Anaerobic Soil Disinfestation and Vermicompost to Manage Bottom Rot in Organic Lettuce.

Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris [Frank] Donk) is an aggressive soilborne pathogen with a wide host range, survives saprophytically between crops presenting a challenge for organic vegetable farmers that lack effective management tools. A two-year field experiment was conducted at two organic farms to compare anaerobic soil disinfestation (ASD) and worm-cured compost (vermicompost) to manage bottom rot caused by R. solani subspecies AG1-IB in field-grown organic lettuce (Lactuca sativa). At each farm, four replicate plots of seven treatments were arranged in a randomized complete block design. Randomization was restricted by grouping treatments to evaluate ASD, and treatments to evaluate vermicompost in starter plugs. ASD experiment treatments were three different ASD carbon sources that are commonly used and widely available to local farmers in Vermont: compost, cover crop residues, and poultry manure fertilizer, and a tarped control. Vermicompost experimental treatments were vermicompost compared to two types of controls: a commercial biocontrol product (RootShield® PLUS+G), and unamended (untarped control). This study demonstrated that the ASD method is achievable in a field setting on Vermont farms. However, neither ASD nor vermicompost produced significant disease suppression or resulted in higher marketable yields than standard growing practices. Given the laborious nature of ASD, it is likely more appropriate in a greenhouse setting with high value crops that could especially benefit from being grown in plastic tarped beds (e.g., tomatoes, strawberries). This study is the first known attempt of field-implemented ASD for soil pathogen control in the northeastern USA.

Rhizoctonia solani AG1-IB, AG1-IC, and AG4-HGII cause bottom rot of field lettuce in Vermont.

Members of Rhizoctonia solani (teleomorph: Thanatephorus cucumeris) species complex cause bottom rot on lettuce (Latuca sativa) and yield losses up to 70% (Subbarao et al. 2017). Severe symptoms include necrosis, stem rot, and/or discoloration especially on the leaf midrib. In Vermont, vegetable farms are small (0.5-30 acres) and grow lettuce concurrently with other vegetable crops in the same field but the AG(s) that causes the disease in Vermont has not been determined. Isolates (n = 157) were collected from 31 fields with reported history of bottom rot between July 10 and October 8, 2019, across Addison, Caledonia, Chittenden, Franklin, Lamoille, and Orleans counties. Isolates were collected from lettuce tissue or potato (Solanum tuberosum), a common rotation crop, or uncropped soil baited using radish (Raphanus sativus). Pieces of tissue (5-10 mm) were cut from the leading margin of lesions, surface disinfested with 0.1% NaClO for 1 min followed by 2 rinses with sterile water, blotted dry, and plated onto acidified 2% water agar (0.085% lactic acid, pH 4.8). After incubation for 48 to 72 h, mycelia resembling Rhizoctonia were examined for morphological characteristics including hyphal branching at ca. 90o angles, a septum near the branching point, multiple nuclei per cell, and lack of both clamp connections and conidia (Sneh et al. 1991). Colonies were white to dark brown, and some produced small sclerotia. Koch's postulates were performed by inoculating nine 8-week-old (9 leaf pairs) romaine lettuce plants (Johnny's Seeds, Winslow, ME, cv. Monte Carlo) per isolate. Isolates were grown on 2% potato dextrose agar for 1 week, from which a 5-mm agar plug was placed on the adaxial leaf surface at the base of a petiole. Plants were enclosed in a plastic bag to maintain high humidity and grown under a 16-hour photoperiod at 24 °C. Disease severity was rated 4 days after inoculation (0: healthy, 1: isolated lesions, 2: lesions across multiple petioles, and 3: systemic disease). Putative AG were determined by Sanger sequencing of the internal transcribed spacer (ITS) region using the ITS1F and ITS4B primer pair (758 bp) (Gardes and Bruns 1993). Contigs were assembled using CAP3 software (Huang and Madan 1999). Taxonomy was assigned to each OTU via the NCBI BLASTn database with criteria as 0.0 E and nucleotide match of at least 97%. Of the 10 isolates sequenced with sufficient coverage (735 to 784 bp alignment length) and definitive resolution (96.7 to 99.9% identity), 5 were putative AG 1-IB (Genbank Accession HG934430.1), 2 AG 1-IC (Genbank Accession AF354058.1), 2 AG 3 (Genbank Accession AF354064.1), and 1 AG 4-HGII (Genbank Accession AF354074.1). Fasta files and metadata are archived at 10.6084/m9.figshare.20301324, 10.6084/m9.figshare.20301375. Putative AG 1-IB was highly virulent on lettuce plants whether it originated from potato (mean 2.6) or lettuce (mean 1.3 to 3). AG 4-HGII and AG 1-IC isolated from lettuce and radish, respectively, were moderately severe (mean 1.4 to 2.2) on lettuce with identical symptoms. The two potato isolates (AG3) were not pathogenic on lettuce. Similarly, higher incidence of AG 1-IB is reported on lettuce in Quebec (Wallon et al. 2021), Ohio (Herr 1993), and Germany (Grosch et al. 2004). Because AG vary in their host range (Sneh et al. 1991), knowing the AG will inform management decisions such as crop rotation and weed control. This is the first report of the causal agent of bottom rot of lettuce or any AG of R. solani in Vermont.

