果表明,次生林转变为农田之后,0～5 cm和5～20 cm 表层土壤有机质含量分别降低33.6%和23.7%;而次生林转变为橡胶园,分别降低28.6%和27.6%。胡敏酸可见-红外光谱结果显示,次生林转变为农田和橡胶园后,0～20 cm表层土壤E4/E6显著降低,这表明胡敏酸化学组成当中芳香族结构增加。傅立叶变换红外光谱结果同样表明,土地利用变化影响土壤有机质的化学组成。次生林转变为农田和橡胶园后,胡敏酸中羧基和酚基结构比例降低,而脂肪族、芳香族和多聚糖比例增加。     

川西亚高山森林凋落物去除对土壤腐殖质动态的影响

森林凋落物作为森林土壤腐殖质的主要来源, 在土壤腐殖质的形成中发挥着重要作用, 但不同森林类型凋落物因其含量、组成等的不同, 对土壤腐殖质的影响也不同。该研究以川西亚高山针叶林、阔叶林和针阔混交林3种不同森林类型为对象, 采用凋落物原位控制实验, 对比研究不同关键期凋落物去除对土壤可提取腐殖质、胡敏酸和富里酸含量及胡敏酸/富里酸、胡敏酸/可提取腐殖质的影响。主要结果: (1)土壤可提取腐殖质、胡敏酸和富里酸含量在不同森林类型中差异显著。土壤可提取腐殖质含量总体表现为针叶林>针阔混交林>阔叶林, 胡敏酸含量总体表现为针阔混交林>针叶林>阔叶林, 而富里酸含量则表现为针叶林>阔叶林>针阔混交林, 其中3种林型中土壤腐殖质的主要成分为富里酸, 总体均表现为富里酸型。不同采样时期也显著影响了土壤可提取腐殖质、胡敏酸和富里酸含量, 总体均表现为先升高后下降的趋势。除个别采样时期外, 凋落物去除总体降低了土壤可提取腐殖质、胡敏酸和富里酸的含量。(2)胡敏酸/富里酸和胡敏酸/可提取腐殖质的结果显示3种林型土壤总体腐殖化程度均较低, 整体表现为针阔混交林>阔叶林>针叶林, 凋落物去除在一定程度上有利于提高阔叶林与针阔混交林的腐殖质品质。(3)相关分析表明不同凋落物处理间土壤可提取腐殖质与土壤有机碳含量、全氮含量和土壤含水量呈显著正相关关系, 与温度呈显著负相关关系。综上所述, 短期的凋落物去除会降低土壤腐殖物质的含量, 但不同林型间由于凋落物类型差异会导致土壤腐殖质的不同变化, 说明土壤腐殖质的动态变化受凋落物类型以及环境因素的综合调控。因此, 关于凋落物变化对土壤腐殖质的影响还需进一步的长期研究。     

Agricultural reclamation of disturbed soils in a lignite mining area using municipal and coal wastes: the humus situation at the beginning of reclamation

Soils disturbed by long-term opencast mining were treated with organic waste materials for reclamation. Humic substances were extracted from waste and soil samples and analysed using pyrolysis-gas chromatography/mass spectrometry and electrofocusing. Furthermore, analytical pyrolysis permits to study all starting materials in situ. According to structural similarities, the statistical evaluation of the pyrolysis results clearly indicates three sample groups. The first group, called compost, implies the waste materials compost and composted sewage sludge. Moreover, pyrolysis revealed that coal humic substances are predominant in brown coal sludge, pure mine soils and mine soils treated with the different organic waste materials. They constitute the second group. The sewage sludge contains a high nitrogen potential, as expected, and represents the third group. Finally, pyrolysis generally showed the specific structural characteristics of humic and fulvic acids, respectively. Electrofocusing yielded for all samples a signal pattern that is typical of humic substances. However, number and ratio of the signals differ according to the special structural features of the samples. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterizing cultivable soil microbial communities from copper fungicide-amended olive orchard and vineyard soils

