Micro-food Web Research Articles

Some materials of the 1st International Earthworm Congress

The scientific works of the 1st International Earthworm Congress in Shanghai (China, 2018) are relevant now. We analyzed case studies about functional roles of earthworm and vermicompost on the functional guilds of soil micro-food web, crop yield and plant resistance against pests. We have selected the most important scientific trends, presented modern methods of gathering and accounting, taxonomy and the molecular-genetic analysis of earthworms. We have described the problems of systematisation of group. The article is also devoted to the problems of new methods of phylogenetic constructions. The Congress gave the new basic of ecological researches and biodiversity of oligochaetes in various regions of Kazakhstan and the world. The information is given in the aspect of environment protection.

Micro-food web interactions involving bacteria, nematodes, and mycorrhiza enhance tree P nutrition in a high P-sorbing soil amended with phytate

Phytate is considered a poorly available plant P source but proved to be useful for particular soil bacteria strains. In soil-free conditions, it has been shown that bacteria locked up the mineralized phosphorus from phytate whereas bacterial grazers like nematodes were able to deliver P to plants. Here, we aimed to determine if the interactions between phytate-mineralizing bacteria, bacterial grazer nematodes, and mycorrhizal fungi could increase plant P acquisition from phytate in high P-adsorbing soils. Pinus pinaster was grown in a Cambisol supplemented with phytate. Plants, whether associated or not associated with the ectomycorrhizal fungus Hebeloma cylindrosporum, were either inoculated or not inoculated with the phytase-releasing bacteria Bacillus subtilis and the bacterial-feeding nematode Rhabditis sp. After 100 days, the dual inoculation of bacteria and nematodes significantly increased net plant P accumulation. We observed that, on average, mycorrhizal plants accumulated more P in their shoots than non-mycorrhizal plants. However, the highest plant P acquisition efficiency was found when the three soil organisms were present in the P. pinaster rhizosphere. We conclude that, in a highly inorganic P-fixing soil, plant P acquisition from phytate strongly depends on the grazing of phytate-mineralizing bacteria. Our results confirm the importance of the soil microbial loop to improve plant P nutrition from phytate, which should be considered a route to improve the utilization of this source of poorly available P by plants.

Grazing simplifies soil micro-food webs and decouples their relationships with ecosystem functions in grasslands.

Livestock grazing often alters aboveground and belowground communities of grasslands and their mediated carbon (C) and nitrogen (N) cycling processes at the local scale. Yet, few have examined whether grazing-induced changes in soil food webs and their ecosystem functions can be extrapolated to a regional scale. We investigated how large herbivore grazing affects soil micro-food webs (microbes and nematodes) and ecosystem functions (soil C and N mineralization), using paired grazed and ungrazed plots at 10 locations across the Mongolian Plateau. Our results showed that grazing not only affected plant variables (e.g., biomass and C and N concentrations), but also altered soil substrates (e.g., C and N contents) and soil environment (e.g., soil pH and bulk density). Grazing had strong bottom-up effects on soil micro-food webs, leading to more pronounced decreases at higher trophic levels (nematodes) than at lower trophic levels (microbes). Structural equation modeling showed that changes in plant biomass and soil environment dominated grazing effects on microbes, while nematodes were mainly influenced by changes in plant biomass and soil C and N contents; the grazing effects, however, differed greatly among functional groups in the soil micro-food webs. Grazing reduced soil C and N mineralization rates via changes in plant biomass, soil C and N contents, and soil environment across grasslands on the Mongolian Plateau. Spearman's rank correlation analysis also showed that grazing reduced the correlations between functional groups in soil micro-food webs and then weakened the correlation between soil micro-food webs and soil C and N mineralization. These results suggest that changes in soil micro-food webs resulting from livestock grazing are poor predictors of soil C and N processes at regional scale, and that the relationships between soil food webs and ecosystem functions depend on spatial scales and land-use changes.