Microplastics in composts, digestates, and food wastes: A review.

Diverting food waste from landfills to composting or anaerobic digestion can reduce greenhouse gas emissions, enable the recovery of energy in usable forms, and create nutrient-rich soil amendments. However, many food waste streams are mixed with plastic packaging, raising concerns that food waste-derived composts and digestates may inadvertently introduce microplastics into agricultural soils. Research on the occurrence of microplastics in food waste-derived soil amendments is in an early phase and the relative importance of this potential pathway of microplastics to agricultural soils needs further clarification. In this paper, we review what is known and what is not known about the abundance of microplastics in composts, digestates, and food wastes and their effects on agricultural soils. Additionally, we highlight future research needs and suggest ways to harmonize microplastic abundance and ecotoxicity studies with the design of related policies. This review is novel in that it focuses on quantitative measures of microplastics in composts, digestates, and food wastes and discusses limitations of existing methods and implications for policy.

Differential Survival of Escherichia coli and Listeria spp. in Northeastern U.S. Soils Amended with Dairy Manure Compost, Poultry Litter Compost, and Heat-Treated Poultry Pellets and Fate in Raw Edible Radish Crops.

ABSTRACT: Composted or heat-treated biological soil amendments of animal origin (BSAAOs) can be added to soils to provide nutrients for fresh produce. These products lower the risk of pathogen contamination of fresh produce compared with the use of untreated BSAAOs; however, meteorological conditions, geographic location, and soil properties can influence the presence of pathogenic bacteria or their indicators (e.g., generic Escherichia coli) and allow potential for produce contamination. Replicated field plots of loamy or sandy soils were tilled and amended with dairy manure compost (DMC), poultry litter compost (PLC), or no compost (NoC) over two field seasons and noncomposted heat-treated poultry pellets (HTPPs) during the second field season. Plots were inoculated with a three-strain cocktail of rifampin-resistant E. coli (rE. coli) at levels of 8.7 log CFU/m2. Direct plating and most-probable-number methods measured the persistence of rE. coli and Listeria spp. in plots through 104 days postinoculation. Greater survival of rE. coli was observed in PLC plots in comparison to DMC plots and NoC plots during year 1 (P < 0.05). Similar trends were observed for year 2, when rE. coli survival was also greater in HTPP-amended plots (P < 0.05). Survival of rE. coli depended on soil type, and water potential and temperature were significant covariables. Listeria spp. were found in NoC plots, but not in plots amended with HTPPs, PLC, or DMC. Radish data demonstrate that PLC treatment promoted the greatest level of rE. coli translocation compared with DMC and NoC treatments (P < 0.05). These results are consistent with findings from studies conducted in other regions of the United States, and they inform northeast produce growers that composted and noncomposted poultry-based BSAAOs support greater survival of rE. coli in field soils. This result has the potential to affect the food safety risk of edible produce grown in BSAAO-amended soils as a result of pathogen contamination.

Organic Amendments Alter Soil Hydrology and Belowground Microbiome of Tomato (Solanum lycopersicum).

