Diana W. Freckman

Assessing nematode communities in agroecosystems of varying human intervention

The effect of disturbance on soil nematode communities was studied in eight treatments varying in intensity of human intervention at the Kellogg Biological Station Long Term Ecological Research site, Hickory Corners, MI. The agricultural treatments ranged from those manipulated with high chemical inputs and heavily impacted by human management to successional treatments that had no chemicals and little human impact. A canonical discriminant analysis of the nematode data separated the treatments into four systems: high chemical input (the conventional tillage and no tillage treatments, both corn/soybean rotations); organic (the low input and zero input treatments, both wheat/corn/soybean rotations); perennial (poplar and alfalfa treatments); successional (abandoned after tillage and never tilled treatments). Nematode abundance was highest in the high input and organic systems and lowest in the poplar treatment. Overall, bacterial feeding, plant parasitic and fungal feeding nematodes dominated the treatments. Species diversity was greatest in the successional treatments. The bacterial feeding trophic group and the modified Shannon index described differences at both the treatment and system levels, while the Shannon index demonstrated diversity at the system and annual and perennial crop level of analysis. Measures that detected differences (P < 0.05) consistently across all treatments, systems, and annual vs. perennial crops were total abundance, the predator trophic group, the maturity index (MI) and the plant parasite index. The minimum analyses needed to detect disturbance reliably were a multivariate analysis and the MI. However, understanding and predicting the impact of the disturbance on the food web and ecosystem functioning would be increased with results from diversity indices and nematode functional groups.

Bacterivorous nematodes and organic-matter decomposition

Abstract Bacterial-feeding nematodes affect organic-matter decomposition in several ways. They include: feeding on microbes and regulating the rate at which organic compounds are degraded into inorganic ions; dispersing microbes throughout the soil and water; feeding on saprophytic and plant pathogenic bacteria and influencing the composition of the microbial community; serving as prey and a source of nutrients for fauna and microflora such as soil nematophagous fungi; affecting the distribution and function of plant symbionts. The influence of bacterial-feeding nematodes in nutrient cycling and their use as ecological indicators is discussed.

Diurnal migration and responses to simulated rainfall in desert soil microarthropods and nematodes

Abstract Diurnal patterns of microarthropod abundance in surface leaf litter were related to its moisture content. Leaf litter moisture was nearly 7% by weight at 0800h but fell to less than 1% by mid-day. Oribatid and tydeid mites moved into litter in the early morning and back into the soil before mid-day. There were no significant differences in numbers of nematodes in litter or soil and 78–98% of the nematodes were anhydrobiotic (coiled) in soil and litter at all times sampled. Following simulated rainfall there were fewer microarthropods in litter at mid-day in the absence of marked decreases in soil and litter moisture content. During drying, there were gradual reductions in numbers and species diversity of litter microarthropods. Nematode numbers did not change as litter dried. Anhydrobiotic nematodes in the soil increased from 14% on day 1 to 85% on day 4. Between 24 and 36 h after simulated rainfall, the proportion of anhydrobiotic litter nematodes increased from 35 to 80%,. Within 1 h after simulated rainfall, there were marked increases in numbers and diversity of microarthropods in surface litter. No collembolans were extracted from dry litter controls but the wet litter was dominated by isotomid, sminthurid and onychiurid collembolans. There were increases in numbers and diversity of oribatid, tydeid and gamasid mites in the wet surface litter within l h after wetting compared to controls.

Distribution, Community Structure, and Microhabitats of Soil Invertebrates along an Elevational Gradient in Taylor Valley, Antarctica

Soils in the Antarctic Dry Valleys have been significantly influenced by soil formation factors such as parent material, climate, and topography. Factors common in more temperate zones, including chemical weathering and leaching of minerals, occur to a much lesser extent in these cold arid soils, leading to an accumulation of salts and bases, which will likely affect the distribution of soil biota. Since the intensity of these factors may vary with topography, this study examined the soil properties and soil invertebrate communities along an elevational gradient in Taylor Valley, Antarctica. We sampled from two spatial scales (1 x 1 m and 10 x 10 m) at three sites (83, 121, and 188 m a.s.1) on the south side of Lake Hoare in Taylor Valley, and examined soil moisture, nitrogen, carbon, pH, and electrical conductivity (which provides an estimation of soil salinity), as well as the distribution and community structure of soil invertebrates. We found significant differences in soil properties with elevation, along with associated differences in soil communities. Biodiversity was greatest at the lowest elevation, closest to the shore of Lake Hoare, where soil moisture, carbon, and nitrogen were highest, and salinity was lowest. Scottnema lindsayae dominated the nematode communities found at all sites. Electrical conductivity was higher and carbon and nitrogen contents were lower at the upper elevations. The distribution of both Eudorylaimus and Plectus appeared to be influenced by soil moisture; electrical conductivity affected the mortality of all three nematode genera found. Soil properties did differ with sampling scale, suggesting that changes in microhabitats not detected at sampling intervals of a meter or more may be more reliably detected by sampling at a smaller scale.

Plant-feeding nematodes in deep-rooting desert ecosystems.

