Cellulase dynamics in a desert soil

A field study was conducted in the Negev Desert over three seasons: June (summer), October (autumn), and April (spring). Cellulose of plant or paper origin was added to the study soils. The concentration of cellulase in the soil was determined by monitoring the rate of solubilization of chromophoric molecules covalently linked to artificial insoluble cellulose (cellulose-azure). The amount of CO₂ evolved from the soil was also evaluated at 60-day intervals.In this paper, we demonstrate that significant differences (p<0.01) in the cellulase concentration in desert soils are mainly due to the time period during which organic matter was incorporated into the soil. Data are presented showing changes in cellulase concentrations in the soil as a response to different cellulose sources (plant and paper origin) throughout the year.The results of our field experiments show that the cellulase concentration in the soil surrounding cellulose (paper) is higher during the summer than during the other seasons. The concentration of cellulase associated with fresh organic matter was found to be double that associated with paper. CO₂ evolution was higher in soil samples supplemented with organic matter than in control samples. This study demonstrates that the concentration of cellulase in desert soil changes over the year and is influenced by the cellulose source and by the quality of the cellulose incorporated in the soil.     

Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass

Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass.     

Soil pH changes during legume growth and application of plant material

During cultivation of legumes soil is acidified due to proton release from roots. As a consequence of proton release, plants accumulate organic anions which may, if returned and decomposed in the soil, neutralize the soil acids. Until now the detailed processes responsible for the change in soil pH after incorporation of plant material have not been completely understood. Using a pot experiment we studied the changes in acid and base in soil during growth of field beans (Vicia faba L. cv. Alfred) and after incorporation of the plant material into the soil. Soil pH was significantly decreased by field beans from 6.00 to 5.64 in a cultivation period of 45 days. Proton release amounted to 32.7 mmol H⁺ pot^-1, which was approximately equivalent to the accumulated alkalinity in the plant shoots (34.4 mmol). Return of field bean shoots caused a significant soil pH increase from 5.64 to 6.29. Within 7 days more than 90% of the added alkalinity was released. After 307 days incubation, soil pH decreased to 5.86 due to nitrification. In a second experiment, maize leaves (Zea mays L.), containing various concentrations of nitrogen and at various alkalinities, were incorporated into the soil. Soil pH change was positively correlated to alkalinity and malate concentration and negatively correlated to total nitrogen and water-soluble organic nitrogen of incorporated leaves. It is concluded that the soil acidification caused by legume cultivation can be partly compensated for if crop residues are returned to the soil. Addition of plant material may initially cause an increase in soil pH due to decomposition of organic anions and organic nitrogen. Soil pH may decrease if nitrification is involved. The concentrations of nitrogen and alkalinity of added plant material are decisive factors controlling soil pH change after incorporation of plant material.Dedicted to Professor J.C.G. Ottow on the occasion of his 60th birthday     

Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass

Net mineralization of N from a range of shoot and root materials was determined over a period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical “quality” components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after 2 months, at which time maximum net N mineralization was recorded for most residues. However, there was no relationship between microbial-N and net N mineralization. Biochemical quality factors controlling the C and N content of the residue remaining at the end of the incubation as light fraction organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellulose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted for by microbial immobilization.     

Assessment of selenium mineralization and availability from catch crops

Selenium (Se) release from four plant species (Indian mustard, fodder radish, Italian ryegrass and hairy vetch) was measured under controlled leaching conditions and in a pot incubation experiment as part of a study of the potential for using these plant species as Se catch crops. Catch crops may reduce Se leaching and, by subsequent release of Se from the plant material, increase the available Se for succeeding crops. Plants grown both without and with Se addition (250 g Se/ha) were tested. In the leaching experiment, frozen plant material was incorporated into soil columns and incubated at room temperature for up to 19 weeks. The results showed that Se concentrations in the leachate were higher when Se‐enriched plant material was incorporated in the soil, indicating Se mineralization. When non‐enriched plant material was added to the soil, Se concentrations in the leachate were generally lower than that in the control, indicating Se immobilization. In the pot incubation experiment, the results were consistent with those from the leaching experiment. The addition of enriched plant material increased Se concentration in Indian mustard plants compared with unamended soil. However, the addition of plant materials grown without Se significantly decreased Se concentrations in plant dry matter, again indicating Se immobilization. Fertilizer application with inorganic Se as selenate did not affect Se concentrations either in the leachates or in the plants grown in the pot incubation. Thus, the results show the potential of catch crops to increase Se mineralization and uptake in succeeding crops.     

