Potapov Anton M, Tiunov Alexei V, Scheu Stefan
University of Göttingen, J.F. Blumenbach Institute of Zoology and Anthropology, Untere Karspüle 2, 37073 Göttingen, Germany.
Russian Academy of Sciences, A.N. Severtsov Institute of Ecology and Evolution, Leninsky Prospect 33, 119071 Moscow, Russia.
Biol Rev Camb Philos Soc. 2019 Feb;94(1):37-59. doi: 10.1111/brv.12434. Epub 2018 Jun 19.
Despite the major importance of soil biota in nutrient and energy fluxes, interactions in soil food webs are poorly understood. Here we provide an overview of recent advances in uncovering the trophic structure of soil food webs using natural variations in stable isotope ratios. We discuss approaches of application, normalization and interpretation of stable isotope ratios along with methodological pitfalls. Analysis of published data from temperate forest ecosystems is used to outline emerging concepts and perspectives in soil food web research. In contrast to aboveground and aquatic food webs, trophic fractionation at the basal level of detrital food webs is large for carbon and small for nitrogen stable isotopes. Virtually all soil animals are enriched in C as compared to plant litter. This 'detrital shift' likely reflects preferential uptake of C-enriched microbial biomass and underlines the importance of microorganisms, in contrast to dead plant material, as a major food resource for the soil animal community. Soil organic matter is enriched in N and C relative to leaf litter. Decomposers inhabiting mineral soil layers therefore might be enriched in N resulting in overlap in isotope ratios between soil-dwelling detritivores and litter-dwelling predators. By contrast, C content varies little between detritivores in upper litter and in mineral soil, suggesting that they rely on similar basal resources, i.e. little decomposed organic matter. Comparing vertical isotope gradients in animals and in basal resources can be a valuable tool to assess trophic interactions and dynamics of organic matter in soil. As indicated by stable isotope composition, direct feeding on living plant material as well as on mycorrhizal fungi is likely rare among soil invertebrates. Plant carbon is taken up predominantly by saprotrophic microorganisms and channelled to higher trophic levels of the soil food web. However, feeding on photoautotrophic microorganisms and non-vascular plants may play an important role in fuelling soil food webs. The trophic niche of most high-rank animal taxa spans at least two trophic levels, implying the use of a wide range of resources. Therefore, to identify trophic species and links in food webs, low-rank taxonomic identification is required. Despite overlap in feeding strategies, stable isotope composition of the high-rank taxonomic groups reflects differences in trophic level and in the use of basal resources. Different taxonomic groups of predators and decomposers are likely linked to different pools of organic matter in soil, suggesting different functional roles and indicating that trophic niches in soil animal communities are phylogenetically structured. During last two decades studies using stable isotope analysis have elucidated the trophic structure of soil communities, clarified basal food resources of the soil food web and revealed links between above- and belowground ecosystem compartments. Extending the use of stable isotope analysis to a wider range of soil-dwelling organisms, including microfauna, and a larger array of ecosystems provides the perspective of a comprehensive understanding of the structure and functioning of soil food webs.
尽管土壤生物群落在养分和能量流动中具有重要意义，但人们对土壤食物网中的相互作用却知之甚少。在此，我们概述了利用稳定同位素比率的自然变化来揭示土壤食物网营养结构的最新进展。我们讨论了稳定同位素比率的应用、标准化和解释方法以及方法上的缺陷。通过分析温带森林生态系统已发表的数据，来概述土壤食物网研究中出现的概念和观点。与地上和水生食物网不同，碎屑食物网基础层面的碳营养分馏较大，而氮稳定同位素的营养分馏较小。实际上，与植物凋落物相比，所有土壤动物的碳含量都更高。这种“碎屑转移”可能反映了对富含碳的微生物生物量的优先摄取，并突出了微生物而非死亡植物材料作为土壤动物群落主要食物资源的重要性。土壤有机质相对于落叶富含氮和碳。因此，栖息在矿质土壤层的分解者可能富含氮，导致土壤栖息的碎屑食性动物和落叶栖息的捕食者之间的同位素比率重叠。相比之下，上层落叶和矿质土壤中的碎屑食性动物之间的碳含量变化不大，这表明它们依赖于相似的基础资源，即少量分解的有机物质。比较动物和基础资源中的垂直同位素梯度，可成为评估土壤中营养相互作用和有机物质动态的宝贵工具。稳定同位素组成表明，土壤无脊椎动物中直接取食活植物材料以及菌根真菌的情况可能很少见。植物碳主要被腐养微生物吸收，并传递到土壤食物网的更高营养级。然而，取食光合自养微生物和非维管植物可能在为土壤食物网提供能量方面发挥重要作用。大多数高级动物类群的营养生态位至少跨越两个营养级，这意味着它们利用多种资源。因此，为了识别食物网中的营养物种和联系，需要进行低等级的分类鉴定。尽管取食策略存在重叠，但高级分类群的稳定同位素组成反映了营养级和基础资源利用方面的差异。不同分类群的捕食者和分解者可能与土壤中不同的有机物质库相关联，这表明它们具有不同的功能作用，并表明土壤动物群落中的营养生态位在系统发育上具有结构。在过去二十年中，使用稳定同位素分析的研究阐明了土壤群落的营养结构，明确了土壤食物网的基础食物资源，并揭示了地上和地下生态系统部分之间的联系。将稳定同位素分析的应用扩展到更广泛的土壤栖息生物，包括小型动物区系，以及更广泛的一系列生态系统，有望全面了解土壤食物网的结构和功能。
