Communities In Agroecosystems Research Articles

Spatial processes structuring riparian plant communities in agroecosystems: implications for restoration

The disruption of hydrological connectivity by human activities such as flood regulation or land-use changes strongly impacts riparian plant communities. However, landscape-scale processes have generally been neglected in riparian restoration projects as opposed to local conditions, even though such processes might largely influence community recovery. We surveyed plant composition of field edges and riverbanks in 51 riparian zones restored by tree planting (565 1-m2 plots) within two agricultural watersheds in southeastern Québec, Canada. Once the effects of environmental variables (hydrology, soil, agriculture, landscape, restoration) were partialled out, three models of spatial autocorrelation based on Moran's eigenvector maps and asymmetric eigenvector maps were compared to quantify the pathways and direction of the spatial processes structuring riparian communities. The ecological mechanisms underlying predominant spatial processes were then assessed by regression trees linking species response to spatial gradients to seed and morphological traits. The structure of riparian communities was predominantly related to unidirectional spatial gradients from upstream to downstream along watercourses, which contributed more to species composition than bidirectional gradients along watercourses or overland. Plant traits selected by regression trees explained 22% of species response to unidirectional upstream-downstream gradients in field edges and 24% in riverbanks, and predominantly corresponded to seed traits rather than morphological traits of the adult plants. Our study showed that even in agriculturally open landscapes, water flow remains a major force structuring spatially riparian plant communities by filtering species according to their seed traits, thereby suggesting long-distance dispersal as a predominant process. Preserving hydrological connectivity at the watershed-scale and restoring riparian plant communities from upstream to downstream should be encouraged to improve the ecological integrity of rivers running through agricultural landscapes.

Eleven years of crop diversification alters decomposition dynamics of litter mixtures incubated with soil

AbstractAgricultural crop rotations have been shown to increase soil carbon (C), nitrogen (N), and microbial biomass. The mechanisms behind these increases remain unclear, but may be linked to the diversity of crop residue inputs to soil organic matter (SOM). We used a residue mixture incubation to examine how variation in long‐term diversity of plant communities in agroecosystems influences decomposition of residue mixtures, thus providing a comparison of the effects of plant diversification on decomposition in the long term (via crop rotation) and short term (via residue mixtures). Three crop residue mixtures, ranging in diversity from two to four species, were incubated for 360 d with soils from five crop rotations, ranging from monoculture corn (mC) to a complex five‐crop rotation. In response, we measured fundamental soil pools and processes underlying C and N cycling. These included soil respiration, inorganic N, microbial biomass, and extracellular enzymes. We hypothesized that soils with more diverse cropping histories would show greater synergistic mixture effects than mC. For most variables (except extracellular enzymes), crop rotation history, or the long‐term history of plant diversity in the field, had a stronger effect on soil processes than mixture composition. In contrast to our hypothesis, the mC soil had nearly three and seven times greater synergistic mixture effects for respiration and microbial biomass N, respectively, compared with soils from crop rotations. This was due to the low response of the mC soils to poor quality residues (corn and wheat), likely resulting from a lack of available C and nutrients to cometabolize these residues. These results indicate that diversifying crop rotations in agricultural systems alter the decomposition dynamics of new residue inputs, which may be linked to the benefits of increasing crop rotation diversity on soil nutrient cycling, SOM dynamics, and yields.

Edaphic properties override the influence of crops on the composition of the soil bacterial community in a semiarid agroecosystem

Annual cropping systems are dynamic environments where soil bacterial communities are responsive to various management practices such as crop rotations, and to seasonal fluctuations of edaphic properties. However, in semiarid regions the impact of crops on the soil bacterial community appears to be modulated or affected by edaphic properties such as soil moisture availability. Therefore the objective of this study was to determine the relative influence of crops (wheat, lentil, field pea, canola, and a fallow treatment), soil chemistry, and edaphic properties on the diversity and composition of soil bacterial communities in a semiarid agroecosystem. We used a temporal sampling strategy and high throughput sequencing approach to monitor changes in the soil bacterial community in relation to crop host identity and phenology and soil properties. Structuring of the bacterial community across the 20 experimental plots was driven primarily by soil pH which varied strongly (min: 5.25, max: 6.97) across the site. Weak temporal shifts in the bacterial community were observed across the growing season and were related to seasonal variation in soil moisture (min: 8.3%, max: 20.3%), soil temperature (min. daily mean: 3.3°C, max. daily mean: 21.1°C), and fluctuation in available phosphate in the soil (min: 0.003μgcm−2day−1, max: 0.290μgcm−2day−1). These temporal and spatial shifts were not influenced by crop host identity as similar bacterial communities were observed among the crops and fallow treatment. Variance partitioning revealed that soil pH and soil moisture accounted for a high proportion of the variation of the soil bacterial community, while crops had no significant impact in the study. Edaphic factors appeared to override or level the effect of crops on the soil bacterial community. These results highlight the importance of accounting for edaphic properties for managing soil bacterial communities in agroecosystems.

