Incorporation of the N-labeled simulated arthropod rain in the soil food web.

Direct trophic links between aboveground and belowground animal communities are rarely considered in food web models. Most invertebrate animals inhabiting aboveground space eventually become prey of soil predators and scavengers forming a gravity-driven spatial subsidy to detrital food webs, but its importance remains unquantified. We used laboratory-grown N-labeled Collembola to trace the incorporation of arthropod rain into soil food webs. Live or euthanized Collembola were supplemented once to field mesocosms in the amount equivalent to the mean daily input of the arthropod rain (19 mg d.w. m). After the addition of live Collembola, the isotopic label was found most often in predatory Trombidiformes (83% of samples) and Mesostigmata mites (85%), followed by Araneae (58%), Chilopoda (45%), and Coleoptera (29%). Among non-predatory groups, the isotopic label was recorded in Thysanoptera (27%), Collembola (24%), and Oribatida (18%). The N-label was also detected in Symphyla, Formicidae, Diplura, Diplopoda, Opiliones, Diptera, Hemiptera, Oligochaeta, and Nematoda. There was a positive correlation between natural N abundance and the frequency of the isotopic label among predators, but not among decomposers. In the non-replicated treatment, in which dead collembolans were added, the label was found in predators and decomposers in approximately equal proportions (21-25%). Unlike other forms of the aboveground subsidy (such as leaf litter, frass, or honeydew) that are primarily processed by microorganisms, arthropod rain is assimilated directly by the animals. The high frequency of consumption of the aboveground subsidy suggests that it plays a significant role in maintaining the abundance of soil predators.