Effects of three biochar types on activity of β-glucosidase enzyme in two agricultural soils of different textures

ABSTRACT

Biochar has attracted significant attention due to the long-lasting nature, and prominent influence on soil characteristics. This study was conducted to evaluate changes in the activity of β-glucosidase enzyme (BG) in loamy and sandy loam texture soils following two winter wheat growing seasons. The experimental design was a randomized complete block with three replicates and four treatments. The treatments were two soils, three biochars (rice husk, corn cob and bean harvest residue), five biochar rates (BR) and five levels of mineral fertilizers (FR) or dairy effluent (DE). The fertilizers were applied at the beginning of each season, while biochars were applied only at the beginning of experiment. Soil samples were collected following the second season and analyzed for the BG activity. Addition of biochars reduced the BG activity and the decline was higher in sandy loam compared to loamy soils. Negative effect of biochar to BG activity was greater at the highest BR (3.0%) than the lower BR. Fertilizer additions along with DE biochar had significant effect on BG activity that increased with higher FR. Given the importance of BG activity in soil organic matter decomposition, biochar application can be considered a sustainable way of increase in carbon sequestration.     

Land use effects on microbial biomass C, β-glucosidase and β-glucosaminidase activities,and availability,storage, and age of organic C in soil

Microbial biomass, β-glucosidase and β-glucosaminidase activities, and availability, storage, and age of soil organic C were investigated after 26 years of conversion from sugarcane (Saccharum officinarum) to forest (Eucaliptus robusta or Leucaena leucocephala), pasture (mixture of tropical grasses), and to vegetable cropping (agriculture) in a vertisol in Puerto Rico. Soil organic C (SOC) at 0–100 cm was similar under Leucaena (22.8 kg C/m²), Eucalyptus (18.6 kg C/m²), and pasture (17.2 kg C/m²), which were higher than under agriculture (13.0 kg C/m²). Soil organic N (SON) at 0–100 cm was similar under the land uses evaluated which ranged from 1.70 (under agriculture) to 2.28 kg N/m² (under Leucaena forest). Microbial biomass C (MBC) and N (MBN) of the 0–15-cm soil layer could be ranked as: pasture > Leucaena = Eucalyptus > agriculture. The percentages of SOC and SON present as MBC and MBN, respectively, were nearly 1% in pasture and less than 0.50% in forest under Leucaena or Eucalyptus and agricultural soil. The activity of β-glucosidase of the 0–15-cm soil layer could be ranked as: Leucaena = Eucalyptus > pasture > agriculture; while β-glucosaminidase activity was ranked as: Eucalyptus > Leucaena = pasture > agriculture. The soil δ ¹³C changed from 1996 to 2006 in forest under Eucalyptus (18.7‰ to 21.2‰), but not under Leucaena (20.7‰ to 20.8‰). The soil under Leucaena preserved a greater proportion of old C compared to the forest under Eucalyptus; the former had an increased soil mineralizable C from the current vegetation inputs. The soil under agriculture had the lowest enzyme activities associated with C cycling, lowest percentage of SOC as MBC, highest percentage of SOC present as mineralizable C, and highest percentage of MBC present as mineralizable C compared to the other land uses.     

Effects of flue gas desulfurization gypsum by-products on microbial biomass and community structure in alkaline–saline soils

Purpose

For an alkaline?Csaline region in Northwest China, we examined the responses of soil microbial communities to flue gas desulfurization gypsum by-products (FGDB), a new ameliorant for alkaline?Csaline soils. In 2009 and 2010, we collected soils from 0?C20?cm and 20?C40?cm depths along an experimental FGDB gradient (0, 0.74, 1.49, 2.25, and 3.00?kg FGDB m^?2).

Materials and methods

As a measure of microbial community composition and biomass, we analyzed phospholipid fatty acids (PLFAs). We used real-time quantitative polymerase chain reaction (qPCR) to measure abundance of bacterial 16?S rRNA copy numbers. Additionally, physicochemical soil parameters were measured by common laboratory methods.

