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This study assessed the carbon (C) budget and the C stocks in major compartments of the soil food web (bacteria, fungi, protists, nematodes, meso- and macrofauna) in an arable field with/without litter addition. The C stocks in the food web were more than three times higher in topsoil (0–10 cm) compared to subsoil (>40 cm). Microorganisms contained over 95% of food web C, with similar contributions of bacteria and fungi in topsoil. Litter addition did not alter C pools of soil biota after one growing season, except for the increase of fungi and fungal feeding nematodes in the topsoil. However, the C budget for functional groups changed with depth, particularly in the microfauna. This suggests food web resilience to litter amendment in terms of C pool sizes after one growing season. In contrast, the distinct depth dependent pattern indicates specific metacommunities, likely shaped by dominant abiotic and biotic habitat properties.

Agroecosystems occupy large areas of the land surface and arable soil food webs are of significant importance for global cycling of carbon (C) and nitrogen (N). In a field experiment we labeled maize plants ( Zea mays L.) in 13 CO 2 atmosphere and by K 15 NO 3 fertilization. During 25 days, the incorporation of 13 C and 15 N was traced in plant compartments, soil and soil arthropods, as well as 13 C in microbial phospholipid fatty acids (PLFAs) and nematodes. Highlighting the importance of root-derived resources in agroecosystems, 13 C was incorporated into all food web compartments, including microorganisms (PLFAs), nematodes and arthropods. The amount of incorporated 13 C (as compared to unlabeled samples) markedly decreased along the food chain with ∆ 13 C decreasing from 500‰ in plant roots and 900‰ in microbial PLFAs, to less than 40‰ in nematodes and arthropods. Incorporation of 13 C into fungal PLFAs considerably exceeded that into bacterial PLFAs, highlighting the importance of soil fungi as compared to bacteria in C cycling. Fertilizer-derived 15 N uniformly increased with time in plant compartments and soil arthropods, indicating that N is distributed homogeneously in the soil food web. High channeling of both root-derived 13 C and fertilizer-derived 15 N to higher trophic levels by fungi, and intensive feeding on fungi by soil animals highlight the central role of saprotrophic fungi in C and nutrient fluxes in soil food webs of arable ecosystems.

Plant-derived carbon (C) transfer to soil is one of the important factors controlling the size and structure of the belowground microbial community. The present study quantifies this plant-derived C incorporation into abiotic and biotic C pools in top- and subsoil in an arable field over five years. Stable isotope analysis was used to determine the incorporation of maize root and shoot litter C into soil organic C (SOC), extractable organic C (EOC), total microbial biomass (Cmic), ergosterol and phospholipid fatty acids (PLFAs). The following treatments were investigated: corn maize (CM), providing root- and shoot-derived C (without corncobs), fodder maize (FM), providing only root-derived C, and wheat plus maize shoot litter amendment (WL), providing only shoot-derived maize C. Wheat plants (W) without maize litter amendment served as control. Soil samples were taken each September directly before harvest from 2009 to 2013. During the experiment, the maize-derived C signal increased in SOC, EOC, Cmic, ergosterol, bacterial and fungal PLFAs in the topsoil (0–10 cm). Although total maize shoot C input was threefold lower than maize root C input, similar relative amounts of maize C derived from shoots and roots were incorporated into the different C pools in the WL and the FM treatments, indicating the importance of shoot-derived C sources for microorganisms in the topsoil. An additive effect of both C sources was found in the CM treatment with almost twice as much maize-derived C in the respective pools. Furthermore, the proportion of maize-derived C varied between the different pools with lower incorporation into the total SOC (17%) and total EOC (24%) pools and higher incorporation ratios of maize C into PLFAs of different microbial groups (29% in Gram-positive (Gr+) bacterial PLFA-C, 44% in Gram-negative (Gr−) bacterial PLFA-C, 69% in fungal PLFA-C and 78% in ergosterol) in the CM treatment in topsoil after five years. After the third and fifth vegetation periods, we also detected maize-derived C in the rooted zone (40–50 cm depth) and the root-free zone (60–70 cm depth). The maize-derived C incorporation was lower in subsoil C pools in comparison to topsoil C pools. In the root-free zone, the maize-derived C was found to be 2% in total SOC, 28% in total EOC, 9% in Gr+ bacterial PLFA-C, 20% in Gr− bacterial PLFA-C and 53% in fungal PLFA-C. Saprotrophic fungi incorporated maize-derived C in all soil depths to a greater degree than Gr+ and Gr− bacteria, indicating the importance of saprotrophic fungi in this agro-ecosystem.