Plant species richness prolongs the growing season of freely assembled riparian herbaceous communities under dry climatic conditions

According to two main and non-exclusive scenarios, plant diversity can influence the growing season of vegetation stands through: (i) functional complementarity among species and, (ii) functional selection of species from the regional pool. The objective of this study was to evaluate whether plant species richness prolongs the active photosynthetic period length (APPL) of freely assembled riparian herbaceous communities in agroecosystems of Eastern Canada. We selected a total of 44 herbaceous communities in riparian buffers of varying species richness but which were comparable in their soil characteristics and cumulative growing degree-days. Independent of abiotic factors, we observed a general positive relationship between species richness and the APPL of riparian herbaceous communities. In the first sampling year, the APPL of species-rich communities was, on average, 30 days longer than the APPL of species-poor communities. Species-rich communities were sheltering herbaceous species with delayed green offset dates in comparison to communities dominated by Phalaris arundinacea, a dominant species in these riparian buffers. This relationship was not observed in the second sampling year. We suggest that the presence of a significant and positive APPL–species richness relationship was related to exceptionally dry climatic conditions in the first sampling year. We estimated that, under certain conditions, restoring the species diversity of herbaceous riparian communities in intensively managed agroecosystems could account for a net carbon uptake gain of up to 900kgCyr−1ha−1 of riparian buffer.

Long‐term change of ant community structure in cacao agroforestry landscapes in Indonesia

Abstract Land‐use change and agricultural intensification can strongly affect biodiversity in agricultural landscapes. Although many studies investigate management impacts, data on the long‐term change of species communities in agroecosystems are scarce. We analysed the long‐term change in diversity and composition of ant communities in cacao agroforestry systems in Central Sulawesi, Indonesia and attempted to disentangle the driving factors of this change. Ant communities were resampled in 2009 from sites for which previous surveys had been conducted either in 2001 (the rainforest‐poor Palolo region) or 2003 (the rainforest‐rich Kulawi region) using insecticide fogging. Ant community composition changed significantly over time in Palolo and Kulawi. The change in ant species richness differed between regions. Species richness increased in Kulawi, probably due to the increasing availability of nest sites and microhabitats as trees grow larger and older. In the Palolo region, species richness decreased, suggesting that the high local intensification and landscape‐wide changes may have counteracted the effects of tree age. Changes in ant communities over time were significant, but were more difficult to explain than expected, despite clear difference in management changes within and between regions. The findings suggest that the landscape‐scale differences between the two study regions play a more important role for species diversity and its composition than changes in local management. This highlights the importance of long‐term studies across contrasting landscapes for better understanding the consequences land‐use intensification has on tropical biodiversity.

Do Species Sorting and Mass Effects Drive Assembly in Tropical Agroecological Landscape Mosaics?

AbstractRecent assessments of biodiversity in tropical agroecosystems have revealed surprisingly high functional and taxonomic diversity in systems with low management intensity. This biodiversity is the product of community assembly. Because agroecosystems are novel ecosystems and occur in landscape mosaics, the assembly processes generating communities in agroecosystems are poorly resolved. Broadly, two models have been proposed to explain landscape assembly: trade‐offs in species performance across habitats (species sorting) and source‐sink dynamics between habitats of differential quality (mass effects). These models are largely untested in tropical agroecosystems. We utilize an extensive data set on a tropical twig‐nesting ant community from five microhabitat types in a shaded coffee agroecosystem to test for species sorting, mass effects, or a mixed model. To test among these models, we used community similarity and a variance decomposition on a focal microhabitat (a moderate‐shade coffee farm) to partition community variance into spatial and environmental components. To identify the source habitat for mass effects and assess their strength, we measured dispersing alates (winged reproductives), artificial nests, and colony and nest size in shade trees and coffee. We found significant environmental and spatial signal and evidence for both species sorting and mass effects. We find sorting occurs among common species, but that mass effects are prevalent among rare species and likely originate in the shade trees. Our results indicate that both metacommunity models occur in tropical landscape mosaics, but they may not apply equally to all species in communities, habitat gradients, or timescales.

