A team of scientists from the Laboratory of Soil Zoology and General Entomology of the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences and the Institute of Biology of the Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences have jointly traced the pathways of invertebrates falling from tree crowns into the soil food web in temperate forests using an isotopically labeled analogue of arthropod rain.
Most invertebrates living in the aboveground layer of forests eventually fall to the soil surface and become prey for soil predators and saprophages. The extent of this vertical subsidy to detrital food chains in various forest types has been previously studied, but its significance for the diet of various groups of soil animals remains unappreciated.
The work used 15N-labeled Collembola grown in the laboratory to trace the pathways of invertebrates falling from tree crowns (“arthropod rain”) into the soil food web. The simulation experiment was conducted in a mixed spruce forest at the Malinki biogeocenological station of the Institute of Ecology and Evolution of the Russian Academy of Sciences (Fig. 1). The design of the experiment included the introduction of several model plots of Folsomia candida springtails from a laboratory culture into the soil, previously labeled with 15N under laboratory conditions (Fig. 2). Live or dead springtails were added once to field mesocosms in an amount equivalent to the average daily flow of arthropod rain in a given forest type (19 mg dry w. m-2).
Trapping of soil populations in experimental and control plots by standard soil-zoological methods was carried out on the 8th and 22nd days after treatment. In total, about 15,000 invertebrate specimens were processed, isotope analysis was performed for 1,251 samples (individual or containing several specimens of the same species).
After adding live springtails, the isotope label was most often detected in predatory trombidiform (83% of samples) and mesostigmatic (85%) mites, spiders (58%), predatory centipedes (45%) and beetles (29%). Among non-predatory groups, the isotope label was recorded in thrips (27%), springtails (24%) and oribatids (18%). An increase in 15N values was also recorded in a number of representatives of the fauna of the litter-soil complex (symphylans, ants, diplurans, millipedes, pholcidae, etc. Fig. 3).
When dead springtails were added to the mesocosms, the label was found in predators and saprophages in approximately equal proportions (21-25%).
This study contributes to the emerging view that soil food webs are not entirely "detrital" but derive a significant portion of their energy from green plants. Unlike other forms of aboveground subsidy in soil food webs, such as litterfall, root exudates, honeydew, etc., which are mainly processed by microorganisms, arthropod rain is directly consumed by animals, mainly higher-order consumers. We found an unexpectedly high frequency of incorporation of aboveground subsidy into the forest soil food web, suggesting that this source of energy and nutrients may contribute significantly to the maintenance of the abundance (and possibly diversity) of soil predators.
The work was supported by the RSF project 22-14-00363
The full results of the study can be found in the article: Rozanova, O.L., Tsurikov, S.M., Kudrin, A.A., Leonov, V.D., Krivosheina, M.G., Fedorenko, D.N., Tanasevitch, A.V., Rybalov, L.B., Tiunov, A.V. Incorporation of the 15N-labeled simulated arthropod rain in the soil food web. Oecologia 205, 587–596 (2024).