Summary 1. The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web. 2. The present study employed 13 CO2 pulse labelling to investigate the incorporation of maize root-derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound-specific (lipids) C isotope ratios were used to quantify pool sizes and 13 C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso- and macrofauna. 3. About 20% of the C assimilated by maize was transferred to below-ground pools. 13 C was predominantly incorporated into rhizosphere micro-organisms rather than in those of the bulk soil. 13 C in phospholipid fatty acid biomarkers revealed that root-derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13 C were transferred to higher trophic levels, predominantly into fungal-feeding nematodes and macrofauna decomposers. 4. Most importantly, C pool size and 13 C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal-feeding nematodes and meso- and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web.

Removal of crop residues has become common practice in arable systems, however, little is known about how soil arthropod communities change in response to reduced resource availability and habitat complexity associated with residue removal. We added maize residues to wheat and maize fields and investigated soil arthropod diversity and abundance over the period of one year. Residue addition did not affect the diversity and little affected the abundance of soil arthropods in wheat and maize fields with the latter being restricted to few taxonomic groups, suggesting that at least in the short-term soil arthropods benefit little from crop residue-mediated increase in food supply and habitat structure. Contrasting the minor effects of residue addition, densities of soil arthropods were much higher in wheat than in maize fields, presumably due to more dense and more continuous coverage by plants, and higher input of root residues. Furthermore, in wheat fields density of arthropods more strongly varied with season, presumably due to more pronounced pulses of root exudates and root residues entering the soil in wheat as compared to maize fields in summer and winter, respectively. Low density and little variation in densities of soil arthropods in maize fields reflect that environmental conditions and resource supply varied little with crop coverage and season. Overall, the results point to low importance of aboveground crop residues for soil arthropod communities and highlight that belowground plant resources, i.e. root exudates and root residues are the major driver of soil arthropod communities of arable systems. Thus, at least in short term removal of crop residues for e.g., biofuel production is likely to be of minor importance for soil arthropod communities. In contrast, changing crop species from wheat to maize markedly reduces the density of soil animals threatening the ecosystem functions they provide.

The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C org ), extractable organic C (EOC), microbial biomass (C mic ) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C org , EOC, C mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.

Soil organic matter (SOM) is the main energy resource of the great diversity of soil invertebrates but knowledge on the contribution of SOM pools of different ages to soil animal nutrition is sparse. In the present study natural stable isotope ratios were used to investigate the incorporation of decade old wheat-borne carbon into soil invertebrates of different trophic levels of a maize field 27 years after the replacement of wheat. For comparison a nearby continuous wheat field was investigated. Although the soil food web of the wheat and the maize field consisted of similar invertebrate taxa, most taxa were significantly more abundant in the wheat than in the maize field confirming that C4 plants are generally of poor food quality to soil animals due to low nutrient and high fibre contents. However, different cultivation strategies may also have contributed to differences in the abundance of invertebrates between both study sites. The mean incorporation of C3 carbon in soil animals in the maize field was 60.6 ± 27.8%, but significantly differed between invertebrate species. Unfortunately, the field size did not entirely exclude incorporation of C3 carbon into the tissue of large and mobile invertebrates by feeding on C3 resources outside of the maize field. However, less mobile species such as endogeic earthworms incorporated high amounts of C3 carbon presumably due to the mobilisation of old carbon pools enclosed in soil aggregates and inaccessible to other soil invertebrates during gut passage. Furthermore, small and less mobile invertebrate species, such as most hemi- and epiedaphic Collembolans, also incorporated high amounts of C3 carbon likely by feeding on saprophytic fungi and microorganisms. In contrast, euedaphic Collembola incorporated only small amounts of C3 carbon suggesting preferentially feeding on maize resources. Overall, our data suggest that decade old carbon resources form an important component of the soil animal food web, but that the exploitation of old carbon resources by animal species varies with their distribution within the soil matrix, trophic position and exploitation of algae, and as an artefact to the small size of the study site, animal mobility.