Location in Soil Pores as Determinant of Resource Accessibility for Microarthropods

Abstract

Trophic interactions between soil-living organisms occur largely in soil pores and are essential for soil functioning. As soil is a three-dimensional environment where resources are distributed in pores of contrasting size, and higher trophic level consumers (grazers) are typically larger than their microbial food sources, the location of microbial resources in soil pores is likely a major determinant of resource accessibility for microbial consumers. However, experimental evidence for this assumption is lacking for many soil organisms, notably microarthropods, the world’s most numerous soil arthropods. Here, we investigated whether the location of microorganisms in pores with different neck diameters influences their accessibility to microarthropod consumers of different body size, and how this affects microbial communities. Using different soil matric potentials, we labelled microbial communities in different pore size classes using 13C sodium pyruvate solution and traced 13C incorporation into three species of Collembola (Insecta). Collembola incorporated labelled C in each of the pore size treatments, indicating that microorganisms are not fully physically protected from consumption by microarthropods even in pores that are one order of magnitude smaller than their body width. However, 13C incorporation increased with pore size and was strongest in small Collembola, reflecting size limitation of resource accessibility. Overall, our study demonstrates that carbon sources in small pores are not fully protected from consumption by microarthropod consumers much larger than the pore necks, but also that the location of carbon resources in soil pores drives resource accessibility to microarthropod consumers. The size restriction of resource availability points to soil structure as an important driver of microarthropod trophic niches.

Keywords: soil physical structure, micro-scale feeding, PLFA, springtail, Collembola, 13C label