Long-Term Balanced Fertilization Decreases Arbuscular Mycorrhizal Fungal Diversity in an Arable Soil in North China Revealed by 454 Pyrosequencing

A balanced fertilization can increase crop yields partly due to stimulated microbial activities and growths. In this study, we investigated arbuscular mycorrhizal fungi (AMF) in arable soils to determine the optimal practices for an effective fertilization. We used pyrosequencing-based approach to study AMF diversity, as well as their responses to different long-term (>20 years) fertilizations, including OM (organic manure) and mix chemical fertilizers of NP (nitrogen-phosphorus), NK (nitrogen-potassium), and NPK (nitrogen-phosphorus-potassium). Results revealed that 124,998 of 18S rRNA gene fragments were dominated by Glomeromycota with 59,611 sequences, generating 70 operational taxonomic units (OTUs), of which the three largest families were Glomeraceae, Gigasporaceae and Acaulosporaceae. In Control and NK plots, AMF diversity and richness significantly decreased under long-term P fertilizations, such as NP, NPK, and OM. Concomitantly, the AMF community structure shifted. Supported by canonical correspondence analysis, we hereby propose that long-term balanced fertilization, especially P fertilizer with additional N fertilizer, helps the build-up of soil nutrients. Consequently, some AMF community constituents are sacrificed, propelled by the self-regulation of plant-AMF-microbes system, resulting in an agro-ecosystem with a better sustainability. This knowledge would be valuable toward better understandings of AMF community in agro-ecosystem, and long-term ecosystem benefits of the balanced fertilization.

Experimental treatment-control studies of ecologically based rodent management in Africa: balancing conservation and pest management

Context Rodent pests severely affect crop production, particularly in monocultures where one or two rodent pest species dominate. We predict higher species richness of native small mammal species in more heterogeneous mosaic (crop–fallow–bush) subsistence agro-ecosystems in Africa. Conservation and agro-ecological imperatives require that such diverse natural communities should be maintained and may benefit crop protection through limiting domination of pest species. Ecologically based rodent-management alternatives to rodenticides are urgently required and one such method (community trapping) is herein advocated. Aims To provide baseline information on rodent and shrew communities in agro-ecosystems in three African countries and to demonstrate efficacy of ecologically based rodent management (EBRM) in Africa (e.g. community household trapping). Methods Removal-trapping in a variety of agro-ecological habitats provided accurate small-mammal species lists. Intensive kill-trapping by rural agricultural communities was carried out experimentally where the efforts of communities were scientifically monitored by kill-trapping to measure impact on rodent numbers and the levels of post-harvest damage to stored grains. Key results Our study revealed a high diversity of endemic species in agricultural habitats in Tanzania and Namibia (but not Swaziland) and the existence of undescribed and possibly rare species, some of which may be at risk of extinction from unchecked habitat transformation for agriculture. Treatment-control studies showed that communities in three African countries could effectively reduce pest rodent populations and rodent damage by intensive trapping on a daily basis in and around the community. Conclusions Community trapping reduced pest rodent populations and damage to stored grains. Unlike the use of indiscriminate rodenticide, this practice is expected to have a negligible effect on beneficial non-target rodent and shrew species. Implications Ecologically based rodent management approaches such as community trapping will conserve beneficial non-pest rodent communities and ultimately improve crop protection.

Factors affecting soil microbial community structure in tomato cropping systems

Soil and rhizosphere microbial communities in agroecosystems may be affected by soil, climate, plant species, and management. The management and environmental factors controlling microbial biomass and community structure were identified in a three-year field experiment. The experiment consisted of a tomato production agroecosystem with the following nine treatments: bare soil, black polyethylene mulch, white polyethylene mulch, vetch cover crop, vetch roots only, vetch shoots only, rye cover crop, rye roots only, and rye shoots only. The following hypotheses were tested: (1) Temperature and moisture differences between polyethylene-covered and cover-cropped treatments are partly responsible for treatment effects on soil microbial community composition, and (2) Different species of cover crops have unique root and shoot effects on soil microbial community composition. Microbial biomass and community composition were measured by phospholipid fatty acid analysis. Microbial biomass was increased by all cover crop treatments, including root only and shoot only. Cover cropping increased the absolute amount of all microbial groups, but Gram-positive bacteria decreased in proportion under cover crops. We attribute this decrease to increased readily available carbon under cover-cropped treatments, which favored other groups over Gram-positive bacteria. Higher soil temperatures under certain treatments also increased the proportion of Gram-positive bacteria. Vetch shoots increased the amount and proportion of Gram-negative bacteria, fungi, and arbuscular mycorrhizal fungi in the rhizosphere of tomato plants. The imposed treatments were much more significant than soil temperature, moisture, pH, and texture in controlling microbial biomass and community structure.