Use of copper-based fungicides has led to an increase in the total Cu content in agriculture soils. The focus of this study was to determine fractionation of Cu and to investigate the structure and the diversity of cultivable bacterial communities in two vineyards (one 25 years old and one 2 years old), one olive orchard and two forest soils. All soils developed on an Oligocene sandstone. The concentration of total Cu in the old vineyard (176.6 mg kg⁻¹) and olive orchard (145.5–296.7 mg kg⁻¹) was from 5 to 10 times higher than in forest soils. The major amount of Cu was found bound to the humic substances in cultivated soils, whereas in forest soils Cu was found in the residual mineral fraction. A relationship was found between the number of cultivable Cu-tolerant bacteria and total Cu content in soil. In the cultivated soils, Cu had a toxicological effect on bacterial community, and thereby Cu-levels > to 145 mg kg⁻¹ could be a risk to soil biota. Microbial communities were analysed by community level physiological profiling (CLPP), using the Biolog system, and by the amplified ribosomal DNA restriction analysis (ARDRA) approach. Only when cell suspensions containing 10⁴ colony-forming units (c.f.u.) were inoculated in each well of Biolog EcoPlates it was possible to discriminate microbial communities from different soil samples. As expected, 16S ARDRA showed that cultivated soils had a lower microbial diversity in respect to forest soils.     

Soil textural class plays a major role in evaluating the effects of land use on soil quality indicators

Inappropriate land use that negatively affects ecological processes and soil quality is generally considered to be the primary cause of soil degradation in tropical agroecosystems. We hypothesized that in addition to land use, soil textural class also has an important effect on ecological processes and soil quality. To test our hypothesis, effects of land use change on soil organic fractions as well as microbial and biochemical indicators were quantified for clayey and sandy-clay-loam soils within the native Cerrado biome, pasture (Brachiaria brizantha) and sugarcane (Saccharum officinarum) agroecosystems in southwestern Brazil (Minas Gerais state). Labile carbon, humic substances, soil microbial respiration (SMR), microbial biomass carbon (MBC), metabolic quotient (qCO₂), hydrolysis of fluorescein diacetate (FDA), beta-glucosidase, urease, phosphatase and arylsulphatase activities were measured for each sample. Labile carbon concentrations were not affected by land use but were lower in sandy-clay-loam soil than clayey soil. Humic substances were at the highest concentrations in the native Cerrado and the lowest in sugarcane agroecosystems. Sandy-clay-loam soil had lower humic acid concentrations than clayey soil. Soil microbial indicators (SMR, MBC and FDA) showed lower values in pasture and sugarcane agroecosystems than in the native Cerrado. FDA was a more sensitive microbial indicator than SMR and MBC for detecting land use and textural class differences. The qCO₂ indices were greater in sugarcane systems than in either pasture or native Cerrado systems. The activity of exocellular hydrolytic enzymes, such as beta-glucosidase, urease, phosphatase and arylsulphatase, was smaller in sugarcane and pasture agroecosystems than in native Cerrado ecosystems. Within the same land use, the activity of these enzymes was always greater in clayey soil than in sandy-clay-loam soil, indicating a higher impact of land uses on enzyme activities in clayey soils. Results for the measured indicators support the hypothesis that soil textural class plays a major role in assessing differences between land use systems in the Brazilian Cerrado biome.     

Soil minerals and humic acids alter enzyme stability: implications for ecosystem processes

In most ecosystems, the degradation of complex organic material depends on extracellular enzymes produced by microbes. These enzymes can exist in bound or free form within the soil, but the dynamics of these different enzyme pools remain uncertain. To address this uncertainty, I determined rates of enzyme turnover in a volcanic soil with and without added enzymes. I also tested whether or not soil minerals and humic acids would alter enzyme activity. In soils that were gamma-irradiated to stop enzyme production, 35–70% of the enzyme activity was stable throughout the 21-day incubation. The remaining enzyme fraction decayed at rates ranging from − 0.032 to − 0.628 day⁻¹. In both the irradiated soils and in soils with added enzymes, addition of the mineral allophane had a strong positive effect on most enzyme activities. Another added mineral, ferrihydrite, had a weak positive effect on some enzymes. Added humic acids strongly inhibited enzyme activity. These findings suggest that minerals, especially allophane, enhance potential enzyme activities in young volcanic soils. However, the actual activity and function of these enzymes may be low under field conditions if stabilization results in less efficient enzyme-substrate interactions. If this is the case, then much of the measured enzyme activity in bulk soil may be stabilized but unlikely to contribute greatly to ecosystem processes.     