Manure-derived organic amendments are a cost-effective tool that provide many potential benefits to plant and soil health including fertility, water retention, and disease suppression. A greenhouse experiment was conducted to evaluate how dairy manure compost (DMC), dairy manure compost-derived vermicompost (VC), and dehydrated poultry manure pellets (PP) impact the tripartite relationship among plant growth, soil physiochemical properties, and microbial community composition. Of tomato plants with manure-derived fertilizers amendments, only VC led to vigorous growth through the duration of the experiment, whereas DMC had mixed impacts on plant growth and PP was detrimental. Organic amendments increased soil porosity and soil water holding capacity, but delayed plant maturation and decreased plant biomass. Composition of bacterial communities were affected more by organic amendment than fungal communities in all microhabitats. Composition of communities outside roots (bulk soil, rhizosphere, rhizoplane) contrasted those within roots (endosphere). Distinct microbial communities were detected for each treatment, with an abundance of Massilia, Chryseolinea, Scedosporium, and Acinetobacter distinguishing the control, vermicompost, dairy manure compost, and dehydrated poultry manure pellet treatments, respectively. This study suggests that plant growth is affected by the application of organic amendments not only because of the soil microbial communities introduced, but also due to a synergistic effect on the physical soil environment. Furthermore, there is a strong interaction between root growth and the spatial heterogeneity of soil and root-associated microbial communities.

Design and implementation of a survey quantifying winter housing and bedding types used on Vermont organic dairy farms.

We conducted a descriptive observational study to quantify the frequency and diversity of winter housing and bedding types used by organic dairy farmers in Vermont. This report describes the survey methods, results, successes, limitations, and lessons learned from administering the survey. Beginning in December 2018, a short questionnaire was administered by web, mail, and telephone to a source population defined as all producers of organic dairy cow milk in Vermont (n = 177) listed in the United States Department of Agriculture Organic Integrity database. Our approach yielded an 82% (n = 145) response from certified organic farms producing cow milk in Vermont at the time of the survey. The 3 most common housing and bedding material combinations used by respondents were tiestall housing with wood (sawdust or shavings) bedding materials (45%), freestall housing with wood bedding materials (14%), and freestall housing with sand bedding (12%). Fifteen percent of respondents reported using more than one type of facility for winter housing of lactating cattle. The median number of lactating cows on farms among respondents was 59.5 (range: 2-400), and the odds of using more than one type of facility to house lactating cows increased positively with the number of lactating cows reported for a herd. Breed distribution was similar across the housing and bedding type categories. An association between frequency of individual cow milk somatic cell count testing and housing type was identified; respondents using freestall sand facilities tested less frequently than respondents using tiestalls with wood bedding. Although the questionnaire length limited the amount of information gathered, the response proportion was exceptional, and overall our survey results provide valuable insight on Vermont organic dairy housing and bedding practices that should inform future extension and outreach efforts for this sector of the dairy industry.

Bacterial Community Dynamics Distinguish Poultry Compost from Dairy Compost and Non-Amended Soils Planted with Spinach.

The aim of this study was to determine whether and how poultry litter compost and dairy manure compost alter the microbial communities within field soils planted with spinach. In three successive years, separate experimental plots on two fields received randomly assigned compost treatments varying in animal origin: dairy manure (DMC), poultry litter (PLC), or neither (NoC). The composition and function of bacterial and fungal communities were characterized by the amplicon sequencing of marker genes and by the ecoenzyme activity, respectively. The temporal autocorrelation within and among years was adjusted by principal response curves (PRC) to analyze the effect of compost on community composition among treatments. Bacteria in the phylum Bacteriodetes, classes Flavobacteriia and Spingobacteriales (Fluviicola, Flavobacteriia, and Pedobacter), were two to four times more abundant in soils amended with PLC than DMC or NoC consistently among fields and years. Fungi in the phylum Ascomycota were relatively abundant, but their composition was field-specific and without treatment differences. The ecoenzyme data verify that the effects of PLC and DMC on soil communities are based on their microbial composition and not a response to the C source or nutrient content of the compost.

Mammary microbiome of lactating organic dairy cows varies by time, tissue site, and infection status.