Results and discussion

Microbial community composition differed along the FGDB gradient; however, the microbial parameters did not follow a linear response. We found that, in 2009, total PLFA concentrations, and concentrations of total bacterial and Gram-negative bacterial PLFAs were slightly higher at intermediate FGDB concentrations. In 2010, total PLFA concentrations, and concentrations of total bacterial, Gram-positive bacterial, Gram-negative bacterial, and fungal PLFAs as well as the fungal:bacterial PLFA ratio were highest at 1.49?kg FGDB m^?2 and 3.00?kg FGDB m^?2. PLFA concentrations often differed between 2009 and 2010; however, the patterns varied across the gradient and across microbial groups. For both years, PLFA concentrations were generally higher at 0?C20?cm depth than at 20?C40?cm depth. Similar results were obtained for the 16?S rRNA copy numbers of bacteria at 0?C20?cm depth. FGDB addition resulted in an increase in soil Ca²⁺ and NO ₃ ^? ?CN and a decrease in pH and electrical conductivity (EC). Shifts in PLFA-based microbial community composition and biomass could partly be explained by pH, soil organic carbon, total nitrogen (TN), soil moisture, EC, inorganic nitrogen, C/N, and Ca²⁺. Indirect effects via shifts in abiotic soil properties, therefore, seem to be an important pathway through which FGDB affect soil microbial communities.

Conclusions

Our results demonstrate that addition of FGDB leads to significant changes in soil physicochemical and microbial parameters. As such, addition of FGDB can have large impacts on the functioning of soil ecosystems, such as carbon and nitrogen cycling processes.     

Changes in nutrient pools,microbial biomass and microbial activity in soils after transit through the gut of three endogeic earthworm species of the genus Postandrilus Qui and Bouché, 1998

Purpose

Endogeic earthworms play a significant role in biogeochemical cycles due to the large amount of soil they ingest, and because after transit through their guts, casts usually show differences in nutrient contents and microbial populations with bulk soil. Here, we studied how three endogeic earthworm species, Postandrilus majorcanus, Postandrilus sapkarevi and Postandrilus palmensis, inhabiting soils in Majorca island (Balearic Islands, W Mediterranean), modify nutrient pools and microbial communities of soil.

Materials and methods

To do this, we analysed C, N and P pools, microbial biomass (phospholipid fatty acids, PLFA) and microbial activity (fluorescein diacetate hydrolysis, FDA) in paired samples of bulk soil and fresh casts.

Results and discussion

The mineral and organic N contents were generally enhanced in casts produced by all three earthworm species. However, inorganic P and organic C contents were only higher in P. sapkarevi (32 %, only P) and P. majorcanus casts (100 % for both soil nutrient pools) than in bulk soil. Bacterial and fungal biomass were only higher than in bulk soil in P. majorcanus casts (65 and 100 %, respectively), but without effects on microbial activity, that was lower in P. palmensis casts (26 %). Earthworm gut transit strongly influenced the soil microbial community structure, resulting in differences between casts and soils.

Conclusions

The increased nutrient mineralization (6-, 1.3- and 1.4-fold for N, C and P, respectively) in casts produced by these earthworm species is of particular importance because of the amount of casts released and the seasonal variations in earthworm activity, which may favour plant growth.     

Isoelectric focusing of β-glucosidase humic-bound activity in semi-arid Mediterranean soils under management practices