Calibration of near infrared spectra for measuring decomposing cellulose and green manure in soils

Plant residues are the major organic input to soils. Therefore, a method for monitoring the degradation of cellulose as a major component of plant residues is expected to be a useful aid in studying their turnover in soil. In order to examine whether the decay of cellulose in soil can be monitored by near infrared spectroscopy (NIRS), we analysed soil samples from an incubation experiment using this rapid and inexpensive method. A soil amended with and without cellulose (2 g cellulose kg⁻¹ soil) was incubated under aerobic conditions in the dark at 15 °C for 70 days. The soil samples, which were taken at the start and twelve times within the incubation period were spectrally analysed with a NIR-spectrometer. The decay of cellulose was simulated using negative exponential functions. These simulated cellulose concentration in the soil was used for the calibration of NIRS-equations. Although the cellulose comprises only a very small part of the total organic carbon in the soil, the decay of cellulose could be clearly monitored by NIRS. Ninety-five percent of the variation in the soil cellulose concentration as simulated by the negative exponential function could be explained by the NIRS-equation (r²=0.95), when using appropriate rate constants in the exponential decay. The spectral signature, represented by this NIRS-equation could be proved to represent cellulose by comparing it with the spectrum of pure powdered cellulose not mixed with soil. We applied the NIRS-equation from the cellulose treatment to soil samples of a green manure treatment. The coefficient of determination for residual green manure in soil, predicted by this NIRS-equation versus residual green manure contents as simulated by a negative exponential function, was r²=0.84 and 0.94 for a sandy and a clay soil, respectively. Our results confirm that NIRS provides a useful tool for keeping track of a specific and relatively small organic fraction among the background of a large amount of total soil organic matter.     

Glucose oxidase activity in soil and its possible interference in assays of cellulase activity

The occurrence of naturally-occurring glucose oxidase activity in a soil collected at a depth of 0–8 cm under tussock has been indicated. This activity was probably responsible for utilization of some of the glucose added to a system of soil, buffer and toluene that was incubated at 30°C; no loss of added glucose was apparent at 50°C.The cellulase activity of this soil was assessed by measuring rates of hydrolysis of added cellulose with Somogyi and anthrone reagents in systems of soil, buffer and toluene at 30 and 50°C. The production of reducing sugars was approximately linear over the first 48 or 72 h of incubation but appeared to decline subsequently at both temperatures. Interference by the activity of the naturally-occurring glucose oxidase at 30°C is suggested. Interference by glucose oxidase appears less likely at 50°C because of the apparent stability of added glucose at this temperature and the instability of added glucose oxidase. An incubation period of 48 h at 30°C appears generally satisfactory for assaying the cellulase activity of this soil.The possible role of glucose oxidase in interfering in assays of other soil enzymes in which the production of glucose is measured is discussed.     

Microbial activity and quality changes during decomposition of Quercus ilex leaf litter in three Mediterranean woods

Changes in enzyme activities during litter decomposition provide diagnostic information on the dynamics of decay and functional microbial succession. Here we report a comparative study of enzyme activities involved in the breakdown of major plant components and of other key parameters (microbial respiration, fungal biomass, N, lignin and cellulose contents) in homogeneous leaf litter of Quercus ilex L. incubated in three evergreen oak woods in Southern Italy (Campania), differing for chemical and physical soil characteristics and microclimatic conditions. The results showed that the litter mass loss rates were similar in the three wood sites. Independently of the incubation sites, cellulase, xylanase and peroxydase activities showed seasonal variations with maximum and minimum levels in wet and dry periods, respectively, and this pattern closely matched microbial respiration. Activities of α- and β-amylase, instead, were high at the beginning of incubation and quickly decreased with decomposition progress because their substrate was rapidly depleted. Laccase activity, in contrast, was low at the beginning of incubation but after 6 months it increased significantly. The increase of laccase activity was correlated to an increase in fungal biomass, probably reflecting a major shift in the litter microbial community. As concerns quality changes, N and lignin content did not significantly change during decay. The cellulosic component started being degraded after about 6 months in the litter incubated in two of the three wood sites and from the start of decomposition in the third site. Apart from minor differences in the levels of certain enzyme activities, the data showed that the functional microbial succession involved in the decomposition of Q. ilex leaf litter did not change appreciably in response to differences in soil and microclimatic conditions in the incubation sites.     