Infections of the cow udder leading to mastitis and reducing milk quality are a critical challenge facing all dairy farmers. Mastitis may be linked to the ecological disruption of an endogenous mammary microbial community, suggesting an ecosystems approach to management and prevention of this disease. The teat end skin represents a first point of host contact with mastitis pathogens and may offer an opportunity for microbially mediated resistance to infection, yet we know little about the microbial community of teat end skin or its potential interaction with the microbial community of intramammary milk of organic dairy cattle. High-throughput sequencing of marker genes for bacterial and fungal communities was used to characterize the skin and milk microbiome of cows with both a healthy and infected gland (i.e., udder quarter) and to assess the sharing of microbial DNA between these tissue habitat sites. The mammary microbiome varied among cows, through time, and between skin and milk. Microbiomes of milk from healthy and infected quarters reflected a diverse group of microbial DNA sequences, though milk had far fewer operational taxonomic units (OTUs) than skin. Milk microbiomes of infected quarters were generally more variable than healthy quarters and were frequently dominated by a single OTU; teat end skin microbiomes were relatively similar between healthy and infected quarters. Commonly occurring genera that were shared between skin and milk of infected glands included Staphylococcus spp. bacteria and Debaryomyces spp. fungi. Commonly occurring genera that were shared between skin and milk of healthy glands included bacteria SMB53 (Clostridiaceae) and Penicillium spp. fungi. Results support an ecological interpretation of the mammary gland and the notion that mastitis can be described as a dysbiosis, an imbalance of the healthy mammary gland microbiome.

Soil Microarthropods and Soil Health: Intersection of Decomposition and Pest Suppression in Agroecosystems.

Two desirable functions of healthy soil are nutrient cycling and pest suppression. We review relevant literature on the contributions of soil microarthropods to soil health through their intersecting roles in decomposition and nutrient cycling and direct and indirect suppression of plant pests. Microarthropods can impact soil and plant health directly by feeding on pest organisms or serving as alternate prey for larger predatory arthropods. Indirectly, microarthropods mediate the ability of crop plants to resist or tolerate insect pests and diseases by triggering induced resistance and/or contributing to optimal nutritional balance of plants. Soil fauna, including microarthropods, are key regulators of decomposition at local scales but their role at larger scales is unresolved. Future research priorities include incorporating multi-channel omnivory into food web modeling and understanding the vulnerability of soil carbon through global climate change models.

Field and greenhouse evaluations of soil suppressiveness to Heterodera glycines in the Midwest corn-soybean production systems.

Soil suppressive to the soybean cyst nematode (SCN), a major yield-limiting pathogen of soybean, plays an important role in biological control. Field and greenhouse experiments were conducted to evaluate the effects of tillage, crop sequence, and biocide application on SCN suppression in corn-soybean cropping systems in Minnesota. The experiment was a split-plot design with no-tillage and conventional tillage as main plots, and six crop-biocide treatments (CRCS, CSCS, SSSS, SSSS + streptomycin, SSSS + captan, and SSSS + formaldehyde - the four letters represent crops in 2009 to 2012, respectively; C is corn, R is SCN-resistant soybean, and S is SCN-susceptible soybean) as subplots with four replicates. Soil samples were taken from each plot at planting, midseason, and harvest each year for SCN egg counts, and soybean yield was determined. In addition, soil samples collected from each plot at midseason were assayed for suppressiveness to SCN. Tillage had minimal effect on SCN population density and soybean yield. Annual rotation with corn reduced SCN population density, but also reduced soil suppressiveness as SCN egg population density increased in the following SCN-susceptible soybean compared with soybean monoculture. Rotation with SCN-resistant soybean and corn was the most effective in reducing SCN population density. The bactericide streptomycin did not affect SCN populations but the fungicide captan increased SCN population density. The biocide formaldehyde was the most effective in reducing the level of suppressiveness to SCN. The greenhouse study confirmed that the soil was suppressive to SCN, but failed to detect effects of tillage, crop sequence, and biocide field treatments. This study demonstrated that the soil in the fields was suppressive to the SCN, and biological agents, especially fungal antagonists, were involved in nematode suppression.