In the Chihuahuan Desert of southern New Mexico, mesquite (Prosopis glandulosa) occurs in a variety of landscape positions where water may accumulate to varying depths. The structure of Chihuahuan Desert landscapes has changed dramatically in the past 100 yr with mesquite expanding from its original distribution (playa, arroyo) into grassland and dune ecosystems. Our objective was to examine spatial and seasonal distribution of plant—feeding nematodes and the potential importance of herbivory in four mesquite communities (playa, arroyo, dunes, grassland) hypothesized to differ in rooting depth and distribution. These sites were compared to a more shallow—rooted creosote bush (Larrea tridentata) community. Intact soil cores were recovered to depths of 13 m beneath the canopies of mesquite or creosote bush using a continuous sampling tube drilling system in the winter, spring, and fall on the Jornada LTER site, Las Cruces, New Mexico. Nematode density and root mass decreased with depth. Nematodes were found to the maximum depth of recovered roots only at the playa (11—12 m) and occurred as deep as 5—6 m at the arroyo, 2—3 m at the dunes, and 1—2 m at the grassland and creosote bush sites. At the playa, 75% of the nematodes and 90% of the roots were below 0.5 m, whereas, in the creosote bush system 79% of the roots and 99.7% of the nematodes were recovered in the upper 0.5 m of soil. Nematode diversity (seven genera) and endoparasite densities were greatest at the playa. Only ectoparasites (three genera) were found at the creosote bush system. Total nematodes across all sites were highly correlated with total soil N, NH4+, and PO4—3, but not NO3—. Nematodes were not positively correlated with soil moisture. Potential herbivory was best described by nematode numbers per gram root or an herbivory index (based on nematode density weighted by an impact factor for each genus), rather than the ratio of endoparasite:ectoparasite numbers. The herbivory index decreased in the order playa > creosote bush > dune > grassland > arroyo. The movement of mesquite into new habitats (grassland, dunes) is associated with shallow rooting, and relative to the playa, a greater allocation of roots to the upper 0.5 m soil layer, a decrease in nematode herbivory and diversity, and, a failure of the nematodes to be distributed throughout the entire rooting zone. The densities of ecto— and endoparasites varied with site, thus, plant—feeding nematodes should not be treated as a single guild since they have different effects on roots. Our study shows that in some ecosystems the majority of herbivory may occur at soil depths rarely studied. Although technically difficult to examine in ecosystems where deep—rooted plants occur, the potential importance of deep soil biota and their effects on plant growth and nutrient cycling should be considered.

Effect of irrigation on nematode population dynamics and activity in desert soils

SummaryThe nematode community in litter and soil was examined for a year in the Chihuahuan desert, before and after supplemental rainfall application. Proportions of nematode-active or anhydrobiotic forms and population densities were determined for 3 treatments: control (natural rainfall), a single, large (25-mm) monthly irrigation pulse, and 4 smaller (6-mm) irrigations spaced at weekly intervals. In litter the greatest nematode abundance was in the 6 mm week−1 treatment (48 nematodes 20 g−1 litter). Bacteriovores and fungivores accounted for approximately 95% of the numbers and biomass in all treatments. In soil, water amendments had no significant effect (P < 0.05) on annual mean densities of total nematodes, fungivores, bacterivores, or omnivore predators. Phytophage densities were greater on both irrigation treatments, with highest densities (9268 m−2) in the 6 mm week−1 soils, which was 5.9% of the total soil nematode density. Total densities of individual trophic groups were not significantly different before or after rainfall. Soil nematode densities fluctuated independently with trophic group, month, and season. Bacterial feeders and omnivore predators were the largest contributor to total soil nematode density and biomass. Prior to irrigation, there were no differences in the percentage of anhydrobiotes on the three treatments. Anhydrobiotes decreased after irrigation in all treatments, and were significantly lower in soils of the larger, monthly irrigation. Nematodes were inactive (anhydrobiotic) and decoupled from decomposition processes when soil water matric potentials reached −0.4 MPa.

Microcosms and soil Ecology: Critical Linkages between Fields Studies and Modelling Food Webs

Soil ecology has a rich tradition of using microcosms as research tools due to the cryptic nature of soils and the ease by which soils and soil organisms can be manipulated in the laboratory. Soil ecologists have accepted microcosms as research tools for two reasons. First, soil organisms are small and are not conducive to the in situ manipulative studies used in other habitats (Connell 1961a, b, Pimm and Kitching 1987, Wooten 1993). However, owing to the small sizes of many soil organisms (microbes to small arthropods of <1 mm), the scale of their habitat (pore spaces to m2), and their generation times (minutes to weeks), whole communities can be brought into the laboratory for study. Second, many soil ecologists have adopted a systems approach whereby, with the aid of models, the interactions among organisms and key ecosystem processes are studied. Microcosm studies have provided models with the details of how soil organisms and processes interact under different environmental conditions (Elliott et al. 1986). We have adopted an approach whereby the tactical and strategic models identified by Holling (1966, 1968) complement each other (May 1973). In the broadest sense, the tactical models represent the system-specific process models used by ecosystem ecologists, while the strategic models represent the population dynamics models more familiar to community ecologists. Our objectives are to demonstrate how microcosms are used in a modelling approach that incorporates information used in ecosystem models into the models developed by community ecologists. The models have proved useful in studying the role of soil biota in nutrient cycling in natural and agricultural ecosystems and the more general questions of how communities are assembled and the factors that affect their stability. We begin by providing a description of Lotka-Volterra-based community models and the process-oriented ecosystem models and a review of how microcosms have aided in their development. Next, we use the models and microcosm studies to address three current topics in ecology. The community models are used to study the factors that limit the length of food chains within communities, the ecosystem models are used to investigate the degree to which communities are compartmentalized into assemblages of species, and both models are used to estimate the interaction strengths among species.