Rate of decomposition of organic matter in soil as influenced by repeated air drying-rewetting and repeated additions of organic material

Repeated air drying and rewetting of three soils followed by incubation at 20°C resulted in an increase in the rate of decomposition of a fraction of ¹⁴C labeled organic matter in the soils. The labeled organic matter originated from labeled glucose, cellulose and straw, respectively, metabolized in the soils during previous incubation periods ranging from 1.5 to 8 years.Air drying and rewetting every 30th day over an incubation period of 260–500 days caused an increase in the evolution of labeled CO₂ ranging from 16 to 121 per cent as compared to controls kept moist continuously. The effect of the treatment was least in the soil which had been incubated with the labeled material for the longest time.Additions of unlabeled, decomposable organic material also increased the rate of decomposition of the labeled organic matter. The evolution of labeled CO₂ during the 1st month of incubation after addition was in some cases 4–10 times larger than the evolution from the controls. During the continued incubation the evolution decreased almost to the level of the controls, indicating that the effect was related to the increased biological activity in the soils during decomposition of the added material.Three additions of organic material during the period of incubation resulted totally in an increase over the controls ranging from 36 to 146 per cent.     

Carbon-nitrogen relationships during the humification of cellulose in soils containing different amounts of clay

¹⁴C-labelled cellulose and ¹⁵N-labelled (NH₄)₂SO₄ were added to four soils with clay contents of 4, 11, 18 and 34%, respectively. Labelled cellulose was added to each soil in amounts corresponding to 1, 2 and 4 mg C g^?1 soil, respectively, and labelled NH₄⁺ at the rate of 1 mg N per 25 mg labelled C.After the first month of incubation at temperatures of 10, 20 and 30°C, respectively, from 38 to 65% of the labelled C added in cellulose had disappeared from the soils as CO₂, and from 60 to nearly 100% of the labelled N added as NH₄⁺ were incorporated into organic forms. The ratio of labelled C remaining in the soils to labelled N in organic forms was close to 25 after 10 days of incubation, decreasing to about 15 after 1 month and about 10 after 4 yr.The retention of total labelled C was largest in the soil with the highest content of clay where after 4 yr it was 25% of that added, compared to 12 in the soil with the lowest content of clay. The incorporation of labelled N in organic forms and its retention in these forms was not directly related to the content of clay in the soils, presumably because the two soils with the high content of clay had a relatively high content of available unlabelled soil-N which was used for synthesis of metabolic material.The proportionate retention of labelled C for a given soil was largely independent of the size of the amendments, whereas the proportionate amount of labelled N incorporated into organic forms increased in the clay-rich soils with increasing size of amendments. Presumably this is because the dilution with unlabelled soil-N was less with the large amendments.From 50 to 70% of the total labelled C remaining in the soils after the first month of incubation was acid hydrolyzable, as compared to 80–100% of the total remaining labelled organic N. This relationship held throughout the incubation and was independent of the size of the amendment and of the temperature of incubation.During the second, third and fourth year of incubation the half-life of labelled amino acid-N in the soils was longer than the half-life of labelled amino acid-C, presumably due to immobilization reactions. Some of the labelled organic N when mineralized was re-incorporated into organic compounds containing increasing proportions of native soil-C. whereas labelled C when mineralized as CO₂ disappeared from the soils.In general, native C and native organic N were less acid hydrolyzable and were accounted for less in amino acid form than labelled C and N.The amount of labelled amino acid-C, formed during decomposition of the labelled cellulose, and retained in the soil, was proportional to the clay content. This amount was about three times as large in the soil with the highest content of clay as in the soil with the lowest content. This difference between the soils was established during the first 10 days of incubation when biological activity was most intense, and it held throughout the 4 yr of incubation; proportionally it was independent of the amount of cellulose added and the temperature.In contrast, the labelled amino acid-N content was not directly related to the amount of clay in the soil, presumably because more unlabelled soil-N was available for synthesis of metabolic material in the two clay-rich soils than in those soils with less clay. The wider ratio between labelled amino acid-C and labelled amino acid-N in the two clay-rich soils as compared with those obtained with the soils with less clay indicates this.The effect of clay in increasing the content of organic matter in soil is possibly caused by newly synthesized matter, extracellular metabolites, as well as cellular material, forming biostable complexes and aggregates with clay. The higher the concentration of clay the more readily the interactions take place. The presence of clay may also increase the efficiency of using substrate for synthesis.     