A Plate Competition Assay As a Quick Preliminary Assessment of Disease Suppression.

The goal was to develop and optimize a simple, affordable, and effective bioassay to detect disease suppressive ability of a specific compost against soilborne fungus Rhizoctonia solani. R. solani is a pathogen of a wide range of plant hosts worldwide. The fungus survives in soils as a saprophyte and grows rapidly on simple water agar media. The plate assay is a rapid method to compare composts for their ability to slow the growth of R. solani. The assay also correlates well with suppression of other soilborne fungal pathogens that survive as saprophytes in soils such as Alternaria early blights, Fusarium wilt, Phytophthora root rot, and Pythium root rot.

Relationship between food waste, diet quality, and environmental sustainability.

Improving diet quality while simultaneously reducing environmental impact is a critical focus globally. Metrics linking diet quality and sustainability have typically focused on a limited suite of indicators, and have not included food waste. To address this important research gap, we examine the relationship between food waste, diet quality, nutrient waste, and multiple measures of sustainability: use of cropland, irrigation water, pesticides, and fertilizers. Data on food intake, food waste, and application rates of agricultural amendments were collected from diverse US government sources. Diet quality was assessed using the Healthy Eating Index-2015. A biophysical simulation model was used to estimate the amount of cropland associated with wasted food. This analysis finds that US consumers wasted 422g of food per person daily, with 30 million acres of cropland used to produce this food every year. This accounts for 30% of daily calories available for consumption, one-quarter of daily food (by weight) available for consumption, and 7% of annual cropland acreage. Higher quality diets were associated with greater amounts of food waste and greater amounts of wasted irrigation water and pesticides, but less cropland waste. This is largely due to fruits and vegetables, which are health-promoting and require small amounts of cropland, but require substantial amounts of agricultural inputs. These results suggest that simultaneous efforts to improve diet quality and reduce food waste are necessary. Increasing consumers' knowledge about how to prepare and store fruits and vegetables will be one of the practical solutions to reducing food waste.

Nitrogen and phosphorus addition impact soil N2O emission in a secondary tropical forest of South China.

Nutrient availability greatly regulates ecosystem processes and functions of tropical forests. However, few studies have explored impacts of N addition (aN), P addition (aP) and N × P interaction on tropical forests N2O fluxes. We established an N and P addition experiment in a tropical forest to test whether: (1) N addition would increase N2O emission and nitrification, and (2) P addition would increase N2O emission and N transformations. Nitrogen and P addition had no effect on N mineralization and nitrification. Soil microbial biomass was increased following P addition in wet seasons. aN increased 39% N2O emission as compared to control (43.3 µgN2O-N m(-2)h(-1)). aP did not increase N2O emission. Overall, N2O emission was 60% greater for aNP relative to the control, but significant difference was observed only in wet seasons, when N2O emission was 78% greater for aNP relative to the control. Our results suggested that increasing N deposition will enhance soil N2O emission, and there would be N × P interaction on N2O emission in wet seasons. Given elevated N deposition in future, P addition in this tropical soil will stimulate soil microbial activities in wet seasons, which will further enhance soil N2O emission.

Changes in bacterial and fungal communities across compost recipes, preparation methods, and composting times.

Compost production is a critical component of organic waste handling, and compost applications to soil are increasingly important to crop production. However, we know surprisingly little about the microbial communities involved in the composting process and the factors shaping compost microbial dynamics. Here, we used high-throughput sequencing approaches to assess the diversity and composition of both bacterial and fungal communities in compost produced at a commercial-scale. Bacterial and fungal communities responded to both compost recipe and composting method. Specifically, bacterial communities in manure and hay recipes contained greater relative abundances of Firmicutes than hardwood recipes with hay recipes containing relatively more Actinobacteria and Gemmatimonadetes. In contrast, hardwood recipes contained a large relative abundance of Acidobacteria and Chloroflexi. Fungal communities of compost from a mixture of dairy manure and silage-based bedding were distinguished by a greater relative abundance of Pezizomycetes and Microascales. Hay recipes uniquely contained abundant Epicoccum, Thermomyces, Eurotium, Arthrobotrys, and Myriococcum. Hardwood recipes contained relatively abundant Sordariomycetes. Holding recipe constant, there were significantly different bacterial and fungal communities when the composting process was managed by windrow, aerated static pile, or vermicompost. Temporal dynamics of the composting process followed known patterns of degradative succession in herbivore manure. The initial community was dominated by Phycomycetes, followed by Ascomycota and finally Basidiomycota. Zygomycota were associated more with manure-silage and hay than hardwood composts. Most commercial composters focus on the thermophilic phase as an economic means to insure sanitation of compost from pathogens. However, the community succeeding the thermophilic phase begs further investigation to determine how the microbial dynamics observed here can be best managed to generate compost with the desired properties.

