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

Summary The 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.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Effects of biological soil crusts on soil nematode communities following dune stabilization in the Tengger Desert,Northern China

Biological soil crusts (BSCs) play an important role in the dune fixation and maintaining soil biota in arid desert systems. Free-living soil nematode communities could be used as significant bioindicators to reflect soil recover regime after sand burial. However, the relationship between BSCs and nematodes is rarely known. To examine the effects of BSCs on soil nematodes, 72 soil samples under cyanobacteria–lichen and moss crusts were collected to analyse nematode communities in the different aged vegetated areas at the southeastern edge of the Tengger Desert. Our results showed the colonization and development of BSCs significantly enhanced nematode diversity. Nematode abundances, generic richness, H′, MI, EI and SI were greater under crusts than those under noncrust. In particular, nematode abundances, generic richness, H′, MI, EI and SI were positively correlated with crust ages. The differences in nematode communities were also dependent upon crust types. Nematode abundances and generic richness under moss crusts were higher than those under cyanobacteria–lichen crusts. This can be contributed to the present and succession of BSCs that increased thickness of topsoil after dunes have been stabilized, namely, creating suitable habitats and providing an essential food source for nematodes.     

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

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.     

科尔沁沙地流动沙丘土壤线虫群落组成与多样性研究

本文对科尔沁沙地流动沙丘不同坡位(丘间地、过渡带、迎风坡中部、坡顶、背风坡上部和下部)和不同土层深度(0 ～ 20、20 ～ 40和40 ～ 60 cm)的土壤线虫群落组成与多样性进行了比较研究.研究结果表明,流动沙丘土壤线虫具有显著的坡位效应,而垂直分布特征不显著.丘间地和背风坡下部具有较高数量和较多种类的土壤线虫,而过渡带和迎风坡中部土壤线虫数量较低、种类也较少.不同坡位土壤线虫总数、食细菌线虫、食真菌线虫和植物寄生线虫数量存在显著差异.除食细菌线虫外,不同土层土壤线虫总数及其各营养类群均无显著差异.土壤含水量和电导率是影响土壤线虫群落空间分布格局的主要因素.土壤线虫群落分布格局的变化反映了流动沙丘土壤微环境的变化,对土壤质量具有一定的指示作用.     

The Vertical Distribution of Soil Seed Bank and Its Restoration Implication in an Active Sand Dune of Northeastern Inner Mongolia,China

Although the functions and characteristics of soil seed banks in topsoil layers have been described for various ecosystems, the spatiotemporal pattern of the seed bank in deep soil and its ecological implications for vegetation restoration of active sand dune have not been fully explored. In 2007 and 2008, seed densities with regard to dune position, soil depth and season were investigated on an active and a stabilized sand dune of northeastern Inner Mongolia, China. Seeds in the 0–10 cm topsoil layer accounted for 60% of total soil seed bank on the stabilized sand dune, while more than 40% of seeds were stored in the 50–100 cm layers on the active sand dune. Seed density declined significantly with soil depth on the stabilized sand dune, but it was relatively constant across the 0–100 cm soil profile on the active sand dune. Seed density fluctuated with soil depth on the active sand dune suggesting that seeds were either relocated upward or downward over time. Seeds of annual non‐psammophytic species accounted for the majority of soil seed bank on the stabilized sand dune, while pioneer psammophytes contributed more to the soil seed bank of the active sand dune. Our data suggest that seeds in the deep soil layers of active sand dunes account for a large proportion of the whole soil seed bank. Because of the effect of wind erosion, seeds in deep soil could be gradually exposed to shallow soil layers and potentially contribute to population recruitment and vegetation restoration on active sand dunes. Copyright © 2015 John Wiley & Sons, Ltd.     

