The multi-response of root foraging strategy to a neighbor, soil heterogeneity and earthworm

Abstract

Despite a broad understanding of behavioral aspects of plant root foraging, most studies have investigated plant responses to single soil factors. Whether plants consistently respond to specific aspects of the soil environment, or whether they express highly contingent foraging responses to combinations of factors is unclear. We conducted a growth room experiment with two plants species (Chenopodium glaucum L. and Chloris virgate Sw.) in which we manipulate plant competition, soil heterogeneity, earthworm presence to answer (1) whether root foraging strategies reflect combinations of these treatments, and (2) whether changes in root foraging are associated with altered competition and pot-level biomass yield. After five weeks of growth, we measured above- and below-ground biomass and root length of each individual in each of four soil zones (shared, owned, and two adjacent zones). These data were used to estimate the degree a plant was suppressed by its neighbors (competitive response), the coefficient of variation in root length among the four zones, and the degree to which a plant concentrated its roots in its home zone vs. in the zone of a neighbor (home aggregation index). Plant neighbors, earthworms, and soil heterogeneity influenced above- and belowground growth and foraging in species-specific ways. The homogenization effect of earthworms on the root distribution of C. glaucum growing alone was changed into heterogenization when it grew with its competitor. Though the root foraging strategy of C. glaucum was relatively conservative while that of C. virgate was relatively aggressive, neither species in this study presented a singular root foraging strategy, and instead, root placement was highly contingent upon local soil factors and planted social context. The avoidance root foraging strategy our plants demonstrated clearly reduced the competitive pressure and increased total productivity per pot which empirically links root foraging to competitive interaction. These findings suggest that the multiple soil-related predictors are integrated to determine the root foraging strategies with a species-specific variation in spatial root distribution, and the shifts in foraging strategy were associated with changes in the strength of plant-plant interactions and impacted total productivity. This work has potential impacts on understanding the contingent root performance, undetermined interspecific relationships and possible species coexistence in a context-specific environment.