Effects of Asian Dust and Phosphorus Input on Abundance and Trophic Structure of Protists in the Southern Yellow Sea

To reveal the effects of Asian dust and phosphorus (P) input on the structure and function of micro-food web in the Yellow Sea, an experiment was conducted onboard the southern Yellow Sea where P was deficient. The response of the abundance and trophic structure of planktonic protists to different concentrations of dust and P were studied. The results showed that the sand-dust deposition presented variable effects on different sizes of protists during incubation periods. At the initial stage of incubation with dust, the amount of all sizes of autotrophic protists, especially 10–20 µm, were improved; on the contrary, the heterotrophic and mixotrophic protists were inhibited. At the late period, the increase of autotrophic protists was restricted, while the 2–5 µm heterotrophic and mixotrophic protists obviously increased. Similarly, adding P demonstrated the obviously positive effect on the 10–20 µm autotrophic protists at the initial period, and then the growth was restricted at the late period. These results were consistent with that of sand-dust deposition. Hence, it could be presumed that the positive effect of sand-dust deposition on autotrophic protists in the southern Yellow Sea was achieved by the release of P from dust. P in the early stage of sand-dust deposition promotes the growth of large-size autotrophic protists, which may accelerate red tides in eutrophic ocean. The stimulation of small-size heterotrophic protists at the late period of sand-dust deposition contributed to the material cycle and food transmission in the ocean. Therefore, the effects of sand-dust deposition on the abundance and trophic structure of different sizes of planktonic protists could change the structure of micro-food web in the southern Yellow Sea and further affected the ecological function of planktonic protists.

Trophic and non-trophic interactions in binary links affect carbon flow in the soil micro-food web

Soil organisms constitute multitrophic food webs, which are important determinants for nutrient and carbon flow through terrestrial systems. However, these complex interactions are not well understood, particular in the most cryptic compartment – the link between microorganisms and microfauna. The present study investigates binary trophic links between microbial prey and nematodes in a controlled semi-natural soil environment. A microcosm system with circulating airflow was filled with γ-irradiated arable soil (Luvisol), inoculated with 13C labelled microbial diet and nematode grazers, and incubated for 72 h. Signatures of 13C and CO2 evolved from microcosms, soil phospholipid fatty acids and nematode densities were determined. Except for the model nematode Caenorhabditis elegans, all organisms joint in the 12 different investigated binary links represent dominant taxa at the arable field site. Studied were the fungal grazers Aphelenchoides saprophilus and Aphelenchus sp. in combination with the fungi Chaetomium globosum, Aspergillus terreus or Cryptococcus terreus as well as the bacterial grazers C. elegans and Acrobeloides buetschlii in combination with the bacteria Pseudomonas putida, Kitasatospora sp. or Sphingomonas trueperi.The results assigned trophic and non-trophic interactions and considerable variation between binary links. The biomass decline of S. trueperi in presence of C. elegans as well as the increased release of 13C in links of C. globosum and A. terreus with Aphelenchus sp. and S. trueperi with A. buetschlii assign nematode grazing and thus a classical predator-prey system. Such direct trophic interactions can induce distinct C fluxes within a binary link as assigned by the increased density (i.e. biomass) of C. elegans when feeding on S. trueperi. In contrast, the numbers of Aphelenchus sp. declined when feeding on A. terreus, and thus reduced C flow to the next trophic level. Grazing can also foster C allocation in microbial biomass via compensatory growth, as indicated e.g. for the link of Aphelenchus sp. with the fungi C. globosum and A. terreus. While these diverse responses to grazing were to be expected, the use of CO2 and 13C measurements further revealed that non-trophic interactions shape C flux in a binary link, particularly in the bacterial channel. Besides the fungal link of C. terreus with Aphelenchus sp., non-trophic interactions were obvious for the bacterial links Kitasatospora sp. with C. elegans or A. buetschlii, and P. putida with A. buetschlii. The presence of nematodes caused a reduction in respiration (evolved CO2), thus a decline in microbial activity. The formation of chemical and/or morphological defense is likely a reason for that reduced prey activity. For further studies the inclusion of such non-trophic interactions is necessary to achieve realistic models for C flow through soil food webs.

Impacts of green manure amendment on detritus micro-food web in a double-rice cropping system

Chinese milk vetch (CMV) is widely used as a green manure due to its efficiency for nutrient preservation and soil fertility improvement, as well for providing more organic substances for soil organisms. Detritus micro-food web drives carbon and nutrient cycling and maintains ecosystem productivity. The effects of CMV amendment on detritus micro-food web were investigated in a six-year field experiment. Compared to the mineral fertilizers (NPK) alone, the fertilizers combined with CMV (NPK + CMV) increased soil organic carbon content, and thus enhanced the biomass of microorganisms, as well as protozoa abundance, while did not change micro-feeding nematode abundance (MN). This indicates a substrate-driven bottom-up effect. However, the green manure application increased omnivores-predators (OP) abundance and the OP/MN ratio, suggesting a top-down effect. Higher F/B ratio indicates a shift of the food web energy structure towards to fungi-dominated channel in the manured soil. Here, the green manure amendment should be encouraged to improve soil detritus micro-food web.