Birch invasion of heather moorland increases nematode diversity and trophic complexity

To determine whether successional changes in plant communities may influence belowground community structure, we quantified nematode abundance, trophic structure and diversity along two separate chronosequences from heather moorland to birch woodland in the Scottish uplands. Tree invasion markedly altered plant community composition, and hence litter inputs, both directly, and indirectly through changes in understorey species. In turn, these changes in detrital inputs were reflected in consistent changes in nematode community structure. Nematode abundance increased from moorland to birch woodland, with moorland soils being dominated by a few taxa, notably root-hair and fungal feeders, compared to the more diverse composition of the birch woodland soils. Trophic structure was altered through an increase in the abundance of bacterial feeding relative to fungal-feeding nematodes, and an increase in the abundance of predatory nematodes. The increase in predators during the succession from moorland to woodland was associated with an increase in soil pH, highlighting that not only changes in the plant community, but also changes in soil properties associated with tree invasion may influence soil nematodes. Nematode diversity increased from moorland to birch woodland, with nematode richness being positively related to both plant species richness and soil pH. These results suggest that trees may control soil community structure through the manipulation of resources and the soil physico-chemical environment, promoting greater nematode diversity and trophic complexity.     

Soil macrofaunal response to sand dune conversion from mobile dunes to fixed dunes in Horqin sandy land,northern China

The response of macrofauna to dune conversion from mobile dunes to fixed dunes was investigated in Horqin sandy land, northern China. Macrofaunal communities were compared in four major stages: mobile dunes (MD), semi-mobile dunes (SMD), semi-fixed dunes (SFD) and fixed dunes (FD). Macrofauna were collected by hand in the field and identified in the lab on the basis of features observed under a magnifying glass. They were then classified into morphotypes at the order and family level. Environmental parameters indicated a significant alteration of the soil environment in the conversion process. A clear increasing trend in abundance, richness, and diversity of the macrofaunal community could be established from mobile dunes to fixed dunes. Data for the different taxa suggested specific responses to dune conversion. Specific groups in the initial stage were predators (Labiduridae and Myrmeleontidae larvae, presumably feeding on Noctuidae and other small flying insects), which in the intermediate stages were replaced by saprophagous taxa (Diptera larvae, feeding on many decaying roots buried in semi-mobile and semi-fixed dunes), and burrowing Tenebrionidae (adults, then larvae) with herbivorous Hemiptera in later fixation stages with higher plant density and cover. Overall, the conversion process, particularly the fixed dunes, provided more suitable habitats with greater soil organic matter and plant cover supporting macrofaunal communities with higher diversity, which further enhanced the stabilization process, benefiting the restoration of soil and vegetation in this semiarid sandy area.     

Effects of repeated drought on soil microarthropod communities in the northern Chihuahuan Desert

 Soil microarthropods were sampled in plots centered on creosotebushes (Larrea tridentata) and in plots centered on mesquite (Prosopis glandulosa) coppice dunes. Nine plots in each area were covered by rain-out shelters with greenhouse plastic roofs which excluded natural rainfall and nine plots received natural rainfall. There were differences in the abundance of several mite taxa in soils from the mesquite coppice dune plots. Some taxa (Stigmaeidae, Nanorchestidae, and Entomobryidae) occurred in significantly lower numbers in the soils of the drought plots. Other taxa (Tarsonemidae and Cunaxidae) were more abundant in the drought plots in the mesquite coppice dunes. There were no significant differences in the abundance of any of the dominant taxa of soil microarthropods in the drought and control plots centered on creosotebush. In the creosotebush habitat, there were significantly fewer Prostigmata in the plots exposed to drought. In an area with both creosotebush and mesquite, there were no significant differences in microarthropod population responses to drought and in recovery from drought. The differences in responses of soil microarthropods to drought in creosotebush and mesquite habitats are attributed to the differences in soil stability, litter accumulations, and microclimate associated with the shrubs. Received: 29 December 1997     

Nematodes associated with saffron II: Bioindication for soil health assessment and impact of agricultural practices