Changes in β-glucosidase enzyme–humic complexes and conventional parameters (pH, total organic C, total N, water-soluble C, and bulk density) were studied in an almond-cropped soil prone to erosion under a rehabilitation practice. The experimental plan included three soil slopes (0%, 2%, and 6%) and two type of fertilization (organic and mineral), with sampling of rhizosphere and inter-row soils. The enzyme humic complexes were extracted by pyrophosphate, purified by ultrafiltration of the organic extracts on molecular mass exclusion membranes (mol wt > 10⁴) and fractionated by isoelectric focusing technique (IEF). The IEF on polyacrylamide rod gels with a restricted pH gradient ranging between 6.0 and 4.0 gave five humic bands on the basis of the little differences of their electric charges (pI). Under both organic and mineral fertilization, β-glucosidase activity bound to the fractionated humic substances, especially in the pH range 4.5–4.2 of the rhizosphere soil, was higher than that of the inter-row soil. This also occurred in 6% slope where the enzyme activity was lower than in soil with lower slopes. The higher number of the β-glucosidase active humic bands in rhizosphere than inter-row soil, particularly for the 0% slope, may be due to the presence of humic molecules capable of preserving the enzyme molecules in the active form, other than to the higher microbial activity synthesizing and releasing the tested enzymes.     

A method for the determination of β-glucosidase activity in soil

A method is described for the rapid and simple assay of soil β-glucosidase activity. It involves colorimetric estimation of ρ-nitrophenol released by β-glucosidase activity when soil is incubated in McIlvaine buffer (pH 4.8) with ρnitrophenyl βd-glucoside and toluene at 30°C for 1 hr. The method has been applied to three different soils. The range of β-glucosidase activity in cultivated soils was from 10.1 to 15.2 mµ mole per min per gram of dried soil. K_m value for ρ-nitrophenyl β-d-glucoside was 3.3 × 10^-4 M. Optimum pH was 4.8.     

Is the decline of soil microbial biomass in late winter coupled to changes in the physical state of cold soils?

During winter when the active layer of Arctic and alpine soils is below 0 °C, soil microbes are alive but metabolizing slowly, presumably in contact with unfrozen water. This unfrozen water is at the same negative chemical potential as the ice. While both the hydrostatic and the osmotic components of the chemical potential will contribute to this negative value, we argue that the osmotic component (osmotic potential) is the significant contributor. Hence, the soil microorganisms need to be at least halotolerant and psychrotolerant to survive in seasonally frozen soils. The low osmotic potential of unfrozen soil water will lead to the withdrawal of cell water, unless balanced by accumulation of compatible solutes. Many microbes appear to survive this dehydration, since microbial biomass in some situations is high, and rising, in winter. In late winter however, before the soil temperature rises above zero, there can be a considerable decline in soil microbial biomass due to the loss of compatible solutes from viable cells or to cell rupture. This decline may be caused by changes in the physical state of the system, specifically by sudden fluxes of melt water down channels in frozen soil, rapidly raising the chemical potential. The dehydrated cells may be unable to accommodate a rapid rise in osmotic potential so that cell membranes rupture and cells lyse. The exhaustion of soluble substrates released from senescing plant and microbial tissues in autumn and winter may also limit microbial growth, while in addition the rising temperatures may terminate a winter bloom of psychrophiles.Climate change is predicted to cause a decline in plant production in these northern soils, due to summer drought and to an increase in freeze-thaw cycles. Both of these may be expected to reduce soil microbial biomass in late winter. After lysis of microbial cells this biomass provides nutrients for plant growth in early spring. These feedbacks, in turn, could affect herbivory and production at higher trophic levels.     

Interelemental relationships in the soil and plant tissue and photosynthesis of field‐cultivated wheat growing in naturally enriched copper soils

Several interelemental relationships have been examined in field‐cultivated wheat (Triticum aestivum L. cv Vergina) growing on naturally enriched copper (Cu) soils. Mean soil Cu concentration per site ranged from 103–394 μg.g^‐1 dry weight (DW). Interrelationships between Cu, iron (Fe), calcium (Ca), potassium (K), zinc (Zn), lead (Pb), and magnesium (Mg) concentrations in the soil and plant tissue (roots, stems, and leaves) were examined using Principle Components Analysis. Soil samples were clustered according to collection site and were primarily differentiated according to their Cu concentrations. Soil Cu concentrations were positively correlated with Zn, Ca, Fe, and K in the soil, with Cu, K, and Ca in the roots, and Cu and Fe in the leaves and negatively correlated with Fe in the roots. The increase in Cu in the roots and leaves was positively correlated with increases in K and Ca in the roots and Fe and Ca in the leaves, but negatively with Fe in the roots. Increases in leaf Ca concentrations were correlated with increases in Mg and decreases in Zn concentrations in the leaf. Plants growing in soil with high Cu concentration exhibited toxicity symptoms with reduced height, decreased total leaf area and lower chlorophyll concentrations. Photosynthesis expressed per unit leaf area was not affected by increasing Cu concentrations in the soil or plant tissue.     

Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauí state,Brazil

The aim of this work was to investigate the response of soil microbial biomass and activity to practices in organic and conventional farming systems. The study was carried out at the Irrigation District of Piauí, Brazil. Five different plots planted with “acerola” orchard (Malpighia glaba) and established at the following management were evaluated: (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24). Soil microbial biomass C (C_mic), basal respiration, organic carbon (C_org), C_mic-to-C_org ratio and metabolic quotient (qCO₂) were evaluated in soil samples collected at 0–10 cm depth. The highest C_org and C_mic levels occurred in organic system plots ORG18 and ORG24 compared to the conventional system. Soil respiration and C_mic-to-C_org ratio were significantly enhanced by the organic system plots. The qCO₂ was greater in conventional than in organic system. These results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C.     

Evaluation of microplate and bench-scale β-glucosidase assays for reproducibility, comparability, kinetics, and homogenization methods in two soils

Enzyme assays that use fluorescently labeled substrates and microplate formats have been incorporated into laboratory protocols to improve sensitivity and reduce the time and labor involved in traditional bench-scale analyses. Microplate protocols vary, and the methods have not been evaluated systematically for comparability and reproducibility. In this study, p-nitrophenol (pNP)-based and 4-methylumbelliferone (MUF)-based microplate methods for estimating β-glucosidase activity were compared in two soils with different properties. Microplate method reproducibility was evaluated in replicate soil suspensions, and Michaelis–Menten kinetics for the microplate assays were compared to those of a standard pNP bench-scale assay. The effect of soil sample sonication on reproducibility was determined for the MUF microplate method. The MUF microplate method was reproducible in five replicate soil suspensions, but the pNP microplate method showed greater variability. The K _m Michaelis–Menten constant was significantly different in the microplate methods compared to the bench method. Enzyme activities measured by the MUF and bench methods were comparable, but the pNP microplate method resulted in more variable measurements and was less sensitive in the soils studied. Sonication of soil at an intensity of 15 W ml^?1 resulted in higher (MUF) measurements, but greater variability. The effects of high background absorbance on the reproducibility, sensitivity, and accuracy of the pNP microplate method do not support this method as a substitute for the standard bench method. A robust comparison study of the MUF microplate method across laboratories is recommended to further validate its use in comparative analyses.     

Impact of soil fumigation practices on soil nematodes and microbial biomass

This study was designed to understand the impact of methyl bromide (MB) (CHaBr) and its alternatives on both free-living and root-knot nematodes in the soil. A randomized complete block experiment with six treatments and 4 replicates (each replicate in a separate greenhouse) was established in Qingzhou, Shandong Province, China. In addition to MB and untreated control (CK) treatments there were four alternative soil fumigation practices including MB virtually impermeable films (VIF), metam sodium (MS), MS VIF and soil solarization combined with selected biological control agents (SS BCA). Two tomato (Lycopersicum esculentum Mill.) cultivars, cv. Maofen-802 from the Xian Institute of Vegetable Science, China, and cv. AF179 Brillante from the Israeli Hazera Quality Seeds, were selected as test crops. The results indicated that Rhabditidae was the most dominant population with percentage abundance as high as 85% of the total number of identified free-living nematodes, followed by that of Cephalobidae. Methyl bromide and its alternatives except for the non-chemical SS BCA treatment controlled the target pest, root-knot nematodes. Also, the impact of the three chemical alternatives on free-living nematode number and functional group abundance was similar to the impact associated with a typical methyl bromide application. Chemical fumigation practices, especially that with MB, significantly reduced the number of nematodes in the soil and simultaneously significantly reduced the number of nematode genera thereby reducing nematode diversity. All the four soil chemical fumigation activities decreased soil microbial biomass and had an obvious initial impact on microorganism biomass. Furthermore, both plant-parasitic and fungivore nematodes were positively correlated with soil microbial biomass.     