Earthworms facilitate carbon sequestration through unequal amplification of carbon stabilization compared with mineralization.

A recent review concluded that earthworm presence increases CO2 emissions by 33% but does not affect soil organic carbon stocks. However, the findings are controversial and raise new questions. Here we hypothesize that neither an increase in CO2 emission nor in stabilized carbon would entirely reflect the earthworms' contribution to net carbon sequestration. We show how two widespread earthworm invaders affect net carbon sequestration through impacts on the balance of carbon mineralization and carbon stabilization. Earthworms accelerate carbon activation and induce unequal amplification of carbon stabilization compared with carbon mineralization, which generates an earthworm-mediated 'carbon trap'. We introduce the new concept of sequestration quotient to quantify the unequal processes. The patterns of CO2 emission and net carbon sequestration are predictable by comparing sequestration quotient values between treatments with and without earthworms. This study clarifies an ecological mechanism by which earthworms may regulate the terrestrial carbon sink.

Nematode Genera in Forest Soil Respond Differentially to Elevated CO2.

Previous reports suggest that fungivorous nematodes are the only trophic group in forest soils affected by elevated CO2. However, there can be ambiguity within trophic groups, and we examined data at a genus level to determine whether the conclusion remains similar. Nematodes were extracted from roots and soil of loblolly pine (Pinus taeda) and sweet gum (Liquidambar styraciflua) forests fumigated with either ambient air or CO2-enriched air. Root length and nematode biomass were estimated using video image analysis. Most common genera included Acrobeloides, Aphelenchoides, Cephalobus, Ditylenchus, Ecphyadorphora, Filenchus, Plectus, Prismatolaimus, and Tylencholaimus. Maturity Index values and diversity increased with elevated CO2 in loblolly pine but decreased with elevated CO2 in sweet gum forests. Elevated CO2 treatment affected the occurrence of more nematode genera in sweet gum than loblolly pine forests. Numbers were similar but size of Xiphinema decreased in elevated CO2. Abundance, but not biomass, of Aphelenchoides was reduced by elevated CO2. Treatment effects were apparent at the genus levels that were masked at the trophic level. For example, bacterivores were unaffected by elevated CO2, but abundance of Cephalobus was affected by CO2 treatment in both forests.

Non-target effects of herbicides on soil nematode assemblages.

BACKGROUND: Herbicides are used extensively to control weeds. However, little is known about the non-target effects of herbicides on soil nematode assemblages. The objective of this study was to determine whether herbicides affect the abundance of nematodes in specific trophic groups. Meta-analysis was performed, and the calculated effect size, lr, quantified the impact of herbicides on the abundance of total nematodes and five trophic groups (bacterivores, fungivores, plant parasites, omnivores and predators). RESULTS: Measurements of lr indicated that herbicides decreased abundance of both fungivores and predators; however, abundance of bacterivores, plant parasites and omnivores increased. Overall, total nematode abundance tended to increase in response to herbicide application. CONCLUSION: The decrease in predator abundance suggests that herbicide application disturbs soil food webs. The increase in bacterivore and decrease in fungivore abundance suggest that bacterivores are more tolerant and both fungivores and predators more sensitive to herbicide applications. Herbicides also have non-target effects on omnivores, which may be due to the increased amount of food resources for omnivores after weed control. Additionally, the use of herbicides may result in a risk of an increase in plant-parasitic nematode abundance.

Roads in northern hardwood forests affect adjacent plant communities and soil chemistry in proportion to the maintained roadside area.

