Role of perennial plants in determining the activity of the microbial community in the Negev Desert ecosystem

Perennial plants are known to be one of the most influential parameters in desert ecosystems affecting microbial activity. In this study, we examined the importance of these perennial shrubs and attempted to determine the most influential factor that contributes the most to the ecosystem by separating the physical part and the organic contribution of perennial plants. The study site is located in the northern Negev Desert, Israel, where 50 Hammada scoparia shrubs and 50 artificial plants were randomly marked to be used as a tool for the above objectives. Soil samples were collected monthly in the vicinity of the canopies of both shrubs while control samples were collected from the open areas between the shrubs. All samples were collected from the upper (0–10 cm) and the deeper (10–20 cm) soil layers. The contribution to microbial activity was measured by evaluation of the microbial community functions in soil. The results of the research showed a trend of a strong influence of the perennial H. scoparia shrubs on microbial community function. The functional aspects of the microbial community that were measured were CO₂ evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community. High values for all parameters were observed under the vicinity of the H. scoparia shrubs, while the artificial plants were found to have a weaker effect on the community according to soil depth. The upper-soil layer at both locations (artificial plant canopy and control samples) showed higher values of the functioning parameters than that of the deeper soil layer. The results indicate the importance of the organic contribution (plant litter) in comparison to the physical part of desert shrubs.     

Changes in soil microbial biomass and functional diversity with a nitrogen gradient in soil columns

Fertilization generates nutrient patches that may impact soil microbial activity. In this study, nitrogen patches were generated by adding ammonium sulfate or urea to soil columns (length 25 cm; internal diameter 7.2 cm). Changes in nitrogen transformation, soil microbial biomass, and microbial functional diversity with the nitrogen gradients were investigated to evaluate the response of microbial activity to chemical fertilizer nutrient patches. After applying of ammonium sulfate or urea, the added nitrogen migrated about 7 cm. Microbial biomass carbon (MBC) was lower in fertilized soil than in the control (CK) treatment at the same soil layers. MBC increased with soil depth while microbial biomass nitrogen (MBN) decreased. BIOLOG analysis indicated that the average well color development (AWCD) and functional diversity indices of the microbial communities were lower in the 1 cm and 2 cm soil layers after application of ammonium sulfate; the highest values were in the 3 cm soil layer. AWCD and Shannon indices from the 1 to 5 cm soil layers were higher than those from other soil layers under urea application. Both principal component analysis and carbon substrate utilization analysis showed significant separation of soil microbial communities among different soil layers under application of ammonium sulfate or urea. Microbial activity was substantially decreased when NH₄⁺-N concentration was higher than 528.5 mg kg⁻¹ (1–3 cm soil layer under ammonium sulfate application) or 536.8 mg kg⁻¹ (1 cm soil layer under urea application). These findings indicated that changes in soil microbial biomass and microbial functional diversity can occur with a nitrogen gradient. The extent of changes depends on the nitrogen concentration and the form of inorganic fertilizer.     

Soil microbial community responses to sulfadiazine-contaminated manure in different soil microhabitats