The influence of clay on the rate of decay of amino acid metabolites synthesized in soils during decomposition of cellulose

¹⁴C-labelled cellulose was added to seven different soils containing silt + clay (particles < 0.02 mm) in amounts which varied from 8 to 75 per cent. The cellulose was allowed to decompose, and the amounts of labelled C transformed into metabolites hydrolyzable into amino acids were determined. The amounts of labelled amino acid C in the soils were proportional to their content of silt + clay. After 30 days of incubation labelled amino acid C remaining in the soil with the lowest content of silt + clay constituted 6 per cent of the carbon added in cellulose, as compared with 18 per cent in the soil with the highest content of silt + clay. These values had decreased to 5 and 13 per cent respectively after 2 years of incubation. The order between the soils in the content of labelled amino acid C established during the first month of incubation, was thus roughly maintained throughout the period of incubation. The biological half-life of the labelled C in amino acids varied in the seven soils during the last year of incubation from 3 to 8 years. The variation was, however, not related to the amount of silt + clay.n the soils had been incubated with the labelled material for 2 years, samples of the soils were exposed to “stress” treatments: air drying-rewetting; increased biological activity caused by addition of glucose, and exposure to chloroform vapour. The treatments resulted in an evolution of labelled C in CO, which was 5–10 times larger than the evolution from untreated samples. The increase in the CO₂ evolution caused by the treatments in the different soils was, however, not related to the amount of silt + clay, and a high content of this material did not protect organic material against the effect of the treatments.is concluded that the silt + clay fraction ensures stabilization of amino acid metabolites produced during the period of intense biological activity that follows the addition of decomposable, energy rich material to the soil. The amount of amino acid metabolites stabilized increased with increasing concentration of silt + clay, but the rate of decay of the amino acid material during later stages was largely independent of the concentration of silt + clay.     

DECOMPOSITION OF SOIL POLYSACCHARIDE

Polysaccharide material was isolated by absorption on charcoal from the acidified, non-humic fraction extracted by alkali from three soils. The polysaccharides were used as substrates in soil incubation, perfusion, and suspension experiments. Concordant results were obtained with freely drained Countess-wells and Insch Association soils derived from acidic and basic igneous parent materials respectively. Polysaccharide material added to soil at low concentration (I per cent) was apparently totally decomposed after 8 weeks when the amounts of polysaccharide in control and amended soils were statistically indistinguishable. At higher concentrations (2-3 per cent) a significant difference in reducing sugar, equivalent to about 30 per cent of the substrate, remained after 32 weeks. Partial neutralization of the polysaccharide material with calcium hydroxide increased the rate of decomposition in Countesswells Association soil but had an opposite, smaller effect in Insch Association soil. Soil polysaccharide material was decomposed slightly faster in perfusion and suspension experiments than in moist soil. Only 20 per cent of the carbohydrate in the unfractionated alkali–soluble organic matter of soil was decomposed during incubation in soil for up to 133 weeks. There was usually little change in the carbohydrate content of soil incubated alone. The soil microbial population showed a marked increase in response to added polysaccharide material but only slight qualitative changes were detected. It is concluded that the persistence of naturally occurring polysaccharide in soil is related to inaccessibility caused by chemical combination, complexing or insolubility but not to a biologically-stable molecular structure.     

Effects of climatic and soil properties on cellulose decomposition rates in temperate and tropical forests

Cellulose decomposition experiments were conducted under field conditions to analyze the effects of climatic and soil properties on rates of organic matter decomposition in temperate and tropical forests. The mass loss rates of cellulose filter papers buried in the soil surface were measured to estimate the respiratory C fluxes caused by cellulose decomposition and mean residence time (MRT) of cellulose. The rates of cellulose decomposition increased with soil temperature, except for during the dry season, while rate constants of decomposition (normalized for temperature) decreased with decreasing pH because of lower cellulase activity. The estimated MRTs of soil cellulosic carbohydrates varied from 81 to 495 days for the temperate forests and from 31 to 61 days for the tropical forests. As a major organic substrate, the C fluxes from cellulose decomposition can account for a substantial fraction of heterotrophic (basal) soil respiration. However, the respiratory C fluxes can be limited by the low substrate availability and low pH in tropical soils, despite high microbial activity. The rate-regulating factors of cellulose decomposition, i.e., temperature, soil pH, and substrate availability, can accordingly influence the rates of heterotrophic soil respiration.     