The influence of heavy metals upon the growth of sitka-spruce in South Wales forests

Summary Experiments on sitka-spruce seedlings grown in acidic peaty gley soils under green-house conditions, where the soils where doped with increasing amounts of Cd, Cu and Pb up to maximum levels of metal added of 16 ppm, 32 ppm and 400 ppm respectively, showed that the levels of Cd and Pb in shoots and roots increased with increasing levels in the soil, whereas levels of copper appeared to be independent. The addition of these three metals to the soils did not influence the uptake of other heavy metals, or of the nutrients potassium or calcium. Increases in the shoot cadmium levels significantly reduced the yields of the plant shoots. However, the plant yields were only affected by the highest level of lead that was added to the soil (400 ppm Pb) and unaffected by all the copper treatments (0–32 ppm Cu in the soil). The lengths of the sitka-spruce roots were reduced when cadmium and lead levels in the soil exceeded certain threshold concentrations (2.5 ppm total Cd, where 0.3 ppm was extractable with 0.5 M acetic acid; and 48 ppm total Pb, where 1.7 ppm was extractable). However, root lengths were not reduced by copper. This was probably related to the fact that copper appears to be relatively unavailable in the type of soil used, as only 1.1. ppm Cu was extractable from a total of 32 ppm Cu added. Root branching was apparently reduced by increases in the soil levels of cadmium, copper and lead. The roots of some control plants had symbiotic mycorrhizal associations (4 out of 19 plants), whereas the roots of all the plants grown in the soils with added heavy metals did not develop these.     

Using hydrogel and clay to improve the water status of seedlings for dryland restoration

The decomposition of soil organic matter is mediated by extracellular enzymes. The aim of this work was to identify the factors determining the activity and size of the mobile fraction of extracellular enzymes (laccase, Mn-peroxidase, endocellulase, cellobiohydrolase, ??-glucosidase, endoxylanase, ??-xylosidase, ??-glucosidase, chitinase, arylsulfatase, phosphatase, phosphodiesterase, alanine and leucine aminopeptidase) using a set of soils covering a wide range of physico-chemical properties. Organic matter content had a major effect on enzyme activity both in forest and grassland soils, while the effects of pH and humic compounds content were only important in forest soils, and the molecular mass of humic compounds and Ca content were only important in grasslands. Specific enzyme activity was either comparable between the soil types or higher in grasslands. With the exception of Mn-peroxidase and ??-glucosidase, the specific activities of all enzymes in arable fields under tillage were similar to those in grasslands. Mobility differed among the enzymes and ranged from <1% for arylsulfatase and phosphodiesterase up to 20?C40% for ??-glucosidase and aminopeptidases, with pH being the most important variable. These results demonstrate that the factors regulating enzyme activity are likely to be different in forest soils and grasslands and that enzyme mobility is a characteristic feature of each individual enzyme.     

Spatial variation of soil quality and pollution assessment of heavy metals in cultivated soils of Henan Province,China

Abstract

Eight chemical and physical characteristics and seven biological properties of the soils from five cultivated areas in the Henan Province were quantified. Concentrations of Cu, Cr, Zn and Mn were determined using the flame atomic absorption spectrometry. In addition available phosphorus, chloride, soluble salt and pH were measured. The activities of enzymes were determined by a literature method. The results showed that the concentrations of Mn, Zn and Cr had close relationships with that of available phosphorus, and their concentrations increased with the prolonged application of organic and mineral fertilisers and pesticides-fungicides. The activities of cellulase have a relationship to the concentrations of Zn and Mn at 0.95 levels respectively, Zn and Mn stimulate many enzyme activities. The relationship between protease and available phosphorus was nearly distinctive at 0.95 levels, relating with the use of nitrogen fertiliser in the farmlands. Local organic manures and urea fertilisers did not change the soil characteristics to a great extent. However, the soils showed moderate and moderate-to-strong contamination by Mn and Zn in the study areas. Wastewater and groundwater irrigation, pesticides-fungicides, and fertiliser may be responsible for the elevated concentrations of Zn and Mn in the soil samples.     