Veterinary antibiotics such as sulfadiazine (SDZ) are applied with manure to agricultural soil. Antimicrobial effects of SDZ on soil microbial community structures and functions were reported for homogenized bulk soils. In contrast, field soil is structured. The resulting microhabitats are often hot spots that account for most of the microbial activity and contain strains of different antibiotic sensitivity or resilience. We therefore hypothesize that effects of SDZ are different in diverse soil microhabitats. We combined the results of laboratory and field experiments that evaluated the fate of SDZ and the response of the microbial community in rhizosphere, earthworm burrow, and soil macroaggregate microhabitats. Microbial communities were characterized by phenotypic phospholipid fatty acid (PLFA) and genotypic 16S rRNA gene patterns (DGGE) and other methods. Data was evaluated by principle component analyses followed by two-way ANOVA with post-hoc tests. Extractable SDZ concentrations in rhizosphere soil were not clearly different and varied by a factor 0.7–1.2 from those in bulk soil. In contrast to bulk soil, the extractable SDZ content was two-fold larger in earthworm burrows, which are characterized by a more hydrophobic organic matter along the burrow surface. Also, extractable SDZ was larger by up to factor 2.6 in the macroaggregate surface soil. The rhizosphere effect clearly increased the microbial biomass. Nonetheless, in the 10 mg SDZ kg⁻¹ treatment, the biomass deceased by about 20% to the level of uncontaminated bulk soil. SDZ contamination lowered the total PLFA concentrations by 14% in the rhizosphere and 3% in bulk soil of the field experiment. Structural shifts represented by Pseudomonas DGGE data were larger in SDZ-contaminated earthworm burrows compared to bulk soils. In the laboratory experiment, a functional shift was indicated by a four-fold reduced acid phosphatase activity in SDZ-contaminated burrows compared to bulk soil. Structural and functional shifts after SDZ contamination were larger by a factor of 2.5 in the soil macroaggregate surface versus interior, but this relation reversed over the long-term under field conditions. Overall, the combined effects of soil microhabitat, microbial community composition, and exposure to SDZ influenced the microbial susceptibility towards antibiotics under laboratory and field conditions.     

Effects of mercury on soil microbial communities in tropical soils of French Guyana

In gold mining regions, the risk of soil pollution by mercury is a major environmental hazard, especially in tropical areas where soil microflora plays a major part in soil functioning, major bio-geochemical cycles and carbon turn-over. The impact of mercury pollution on soil microflora should thus be carefully assessed in such environments while taking into consideration the specificities of tropical soils. The aim of this study was to compare the effects of mercury (0, 1 and 20 μg of inorganic mercury per gram of soil) on the functional diversity and genetic structure of microbial communities in a tropical soil. We investigated the effects of mercury on tropical soil microflora using soil microcosms spiked with mercury and incubated at 28 °C for 1 month. Microcosm flora, its biomass and its activity, as well as its functional and genetic structure, were followed by cultural methods, measures of respiration, ECOLOG plates, and DGGE (denaturing gel gradient electrophoresis), respectively. Fate of total and bioavailable mercury was estimated by CVAFS (cold vapor atomic fluorescence spectrometry). Results obtained for the microcosms enriched with only 1 μg g^?1 mercury were indistinguishable from controls. Conversely, in the presence of high mercury contents (20 μg g⁽¹), an immediate effect was measured on soil respiration, functional diversity (ECOLOG plates) and genetic structure (DGGE), although no significant effect was observed on plate counts or microbial biomass. In addition, whereas microbial activities (respiration and functional diversity) rapidly regained control values, a lasting effect of the high mercury concentration was observed on the genetic structure of the soil microbial community. These modifications took place during the first week of incubation when total mercury concentration was declining and bioavailable mercury was at its highest.This multiple approach study is one of the first attempts at investigating the effects of mercury on soil microbial communities in tropical soils. Our results demonstrate that in the tropical soil under study, mercury affects the soil microbial communities in a different manner than was previously reported in temperate soils. Furthermore, mercury toxicity on soil microbes may be modulated by typical tropical soil characteristics.     

Influence of organic and mineral amendments on microbial soil properties and processes

Microbial diversity in soils is considered important for maintaining sustainability of agricultural production systems. However, the links between microbial diversity and ecosystem processes are not well understood. This study was designed to gain better understanding of the effects of short-term management practices on the microbial community and how changes in the microbial community affect key soil processes. The effects of different forms of nitrogen (N) on soil biology and N dynamics was determined in two soils with organic and conventional management histories that varied in soil microbial properties but had the same fertility. The soils were amended with equal amounts of N (100 kg ha⁻¹) in organic (lupin, Lupinus angustifolius L.) and mineral form (urea), respectively. Over a 91-day period, microbial biomass C and N, dehydrogenase enzyme activity, community structure of pseudomondas (sensu stricto), actinomycetes and α proteobacteria (by denaturing gradient gel electrophoresis (DGGE) following PCR amplification of 16S rDNA fragments) and N mineralisation were measured. Lupin amendment resulted in a two- to five-fold increase in microbial biomass and enzyme activity, while these parameters did not differ significantly between the urea and control treatments. The PCR–DGGE analysis showed that the addition of mineral and organic compounds had an influence on the microbial community composition in the short term (up to 10 days) but the effects were not sustained over the 91-day incubation period. Microbial community structure was strongly influenced by the presence or lack of substrate, while the type of amendment (organic or mineral) had an effect on microbial biomass size and activity. These findings show that the addition of green manures improved soil biology by increasing microbial biomass and activity irrespective of management history, that no direct relationship existed among microbial structure, enzyme activity and N mineralisation, and that microbial community structure (by PCR–DGGE) was more strongly influenced by inherent soil and environmental factors than by short-term management practices.     

