Human and Environmental Factors Influence Soil Faunal Abundance-Mass Allometry and Structure

We examined structural variation in soil faunal communities from 146 agroecosystems in The Netherlands, using a unique database of taxonomically highly resolved soil samples gathered by uniform methods. For each site, data included measurements of average body mass (M) and population density (N) of each detected taxon and environmental and human-use factors. We used three descriptors of soil faunal community structure: abundance-mass slope, which is the slope of the regression line through all faunal taxa in a site plotted on log(N)-versus-log(M) coordinates (all logarithms were base 10); the taxonomic diversity of each community's fauna (number of animal taxa at the finest available level of taxonomic resolution); and the total biomass of all fauna. The goal of the study was to account for variation in these descriptors and to develop causal hypotheses. These structural descriptors varied systematically. More than half of the variation in each descriptor was explained by external human, environmental, and biotic influences. Few predictors were needed to explain structural variation: above-ground ecosystem type (ET, describing the kind of human management); soil bacterial biomass; and a measure of precipitation. ET was the most important predictor of below-ground faunal community structure. Abundance-mass slopes ranged from -0.85 to -0.07 with mean -0.51; only four slopes were more negative than -3/4 (i.e., the log(N)-versus-log(M) regression line was steeper than -3/4). Slopes less negative than -1 (respectively, -3/4) indicated that, on average, taxon biomass (respectively, taxon energy consumption) increased with taxon body mass. Abundance-mass slope was more negative in more disturbed sites than in less disturbed sites. Disturbance may have produced this pattern by affecting populations of large-M taxa, which are slower to reproduce, more than small-M taxa. Across some types of site (super-intensive farms and possibly intensive farms), greater soil bacterial biomass was associated with less-negative abundance-mass slope, suggesting top-down control of bacterivorous taxa. ET and soil bacterial biomass were sufficient to explain most of the variation in the whole abundance-mass allometric relationship, including slope and intercept. Total faunal biomasses were higher in recently fertilized sites. Greater soil bacterial biomass was associated with the same increase in log faunal biomass between sites, on average, for all ET. Taxonomic diversity differed in sites of different ET in a way related to human disturbance. Precipitation was associated oppositely with diversity in sites of different types. Testable mechanistic hypotheses for the patterns observed here are discussed.