Deborah A. Neher

DISTINGUISHING SENSITIVITY OF FREE‐LIVING SOIL NEMATODE GENERA TO PHYSICAL AND CHEMICAL DISTURBANCES

During the past 50 yr, ecological and agricultural scientists have pursued an integrated definition and metric of soil quality. In the past 20 yr, considerable attention has been paid to nematodes, demonstrating that these ubiquitous members of the soil community reflect change in ecological structure and function of soils in ways more predictable and efficient than for other soil flora or fauna. With the help of multivariate analysis, we studied the application of free-living nematode communities as model indicators of physical and chemical disturbance of agricultural soil. We used canonical correspondence analysis (CCA) and partial CCA to segregate effects of tillage and chemical/nutrient treatments. With the results of CCA, we assigned relative direct and indirect tillage sensitivity and chemical/nutrient sensitivity ratings to soil genera found in two test data sets, each containing three matrices: (1) sites by species or genera, (2) sites by soil properties, and (3) sites by management practices. Of ...

Nematode communities and microbial biomass in soils with annual and perennial crops

Abstract Soils from medium- (alfalfa) or long-term (tall-fescue pasture) perennial cropping systems, which represent systems with relatively little disturbance, were collected across the three geographic regions of North Carolina and evaluated as potential reference sites for monitoring the ecological condition of soils for annual crops (e.g. soybeans). Maturity (based on life history characteristics) and trophic diversity indices of soil nematode communities were quantified for all soils to determine the successional status and relative abundance of trophic groups, respectively, in agricultural soils. The distribution and range of maturity index values for plant-feeding nematodes were greater for perennial than annual crops. The relative distribution of nematode trophic groups (bacterial-feeders, fungal-feeders, plant-feeders, omnivores, and predators) was similar among annual and perennial cropping systems. The ratios of fungal-feeding to bacterial-feeding nematodes were greater in perennial than annual cropping systems indicating that the decomposition pathway was dominated more by fungi and fungal-feeding nematodes in alfalfa and pastures than soybean cropping systems. Ratios of total fungal- to total bacterial-biomass were greater for pasture and soybean than for alfalfa cropping systems and, therefore, did not clearly differentiate perennial and annual crops. Based on the maturity index for plant-feeding nematodes, and the ratio of fungal-feeding to bacterial-feeding nematodes, fields with alfalfa or in pasture may be suitable for use as reference sites in monitoring the ecological condition of soil associated with annual crops for the Agroecosystem component of the Environmental Monitoring and Assessment Program (EMAP ), a national monitoring program initiated and sponsored by the U.S. Environmental Protection Agency.

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.

Soil community composition and ecosystem processes: Comparing agricultural ecosystems with natural ecosystems

Soil organisms play principal roles in several ecosystem functions, i.e. promoting plant productivity, enhancing water relations, regulating nutrient mineralisation, permitting decomposition, and acting as an environmental buffer. Agricultural soils would more closely resemble soils of natural ecosystems if management practices would reduce or eliminate cultivation, heavy machinery, and general biocides; incorporate perennial crops and organic material; and synchronise nutrient release and water availability with plant demand. In order to achieve these goals, research must be completed to develop methods for successful application of organic materials and associated micro-organisms, synchronisation of management practices with crop and soil biota phenology, and improve our knowledge of the mechanisms linking species to ecosystem processes.

Soil microbial activity in a Liquidambar plantation unresponsive to CO2-driven increases in primary production

The indirect responses of soil microbiota to changes in plant physiology effected by elevated atmospheric carbon dioxide have the potential to alter nutrient availability and soil carbon storage. We measured fine root density, microbial biomass nitrogen, rates of nitrogen mineralization and nitrification, substrate utilization by soil bacteria and extracellular enzyme activities (EEA) associated with bulk soil and fine root rhizoplanes within a 3-year period at the Oak Ridge National Laboratory (ORNL) Free Air Carbon Enrichment (FACE) experiment, situated in a Liquidambar styraciflua plantation. Rhizoplane EEA was similar to that of bulk soil. Prior studies have reported a 21% increase in net primary production (NPP) in the enrichment plots and evidence that additional carbon is reaching the soil system, however we observed no response in any of the variables we measured. These results, which contrast with those from other temperate forest FACE sites, suggest that soil characteristics can influence the magnitude and timing of belowground responses.

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.

Soil invertebrate and microbial communities, and decomposition as indicators of polycyclic aromatic hydrocarbon contamination.