Spatial variability of enzyme activities and microbial biomass in the upper layers of Quercus petraea forest soil

Extracellular lignocellulose-degrading enzymes are responsible for the transformation of organic matter in hardwood forest soils. The spatial variability on a 12 × 12 m plot and vertical distribution (0–8 cm) of the ligninolytic enzymes laccase and Mn-peroxidase, the polysaccharide-specific hydrolytic enzymes endoglucanase, endoxylanase, cellobiohydrolase, 1,4-β-glucosidase, 1,4-β-xylosidase and 1,4-β-N-acetylglucosaminidase and the phosphorus-mineralizing acid phosphatase were studied in a Quercus petraea forest soil profile. Activities of all tested enzymes exhibited high spatial variability in the L and H horizons. Acid phosphatase and 1,4-β-N-acetylglucosaminidase exhibited low variability in both horizons, while the variability of Mn-peroxidase activity in the L horizon, and endoxylanase and cellobiohydrolase activities in the H horizon were very high. The L horizon contained 4× more microbial biomass (based on PLFA) and 7× fungal biomass (based on ergosterol content) than the H horizon. The L horizon also contained relatively more fungi-specific and less actinomycete-specific PLFA. There were no significant correlations between enzyme activities and total microbial biomass. In the L horizon cellulose and hemicellulose-degrading enzymes correlated with each other and also with 1,4-β-N-acetylglucosaminidase and acid phosphatase activities. Laccase, Mn-peroxidase and acid phosphatase activities correlated in the H horizon. The soil profile showed a gradient of pH, organic carbon and humic compound content, microbial biomass and enzyme activities, all decreasing with soil depth. Ligninolytic enzymes showed preferential localization in the upper part of the H horizon. Differences in enzyme activities were accompanied by differences in the microbial community composition where the relative amount of fungal biomass decreased and actinomycete biomass increased with soil depth. The results also showed that the vertical gradients occur at a small scale: the upper and lower parts of the H horizon only 1 cm apart were significantly different with respect to seven out of nine activities, microbial biomass content and community composition.     

Differences in soil enzyme activities,microbial community structure and short-term nitrogen mineralisation resulting from farm management history and organic matter amendments

Changes in soil microbial biomass, enzyme activities, microbial community structure and nitrogen (N) dynamics resulting from organic matter amendments were determined in soils with different management histories to gain better understanding of the effects of long- and short-term management practices on soil microbial properties and key soil processes. Two soils that had been under either long-term organic or conventional management and that varied in microbial biomass and enzyme activity levels but had similar fertility levels were amended with organic material (dried lupin residue, Lupinus angustifolius L.) at amounts equivalent to 0, 4 and 8 t dry matter lupin ha^?1. Microbial biomass C and N, arginine deaminase activity, fluorescein diacetate hydrolysis, dehydrogenase enzyme activity and gross N mineralisation were measured in intervals over an 81-day period. The community structure of eubacteria and actinomycetes was examined using PCR–DGGE of 16S rDNA fragments. Results suggested that no direct relationships existed between microbial community structure, enzyme activities and N mineralisation. Microbial biomass and activity changed as a result of lupin amendment whereas the microbial community structure was more strongly influenced by farm management history. The addition of 4 t ha^?1 of lupin was sufficient to stimulate the microbial community in both soils, resulting in microbial biomass growth and increased enzyme activities and N mineralisation regardless of past management. Amendment with 8 t lupin ha^?1 did not result in an increase proportional to the extra amount added; levels of soil microbial properties were only 1.1–1.7 times higher than in the 4 t ha^?1 treatment. Microbial community structure differed significantly between the two soils, while no changes were detected in response to lupin amendment at either level during the short-term incubation. Correlation analyses for each treatment separately, however, revealed differences that were inconsistent with results obtained for soil biological properties suggesting that differences might exist in the structure or physiological properties of a microbial component that was not assessed in this study.     