Effect of paclobutrazol on microbial biomass,respiration and cellulose decomposition in soil

Paclobutrazol is a plant growth regulator largely utilized in mango cultivation and usually applied directly to soil. The aim of this study was to examine the effect of paclobutrazol on soil microbial biomass, soil respiration and cellulose decomposition in Brazilian soils under laboratory conditions. Soil samples were collected from fields with and without a reported history of paclobutrazol application. A solution of paclobutrazol (8 mg of active ingredient kg^?1 of soil) was added to soils, which were then incubated at 28 °C for 30 days. Paclobutrazol decreased soil microbial biomass, soil respiration and cellulose decomposition in soil with and without a report of paclobutrazol application, while significant increase was observed in the respiratory quotient (qCO₂). Our results show that the soil microbiological attributes were negatively affected by paclobutrazol in short-term experiment.     

不同剂量外源纤维素酶对设施土壤生物活性与番茄生长的影响

【目的】土壤纤维素酶活性在一定程度上反映土壤生物化学过程的强度及土壤肥力水平。本研究主要探讨了添加外源纤维素酶对设施土壤环境及栽培作物的积极影响,以期为设施土壤改良和质量提升提供参考。 【方法】以番茄‘芬达’为试材进行了盆栽试验。在设施土壤上设置添加外源纤维素酶:0、3、6、9、12、15 kg/hm2,分别用CK、T1、T2、T3、T4和T5表示,共6个处理。结果初期测定了番茄叶片光合指标,结果初期、结果盛期、采收盛期分别取土样测定了土壤脲酶、蔗糖酶、SOD、碱性磷酸酶活性,以及土壤细菌、真菌、放线菌数量,果实成熟期分批测产。 【结果】同一生育期,随酶制剂用量的增加,土壤微生物数量及酶活性均先增加后降低。与CK相比,细菌、真菌、放线菌最高分别增加了996.8% (结果盛期的T4)、801.4% (采收盛期的T3) 和314.6% (坐果初期的T3);坐果初期T3的土壤脲酶、蔗糖酶、SOD及采收初期T3的碱性磷酸酶活性提高较多,分别较CK增加了214.3%、424.3%、254.0%和44.0%;同时添加外源纤维素酶对番茄株高、茎粗以及Pn、Tr、Gs、Ci等光合指标的提高有促进作用,增加了番茄产量,T3的番茄产量最高,达到55188 kg/hm2。 【结论】适当添加外源纤维素酶,在提高土壤本身纤维素酶活性的同时也增强了其他土壤酶活性,促进了土壤中微生物的积累和繁殖,从而改善了土壤环境,并促进蔬菜作物健康生长,提高了作物产量。9 kg/hm2为本试验推荐的应用于设施土壤的纤维素酶最佳使用量。     

The influence of sward species composition on the rate of organic matter decomposition in grassland soil

To examine the influence of different plant materials on the rate of organic matter (OM) decomposition in soil, respiration and N mineralization/immobilization were measured during incubation of a test soil to which the plant materials were added. Amendments consisted of sward material either from grassland plots with different amounts of OM accumulation, or material from plants associated with soils having markedly different OM contents. Evolution of CO₂ from the test soil plus herbage from an experimental plot showing OM accumulation was greater than that from the same soil amended with herbage from plots without accumulation. Apparently, grass species had no significant effect on OM turnover; differences in accumulation in grassland soils must be related to other environmental factors. Decomposition of young Calluna vulgaris was, however, slower than that of young grass material and in this case species could affect organic matter accumulation.     

A VISUAL RECORD OF THE DECOMPOSITION OF 14C-LABELLED FRAGMENTS OF GRASSES AND RYE ADDED TO SOIL

The decomposition of grasses and rye labelled with ¹⁴C was studied using ground material and also fragments cut from intact leaves or roots either placed on the soil surface or buried in the soil incubated under various conditions. Autoradiography was used to observe the changes in the decaying plant tissue with a minimum of disturbance. Autoradiograms prepared before incubation and subsequently at intervals reveal an over-all fall in density of the images, a complete disappearance of ¹⁴C in small discrete sites, as well as a dispersion of ¹⁴C over distances of several cm from the plant residues. A photoelectric technique was devised by which changes in density could be expressed quantitatively. The log density of autoradiographic images of pellets of ground grasses shows a predominantly, though not completely, linear regression on time of incubation. The method has shown that the process of decomposition is very slow, that in the first stages of decomposition ryegrass decays faster than cocksfoot, and that ground material tends to behave in a different manner to fragments cut from intact tissues. Changes in the area of autoradiographic images with time of incubation could be used as an additional but less sensitive measure of the rate of decomposition. The participation of micro-organisms (especially fungi) in the breakdown of the plant tissues has been demonstrated by the presence of labelled organisms in the vicinity of plant residues. Labelled fungi are more numerous in the first 3 months of incubation, during which a marked fall in image density of the plant residues occurs. A further decrease in image density is frequently associated with the appearance of fungal resting structures with a greater concentration of ¹⁴C than the surrounding plant fragments. Because of their longevity these structures contribute to the fixing of ¹⁴C in a different fraction of the biomass. Faecal pellets of soil mesofauna also concentrate ¹⁴C and resist decomposition for very long periods of time.     