Effect of temperature on EDTA‐extractable copper in soils

Abstract

Two contrasting soils were extracted with 0.05 M EDTA in 1 M CH₃ COONH₄ at pH 6, before and after incubation for 4 weeks at constant (10, 20 or 30°C) or fluctuating (10/30, mean 20°C) temperatures. Less copper was extracted from soils which were incubated at fluctuating temperature than from those maintained at a constant 20 C. Where incubation temperature was constant, extractable copper increased or decreased with increasing temperature depending on the soil and how it was treated. Recovery of added copper was low initially but increased during the incubation. Maximum recovery was associated with low incubation temperature in one soil, but high temperature in the other. The amounts of copper extracted were slightly increased by γ‐irradiation of the soils. Extractable copper was also increased by increasing the temperature at which the extraction was performed.     

Effects of water‐soluble constituents of plant residues on water uptake by seeds

Abstract

There has been strong support for the hypothesis that the adverse effects of plant residues on crop yields are due to phytotoxic compounds derived from these residues. This hypothesis is based largely on studies showing that, when compared with distilled water, aqueous extracts of plant residues have an adverse effect on seed germination and seedling growth. Because seed germination and seedling growth are reduced by a delay in germination resulting from slow uptake of water by seeds, we studied the possibility that the adverse effects of aqueous extracts of plant residues on seed germination and seedling growth might be at least partly due to water uptake by seeds being retarded by water‐soluble constituents of these residues. To test this possibility, we compared the rates of water uptake and germination of seeds of corn (Zea mays L.), soybean [Glycine max. (L.) Merrill], and wheat (Triticum aestivum L.) when these seeds were treated with distilled water and with aqueous extracts of corn, sorghum [Sorghum bicolor (L.) Moench], and wheat residues. We found that the rates of water uptake and germination of seeds treated with aqueous extracts of plant residues were appreciably slower than the corresponding rates for seeds treated with distilled water. This may be due to the water potentials of these extracts (ca. ‐50 kPa) because when seeds of corn, sorghum, and wheat were treated with a solution of polyethylene glycol 8000 having a water potential similar to that of the extracts of plant residues tested, the rates of water uptake and germination were also slower than the corresponding rates for seeds treated with distilled water. These observations suggest that the adverse effects of aqueous extracts of plant residues on seed germination and seedling growth when compared with distilled water may be partly due to constituents of these extracts inducing water potential effects that reduce water uptake by germinating seeds.     

Vineyard soils under organic and conventional management—microbial biomass and activity indices and their relation to soil chemical properties

Eight vineyards in Pfaffenheim (P) and Turckheim (T) close to Colmar, France, forming four pairs of organic and conventional vineyards, were analyzed for microbial biomass and activity indices in relation to important soil chemical properties (carbon, nutrient elements, heavy metals) and also to differences between the bottom and top positions on the vineyard slope. The question was whether the vineyard management affects especially the soil microbiological indices. Three locations were on limestone (P-I, P-II, T-II), one on granite (T-I). The gravel content (>2 mm) ranged from 9 to 47%. The management systems had no significant main effect on the contents of organic C, total N, P, and S. The mean total contents of man-derived heavy metals decreased in the order Cu (164 μg g⁻¹ soil) > Zn (100 μg g⁻¹ soil) > Pb (32 μg g⁻¹ soil). The contents of microbial biomass C varied between 320 and 1,000 μg g⁻¹ soil. The significantly highest content was found at location P-II, the significantly lowest at the moderately acidic location T-I. The contents of microbial biomass N and adenosine triphosphate showed a similar trend. At location T-I, the fungal ergosterol-to-microbial biomass C ratio and the metabolic quotient qCO₂ were significantly highest, whereas the percentage of soil organic C present as microbial biomass C was lowest. Highest percentages of soil organic C present as microbial biomass C and lowest qCO₂ values were found in the organic in comparison with the conventional vineyards. None of the soil microbiological indices was significantly affected by the position on the slope, but all were significantly affected by the management system. This was mainly due to the highest index levels in the organic vineyard location P-II with the longest history in organic management.     