Humic substances biological activity at the plant-soil interface: From environmental aspects to molecular factors

Humic substances (HS) represent the organic material mainly widespread in nature. HS have positive effects on plant physiology by improving soil structure and fertility and by influencing nutrient uptake and root architecture. The biochemical and molecular mechanisms underlying these events are only partially known. HS have been shown to contain auxin and an “auxin-like” activity of humic substances has been proposed, but support to this hypothesis is fragmentary. In this review article, we are giving an overview of available data concerning molecular structures and biological activities of humic substances, with special emphasis on their hormone-like activities.Key words: auxin, humic substances, root, soil, sustainable agriculture     

Long-term Effect of Municipal Solid Waste Amendment on Microbial Abundance and Humus-associated Enzyme Activities Under Semiarid Conditions

Microbial ecology is the key to understanding the function of soil biota for organic matter cycling after a single amendment of organic waste in semiarid soils. Therefore, in this paper, the long-term effect (17 years) of adding different doses of a solid municipal waste to an arid soil on humus–enzyme complexes, a very stable and long-lasting fraction of soil enzymes, as well as on microbial and plant abundance, was studied. Humic substances were extracted by 0.1 M pH 7 sodium pyrophosphate from soil samples collected in experimental plots amended with different doses of a solid municipal waste (0, 65, 130, 195, and 260 t/ha) 17 years before. The activity of different hydrolases related with the C (β-glucosidase), N (urease), and P (alkaline phosphatase) cycles and with the formation of humic substances (o-diphenol oxidase) were determined in this extract. The density and diversity of plant cover in the plots, as well as the fungal and bacterial biomass (by analyzing phopholipid fatty acids) were also determined. In general, the amended plots showed greater humic substance-related enzymatic activity than the unamended plots. This activity increased with the dose but only up to a certain level, above which it leveled off or even diminished. Plant diversity and cover density followed the same trend. Fungal and bacterial biomass also benefited in a dose-dependent manner. Different signature molecules representing gram+ and gram− bacteria, and those corresponding to monounsaturated and saturated fatty acids showed a similar behavior. The results demonstrate that organic amendment had a noticeable long-term effect on the vegetal development, humic substances-related enzyme activity and on the development of bacteria and fungi in semiarid conditions.     

新疆绿洲农田不同连作年限棉花根际土壤微生物群落多样性

以南北疆不同连作年限棉花根际土壤为研究对象,采用Biolog技术,并结合传统平板培养法和土壤酶的测定,研究连作对棉花根际土壤微生物群落多样性的影响。Biolog分析结果表明,不同连作年限棉花根际土壤微生物碳源利用和功能多样性差异显著。荒地土壤微生物活性较低;在连作年限较短时(5—10a),根际土壤微生物群落的平均颜色变化率(AWCD)和Shannon指数较高;长期连作(15—20a),则呈明显下降趋势。主成分分析表明,不同连作年限的棉花根际土壤微生物碳源利用特征有明显不同。第一、二组开垦与未开垦土壤分别在PC1和PC2上出现差异,未开垦土壤得分均为负值,开垦土壤均为正值;而正茬与连作多年的棉花土壤在PC1上差异显著。其中在PC1上起分异作用的碳源主要是羧酸类和聚合物类,这两类碳源可能是影响连作棉花根际土壤微生物的主要碳源。可培养微生物数量的测定结果表明,荒地细菌数量最少;在连作年限较低时(5—10a左右),细菌数量呈上升趋势;而长期连作(>15a)后,细菌数量呈现下降趋势。真菌数量在连作多年后(10—15a)也开始增加。放线菌变化趋势不明显。四种土壤酶活性在连作的初中期(5—15a),连作障碍表现明显,土壤酶活性呈下降(过氧化氢酶和磷酸酶)或先升高后下降(脲酶和蔗糖酶)趋势,但随着连作年限的延长(15—20a),这4种土壤酶活性均表现出增高趋势。综上所述,棉花长期连作使棉花根际土壤微生物群落多样性降低,发生连作障碍,进而导致棉花产量降低。     