Life cycle of the microbivorous Antarctic Dry Valley nematode Scottnema lindsayae (Timm 1971)

The life cycle of the Antarctic Dry Valley soil nematode, Scottnema lindsayae (Timm 1971) was studied in laboratory culture at two temperatures, 10°C and 15°C. Soil yeast and bacteria isolated with the nematodes were used as the food source. The species reproduced sexually. The higher temperature had a negative effect on the life cycle. The number of eggs per female and the number of juveniles developing per female were greater at 10°C than at 15°C. Juveniles developed faster at 10°C and four juvenile stages were observed outside of the egg at both temperatures. The unusually long life cycle (218 d at 10°C) suggests that more than one austral summer may be required for successful completion. An increase in Dry Valley soil temperatures associated with potential global environmental change may have detrimental effects on soil nematodes.

Soil biota and soil properties in the surface rooting zone of mesquite (Prosopis glandulosa) in historical and recently desertified Chihuahuan Desert habitats

SummaryThe woody legume, mesquite (Prosopis glandulosa) has expanded from its historical habitats (playas and arroyos) to recently occupied grassland and dune habitats during the desertification of perennial grasslands in the Chihuahuan Desert. We studied historical and recently occupied sites, having hypothesized that the trophic structure and population density of soil microarthropods and nematodes associated with the surface root system of mesquite would differ in sites representing historical and recent habitats, and that the N mineralization potential would be lower in the recent habitats. Our results showed that net N mineralization potential did not differ significantly among the sites, even though soil nutrient concentrations and texture varied widely. Concentrations of organic C, N, and P were lowest in the recent dune habitat and highest at the playa. Very low concentrations of P in the dune and grassland soils implicated P as a limiting factor in these systems. The bacterial-feeding and omnivore-predator functional groups made up the largest fraction of the nematode community at most of the sites. The high density of plant-feeding nematodes at the playa indicated that herbivory is potentially most important at this site. Total microarthropod densities did not vary significantly among habitats, with Collembola densities highest in the mesquite dunes. Grazers were the dominant microarthropod functional group. While both C and N pool sizes were higher in the historical habitats, a higher substrate lability in the recent habitats appeared to support biota populations and N mineralization rates equivalent to those in the playa and arroyo. Differences in soil properties and biota among historical and recent mesquite habitats may be important for understanding the changes that have occurred in Chihuahuan Desert ecosystems during desertification.

Nematode Density and Biomass in an Annual Grassland Ecosystem

The nematode community structure was examined in grazed and ungrazed annual-plant rangeland on the US/IBP Grassland Biome San Joaquin Site located in the foothill-grasslands of central California. Nematode numbers and biomass were estimated from early growth to mature stages of the annual-plant vegetation. Nematode density was greater on the grazed area, predominately forbs, than on the ungrazed, mainly grass, area. A lower than normal precipitation appeared to be a limiting factor of nematode population density. The nematode trophic structure differed between the two sites, fimgivores and microbivores predominating on the grazed and ungrazed sites, respectively. Indications are that the critical factors controlling nematode density and community structure on this annual grassland are not grazing but soil moisture and temperature. Yuen (1966), Schmitt and Norton (1972), Schmitt (1973), Stanton (1974), Yeates (1974), and Smolik and Rogers (1976) have studied the function of nematodes in belowground grassland ecosystems; however, knowledge of nematodes from the California annual grassland ecosystem is almost nonexistent. Recent investigations in the mixed-grass prairie of South Dakota (Smolik 1974) provided evidence that nematodes constituted a significant pathway of energy flow, and that the application of nematicides significantly reduced nematode populations resulting in large increases in herbage production. Also, phytophagic nematode biomass was significantly higher in an ungrazed pasture than in a grazed pasture. A preliminary study in the shrub-steppe area of south-central Washington (Smolik and Rogers 1976) revealed large numbers of soil-dwelling nematodes but no consistent differences in nematode density or biomass between grazed, ungrazed, and burned areas. In 1972, the annual grassland site of the United States International Biological Program (US/IBP), Grassland Biome, was established at the U.S. Forest Service’s San Joaquin Experimental Range, 40 km northeast of Fresno, California. For a 3-year period, data were collected on abiotic, producer, consumer, and decomposer components of the ecosystem (Duncan 1975). A preliminary soil sampling program was undertaken in 1974 to estimate the importance of nematodes at the San. Joaquin site. Soil samples were taken on two dates (March 13 and May 1, 1974) in the grazed and ungrazed