Effects of ecological restoration on microbial activity,microbial functional diversity,and soil organic matter in mixed-oak forests of southern Ohio,USA

As a result of many decades of fire suppression and atmospheric deposition the deciduous forests of eastern North America have changed significantly in stem density, basal area, tree size-frequency distribution, and community structure. Consequently, soil organic matter quality and quantity, nutrient availability, and microbial activity have likely been altered. This study evaluated the effects of four alternative forest ecosystem restoration strategies on soil microbial activity, microbial functional diversity, soil organic C, and soil N status in two mixed-oak (Quercus spp.) forests in southern Ohio, USA. The soils of these forests were sampled during the fourth growing season after application of (1) prescribed fire, (2) thinning of the understory and midstory to pre-settlement characteristics, (3) the combination of fire and thinning, and (4) an untreated control. Prescribed fire, with or without thinning, resulted in increased bacterial but not fungal activity when assessed using Biolog^®. In contrast, assays of acid phosphatase and phenol oxidase activity indicated greater microbial activity in the thinning treatment than in the other three treatments. Functional diversity of both bacteria and fungi was affected by restoration treatment, with the bacterial and fungal assemblages present in the thin + burn sites and the fungal assemblage present in the thinned sites differing significantly from those of the control and burned sites. Treatments did not result in significant differences in soil organic C content among experimental sites; however, the soil C:N ratio was significantly greater in thinned sites than in sites given the other three treatments. Similarly, there were no significant differences in dissolve inorganic N, dissolved organic N, or microbial biomass N among treatments. Bacterial and fungal functional diversity was altered significantly. Based on Biolog^® utilization treatments the bacterial assemblage in the thin-only treatment appeared to be relatively N-limited and the fungal assemblage relatively C-limited, whereas in the thin + burn treatment this was reversed. Although effects of restoration treatments on soil organic matter and overall microbial activity may not persist through the fourth post-treatment year, effects on microbial functional diversity are persistent.     

The effects of fresh and stabilized pruning wastes on the biomass,structure and activity of the soil microbial community in a semiarid climate

The incorporation of organic amendments from pruning waste into soil may help to mitigate soil degradation and to improve soil fertility in semiarid ecosystems. However, the effects of pruning wastes on the biomass, structure and activity of the soil microbial community are not fully known. In this study, we evaluate the response of the microbial community of a semiarid soil to fresh and composted vegetal wastes that were added as organic amendments at different doses (150 and 300 t ha⁻¹) five years ago. The effects on the soil microbial community were evaluated through a suite of different chemical, microbiological and biochemical indicators, including enzyme activities, community-level physiological profiles (CLPPs) and phospholipid fatty acid analysis (PLFA). Our results evidenced a long-term legacy of the added materials in terms of soil microbial biomass and enzyme activity. For instance, cellulase activity reached 633 μg and 283 μg glucose g⁻¹ h⁻¹ in the soils amended with fresh and composted waste, respectively. Similarly, bacterial biomass reached 116 nmol g⁻¹ in the soil treated with a high dose of fresh waste, while it reached just 66 nmol g⁻¹ in the soil amended with a high dose of composted waste. Organic amendments produced a long-term increase in microbiological activity and a change in the structure of the microbial community, which was largely dependent on the stabilization level of the pruning waste but not on the applied dose. Ultimately, the addition of fresh pruning waste was more effective than the application of composted waste for improving the microbiological soil quality in semiarid soils.     