Exotic earthworms maintain soil biodiversity by altering bottom-up effects of plants on the composition of soil microbial groups and nematode communities.

Bottom-up effects of plants on soil communities can be modified by the activity of exotic earthworms, by altering resource availability for soil food webs through feeding, burrowing, and casting activities. The present study explored effects of plants (planting of shrubs) on soil micro-food webs (composition of soil microbial and nematode communities), and whether these effects were altered by the activity of exotic earthworms (exotic earthworms addition). Planted shrubs resulted in a non-significant increase of bacterial biomass and significantly increased the abundance of different nematode trophic groups and total nematode biomass, indicating that planted shrubs had significant bottom-up effects on soil bacteria and nematodes. Planted shrubs decreased nematode diversity, evenness, and richness, but increased nematode dominance in the plots where the abundance of exotic earthworms was not amended. By contrast, these effects of shrub presence on soil biodiversity were not found in the plots that received exotic earthworms. In addition, planted shrubs increased the total energy flux to the nematode community. By contrast, the elevated activity of exotic earthworms mitigated the increase in total energy flux to nematodes in the presence of shrubs, and increased the ratio of fungal to bacterial PLFAs. Both of these changes indicate reduced energy flux in the plots with added exotic earthworms. Nematode diversity decreased, while nematode dominance increased with increasing total energy flux to nematodes, probably because few species benefited from high energy flux. Our study indicates that exotic earthworms can maintain soil biodiversity by reducing the energy flux through soil food webs.

Soil micro-food web interactions and rhizosphere priming effect

The rhizosphere priming effect (RPE) is the stimulation or suppression of soil organic matter decomposition by living roots and associated rhizosphere organisms. The RPE is pivotal in regulating biogeochemical cycles in terrestrial ecosystems. However, biological mechanisms, especially soil micro-food web interactions, behind the RPE remain largely unknown. We quantified the RPE of soybean and cottonwood at three growth stages using a natural 13C tracer method, measured soil microbial and nematode community composition, and investigated their relations with the RPE. The magnitude of the RPE varied widely at different growth stages. Soybean produced a greater cumulative RPE than cottonwood. The plant species effect was also observed in the bacterial PLFA with higher values found in the soybean treatment. Mantel test analysis suggested that the variations in microbial community were closely related with the RPE, soil and plant characteristics. The nematode community affected the RPE indirectly through altering the structure of the microbial community. We demonstrated that the RPE was connected with interactions of soil micro-food webs. This connection indicates that soil micro-food web interactions in the rhizosphere may either regulate microbial turnover and/or microbial community composition, subsequently modulating the RPE.

Long-term fertilization management affects the C utilization from crop residues by the soil micro-food web

Crop residue decomposition is a major component of carbon (C) cycling and provides energy and nutrients to the soil micro-food web. An in-situ field experiment was conducted to examine how exogenous organic C is incorporated into the soil micro-food web and how this is influenced by four different fertilization treatments: organic manure (M), urea fertilizer (U), the combined application of organic and urea fertilizer (MU) and unfertilized control. The amount of 13C-enriched maize remained was traced in microbial phospholipid fatty acids (PLFAs) and in different trophic groups of the soil nematode community after a 183-day decomposition period. The fertilization type influenced the incorporation of residue into the microbial community. Soil bacterial and fungal PLFAs utilized the least 13C-labeled crop residues in the U treatment. Both the nematode bacterial and the fungal pathways utilized more crop residues in the M treatment than in other treatments. Given the ecological services provided by the soil organisms, our results suggest that long-term manure application increases the soil C pool directly. This also leads to more C from crop residues utilized by the soil food web, which in turn, can benefit crop growth or C accumulation in agroecosystems.