BackgroundSaffron cultivation is vital in the Taliouine-Taznakht regions, but the influence of agricultural practices on soil nematode communities, critical for soil health and plant productivity, is not well understood.This study characterizes nematode communities in saffron fields of the Taliouine-Taznakht regions, assessing the impact of various agricultural practices on these communities, with a focus on their diversity, functional roles, and potential as bio-indicators of soil health.A total of 163 soil samples were collected from saffron fields in Taliouine-Taznakht. Nematode communities were identified, quantified, and their functional diversity analyzed. Principal Component Analysis (PCA) was used to visualize relationships between nematode communities and sampling sites. Co-inertia analysis assessed the impact of agricultural practices on nematode diversity.The nematode communities were diverse and varied across regions. PCA identified unique nematode community compositions in different saffron fields. Omnivorous nematodes were strongly linked with Taouyalte (TA), and herbivorous ones were prevalent at Agadir Melloul (AM) and Sidi Hssaine (SH). Modern crop types, high-frequency irrigation, and alfalfa-barley rotation were positively correlated with predator nematode abundance, potentially controlling plant parasitic nematodes and encouraging nutrient cycling. Conversely, monocropping, traditional irrigation, and long plantation age correlated with reduced structure and maturity indices, suggesting a less stable ecosystem.This study unveils the intricate relationships between nematode communities in Taliouine-Taznakht saffron fields and agricultural practices. Findings indicate that specific practices, such as crop rotation and modern irrigation techniques, can foster beneficial nematode groups that improve soil health and potentially regulate harmful plant parasitic nematodes. This knowledge is crucial for crafting sustainable and effective saffron cultivation strategies.     

Influence of clear-cutting and forest age on the nematode fauna in a Swedish pine forest soil

The abundance and composition of the soil nematode fauna in three seral stages of a pine forest chronosequence located in central Sweden was investigated. The aim of the study was to investigate faunal development during the forest cycle (120–160 years). Three sites, clear-cut 3–12 years before sampling but still without pine trees were compared to three sites with 10–130 year-old trees. Together, these provided sites that could be classified as clear-cut, young stand and mature forest. In most sites without trees and with a poor field layer vegetation, nematode abundance was much lower than in the forested plots. Similar major species/genera of nematodes were found in all age stages of the forest development. The development of the fauna could be related both to climatic shifts and to changes in production of food sources.The ratio of fungal to bacterial feeders and the proportions of two groups of bacterial feeders (Rhabditida BF and Adenophorea BF) varied in distinct ways related to soil layer and seral stage. Thus, in the clear-cut sites the proportion of fungal feeders was quite low. This was also the case with Rhabditida BF. These two groups also contributed a rather low proportion in the litter layer, whereas, their importance successively increased with increasing depth of the soil layers. Proportions of fungal feeding members of the Dorylaimida were highest in the 10–20 year-old stands. The effect of forest trees on nematodes was quite large and the faunal structure in sites with pine trees differed quite markedly from the sites without trees.Apparently, there are certain nematode species whose contribution to the fauna is greatest in the early tree-free stages, others are more abundant in the young forest and still others have their greatest abundance in the mature forest. There are also species whose abundance seems to be unaffected by forest age. Nematode abundance was correlated with the abundance of some other groups of soil organisms.     

Cover crops alter the soil nematode food web in banana agroecosystems

Cover crops are increasingly being used in agriculture, primarily for weed or erosion management. The addition of cover crops increases the primary productivity of the system and diversifies basal resources for higher trophic levels. How increases in the quality and quantity of basal resources affect bottom-up and top-down control remains a key question in soil food web ecology. We evaluated the response of the nematode community to the introduction of cover crops between rows of a banana plantation. We measured changes in nematode food web structure and inferred the prevalence of bottom-up and top-down effects on the abundance of phytophagous nematodes (i.e., plant-feeding and root-hair-feeding species) 1.5 years after plots with cover crops (Poaceae or Fabaceae species) or bare soil were established. The addition of a cover crop greatly affected the structure and the abundance of the soil nematode community 1.5 years after planting. The abundance of all trophic groups except for plant-feeding nematodes tended to increase with the addition of cover crops. The Shannon–Weaver diversity index and the enrichment index increased with the addition of cover crops, indicating that opportunistic, bacterivorous and fungivorous nematodes benefited from the added resources. Plant-feeding nematodes were least abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous, and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affected soil food web structure. Bottom-up effects on all trophic groups other than plant-feeding nematodes were evident with Poaceae cover crops, suggesting an top-down control of plant-feeding nematodes by omnivorous nematodes. Conversely, plant-feeding nematodes were evidently not suppressed in Fabaceae cover crops, perhaps because bottom-up effects on omnivorous nematodes were weaker (hence, top-down control by omnivorous nematodes was weaker), and because Fabaceae cover crops probably served as good hosts for some plant-feeding nematodes.     