半干旱黄土丘陵区纯林土壤腐殖质异化特征及与其他性质的关系

纯林长期生长或多代连栽必然会导致土壤腐殖质含量和构成发生异化,探究这种异化特征及其与土壤其他性质的关系可以为纯林管理或混交改造提供科学依据。通过对半干旱黄土丘陵区南泥湾林场8种典型纯林土壤腐殖质及其他性质进行系统检测,结果表明:(1)侧柏林土壤腐殖质含量最高(34.61 g/kg),腐殖化程度和稳定性一般;白榆和白桦林土壤的腐殖质含量中等(19.69—23.58 g/kg)、腐殖化程度和稳定性最佳;茶条槭和小叶杨林土壤的腐殖质含量(20.59—22.53 g/kg)和构成均为中等水平;油松、沙棘和刺槐林土壤的腐殖质质量较低(11.77—13.81 g/kg),且腐殖化程度较低,稳定性相对最差;(2)与胡敏酸含量存在显著相互促进作用(P0.05)的土壤性质为CEC、N、微生物量和蛋白酶活性(相关系数0.769—0.926,下同),存在显著相互抑制作用的为有效Cu(-0.793);与富啡酸存在显著相互促进作用的为N、CEC、微生物量、蔗糖酶和磷酸酶活性(0.836—0.955),存在显著相互抑制作用的为有效Cu(-0.822);与胡敏素存在显著相互促进作用的为N、CEC、微生物量、磷酸酶活性和有效Zn(0.766—0.951),存在显著相互抑制作用的为脱氢酶活性(-0.784)。(3)腐殖质构成与其他性质的相关性均不显著(P0.05),其中相对有利于提高胡敏酸/腐殖酸含量之比的土壤性质为蛋白酶、蔗糖酶和过氧化氢酶活性,而不利的是脱氢酶活性;相对有利于提高胡敏酸/富啡酸含量之比的为速效K、CEC和脲酶活性,而不利的是脱氢酶活性。(4)总体而言土壤腐殖质含量较之腐殖质构成与其他性质之间具有更大的相关性;向土壤增施N肥可以促进腐殖质的形成,增加K肥则有利于腐殖质构成的改善,而通过混交改造或增加林下植被是促进纯林土壤腐殖质化过程和解决土壤退化的根本措施。     

Water repellency of Casuarina windbreaks (Casuarina equisetifolia Forst.) caused by fungi in central Taiwan

The water repellent layer (WRL) in the Casuarina plantation near Taichung Harbor in central Taiwan is mainly due to the development of filamentous fungi. The hyphae of the isolated fungi and the metabolites of the TCHC-5 and TCHC-20 fungi are also significantly hydrophobic. In the soil layers, humic substances decrease the phosphorus fixation and contribute to the formation of WRL. The hydrophobic properties of humic substances are unfavorable for the nutrient cycling in this area. The water contact angles of fulvic acids and humic acids are pH-dependent. Increasing the solution pH value reduces the hydrophobic strength for fulvic acids and/or humic acids. TCHC-15 and TCHC-16 isolated fungi exude strong acidic metabolites (pH 2.7–3.0). This may result in polymerization and/or precipitation of fulvic acids and humic acids and increase the hydrophobic strength of the soil layers. Humic substances with aliphatic chains are the main components that form WRL in soils. Soil pH may be an indicator of the hydrophobic potential for organic matters.     

Hydrolytic enzyme activities of extracted humic substances during the vermicomposting of a lignocellulosic olive waste

Humic substances and three hydrolytic enzymes (beta-glucosidase, phosphatase and urease) were extracted by neutral sodium pyrophosphate from an olive waste (dry olive cake), alone or mixed with municipal biosolids, during a nine month vermicomposting process. Easily degradable compounds decreased during the vermicomposting process because of microbial consumption. When municipal biosolids were added to dry olive cake, microbial activity increased and the amounts of compounds extracted by pyrophosphate were three times lower than olive cake alone. In both instances, beta-glucosidase, phosphatase and urease activities of the organic extracts either increased or remained the same after a nine month period of vermicomposting, thus suggesting that the humus enzyme complexes resisted microbial and earthworm attack. It is known that humus immobilised enzymes also remain active in soil environments, reactivating the nutrient cycles in soil. The use as amendments of vermicomposted olive cake, alone or when mixed with biosolids, could be a good alternative to reactivate the C, P and N-cycles in degraded soils for regeneration purposes.     

The influence of topography on the nature of humic substances in soil organic matter at a site in the Atlantic Coastal Plain of South Carolina

The effect of topography on the nature of humic substances, isolated as water soluble organic carbon (WSOC), fulvic acid (FA), and humic acid (HA) was evaluated by comparing relative proportion and chemical characteristics of these fractions in upland and bottomland Coastal Plain soils in South Carolina. The fractions were characterized by elemental analysis and¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The majority of humic substances occur as humic acids, with bottomland soils having higher HA/FA ratios when compared to upland soils. We found no significant differences between upland and bottomland humic substances with respect to yields of WSOC and fulvic acids, and in the C and N content of humic and fulvic acids. Carbon-13 CPMAS NMR spectroscopy revealed that the WSOC and fulvic acid fractions were composed largely of O-alkyl-C structures with bottomland soils having higher amounts of these groups. Humic acid C distribution was similar between upland and bottomland soils and was largely composed of aromatic groups. Our results demonstrate that topography influences the formation of humic acid and the structural and chemical properties of the various humic fractions.requests for offprints     