Soil enzyme activities,microbial community composition and function after 47 years of continuous green manuring

Green manuring practices can influence soil microbial community composition and function and there is a need to investigate the influence compared with other types of organic amendment. This study reports long-term effects of green manure amendments on soil microbial properties, based on a field experiment started in 1956. In the experiment, various organic amendments, including green manure, have been applied at a rate of 4 t C ha⁻¹ every second year. Phospholipid fatty acid analysis (PLFA) indicated that the biomass of bacteria, fungi and total microbial biomass, but not arbuscular mycorrhizal (AM) fungi, generally increased due to green manuring compared with soils receiving no organic amendments. Some differences in abundance of different microbial groups were also found compared with other organic amendments (farmyard manure and sawdust) such as a higher fungal biomass and consequently a higher fungal/bacterial ratio compared with amendment with farmyard manure. The microbial community composition (PLFA profile) in the green manure treatment differed from the other treatments, but there was no effect on microbial substrate-utilization potential, determined using the Biolog EcoPlate. Protease and arylsulphatase activities in the green manure treatment were comparable to a mineral fertilized treatment receiving no additional C, whereas acid phosphatase activity increased. It can be concluded that green manuring had a beneficial impact on soil microbial properties, but differed in some aspects to other organic amendments which might be attributed to differences in quality of the amendments.     

The effect of glyphosate on soil microbial activity,microbial community structure,and soil potassium

The herbicide, glyphosate [N-(phosphonomethyl) glycine] is extensively used worldwide. Long-term use of glyphosate can cause micronutrient deficiency but little is known about potassium (K) interactions with glyphosate. The repeated use of glyphosate may create a selection pressure in soil microbial communities that could affect the nutrient dynamics such as K. The objective of this study was to determine the effect of single or repeated glyphosate applications on microbial and K properties of soils. A 54 day incubation study (Exp I) had a 3 × 5 factorial design with 3 soils (silt loam: fine, illitic, mesic Aeric Epiaqualf) of similar physical and chemical characteristics, that varied in long-term glyphosate applications (no, low, and high glyphosate field treatments) and five glyphosate rates (0, 0.5×, 1×, 2×, and 3× recommended field rates applied once at time zero). A second 6 month incubation study (Exp II) had a 3 × 3 factorial design with three soils (as described above) and three rates of glyphosate (0, 1×, and 2× recommended field application rates applied monthly). For each study microbial properties [respiration; community structure measured by ester linked fatty acid methyl ester (EL-FAME) analysis and microbial biomass K] and K fractions (exchangeable and non-exchangeable) were measured periodically. For Exp I, glyphosate significantly increased microbial respiration that was closely related to glyphosate application rate, most notably in soils with a history of receiving glyphosate. For Exp II, there was no significant effect of repeated glyphosate application on soil microbial structure (EL-FAME) or biomass K. We conclude that glyphosate: (1) stimulates microbial respiration particularly on soils with a history of glyphosate application; (2) has no significant effect on functional diversity (EL-FAME) or microbial biomass K; and (3) does not reduce the exchangeable K (putatively available to plants) or affect non-exchangeable K. The respiration response in soils with a long-term glyphosate response would suggest there was a shift in the microbial community that could readily degrade glyphosate but this shift was not detected by EL-FAME.     

Relative strengths of relationships between plant,microbial, and environmental parameters in heavy-metal contaminated floodplain soil

We used a combination of sampling and statistical approaches to investigate the relative influence of metals, soil acidity, and organic matter on a suite of analogous plant and microbial community parameters in floodplain soils contaminated by mine wastes in the early twentieth century. We compared the sensitivity of plant and microbial communities to environmental variables and to one another using constrained ordination analyses. Environmental factors accounted for a larger percentage of the total variance in microbial communities (56.2%) than plant communities (22.0%). We also investigated biological and geochemical changes that occurred along a short transect (64 cm) that spanned a transition from productive grassland to an area of barren wasteland representing a total functional collapse of the grassland/soil ecosystem. Along this small-scale transect we quantified geochemical parameters and biological parameters in two soil layers, an upper layer (0–10 cm) and a lower layer (10–20 cm). Results from the short transect indicated that soil respiration was not a strong indicator of underlying metal concentrations, but soil acidity was correlated in the upper and lower layers. PLFA profiles changed with distance along the gradient in the upper, but not the lower layer. Implications for remediation of contaminated floodplain soils are discussed.     