Soil organisms are useful for quantification of ecological impact of chemical contamination of soils. This study examined the effects of creosote (complex mixture of polycyclic aromatic hydrocarbons) on composition and abundance of soil invertebrates (nematodes, collembolans and mites) and decomposition processes. Thirty intact soil cores and adjacent litter samples were collected each of three times during the 1998 growing season from soil contaminated with creosote for 50 years. Each core was divided evenly into two subsamples. Abundance of nematodes (by family), Collembola (by family), mites (by Oribatida and others), total bacterial biomass, and active fungal biomass were quantified in the first subsample; soil properties including polycyclic aromatic hydrocarbon (PAH) concentration, organic carbon, pH, electrical conductivity (EC), bulk density, soil moisture and soil texture were measured in the second subsample. Creosote affected soil food webs and decomposition more by altering habitat of microinvertebrates and their prey, fungi and bacteria, than by direct toxicity. We hypothesize that nematodes were affected directly by PAH, more than collembolans or mites, because of their intimate contact with contaminated soil particles and permeable cuticles. Collembola and mites explained decomposition of 100% cellulose and mixed cellulose/lignin substrates better than nematodes because of their co-location in the litter layer. This is the first study to examine effects of PAH contamination on soil food webs and ecological processes.

Biogeophysical factors influencing soil respiration and mineral nitrogen content in an old field soil

Microbivorous grazers are thought to enhance nutrient mineralization. The predicted effect of microbivory on nutrient cycling depends on the pore habitat model used. We evaluated CO2 evolution and mineral N content of an old field soil to test two alternative habitat hypotheses. The exclusion hypothesis predicts that nematodes are separated from their microbial food resources in water-filled pores when soils dry, resulting in slower rates of biogeochemical transformations. The enclosure hypothesis predicts that nematode densities increase relative to their forage in smaller, isolated water volumes when soils dry, accelerating rates of biogeochemical transformations. We investigated the effect of soil moisture on the relationship between microbial biomass, microbivorous and predaceous nematodes, soil respiration and mineral N concentrations in an old field five times during the course of a year. We could evaluate the validity of the two habitat hypotheses for the entire field only in August 1997 because that was the only sampling date when maximum water-filled pore diameters were smaller than microbivorous nematode body diameters in all sampled field locations. The mean microbivorous and predaceous nematode abundances for the field in August were greater than 6300 kg 21 and 80,000 kg 21 , respectively. Accordingly, the exclusion hypothesis was rejected. Predaceous nematode abundance was markedly higher in August than at any other sampling date. The high abundance of predators present suggests that detrital resources were not limiting productivity and that predators and microbivores were in enclosures, allowing predators to efficiently access their prey. Spatial maps, in agreement with linear correlation analyses, suggest that under our driest sampling conditions, soil respiration and mineral N content were controlled by microbivory and predation. q 2001 Elsevier Science Ltd. All rights reserved.

Effects of disturbance and ecosystem on decomposition

Decomposition of organic matter integrates collective activities of organisms within the soil food web. We compared decomposition of museum board (predominantly cellulose) and balsa wood substrates in 18 sites chosen to represent a completely nested design with two disturbance levels nested within three ecosystems (agriculture, wetland, and forest) and ecosystems nested within three land resource regions (LRR) in North Carolina. Percentage mass remaining and daily rate of mass loss of museum board and balsa wood substrates were analyzed using repeated measures analysis of covariance with soil physical and chemical properties as covariates. At the end of the two-year monitoring period, the percentage of museum board and balsa wood substrates remaining was least in agricultural and wetland and greatest in forest ecosystems. Soil pH influenced the percentage of substrate remaining based on days of incubation, and its effects were greater than electrical conductivity, percentage soil organic matter, and total available soil nitrogen (N). Percentage of substrate remaining (museum board or balsa wood) was correlated negatively with pH for all sites, suggesting that pH should be included as a covariate if measures of decomposition are used as environmental indicators. Overall, rate of decomposition of museum board substrates distinguished between relative levels of disturbance in agricultural and wetland but not forest ecosystems. The rate of balsa wood decomposition distinguished between relative levels of disturbance in wetland but not forest or agricultural sites. Forest soils had consistently lower total N and electrical conductivity, and sometimes lower pH, associated with slower decomposition than disturbed wetlands or agricultural lands. We conclude that for short-term monitoring, measures of decomposition of predominantly cellulose substrates can be used to distinguish between relative levels of disturbance in agricultural and wetland but not forest systems. Differences in decomposition may signal either a change in decomposer community or condition of biotic and abiotic resources at a site.

COMPARISON OF NEMATODE COMMUNITIES IN AGRICULTURAL SOILS OF NORTH CAROLINA AND NEBRASKA

Samples of agricultural soils were collected across North Carolina in 1992 and Nebraska in 1993 to determine which indices of nematode communities could be applied to distinguish ecological pattern at regional geographic scales. Sampling density was proportional to the area of agriculture in each region of each state. Maturity indices (based on life-history characteristics) were calculated to determine the successional status of nematode communities, and diversity indices were calculated to estimate relative abundance of nematode trophic groups. Population densities of nematode families were also compared between states and among regions within states. The range of maturity-index values for free-living and plant-parasitic nematodes was greater for soils in North Carolina than in Nebraska. The relative distribution of nematode trophic groups (bacterial feeders, fungal feeders, plant parasites, omnivores, and predators) was similar in the two states. Significant differences in maturity-index values for free...