Community composition,diversity and metabolic footprints of soil nematodes in differently-aged temperate forests

Soil nematode communities can provide important information about soil food web structure and function. However, how soil nematode communities and their metabolic footprints change over time in temperate forests is not well known. We examined the changes in the composition, diversity and metabolic footprints of soil nematode communities in three differently-aged (young, mid and old) forests of the Changbai Mountains, China. Carbon flows through different nematode trophic groups were also quantified based on nematode biomasses. The results showed that the highest abundance and diversity of total nematodes was found in the mid forest. Nematode communities were characterized by the replenishment in abundance but not the replacement of dominant genera. A low enrichment footprint in the young forest suggests a decline in available prey, while a high enrichment footprint in the mid forest indicates an increase in resource entry into soil food web. The relationship between the carbon flows of omnivores-predators and fungivores was stronger than that among other trophic groups. Our study shows that bottom-up effects of the vegetation, the soil environment and the connectedness of nematode trophic groups are all important driving forces for nematode community structure in temperate forests.     

Effect of vegetation restoration on ant nest‐building activities following mobile dune stabilization in the Horqin Sandy land,Northern China

Measurements of the density and characteristics (size and height) of ant mounds were carried out on mobile, semi‐mobile, semi‐fixed and fixed dunes in the Horqin Sandy Land, Northern China. The relationships between the distribution of ant mounds and the plant community (abundance and cover) were investigated, for areas within and across the dune stages. Changes in spatial pattern of ant mounds were also analysed following mobile dune stabilization. The results showed no ant mounds on mobile dunes; the mound density was significantly higher on fixed dunes than on semi‐fixed or semi‐mobile dunes. The mound density depended on plant density and cover across all dune stages, but not within any dune stage. The mound diameter increased but the mound height decreased from semi‐mobile dunes to fixed dunes. However, the mound diameter and height were not related to plant cover and density within any stabilization stage or across all dune stages. The spatial pattern of ant mounds tended to change from more clumped (semi‐mobile and semi‐fixed dunes) to less clumped and approached a random pattern (fixed dunes) along the dune stabilization gradient, which was related to the changing vegetation pattern during succession. Although the quantification of the number of ant species present was not attempted on each dune, the observed differences in ant colour and size suggest at least eight species were present. Fixed dunes were more attractive for different kinds of ant species, but the mound distribution exhibited a more random pattern with more continuous vegetation. Thus, different environmental conditions, especially in terms of the plant communities present at different dune stages, affected the activities and behaviour of the ants (including the distribution of mounds), but did not affect mound size and height. Copyright © 2009 John Wiley & Sons, Ltd.     

Ciliates in coastal dune soils of different stages of development

Abundance and diversity of soil ciliates were studied in the dunes of the North Sea island Norderney (D). Live counts of ciliates were carried out in spring and fall 1999. The eight examined sites show a sequence from freshly thrown sand beach to heath, including primary dunes, white dunes, gray dunes, bushes and forest. Ciliate abundance was low (5–13 ind g^–1 dm) in the young sites without humus horizon (beach, primary and white dune), with dominance of hymenostomes and hypotrichs. The dune soils in older stages showed high abundance, between 39 and 264 ind g^–1 dm. Here, the Colpodea were the dominant group. Bacterial feeders were highly dominant at all sites. While in young sites nano- and microtrophic ciliates were important, in the older sites the community was much more diverse with fungivorous and omnivorous ciliates in addition. By using ordination methods, the young soil sites separated as a group, but the older sites, which differ clearly in community structure, did not. The results show the importance of organic matter as a factor for abundance and species richness of soil ciliates.     