Physico-chemical characteristics of humic substances of major soil associations of Bihar (India)

Humic acid (HA) and fulvic acid (FA) extracted from fourteen surface soil samples (0–20 cms) belonging to nine major Soil Association Groups scattered over different agro-climatic situations, were characterized by elemental and functional group analysis, E₄/E₆ ratios, coagulation behaviour and distribution of carbon in different soil humus fractions. The E₄/E₆ ratio of FA extracted from different soils was wider than that of HA. The coagulation behaviour of HA and FA fractions and also of cultivated and forest soils showed marked differences. The variations in the ratios of HAC:FAC (0.31 to 1.0) among different soils were indicative of the degree of humification under the influence of vegetation and agroclimatic conditions. The elemental composition of HA and FA, in general, indicated a higher carbon and nitrogen content and C/N ratio in the former than in the latter fraction. On the contrary, the oxygen content of FA was higher compared to that of HA. The carbon contents of HA extracted from the cultivated and forest soils of Hazaribagh were almost equal, as were also the carbon contents of HA from the cultivated and forest soils from Ranchi. Total acidity of FA of the soils selected in the present study was higher than that of HA. The functional groups such as carboxyl, phenolic hydroxyl and carbonyl were present in the two fractions in varying proportions.     

Comparative Resistance and Resilience of Soil Microbial Communities and Enzyme Activities in Adjacent Native Forest and Agricultural Soils

Degradation of soil properties following deforestation and long-term soil cultivation may lead to decreases in soil microbial diversity and functional stability. In this study, we investigated the differences in the stability (resistance and resilience) of microbial community composition and enzyme activities in adjacent soils under either native tropical forest (FST) or in agricultural cropping use for 14 years (AGR). Mineral soil samples (0 to 5 cm) from both areas were incubated at 40°C, 50°C, 60°C, or 70°C for 15 min in order to successively reduce the microbial biomass. Three and 30 days after the heat shocks, fluorescein diacetate (FDA) hydrolysis, cellulase and laccase activities, and phospholipid-derived fatty acids-based microbial community composition were measured. Microbial biomass was reduced up to 25% in both soils 3 days after the heat shocks. The higher initial values of microbial biomass, enzyme activity, total and particulate soil organic carbon, and aggregate stability in the FST soil coincided with higher enzymatic stability after heat shocks. FDA hydrolysis activity was less affected (more resistance) and cellulase and laccase activities recovered more rapidly (more resilience) in the FST soil relative to the AGR counterpart. In the AGR soil, laccase activity did not show resilience to any heat shock level up to 30 days after the disturbance. Within each soil type, the microbial community composition did not differ between heat shock and control samples at day 3. However, at day 30, FST soil samples treated at 60°C and 70°C contained a microbial community significantly different from the control and with lower biomass regardless of high enzyme resilience. Results of this study show that deforestation followed by long-term cultivation changed microbial community composition and had differential effects on microbial functional stability. Both soils displayed similar resilience to FDA hydrolysis, a composite measure of a broad range of hydrolases, supporting the concept of high functional redundancy in soil microbial communities. In contrast, the resilience of the substrate-specific activities of laccase and cellulase were lower in AGR soils, indicating a less diverse community of microorganisms capable of producing these enzymes and confirming that specific microbial functions are more sensitive measurements for evaluating change in the ecological stability of soils.     

Degradation and transformation of humic substances by saprotrophic fungi: processes and mechanisms

Humic substances represent the main carbon reservoir in the biosphere, estimated at 1600 × 10¹⁵ g C. Due to their crucial role in reductive and oxidative reactions, sorption, complexation and transport of pollutants, minerals and trace elements, sustaining plant growth, soil structure and formation, and control of the biogeochemistry of organic carbon in the global ecosystem, humic substances are extremely important to environmental processes. Saprotrophic fungi active in the decomposition process of humic substances include mainly ascomycetes and basidiomycetes, which are both common in the upper layers of soils. White rot and litter decomposing fungi are the most important organisms in the degradation and mineralization of refractory organic matter (OM), whereas ascomycetes are mainly involved in the modification and polymerization of humic substances. The mechanisms of degradation probably involve mainly a variety of non specific oxidizing enzymes. This review provides an overview of the subject, while bridging two main disciplines: soil OM chemistry and fungal microbiology. It is aimed to highlight problems, unsolved questions and hypotheses.