Resilient populations of root fungi occur within five tomato production systems in southeast Florida

Farming practices are known to impact arbuscular mycorrhizal (AM) fungi and other soil microbial communities in agroecosystems. The effects of divergent land management strategies on the incidence and infectivity of AM and other fungal root endophytes were evaluated in a 5-year tomato ( Lycopersicon esculentum Mill.) cropping systems study. Two of the five treatments utilized farming practices considered detrimental to AM fungal populations, including the tillage-mediated elimination of vegetation and soil fumigation. The remaining three treatments used practices thought to be more conducive to the presence of AM fungi, including organic production methods, bahiagrass pasture and undisturbed weed fallow. In years four and five of the study tomato roots and rhizosphere soil were collected. Roots were examined for colonization by AM and other fungal root endophytes, and rhizosphere soil was assayed to measure the amount of infective inoculum present based on maize ( Zea mays L.) seedling infection. Tomato roots and rhizosphere soil were also analyzed for the AM fungal fatty acid biomarker 16:1ω5c. Sudangrass ( Sorghum sudanense (Piper) Stapf) trap cultures were initiated using field soil to assess the diversity of AM fungal spore morphotypes. Soil disturbance and phosphorus (P) levels had the greatest influences on AM fungal infectivity and abundance. All plots had high levels of available soil P, resulting in low levels of colonization across treatments. Bahiagrass ( Paspalum notatum Flugge) pasture was the only treatment without repeated, intensive soil mixing, and had the highest level of field root colonization by AM fungi. Field roots were more heavily colonized by other fungal endophytes than by AM fungi in all treatments. Tomato roots from organic plots were apparently unique in encouraging colonization by fungi that appeared to be Microdochium bolleyi (R. Sprague) de Hoog & Herm.-Nijh. Infection by AM and other fungal root endophytes were positively correlated in all studies. Flooding and a shortened growing season likely contributed to reduced infection potentials in all treatments except for organic plots in year five compared to year four. Areas of high disturbance from frequent tillage had the lowest levels of primary inoculum, but recovery to levels comparable to less disturbed treatments occurred after a single season of host root growth. Diversity of AM fungal morphotypes was typical of agricultural fields, with at least 10 spore morphotypes present across treatments; Glomus spp. were the dominant spore type recovered in all treatments.

Using Species-Area Curves to Examine Weed Communities in Organic and Conventional Spring Wheat Systems

The ecological significance of species diversity within agroecosystems has become a point of interest in recent years. Although the time and spatial scale at which diversity is measured may influence the interpretation of its functional importance, little research has been conducted on methodological approaches to assess the number and relative abundance of agricultural plant species. In this study, we (1) evaluated the applicability of the species-area curve to examine plant species richness and α and β diversity in conventional no-tillage and organically managed spring wheat systems, and (2) assessed temporal changes in plant species richness across systems. Measurements were obtained at three times during two growing seasons in experimental plots and at three times during one growing season on commercial farms in Montana. In accordance with previous studies, management system affected species richness and diversity. In eight of nine studied cases, we detected a positive relationship between species richness and sampled area. In these eight cases, intercepts (α diversity) were higher for the organic systems than for the conventional no-tillage systems. Slopes (β diversity) were higher for the organic system in six of nine cases studied. Species richness declined as the season progressed for both systems, with the organic systems consistently having more species than the conventional no-tillage systems. Despite differences in the species composition and between the experimental plots and commercial farm field size, the patterns of species richness and α and β diversity were relatively constant, suggesting that the processes responsible for the assembly of plant communities in agroecosystems of the Northern Great Plains are similar at a range of spatial scales.