Rapid Changes in Soil Nematodes in the First Years after Technosol Construction for the Remediation of an Industrial Wasteland

Technosol construction is an emergent technology that uses an assemblage of technogenic materials for the ecological reclamation of derelict land and waste recycling. Knowledge about the colonisation of Technosols by soil biota is limited, despite the latter’s central role in ecosystem functioning. In this four-year field (2008 to 2011) study, we characterized the development over time of the diversity and the abundance of soil nematodes in two types of Technosols in North-Eastern France. We also studied the nematode community structure, abundance of taxa and functional groups in both Technosol profiles in the third year of the study. Samples were collected from the top soil layer (0–20 cm) each year in the spring (April), on a one ha. field experiment that had spatially divided in 24 sampling areas. For soil profiles, three samples were collected in three horizons within six pits (three pits per Technosol). Nematodes were extracted from soil and identified at the family or genus level and then classified into functional feeding guilds. In the first year, the community was dominated by opportunistic bacterial feeders. The taxonomic and functional nematode diversity increased with time, with a dominance of non-opportunistic bacterial feeders after four years, but also the significant presence of fungal feeders, omnivorous and carnivorous, as well as plant parasites and insect parasites. No significant difference was observed between the two Technosols. Each layer showed distinct communities, with nematode diversity and abundance decreasing with depth. Abundance and diversity, coupled with the analysis of several indexes, commonly used for nematodes, including Maturity index (MI), Enrichment index (EI), Structure index (SI) and Nematode channel ratio (NCR), lead to the conclusion that the high organic matter content, particularly in the upper horizon of both Technosols, guaranteed nematode colonization and progressive diversification, and is likely to be the key for successful biodiversity reclamation.     

Interactions between nematodes and microbial communities in a tropical soil following manipulation of the soil food web

The carrying capacity for microflora and nematofauna was manipulated (using a bactericide, a fungicide, manure or a growing millet plant) in a poor tropical soil, in order to identify relationships between the soil microbes and nematodes and to assess the influences of these organisms on nitrogen flux. The experiment was conducted for 4 months in containers under greenhouse conditions, with analyses of soil, nematofauna and microbial characteristics at regular intervals. Manure input and initial bactericide application led to a significant increase in bacterial-feeding and fungal-feeding nematodes of coloniser-persister classes 1 and 2, respectively, whereas high manure input stimulated omnivorous nematodes (i.e. Microdorylaimus rapsus) which became the dominant trophic group. Changes in abundance of the different bacterial-feeding nematode taxa between treatments seemed to be more related to changes in the structure of the microbial communities than to the total amount of micro-organisms, as suggested by the RISA fingerprint analysis of the bacterial communities. Canonical analysis of nematode feeding guilds, combined with soil microbial and mineral nitrogen parameters as well as multiple regression showed that the bacterial-feeding nematodes influenced the inorganic N content in the soil whereas microbial biomass was determined by total nematode abundance and not by any specific trophic group.     

Effect of subsurface clay on nematode communities in a sandy soil

A variety of soil properties can directly or indirectly affect nematode community structure. The effects of subsurface clay content (at 20–40 cm depth) on nematodes in the surface layer (0–20 cm depth) of a sandy soil were examined in field experiments in Florida, USA. Plots were established in a site with a relatively uniform sandy upper soil layer (88–91% sand and 5–7% clay at 0–20 cm depth) but with varying levels of clay in the subsurface layer (3–35% clay at 20–40 cm depth). Nematode numbers in the surface soil layer were affected by the amount of clay in the subsurface layer. Population densities of a number of different nematode genera were greater in the surface layer of plots with 35% subsurface clay than in plots with 3% subsurface clay. Indices of nematode community structure were largely unaffected, since effects of subsurface clay were observed across all nematode groups. Most nematodes (70–80% of total numbers) occurred at 0–20 cm depth, although Teratocephalus was more common at 20–40 than at 0–20 cm. Subsurface clay content indirectly affected soil moisture and other environmental factors in the upper soil layer in which most nematodes reside.     