Micro-Food Web Structure Shapes Rhizosphere Microbial Communities and Growth in Oak

The multitrophic interactions in the rhizosphere impose significant impacts on microbial community structure and function, affecting nutrient mineralisation and consequently plant performance. However, particularly for long-lived plants such as forest trees, the mechanisms by which trophic structure of the micro-food web governs rhizosphere microorganisms are still poorly understood. This study addresses the role of nematodes, as a major component of the soil micro-food web, in influencing the microbial abundance and community structure as well as tree growth. In a greenhouse experiment with Pedunculate Oak seedlings were grown in soil, where the nematode trophic structure was manipulated by altering the proportion of functional groups (i.e., bacterial, fungal, and plant feeders) in a full factorial design. The influence on the rhizosphere microbial community, the ectomycorrhizal symbiont Piloderma croceum, and oak growth, was assessed. Soil phospholipid fatty acids were employed to determine changes in the microbial communities. Increased density of singular nematode functional groups showed minor impact by increasing the biomass of single microbial groups (e.g., plant feeders that of Gram-negative bacteria), except fungal feeders, which resulted in a decline of all microorganisms in the soil. In contrast, inoculation of two or three nematode groups promoted microbial biomass and altered the community structure in favour of bacteria, thereby counteracting negative impact of single groups. These findings highlight that the collective action of trophic groups in the soil micro-food web can result in microbial community changes promoting the fitness of the tree, thereby alleviating the negative effects of individual functional groups.

Structure and ecological functions of soil micro-food web.

Soil micro-food web, as a part of the detritus food webs, directly or indirectly participates in nutrient cycling by feeding on substrate and microorganisms and consequently influences the functions of terrestrial ecosystems. Here, we reviewed the research progress of soil micro-food web in recent years by focusing on its composition, structure and ecological function in soil ecosystem. The important roles of soil micro-food web in driving carbon (C) and nitrogen (N) transformation, organic matter decomposition and plant growth were reviewed through the description of energy channel and trophic cascade effects of soil micro-food web. At last, the research perspectives were put forward. Future researches should be combined with high-throughput sequencing and stable isotope techniques for better understanding the belowground ecological process and their feedback mechanisms to plant growth through modeling analysis.

The relative importance of the bacterial pathway and soil inorganic nitrogen increase across an extreme wood‐ash application gradient

AbstractAsh from combustion of biofuels, for example wood chips, is often deposited as waste, but due to its high content of essential plant nutrients and alkalinity, it has been proposed to recycle ash as a fertilizer and liming agent in biofuel production forest. However, current legislation sets rather strict limitations for wood‐ash application in biofuel production systems. The soil microfood web, that is microorganisms and their microfaunal grazers, protozoa and nematodes, is pivotal for essential ecosystem processes such as decomposition and plant nutrient release. Therefore, a thorough assessment of the impacts on microfood web structure and functioning must precede actions towards raising the currently allowed application rates. In a Danish Norway spruce plantation, we evaluate the impact of wood ash applied at dosages from 0 to the extreme case of 90 t ash ha−1 on the microfood web, the bacterial community structure, soil content of inorganic nitrogen, organic matter, dissolved organic carbon and nitrogen. Using structural equation modelling (SEM), we disentangled the direct effect of the disturbance imposed by ash per se, the associated pH increase and changes in prey abundance on individual organism groups in the microfood web. The SEM showed that the pH rise was the main driver of increasing abundances of culturable heterotrophic bacteria with increasing ash doses, and via trophical transfer, this also manifested as higher abundances of bacterial grazers. Fungal‐feeding nematodes were unaffected by ash, whereas carnivorous/omnivorous nematodes decreased due to the direct effect of ash. Increasing ash doses enhanced the difference between bacterial communities of control plots and ash‐amended plots. The ash‐induced stimulation of culturable heterotrophic bacteria and bacterial grazers increased inorganic nitrogen availability at ash doses of 9 t ha−1 and above. Hence, raised limits for ash application may potentially benefit tree growth via enhanced N mineralization activity of the soil food web.