The spatial extent of the transported materials from three road types was studied in forest soil and vegetative communities in Vermont. Hypotheses were two-fold: 1) soil chemical concentrations above background environment would reflect traffic volume and road type (highway>2-lane paved>gravel), and 2) plant communities close to the road and near roads with greater traffic will be disturbance-tolerant and adept at colonization. Soil samples were gathered from 12 randomly identified transects for each of three road types classified as "highway," "two-lane paved," and "gravel." Using GIS mapping, transects were constructed perpendicular to the road, and samples were gathered at the shoulder, ditch, backslope, 10 m from the edge of the forest, and 50 m from road center. Sample locations were analyzed for a suite of soil elements and parameters, as well as percent area coverage by plant species. The main effects from roads depended on the construction modifications required for a roadway (i.e., vegetation clearing and topography modification). The cleared area defined the type of plant community and the distance that road pollutants travel. Secondarily, road presence affected soil chemistry. Metal concentrations (e.g., Pb, Cd, Cu, and Zn) correlated positively with road type. Proximity to all road types made the soils more alkaline (pH 7.7) relative to the acidic soil of the adjacent native forest (pH 5.6). Roadside microtopography had marked effects on the composition of plant communities based on the direction of water flow. Ditch areas supported wetland plant species, greater soil moisture and sulfur content, while plant communities closer to the road were characteristic of drier upland zones. The area beyond the edge of the forest did not appear to be affected chemically or physically by any of the road types, possibly due to the dense vegetation that typically develops outside of the managed right-of-way.

Responses of soil microbial and nematode communities to aluminum toxicity in vegetated oil-shale-waste lands.

Both soil nematodes and microorganisms have been shown to be sensitive bioindicators of soil recovery in metal-contaminated habitats; however, the underlying processes are poorly understood. We investigated the relationship among soil microbial community composition, nematode community structure and soil aluminum (Al) content in different vegetated aluminum-rich ecosystems. Our results demonstrated that there were greater soil bacterial, fungal and arbuscular mycorrhizal fungal biomass in Syzygium cumini plantation, greater abundance of soil nematodes in Acacia auriculiformis plantation, and greater abundance of soil predatory and herbivorous nematodes in Schima wallichii plantation. The concentration of water-soluble Al was normally greater in vegetated than non-vegetated soil. The residual Al and total Al concentrations showed a significant decrease after planting S. cumini plantation onto the shale dump. Acid extractable, reducible and oxidisable Al concentrations were greater in S. wallichii plantation. Stepwise linear regression analysis suggests the concentrations of water-soluble Al and total Al content explain the most variance associated with nematode assembly; whereas, the abundance of early-successional nematode taxa was explained mostly by soil moisture, soil organic C and total N rather than the concentrations of different forms of Al. In contrast, no significant main effects of either Al or soil physico-chemical characteristics on soil microbial biomass were observed. Our study suggests that vegetation was the primary driver on soil nematodes and microorganisms and it also could regulate the sensitivity of bio-indicator role mainly through the alteration of soil Al and physico-chemical characteristics, and S. cumini is effective for amending the Al contaminated soils.

Ecology of plant and free-living nematodes in natural and agricultural soil.

Nematodes are aquatic organisms that depend on thin water films to live and move within existing pathways of soil pores of 25-100 mum diameter. Soil nematodes can be a tool for testing ecological hypotheses and understanding biological mechanisms in soil because of their central role in the soil food web and linkage to ecological processes. Ecological succession is one of the most tested community ecology concepts, and a variety of nematode community indices have been proposed for purposes of environmental monitoring. In contrast, theories of biogeography, colonization, optimal foraging, and niche partitioning by nematodes are poorly understood. Ecological hypotheses related to strategies of coexistence of nematode species sharing the same resource have potential uses for more effective biological control and use of organic amendments to foster disease suppression. Essential research is needed on nematodes in natural and agricultural soils to synchronize nutrient release and availability relative to plant needs, to test ecological hypotheses, to apply optimal foraging and niche partitioning strategies for more effective biological control, to blend organic amendments to foster disease suppression, to monitor environmental and restoration status, and to develop better predictive models for land-use decisions.