Effects of soil compaction and water‐filled pore space on soil microbial activity and N losses

Abstract

Soil compaction is a significant production problem for agriculture because of its negative impact on plant growth, which in many cases has been attributed to changes in soil N transformations. A laboratory experiment was conducted to study the effect of soil compaction and water‐filled pore space on soil microbial activity and N losses. A hydraulic soil compaction device was used to evenly compress a Norfolk loamy sand (fine‐loamy, siliceous, thermic Typic Kandiudults) soil into 50 mm diameter by 127 mm long cores. A factorial arrangement of three bulk density levels (1.4, 1.6, and 1.8 Mg/m³) and four water‐filled pore space levels (60, 65, 70, 75%) was used. Fertilizer application of 168 kg N/ha was made as 1.0 atom % ¹⁵N as NH₄NO₃. Soil cores were incubated at 25°C for 21 d. Microbial activity decreased with both increasing water‐filled pore space and soil bulk density as measured by CO₂‐C entrapment. Nitrogen loss increased with increasing bulk density from 92.8 to 334.4 g N/m³ soil at 60% water‐filled pore space, for 1.4 and 1.8 Mg/m³, respectively. These data indicate that N loss and soil microbial activity depends not only on the pore space occupied by water, but also on structure and size of soil pores which are altered by compaction.     

Hydrolysis of cellobiose by β-glucosidase in the presence of soil minerals - Interactions at solid-liquid interfaces and effects on enzyme activity levels

Extracellular enzymatic activities in soils are essential for the cycling of organic matter. These activities take place in multiphase environments where solid phases profoundly affect biocatalytic activities. Aspergillus niger is ubiquitous in soils; its β-glucosidase plays an important role in the degradation of cellulose, and therefore in the global carbon cycle and in the turnover of soil organic matter. However, the information on the interactions of this protein with soil minerals is very limited, and even less is known about their consequences for the hydrolysis of the natural substrate cellobiose. We therefore characterised the sorptive interactions of this enzyme with the soil minerals montmorillonite, kaolinite and goethite and quantified the resulting changes in the hydrolysis rate of cellobiose. Fractions of adsorbed protein, and the resulting catalytic activity loss, were lower for montmorillonite than for kaolinite and goethite at given experimental conditions; adsorption was 9.7 ± 7.3% for montmorillonite, 70.3 ± 3.1% for kaolinite and 71.4 ± 1.8% for goethite, respectively. Adsorption of the protein to the minerals caused a total decrease in the catalytic activity of 18.8 ± 3.4% for kaolinite and 17.9 ± 4.7% for goethite whereas it was not significant for montmorillonite. The average catalytic activity lost by the pool of adsorbed molecules was 26.8% for kaolinite and 25.0% for goethite. Both the amount of adsorbed protein and the resulting loss of catalytic activity were found to be independent of the specific surface areas yet were influenced by the electrical properties of the mineral surfaces. Under the experimental conditions, montmorillonite and kaolinite are negatively charged whereas goethite is positively charged. However, because of the adsorption of phosphate anions from the buffer, a charge reversal took place at the surface of goethite. This was confirmed by zeta (ζ)-potential measurements in phosphate buffer, revealing negative values for all the tested minerals. Indeed goethite interacted with the enzyme as a negatively charged surface: the amount of adsorbed protein and the resulting catalytic activity loss were very similar to those of kaolinite. Our results show that, even if an important fraction of β-glucosidase is adsorbed to the minerals, the catalytic activity is largely retained. We suggest that this strong activity retention in presence of soil minerals results from a selective pressure on A. niger, which benefits from the activity of the adsorbed, and thus stabilized, enzyme pool.     

