Tracking the flow of bacterially derived¹³C and¹⁵N through soil faunal feeding channels

Abstract

The soil food web has been referred to as a 'black box', a 'poor man's tropical rainforest' and an 'enigma', due to its opacity, diversity and the limited insight into feeding specificity. Here we investigate the flow of C and N through the soil food web as a way to gain understanding of the feeding interactions occurring. A bacterium, Pseudomonas lurida, was introduced to soil cores from two different habitats, a grassland and a woodland with the same soil type, enriched to 99 atom% in (13)C and (15)N, to trace the flow of bacterial C and N through the soil food web. Throughout the experiment the soil remained enriched in (13)C and (15)N. Almost all the invertebrates tested gained C and N enrichment indicative of the labelled bacteria, implying that bacterial feeding is a common mechanism within the soil. Only three groups were significantly enriched in both (13)C and (15)N in both habitats. These were Collembola (Entomobryomorpha), Acari (Oribatida), and Nematoda, indicating that these organisms are consuming the most bacteria within both systems. When the invertebrates were grouped into hypothesised trophic levels, those considered secondary decomposers were gaining the most enrichment across all invertebrates tested. This enrichment was also high in the micro-predators within the soil, implying that their main food source was the secondary decomposers, particularly the Collembola. Using an enriched bacterium to track the trophic transfer between organisms within the soil food web is a novel way of empirically showing that interactions are occurring, which normally cannot be seen.

Protozoan Pulses Unveil Their Pivotal Position Within the Soil Food Web

“…The grassland site had received no inorganic-N fertiliser for over 30 years although it had been grazed annually by cattle and the floral assemblage was predominantly grass species. The willow woodland was planted approximately 25 years ago and although coppiced regularly in the first 5 years after planting, it has not been coppiced or disturbed in the last 10 years; the botanical composition of the cores taken consisted of under-canopy woodland forbs (for full site descriptions, see Crotty et al [13] and also Table 1). Ten intact soil cores (10-cm diameter, 10-cm deep) were collected in April 2010, from each site.…”

“…Gas samples were analysed for 13 CO 2 only, using a trace gas analyser (TGII, PDZ-Europa, Crewe, UK) linked to a mass spectrometer (20/20 PDZ-Europa, Crewe, UK). The isotope abundance in the plant, soil, headspace gas and invertebrate samples are expressed as atom% excess (APE) 13 C or 15 N and calculated as Eqs. 1 and 2.…”

“…In the grassland soil, invertebrate taxa with the greatest proportion of total biomass were Nematodes (27%), Lumbricina (22%) and Diptera larvae (14%) ( Table 2), but the invertebrates that incorporated the greatest proportion of 13 C of protozoan origin were Lumbricina (30%), Entomobryomorpha (21%) and Mesostigmata (9%), whilst the Nematodes and Diptera larvae accumulated less than 5% (Fig. 5).…”

“…Methods have been developed to enrich bacteria [38], fungi [12], earthworms [16], slugs [22] and plant litter [48], to trace the passage of 13 C and 15 N through soil ecosystems and act as an isotopic baseline [49]. Our aim of this study was to extend the methods developed to determine the uptake of bacterially derived C and N into the soil food web [13,38], using protozoa. Here, for the first time, we can report the use of stable isotopes to identify the trophic utilisation of indigenous protozoa and trace linkages within the soil food web.…”

Oribatid Mites (Acari: Oribatida) in Microcosms - A Review

“…Remén et al (2010) and Crowther et al (2005Crowther et al ( , 2010 carried out soil microcosms with only one oribatid species and with a single micorrhiza or saprotrophic fungi species and the decrease of fungus biomass consumed by mites was monitored. Crotty et al (2010) introduced bacteria enriched to 99 atom% in 13 C and 15 N to soil cores in laboratory and traced the flow of isotopes through the food web. They found that some of the oribatid mite species were enriched both in 13 Anderson (1978) demonstrated in a microcosm experiment that Nothrus silvestris and Hermanniella granulata Nicolet, 1855 compete with each other because the presence of both species in mini ecosystems affected their feeding and distribution over microcosm layers.…”