Distinct macroinvertebrate soil food webs at one-meter scale in a Mediterranean agroecosystem

Abstract

Soil macroinvertebrates play a central role in ecosystem processes such as decomposition and nutrient cycling, but the extent to which macroinvertebrate food web structure varies within ecosystems is basically unknown. Here, we examined changes in plant and macroinvertebrate community composition at increasing distances from trees within a citrus grove, following a gradient in organic matter and water availability created by the localized fertilization and irrigation of citrus trees. In addition, we analysed stable isotope signatures (δ13C and δ15N) of soil, plants, and macroinvertebrates to investigate changes in trophic structure within the grove. The macroinvertebrate community near the tree trunks had a higher number of detritivores and predators than the community located farther away. After correcting the isotope values of macroinvertebrates by their isotopic baseline, a spatial shift in trophic structure emerged: macroinvertebrates near the trunks had lower δ13C-values than those located farther away (i.e., they apparently relied more on plant- based resources). This pattern is attributable to species only found near the trunks. In contrast, species distributed across the studied area showed no differences in their isotopic signatures across space, suggesting that they are connecting otherwise two compartmentalized food webs. A reduction in organic matter and water availability at increasing distances from trees is the most parsimonious explanation of the observed isotopic gradient in soil and plants, and this may, in turn, be responsible for the observed changes in food web composition and structure. This study shows that spatial variability in macroinvertebrate soil food webs can be present at scales as small as one meter if spatial variability in environmental factors is high enough.