Mineral phosphate solubilizing bacterial community in agro-ecosystem

The present communication deals with the assessment of phosphate solubilizing bacterial community structure across artificially created fertility gradient with regards to N, P and K status of soil in the experimental site. 20 randomly phosphate solubilizing bacteria from each fertility gradient were isolated, purified and characterized through their insoluble mineral phosphate-source utilization patterns. Four insoluble phosphate sources; purulia rock phosphate (PRP), mussourie rock phosphate (MRP), crystalline iron and aluminum phosphate were charged in basic Pikovskaia solid medium. Growth pattern of the isolates on those phosphate sources was recorded. Different communities utilized different P-sources in different magnitudes. Size of isolates of low fertility gradient showing greater utilization efficiency towards diverse phosphate sources gradually reduced along the gradient of fertility. Data thus obtained from the experiment were filed to compute community structure of phosphate solubilizing bacteria through principal component analysis (PCA). On the whole, the community composition of phosphate solubilizing bacteria reduced gradually under the influence of cultivation and fertilization in different fertility gradient soils.

What is the effect of soil use on ant communities?

Studies on ant communities in agroecosystems have contributed to the knowledge of the effect of agricultural activities on biological communities. The aim of this study is to explain the effect of soil use on ant communities. We tested the hypothesis that there was a decrease in ant species richness and a change in the species composition at habitats with more intense soil use. We collected ants using sardine baits, subterranean traps and direct sampling at four habitats with different soil use (secundary forest, Acacia forestry, initial stage of succession and mixed crops). The ant species richness did not decrease with intensity of soil use. In successional habitat the species numbers collected using sardine baits and subterranean traps were significantly different. Species composition of communities had a pronounced variation, with the epigaeic and hypogaeic ant faunas of the habitat with high intense soil use (mixed crops) had low similarity with ant communities of the three other habitats. The predator species were restricted to habitats with low intensity of soil use. Then, species composition could better reflect the functional changes on ant communities than species richness. Our data can help to choose the component of ant community that better reflect the response of biodiversity to agricultural impacts.

Biodiversity, the ultimate agri-environmental indicator?: Potential and limits for the application of faunistic elements as gradual indicators in agroecosystems

Most political statements (e.g. convention on biodiversity), referring to existing approaches regarding the development of biotic agrienvironmental indicators focus mainly and nearly exclusively on “biodiversity”, the enhancement of which is almost the overall target for the development of agricultural landscapes towards sustainability. In this regard the understanding of the term “biodiversity” is rather different: mostly it is interpreted as species richness, only occasionally as richness of varieties, cultivars or genetical expressions (e.g. micro-organisms). In this contribution the approach of evaluating the quality of species communities in agroecosystems mainly on the basis of “biodiversity” (in the sense of species richness), will be analysed critically. General difficulties of an exact sampling or establishment of biodiversity especially of invertebrate communities in agricultural landscapes will be highlighted. Arising from this synthesis, it is demonstrated that biotic agrienvironmental indicators should not only address a maximising of species numbers in agricultural habitats, but also a sustainable and long-term stability, and the reproductive potential of populations of species typical of agroecosystems. In this regard alternative biotic indicators for the assessment of the effects of husbandry practices are required that are based on existing knowledge of the reactions of populations and species communities to environmental stress, and which give information about the physiological fitness of populations of species that are relevant members of the agrobiocoenosis from both the qualitative (rarity, endangerment status) and functional point of view (natural enemies, parasitoids, decomposers). Regarding the sensitivity of those indicators it will be indicated that in contrast to many recent scientific experiments it is usually necessary to evaluate and compare cultivation intensities, that are quite similar (e.g. gradual differences in conventional managed farms and landscapes) as well as those displaying much larger differences (e.g. organic farming vs. conventional farming), because conventional farms currently cover nearly 90% of the area under plough in Europe. Finally an assessment of the practicality, the power of indication statements and the remaining work required to validate indicators will be provided for discussion, as well as suggestions for a simplification of indicator systems in order to minimise the input needed for data recording.

Composition, abundance and pest control potential of spider communities in agroecosystems: a comparison of European and US studies