Effects of wood char and activated carbon on the hydrolysis of cellobiose by β-glucosidase from Aspergillus niger

Biochar amendments to soils have been suggested as a strategy to sequester carbon and therefore mitigate global climate change. The enrichment of soils with charred materials also increases their fertility. This fertilising effect of biochar may be caused by various mechanisms; an acceleration of nutrient cycling has been suggested as one such mechanism. The rate-limiting step in nutrient cycling is thought to be the extracellular enzymatic attack on biological macromolecules. In this study, therefore, the effects of chestnut wood char (specific surface area 2.0 m² g⁻¹) and of activated carbon (specific surface area approximately 900 m² g⁻¹) on an extracellular enzymatic reaction involved in the degradation of cellulose (i.e., hydrolysis of cellobiose by β-glucosidase from Aspergillus niger) were investigated. Cellobiose was not adsorbed by chestnut wood char, whereas activated carbon absorbed more than 97% of it. Both charred materials adsorbed more than 99% of β-glucosidase. For chestnut wood char, adsorption of the enzyme caused a decrease of approximately 30% in the reaction rate, whereas for activated carbon, the nearly complete absorption of both substrate and enzyme entirely inhibited the reaction. These results show that β-glucosidase from A. niger retains most of its activity when adsorbed to chestnut wood char and that the reaction it catalyses in nature is only slightly affected by this charred material. On the other hand, a material characterised by a high specific surface area and high porosity, such as activated carbon, can make even a highly soluble substrate unavailable for soil enzymes and therefore completely inhibit the reaction. Thus, charred materials may affect nutrient cycling mainly by regulating the availability of substrates: the degradation of highly soluble substrates may be accelerated by materials with low specific surface area, which maintain an active and protected enzyme pool, whereas materials with high specific surface and high porosity may slow down the degradation by making substrates unavailable.     

The effect of sewage‐sludge on the heavy metal content of soils and plant tissue

Abstract

Domestic sewage sludge applied to farm fields at a rate of 44.9 kg/ha in a mixture with lime and sawdust was found to increase the soil levels of cadmium, chromium, copper, lead, mercury, nickel and zinc. The average levels in sludge treated soil were: 0.11, 0.56, 3.59, 2.72, 0.068, 1.49 and 2.57 ppm, respectively. The increases were small and the overall loading factors were well below recommended maximums. The uptake of these heavy metals by grass and legume plants was variable with cadmium, copper and zinc levels being higher in those plants growing in the sludge treated soils but only copper was significantly higher. The heavy metal contents found were all within the levels normally found in grass and legume plants. The higher mean concentration in plants growing on the sludge treated soils were cadmium 0.495; chromium 1.22; copper 12.3; lead 1.54; mercury 0.022; nickel 4.08 and zinc 28.4 ppm.     

Spatial distribution of glomalin-related soil protein and its relationships with root mycorrhization,soil aggregates,carbohydrates, activity of protease and β-glucosidase in the rhizosphere of Citrus unshiu

Relationships between the spatial distributions of glomalin-related soil protein (GRSP) and soil aggregates, carbohydrates or relevant enzymes are poorly studied. We found that two categories of GRSP, the easily extractable Bradford-reactive soil protein (EE-BRSP) and total BRSP (T-BRSP), respectively ranged between 0.3–0.6 and 0.5–0.8 mg/g DW soil, and these two BRSPs decreased with the increase of soil depth (0–40 cm) in the rhizosphere of a 22-year-old Citrus unshiu orchard. Both EE-BRSP and T-BRSP were significantly positively correlated with mycorrhization, 0.25–0.50 mm soil water-stable aggregates, water-extractable or hydrolyzable carbohydrates, and β-glucosidase, but significantly negatively correlated with protease. Our results demonstrate that the spatial distribution of GRSP is significantly affected by mycorrhization, soil carbohydrate, β-glucosidase and protease.