Responses of rice paddy micro-food webs to elevated CO2 are modulated by nitrogen fertilization and crop cultivars

Elevated atmospheric CO2 concentrations (eCO2) often increase plant growth but simultaneously lead to the nitrogen (N) limitation in soil. The corresponding mitigation strategy such as supplementing N fertilizer and growing high-yielding cultivars at eCO2 would further modify soil ecosystem structure and function. Little attention has, however, been directed toward assessing the responses of soil food web. We report results from a long-term free air CO2 enrichment (FACE) experiment in a rice paddy agroecosystem that examined the responses of soil micro-food webs to eCO2 and exogenous nitrogen fertilization (eN) in the rhizosphere of two rice cultivars with distinctly weak and strong responses to eCO2. Soil micro-food web parameters, including microfauna (protists and nematodes) and soil microbes (bacteria and fungi from phospholipid fatty acid (PLFA) analysis), as well as soil C and N variables, were determined at the heading and ripening stages of rice. Results showed that eCO2 effects on soil micro-food webs depended strongly on N fertilization, rice cultivar and growth stage. eCO2 stimulated the fungal energy channel at the ripening stage, as evidenced by increases in fungal biomass (32%), fungi:bacteria ratio (18%) and the abundance of fungivorous nematodes (64%), mainly due to an enhanced carbon input. The eN fueled the bacterial energy channel by increasing the abundance of flagellates and bacterivorous nematodes, likely through alleviating the N-limitation of plants and rhizosphere under eCO2. While eCO2 decreased the abundance of herbivorous nematodes under the weak-responsive cultivar by 59% and 47% with eN at the heading and ripening stage, respectively, the numbers of herbivorous nematodes almost tripled (×2.9; heading) and doubled (×1.6; ripening) under the strong-responsive cultivar with eCO2 at eN due to higher root quantity and quality. Structural equation model (SEM) showed that lower trophic-level organisms were affected by bottom-up forces of altered soil resources induced by eCO2 and eN, and effects on higher trophic level organisms were driven by bottom-up cascades with 69% of the variation being explained. Taken together, strategies to adapt climate change by growing high-yielding crop cultivars under eCO2 may face a trade-off by negative soil feedbacks through the accumulation of root-feeding crop pest species.

Disentangling the root- and detritus-based food chain in the micro-food web of an arable soil by plant removal.

Soil food web structure and function is primarily determined by the major basal resources, which are living plant tissue, root exudates and dead organic matter. A field experiment was performed to disentangle the interlinkage of the root-and detritus-based soil food chains. An arable site was cropped either with maize, amended with maize shoot litter or remained bare soil, representing food webs depending on roots, aboveground litter and soil organic matter as predominant resource, respectively. The soil micro-food web, i.e. microorganisms and nematodes, was investigated in two successive years along a depth transect. The community composition of nematodes was used as model to determine the changes in the rhizosphere, detritusphere and bulk soil food web. In the first growing season the impact of treatments on the soil micro-food web was minor. In the second year plant-feeding nematodes increased under maize, whereas after harvest the Channel Index assigned promotion of the detritivore food chain, reflecting decomposition of root residues. The amendment with litter did not foster microorganisms, instead biomass of Gram-positive and Gram-negative bacteria as well as that of fungi declined in the rooted zone. Likely higher grazing pressure by nematodes reduced microbial standing crop as bacterial and fungal feeders increased. However, populations at higher trophic levels were not promoted, indicating limited flux of litter resources along the food chain. After two years of bare soil microbial biomass and nematode density remained stable, pointing to soil organic matter-based resources that allow bridging periods with deprivation. Nematode communities were dominated by opportunistic taxa that are competitive at moderate resource supply. In sum, removal of plants from the system had less severe effects than expected, suggesting considerable food web resilience to the disruption of both the root and detrital carbon channel, pointing to a legacy of organic matter resources in arable soils.

Differentiating between root- and leaf-litter controls on the structure and stability of soil micro-food webs

The dynamics of soil food webs play a key role in determining the ecological response of terrestrial ecosystems to current and future environmental change. Both root and leaf litter fuel the micro-food webs (microorganisms and nematodes) in soil, but whether the root and leaf litter would favor different components of the micro-food webs is not clear, especially the relative importance of these two litter sources on the stability of the micro-food web.We compared micro-food webs in mineral soils without added litter, with added leaf litter and with added root litter and investigated the effect of the litter carbon source on the resistance of the micro-food webs to added nitrogen and added nitrogen and phosphorus.The nematode community was more sensitive than the microbial community to the carbon source. Root litter was favored by bacteria, and leaf litter was preferred by fungi. Herbivorous nematodes were controlled more by the input of leaf litter than the input of root litter. Fertilization increased the effects of the root litter on the structure of the nematode community and thus moderated the effects of the litter source on the micro-food web. When both nitrogen and phosphorus were added, the root litter fueled the more resistant first trophic level (microorganisms), and the leaf litter fueled the more adaptable second trophic level (nematodes).Our results suggest that the input of leaf litter would be more important than the input of root litter for promoting a conservative cycling of nutrients and more positive soil feedback effects on plant growth. Our results also indicate that leaf litter and root litter help micro-food webs to adapt to exogenous disturbance in different ways, which would aid our mechanistic understanding of how soil food webs and their associated ecosystem processes would respond to current and future environmental changes.