Immobilization and mineralization of nitrogen in a saline and alkaline soil during microbial use of sugarcane filter cake amended with glucose

A 42-day incubation was conducted to study the effect of glucose and ammonium addition adjusted to a C/N ratio of 12.5 on sugarcane filter cake decomposition and on the release of inorganic N from microbial residues formed initially. The CO₂ evolved increased in comparison with the non-amended control from 35% of the added C with pure +5 mg g⁻¹ soil filter cake amendment to 41% with +5 mg g⁻¹ soil filter cake +2.5 mg g⁻¹ soil glucose amendment to 48% with 5 mg g⁻¹ soil filter cake +5 mg g⁻¹ soil glucose amendment. The different amendments increased microbial biomass C and microbial biomass N within 6 h and such an increase persisted. The fungal cell-membrane component ergosterol initially showed a disproportionate increase in relation to microbial biomass C, which completely disappeared by the end of the incubation. The cellulase activity showed a 5-fold increase after filter cake addition, which was not further increased by the additional glucose amendment. The cellulase activity showed an exponential decline to values around 4% of the initial value in all treatments. The amount of inorganic N immobilized from day 0 to day 14 increased with increasing amount of C added, in contrast to the control treatment. After day 14, the immobilized N was re-mineralized at rates between 1.3 and 1.5 μg N g⁻¹ soil d⁻¹ in the treatments being more than twice as high as in the control treatment. This means that the re-mineralization rate is independent of the actual size of the microbial residues pool and also independent of the size of the soil microbial biomass.     

Microbial enzyme activities,fungal biomass and quality of the litter and upper soil layer in a beech forest of south Italy

Forest soils contain a large amount of organic matter (OM) and therefore represent a considerable carbon reserve. The amount of OM sequestered in the soil is dependent on annual input of litter and its quality. The aim of this study was to investigate the quantity and quality of OM, the microbial capacity to degrade it and its recalcitrance to further degradation, by considering some extracellular enzyme activities in a beech (Fagus sylvatica L.) forest in south Italy (Mediterranean area). Our attention was focused on the decomposition continuum of the litter horizon and upper soil layer. Because fungi are the major decomposers of plant material, fungal biomass was also measured and its relationship with enzyme activities was tested. The results showed that: (i) the litter horizon and the upper soil layer differed in chemical characteristics and biological activities; (ii) within the litter horizon, the three layers detected for their different degree of degradation (L, recently fallen, not decomposed and not compressed material; F, partially decomposed and fragmented but macroscopically recognizable material; H, compressed and strongly fragmented) differed more in chemical characteristics than in biological activities; (iii) the enzyme activities and fungal biomass changed during the study period but a clear relationship with succession of seasons was evident only for cellulase, laccase, peroxidase and fungal biomass; and (iv) the upper soil layer included 42% OM and less than 50% of that was susceptible to further decomposition. This percentage was 30% in the OM of L.     

N-mineralization in polyphenol-rich plant residues and their effect on nitrification of applied ammonium sulphate

Soil was incubated under greenhouse conditions with plant residues having varying phenolic and nitrogen contents. The total plant material added in staggered applications every 4 months was 15 g kg^?1 soil and the total incubation period was 12 months.The N-mineralization in these plant residues as influenced by their phenol and N contents was examined. The nitrification of applied (NH₄)₂SO₄ in these amended soils was also investigated under optimum conditions of pH.A high plant-N content resulted in increased N-mineralization of plant residue, but this effect was lowered by the presence of high concentrations of polyphenols in the decomposing residue, most probably due to increased participation of N with polyphenols in the formation of humus fractions.Soils amended with phenol-rich residues did not show any inhibition of nitrification of applied (NH₄)₂SO₄. Possible reasons are discussed. In organic matter decomposition, the quality of the leaf polyphenols appears to determine the degree of inhibition to soil nitrification.