Current knowledge of spiders in agroecosystems (excluding tree crops) reported in European and US literature is discussed comparatively, in an attempt to relate spider community structure to pest control potential. The spider fauna of agroecosystems in the northern-temperate zone of Europe is strongly dominated by small linyphiid spiders that capture tiny insects in their sheet webs, including large numbers of pest aphids. In the US, spider guild structure is more complex, and hunters (especially, Oxyopidae, Salticidae, Clubionidae, Thomisidae, and Lycosidae), that have broader diets (including lepidopteran and heteropteran pests), numerically prevail in many locations. Spider populations increase to high densities (2–600 m −2) in European field crops, but densities are typically much lower (0.02–14 m −2) in US annual crops. Agroecosystem spiders, in both Europe and the US, feed rather infrequently, but they contribute to pest control as part of larger assemblages of natural enemies, and there is potential for increasing their density and impact in both continents. Many of the differences between continents in spider guild structure, density and feeding patterns highlighted in this paper are likely to be attributable to climatic differences. Most of the US data originate from more southern latitudes (i.e., subtropical and Mediterranean climates) with distinctly higher mean annual temperatures compared to the European study areas, which are in the northern-temperate zone. Spider communities may respond to climate directly, and also indirectly via food availability and antagonists. In addition, differences in crop structure and cultural practices (including habitat diversification and the provision of ground cover) could influence spider density and community organisation. Mean farm size is an order of magnitude less in Europe than in the US and this is likely to be associated with greater habitat diversity, which is known to increase spider abundance. Currently, there is a dearth of field studies from southern Europe (Mediterranean climate) and the northern regions of the US (humid continental climate). The few data available from such regions indicate that the patterns of spider predation may differ less between the two continents if sufficient study areas with similar climatic conditions could be compared. The conclusions in terms of biological control are, however, widely applicable, because a large proportion of the productive agricultural land area of Europe is located in more northern latitudes and the reverse is true in the US.

Bumblebees in the agricultural landscape of Estonia

Bumblebees are valuable pollinators of crops as well as native plants but their populations have declined in many parts of western Europe during recent decades. Their decline has been greatest in intensively managed agricultural areas and associated with the decline in suitable forage plants. The objective of this study was to compare the abundance and diversity of forage plants and bumblebees in agricultural and semi-natural habitats in Estonia, where land management for agriculture is less intensive than in western Europe. Six localities were surveyed, each by two census transects (1000 m long×2 m wide), one passing through a semi-natural habitat and the other through an agricultural habitat. Flower abundance and bumblebee visitors to flowers along the transects were recorded on 4 days per transect during late July/early August in each of 3 years. No significant difference in the abundance of either flowering forage plants or bumblebees between agricultural and semi-natural habitats was found. However, the number of bumblebee species observed in agricultural areas was significantly smaller than the number in adjacent semi-natural areas. The smaller proportion of preferred forage plants in the agricultural habitat was a possible reason for its reduced bumblebee species richness. For example, in the agricultural habitat, the annual and biennial plants tended to be more common and the hemiparasitic Melampyrum nemorosum significantly less common, among the flowers visited by bumblebees, than in the semi-natural habitat. The proportion of stable micro-habitats, which are not subjected to ploughing and root disturbance and therefore allow establishment of highly preferred forage plants, rather than the overall quantity of any kinds of refuge areas between cultivated lands, may be critical for the species richness of bumblebee communities in agro-ecosystems.

Population dynamics of earthworm communities in corn agroecosystems receiving organic or inorganic fertilizer amendments

The dynamics of earthworm populations were investigated in continuously-cropped, conventional disk-tilled corn agroecosystems which had received annual long-term (6 years) amendments of either manure or inorganic fertilizer. Earthworm populations were sampled at approximately monthly intervals during the autumn of 1994 and spring and autumn of 1995 and 1996. The dominant earthworm species were Lumbricus terrestris L. and Aporrectodea tuberculata (Eisen), which comprised 50–60% and 8–13%, respectively, of the total annual earthworm biomass. Lumbricus rubellus (Hoffmeister) and Aporrectodea trapezoides (Duges) were much less abundant and contributed a small fraction of total earthworm biomass. Earthworm numbers and biomass were significantly greater in manure-amended plots compared to inorganic fertilizer-treated plots during the majority of the study period. Seasonal fluctuations in earthworm numbers and biomass were attributed to changes in soil temperature and moisture, and cultivation. Unfavorable climatic conditions in the summer and autumn of 1995 caused earthworm abundance and biomass to decline significantly. Mature L. terrestris, L. rubellus and A. tuberculata were most abundant in May and June of 1995 and 1996, and cocoon production was greatest in June and July 1995 and June 1996. Recruitment of juveniles of Lumbricus spp. and Aporrectodea spp. into earthworm communities occurred primarily in the autumn. Long-term amendments of manure or inorganic fertilizer did not change the species composition of earthworm communities in these agroecosystems. The earthworm populations in both manure and inorganic fertilizer plots have declined significantly after 5 years of continuously-cropped corn.

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.