Effects of atrazine on microbial activity in semiarid soil

The effect of an atrazine formulation on microbial biomass, microbial respiration, ATP content and dehydrogenase and urease activity in a semiarid soil and the influence of time on the response of soil microbial activity to the herbicide treatment were assessed. The atrazine formulation was added to soil as aqueous solutions of different concentrations of active ingredient to obtain a range of concentrations in the soil from 0.2 to 1000 mg kg⁻¹. Microcosms of soil with the different herbicide concentrations and untreated control soil were incubated for 6 h, 16 and 45 days. In general, an increase in the measured microbiological and biochemical parameters with atrazine concentration in soil was observed. The increase in microbial activity with atrazine pollution was noticeable after lengthy incubation.     

Soil organic carbon distribution drives microbial activity and functional diversity in particle and aggregate-size fractions

Chemical and functional characterizations of particle-size and aggregate fractions of soils were performed to investigate whether accessibility and decomposability of organic matter regulate functions and diversity of the soil microbial community at the micro-habitat scale. Soils were physically fractionated into particle size fractions, free-particulate organic matter (F-POM), macro-aggregates (250–2000 μm) and micro-aggregates (53–250 μm). Organic C was enriched in silt and clay, micro-aggregates and F-POM fractions. Enzymes showed the greatest activity in the fine fractions (silt and clay) and F-POM, and were largely influenced by organic C content. MicroResp-CLPP (Community Level Physiological Profile) showed the lowest catabolic responses in the sand and the highest in the fine fraction and micro-aggregates. In general, organic C availability drove soil activity and functional diversity: soils with the higher amount of organic C showed the higher catabolic activity. However, this response was variable within soil fractions, where organic C accessibility, as well as microbial selection and distribution, affected functional diversity.     

Structural and functional diversity of bacterial community in soil treated with the herbicide napropamide estimated by the DGGE,CLPP and r/K-strategy approaches

Napropamide is one of the most commonly used herbicide in agricultural practice and can exhibit toxic effect to soil microorganisms. Therefore, the main objective of this study was to examine the genetic and functional diversity of microbial communities in soil treated with napropamide at field rate (FR, 2.25 mg kg⁻¹ of soil) and 10 times the FR (10 × FR, 22.5 mg kg⁻¹ of soil) by the denaturing gradient gel electrophoresis (DGGE) and the community level physiological profile (CLPP) methods. In addition, the r/K-strategy approach was used to evaluate the effect of this herbicide on the community structure of the culturable soil bacteria. DGGE patterns revealed that napropamide affected the structure of microbial community; however, the richness (S) and genetic diversity (H) values indicated that the FR dosage of napropamide experienced non-significant changes. In turn, the 10 × FR dosage of herbicide caused significant changes in the S and H values of dominant soil bacteria. DGGE profiles suggest an evolution of bacteria capable of degrading napropamide among indigenous microflora. Analysis of the CLPPs indicated that the catabolic activity of microbial community expressed as AWCD (average well-color development) was temporary positively affected after napropamide application and resulted in an increase of the substrate richness (S_R) as well as functional biodiversity (H) values. Analysis of the bacterial growth strategy revealed that napropamide affected the r- or K-type bacterial classes (ecotypes). In treated-soil samples K-strategists dominated the population, as indicated by the decreased ecophysiological (EP) index. Napropamide significantly affected the physiological state of culturable bacteria and caused a reduction in the rate of colony formation as well as a prolonged time of growth rate. Obtained results indicate that application of napropamide may poses a potential risk for soil functioning.     

