Resilience of soil biota in various food webs to freezing perturbations

We tested three predictions of food web behavior: (1) inclusion of a third trophic level will reduce populations on the second trophic level, allowing species on the first trophic level to increase; (2) food chains with an odd number of trophic levels release nutrients to the environment at a slower rate than food chains with an even number of trophic levels;' (3) the longer the food chain, or more complex the food web, the longer the community will take to recover from disturbance. Bacteria (Pseudomonas fluorescens, P. maltophilia, P. paucimobilis, P. stutzeri, and Alcaligenes sp.), and fungi (Fusarium oxysporum) were used for the first trophic level, and nematodes were used for the second (Acrobeloides sp. and Aphelenchus avenae) and third (Mononchus tunbirdgensis) trophic levels. Microorganisms were assembled to form food chains and food webs of different structures in sterile-soil microcosms. The microcosms were designed to allow repeated sampling without losing sterility. Soil samples were removed every 2 wk for 3 mo, then either the experiment was terminated, or samples were taken less frequently. Carbon was added when each sample was taken and the soil removed for analyses was replenished in sterile soil. Population abundance, soil ammonium-nitrogen, and carbon dioxide were measured throughout the course of the experiments. Once constant population levels were established (days 30-40), the systems were frozen to -1@?C for 7 d. Sampling continued after the freeze for up to 100 d. The populations on the first trophic level were lower with increased food web complexity or food chain length, refuting the first predict ion. Net nitrogen mineralization and carbon release were greater with increased food web complexity, refuting the second prediction. We speculate that these results are closely linked to the organisms' generation times and species-specific traits, such as rapid grown following a stress (Acrobeloides sp.). Recovery after freezing was highly variable and was more a function of the species than of food web structure. Nutrient cycling and recovery from stress were more dependent upon the species that make up the community than upon the community structure itself.