Responses of Soil Micro-Food Web to Land Use Change from Upland to Paddy Fields with Different Years of Rice Cultivation

Land use changes affect belowground ecosystems. During the past few decades, land use in Northeast China has changed considerably, and the area of paddy fields has increased rapidly from upland. In this study, soil characteristics and soil biotic community in paddy fields with different years of rice cultivation were measured to examine the effects of land use change from upland to paddy fields on soil micro–food web. The upland maize fields were selected as control and the microbial community composition was characterized using phospholipid fatty acids (PLFAs) analysis. The microbial biomass (total PLFA), bacteria biomass, and fungi biomass were higher in the 20–40–year (late–stage) than 1–10–year (early–stage) paddy fields. The abundances of total nematodes and bacterivores were lower in the early–stage than late–stage paddy fields. The abundance of herbivores was the highest in the early–stage paddy fields but that of omnivore–predators was the highest in the late–stage paddy fields. Structural equation model indicated that soil food web was developed and structured after 20 years of paddy cultivation. Our results suggested that soil micro–food web may be a good indicator for soil development and stabilization of paddy fields following land use change.

Tidal flooding diminishes the effects of livestock grazing on soil micro-food webs in a coastal saltmarsh

Livestock grazing not only has a direct impact on plant productivity but also exerts an indirect influence on soil biota via various pathways. However, little is known about the effects of livestock grazing on soil food webs in saltmarsh ecosystems that are subject to regular tidal inundation stress. By exclosure experiments established at a frequently inundated middle marsh and a less inundated high marsh of Chongming Island (China), the responses of soil micro-food web components (microorganisms, protozoa, and nematodes) to cattle grazing in intertidal marshes were investigated. In the high marsh, cattle grazing significantly increased the biomass of soil microorganisms, protozoa, and the abundance of total nematodes by 30.0%, 97.3% and 76.2%, respectively, but did not significantly affect their biomass or abundance in the middle marsh. For low-trophic-level nematodes, the abundance of bacterial-feeding and algal-feeding nematodes increased more in the high marsh than in the middle marsh, and that of plant-feeding nematodes decreased more in the high marsh than in the middle marsh under grazing. In contrast, carnivorous and omnivorous nematodes at high trophic levels did not respond to cattle grazing along an elevational gradient. The nematode maturity index and structure index based on nematode functional guilds significantly decreased under grazing along the elevational gradient, suggesting that cattle grazing caused a more simplified and unstable soil micro-food web structure. Overall, low trophic levels in soil micro-food webs were most vulnerable under grazing and the response was strongest in the less inundated high marsh. Thus, cattle grazing leads to different changes in soil ecosystem processes at different elevations. These results indicate that the strength of the biotic grazing effect on soil micro-food webs and ecological functions might also depend on local abiotic disturbance such as tidal inundations in the saltmarsh.

Fluxes of root-derived carbon into the nematode micro-food web of an arable soil

Organic carbon (C) released from living roots forms a major resource for microorganisms controlling energy, C pathways and, hence, food web structure and dynamics. However, knowledge on quantitative C fluxes into food web compartments is scarce. Nematodes, with functional groups at each trophic level, served as a model community for assessing root C fluxes into the micro-food web. Maize, grown on soil cores from an arable field, was pulse-labeled with 14CO2 followed by sampling 2, 5, 10 and 16days after labeling. Nematode population density, community structure, trophic groups and their 14C activities were analyzed.Overall, 55 genera of 22 families were detected. Plant-feeders, which had the highest density, showed the fastest and highest incorporation of root C. Bacterial-feeders incorporated more root-derived 14C than fungal-feeders. This was consistent with a bacterial- to fungal-feeder-ratio of 0.63 and a moderate to low Channel Index (average 38), a nematode faunal index that assigns the magnitude of carbon flow via the bacterial or fungal channel, both indicating a major energy flux in the bacterial decomposition pathway. Predators and omnivores showed low incorporation of root-derived C, pointing to a basal food web structure with short food chains and low energy transfer to higher trophic levels.Combining 14C tracing with taxonomic identification of nematodes allowed quantification of root C fluxes into food web compartments. The incorporation of root C into nematodes was small (~0.1% of that in microbial biomass), yet forms an important part of belowground C channeling as it links microbial and faunal food web.