Response of microbial communities to different doses of chromate in soil microcosms

The toxic effect of chromate on soil microbial communities is not well documented, although microorganisms control biogeochemical cycling, contribute to formation of soil structure, regulate the fate of organic matter applied to soil. In this study the effects of short- and middle-term chromate on the soil microbial community were investigated. The shifts in the size and in the diversity of culturable heterotrophic bacterial community, the resistance to Cr(VI) of heterotrophic bacteria, the presence of cyanobacteria, the activity of 19 enzymes, and the ATP content were monitored over time (120 days) in soil microcosms artificially contaminated with three concentrations of chromate (50, 250 and 1000 mg kg⁻¹ soil). The chromate contamination affected the structure and the diversity of the soil bacterial community. Bacterial strains isolated from the microcosm contaminated with the highest concentration of chromate were identified by 16S rDNA gene sequencing. All isolates belonged to the genus Pseudomonas, were able to reduce Cr(VI), and showed a high resistance to chromate. To our knowledge, this is the first report that shows Pseudomonas strains having the capability to resist up to 40 mM of Cr(VI) on minimal medium. The cyanobacterial group was more sensitive to chromate contamination than culturable heterotrophic bacteria. No cyanobacterial growth was detected in enrichment cultures from the soil polluted with the highest chromate concentration. Some enzymes were inhibited by high concentrations of chromate, whereas others were stimulated. The ATP content in microcosms was strongly affected by chromate. We conclude that the soil microbial community responds to chromate pollution through changes in community structure, in metabolic activity, and in selection for Cr(VI)-resistance.     

Soil microbial biomass,crop yields,and bacterial community structure as affected by long-term fertilizer treatments under wheat-rice cropping

Soil microbial biomass carbon (SMBC) and nitrogen (SMBN), soil microbial community structure, and crop yields were studied in a long-term (1982–2004) fertilization experiment carried out in Suining, Sichuan province of PR China. Eight treatments included three chemical fertilizer (CF) treatments (N, NP, NPK), three CF + farmyard manure (M) treatments (NM, NPM, NPKM), M alone and no fertilizer (CK) as control. The results showed that the soil microbial biomass was higher in soil treated with CFM than in soil treated with CF alone, and that NPKM gave the highest rice and wheat yields. The SMBC and SMBN were higher after rice than those after wheat cropping. SMBC correlated closely with soil organic matter. Average yields of wheat and rice for 22 years were higher and more stable in the fertilized plots than in control plots. Bacterial community structure was analyzed by PCR-DGGE targeting eubacterial 16S rRNA genes. A higher diversity of the soil bacterial community was found in soil amended with CFM than in other fertilizer treatments. Some specific band emerged in the soil amended with M. The highest diversity of bacterial communities was found in the NPKM treated soil. The bacterial community structures differed in rice and wheat plots. Sequencing of PCR products separated in DGGE showed that some of the common and dominant bands were closely related to Aquicella lusitana and to Acidobacteria. This study demonstrated that mixed application of N, P, and K with additional M amendment increased soil microbial biomass, diversified the bacterial communities and maintained the crop production in the Calcareous Purplish Paddy soil.     

Why do cereal–legume intercrops support large earthworm populations?

The relative importance of reduced soil disturbance and increased food supply in supporting large earthworm populations in cereal–legume intercropping systems was investigated in two replicated field experiments over a 3-year period. In the first experiment, the effects of the absence of tillage and the presence of a permanent white clover understorey on earthworm populations were assessed in three winter wheat cropping systems. The mean earthworm abundance as assessed by electrical extraction in conventional wheat, direct drilled wheat and direct-drilled wheat–clover intercrops was 211, 280 and 572 individuals m⁻² and the corresponding earthworm biomass was 62, 92 and 203 g m⁻². In the second experiment, the effects of dairy cattle slurry and mineral fertilizer applications on earthworm populations in wheat–clover intercropping systems were assessed. Neither the input of additional organic matter (as slurry) nor mineral N fertilization increased earthworm population levels which were already remarkably high, averaging 1097 individuals m⁻² and 266 g biomass m^–2 in the third year of the experiment. These results suggest that while the absence of ploughing alone had only a modest effect, the combination of absence of ploughing and presence of a clover understorey increased earthworm populations greatly. It is concluded that cereal–legume intercrops support large earthworm populations primarily because the organic matter input from such systems is favourable for earthworms in terms of quantity, quality and continuity of food supply throughout the year.     