科尔沁不同类型沙丘土壤水分时空变化特征及其环境影响因子

为了深入探究半干旱地区沙丘土壤水分时空变化特征及其与环境因子关系，以科尔沁沙地为研究区，综合运用原位观测、数值模拟和冗余分析等方法，对沙丘土壤水分的时空变化特征、变异性、水平衡及其与环境因子的定量关系进行研究。结果表明：沙丘土壤水分在垂直剖面呈现出由半流动沙丘"镜像S "形逐渐过渡为阴坡固定沙丘" S"形的趋势，在时间尺度上呈"正态分布"形；半流动、半固定和阴坡固定沙丘土壤水分变异性随深度的增加逐渐减弱，半阳坡固定沙丘呈"S"形分布，最大变异系数为75.45%，均属中等变异；半流动和半固定沙丘水分主要消耗于深层渗漏，分别占总水量的57.35%和54.56%，半阳坡固定和阴坡固定沙丘水分主要消耗于植被蒸腾，分别占总水量的77.15%和54.88%；沙丘土壤水分影响因子具有差异性，容重、砂粒含量、粉粒含量和饱和导水率是影响半流动、半固定和半阳坡固定沙丘土壤水分的主要环境因子，而有机质、砂粒含量、粉粒含量和饱和导水率是阴坡固定沙丘土壤水分的主要影响因子。研究表明，半阳坡固定沙丘小叶锦鸡儿易消耗深层土壤水，造成土壤干燥化，草本和半灌木有利于深层土壤水分保持。     

Abundance,diversity and connectance of soil food web channels along environmental gradients in an agricultural landscape

Soil food webs respond to anthropogenic and natural environmental variables and gradients. We studied abundance, connectance (a measure of the trophic interactions within each channel), and diversity in three different channels of the soil food web, each comprised of a resource-consumer pair: the microbivore channel (microbes and their nematode grazers), the plant–herbivore channel (plants and plant-feeding nematodes), and the predator–prey channel (predatory nematodes and their nematode prey), and their associations with different gradients in a heterogeneous agricultural landscape that consisted of intensive row crop agriculture and grazed non-irrigated grasslands in central California. Samples were taken at three positions in relation to water channels: water’s edge, bench above waterway, and the adjacent arable or grazed field. Nematode communities, phospholipid fatty acid (PLFA) biomarkers, and soil properties (NH₄⁺-N, NO₃⁻-N, total N, total C, pH, P, bulk density and soil texture) were measured, and riparian health ratings were scored. Environmental variables were obtained from publicly-available data sources (slope, elevation, available water capacity, erodability, hydraulic conductivity, exchangeable cation capacity, organic matter, clay and sand content and pH).The abundance and richness in most food web components were higher in grazed grasslands than in intensive agricultural fields. Consumers contributed less than their resources to the abundance and richness of the community in all channels. The association between richness and abundance for each component was strongest for the lowest trophic links (microbes, as inferred by PLFA) and weakest for the highest (predatory nematodes). The trophic interactions for the predator–prey and plant–herbivore channels were greater in the grassland than in the cropland. Fields for crops or grazing supported more interactions than the water’s edge in the plant–herbivore and microbivore channels. Connectance increased with the total richness of each community. Higher connectance within the microbivore and predator–prey soil food web channels were associated with soil NO₃⁻-N and elevation respectively, which served as surrogate indicators of high and low agricultural intensification.     

Responses of soil nematode community structure to different long-term fertilizer strategies in the soybean phase of a soybean–wheat–corn rotation

The impact of long-term application of fertilizers in soybean fields on soil nematode community structure was studied. The long-term application model of fertilizers lasted 13 years in a soybean–wheat–corn rotation, and included three treatments: no fertilizer (NF), chemical fertilizer (urea and ammonium phosphate, CF), and pig manure combined with chemical fertilizer (MCF). The soil nematode community structures and ecological indices were determined from soil samples taken at five soybean growth stages from May to October in the soybean phase of the rotation. Fertilizer application had significant effects on abundance of plant parasites, bacterivores and fungivores (P < 0.05), but had no significant effects on total nematodes and omnivores-predators. Abundance of plant parasites was higher in NF than in MCF and CF, and abundance of bacterivores was highest in MCF. Fertilizer application significantly affected Plant-parasitic Nematode Maturity Index (PPI) and Nematode Channel Ratio (NCR) ecological indices (P < 0.05). Shannon–Weaver Index (H′) and Species Richness (SR) indices were higher in MCF than in either NF or CF. The abundances of total nematode and plant parasites showed increasing trend with soybean growth in all three treatments. This is probably due to soil environment being more suitable for soil nematode survival with more food available for plant parasites as the soybean grows. Soybean growth stage significantly affected the H′, Free Living Nematode Maturity Index (MI) and PPI. Bacterivores significantly correlated with soil nutrient status suggesting that they could be used as a potential indicator of soil fertility.