Spatial and temporal variation of resource allocation in an arable soil drives community structure and biomass of nematodes and their role in the micro-food web

The assemblages of organisms residing at different soil horizons are important drivers for carbon and nutrient cycle in arable systems, yet knowledge on food web dynamics below the plough zone is sparse. The present study investigated the effects of management practice on the nematode community structure and biomass in the top soil, rooted zone below plough layer, and root free deeper soil in two successive vegetation periods. Resource quality was manipulated by crop plant (maize and wheat) and organic amendment (with/without maize litter), to differently fuel the major soil carbon pathways based on roots, bacteria and fungi. Nematode assemblages were used as model to determine changes in the structure, function and carbon allocation of the soil micro-food web.The biomass of nematode families as well as the fungal to bacterial feeder ratio F/(F+B) assigned a predominance of the bacterial carbon channel in the arable soil. The differences in spatial and temporal availability of major plant resources, i.e. litter (recalcitrant substrate) and rhizodeposits (labile carbon), separated the nematode fauna into meta-communities along the depth profile. The seasonal pattern in population density, and the positive effects of litter amendment predominantly in the top soil, indicated low migration between these patches during crop growth. However, mass-flow as resource subsidy for communities at depth takes place under fallow during winter. Crop type was most important for carbon allocation, with wheat generally resulting in a greater biomass build-up of nematodes. The biomass distribution of trophic groups differed with depth, pointing to a strong bottom-up effect in the root channel, whereas predation likely was the primary regulating force in the fungal channel. Overall, combining nematode community composition with biomass revealed spatially separated micro-food web assemblages, differing in structure and carbon allocation. Thereby transport processes cross community boundaries, linking top soil and subsoil, notably in periods without a crop.

The dynamics of soil micro-food web structure and functions vary according to litter quality

Anthropogenic pressures on agricultural soils are known to alter the properties of soil food webs, which may affect ecosystem functions and the capacity to deliver services. Cropping systems fuel belowground biota by supplying litter, but litter quality varies from year to year. In this study, we hypothesized that (i) the structure and function of the soil biota may respond strongly to the quality of crop residue, and (ii) this response may vary with the complexity of the food web. To test this hypothesis, a 3-month incubation experiment was performed using soil columns filled with a silty loam agricultural soil (Estrées-Mons, Northern France), and plant litter was incorporated into the 0–5-cm layer. The quality of the litter was modulated by using either high quality (maize leaves) or low quality materials (maize roots), and two levels of biotic complexity were tested by introducing or not introducing an ecosystem engineer (Lumbricus terrestris, anecic earthworm) into the columns. We then investigated litter decomposition and enzymatic activities, as well as the biomass and diversity of the bacterial, fungal and nematode communities. The C mineralization and enzymatic profiles varied according to litter quality, and the most obvious differences were observed in the hydrolytic enzymes and phenol oxidase. Micro-food web interactions and structure were mainly impacted by litter type and the time since litter incorporation, and as expected, we observed a temporal succession of cellulolytic organisms followed by lignolytic organisms. This progression was mirrored by the nematode community; the ratio of fungal feeders/bacterial feeders increased between 35 and 91 days. Initial litter quality primarily impacted bacterial and fungal community structure, and the development of the bacterial channel was greater with the addition of leaf litter. This was related to an increase in specific, favored phyla, Bacteroidetes and Proteobacteria, whereas Actinobacteria increased more with the addition of root litter. However, we could not statistically link the changes in the structure of the microbial communities over time with those in enzyme activity, which led us to assume that other factors, such as top-down regulation, are more important than microbial community structure in the determination of the enzyme secretions in the soil. Finally, the addition of earthworms did not significantly impact litter decomposition and had a low impact on micro-food web structure and interactions. However, the presence of L. terrestris did modify the enzymatic secretions, probably through its endemic microbial population, as well as the stimulation of microbial activity through predation.