The combined effects of earthworms and arbuscular mycorrhizal fungi on microbial biomass and enzyme activities in a calcareous soil spiked with cadmium

Earthworms and arbuscular mycorrhizal fungi (AMF) are known to independently affect soil microbial and biochemical properties, in particular soil microbial biomass (SMB) and enzymes. However, less information is available about their interactive effects, particularly in soils contaminated with heavy metals such as cadmium (Cd). The amount of soil microbial biomass C (MBC), the rate of soil respiration (SRR) and the activities of urease and alkaline phosphatase (ALP) were measured in a calcareous soil artificially spiked with Cd (10 and 20 mg Cd kg⁻¹), inoculated with earthworm (Lumbricus rubellus L.), and AMF (Glomus intraradices and Glomus mosseae species) under maize (Zea mays L.) crop for 60 days. Results showed that the quantity of MBC, SRR and enzyme activities decreased with increasing Cd levels as a result of the elevated exchangeable Cd concentration. Earthworm addition increased soil exchangeable Cd levels, while AMF and their interaction with earthworms had no influence on this fraction of Cd. Earthworm activity resulted in no change in soil MBC, while inoculation with both AMF species significantly enhanced soil MBC contents. However, the presence of earthworms lowered soil MBC when inoculated with G. mosseae fungi, showing an interaction between the two organisms. Soil enzyme activities and SRR values tended to increase considerably with the inoculation of both earthworms and AMF. Nevertheless, earthworm activity did not affect ALP activity when inoculated with G. mosseae fungi, while the presence of earthworm enhanced urease activity only with G. intraradices species. The increases in enzyme activities and SRR were better ascribed to changes in soil organic carbon (OC), MBC and dissolved organic carbon (DOC) contents. In summary, results demonstrated that the influence of earthworms alone on Cd availability is more important than that of AMF in Cd-polluted soils; and that the interaction effects between these organisms on soil microorganism are much more important than on Cd availability. Thus, the presence of both earthworms and AMF could alleviate Cd effects on soil microbial life.     

Identifying earthworm's organic matter signatures by near infrared spectroscopy in different land-use systems in Tabasco,Mexico

In the state of Tabasco, South-eastern, Mexico, land-use changes such as the conversion of natural into agricultural systems, modify soil quality and the abundance of soil macrofauna, including earthworms. The aim of this study was to characterize by near-infrared spectroscopy (NIRS) the earthworms’ fingerprint in soil, in six sites including natural and agricultural ecosystems with low and high earthworm biomass and low and high earthworm diversity, in order to identify specific wavelengths that discriminate the presence/abundance of earthworm species and functional groups. The spectral region of 1860–1870 nm was significantly correlated with total earthworm density, particularly at one of the sites (Cedar polyculture; r = 0.8, p < 0.05). Earthworm biomass had a specific NIRS wavelength according to the earthworm species and feeding category: 1820 and1860–1870 nm wavelengths were significantly correlated with Polypheretima elongata (r² = 0.7, p < 0.05; mesohumic species) biomass and 2090 nm for biomass of all Lavellodrilus species (polyhumics). Two species had a much wider spectral range: L. bonampakensis and Dichogaster saliens (an epigeic worm; 1690–2300 nm, r² = 0.7, p < 0.05). Biomasses of Periscolex brachysistis and Diplotrema murchiei were not significantly correlated with any near infrared wavelength spectra analyzed. Combining a maximum of 4 species per wavelength, mesohumic earthworms had a wider wavelength spectrum than polyhumics. Therefore, earthworm species diversity, biomass and abundance are associated with soil quality (as measured by NIR spectra) and this relationship varies with species and ecological category. Sites with lower and higher earthworm diversity have lower and higher soil organic matter quality, respectively, as observed by the wider or narrower spectral range with which earthworm biomasses are correlated.