Spatio-Temporal Plasticity of Root Exudation in Three Temperate Tree Species: Effects of Season, Site, and Soil Characteristics.

Root exudation provides a constant carbon input to the rhizosphere and is therefore a very important factor in shaping this hotspot of biological activity. Nonetheless, data on root exudation and its spatio-temporal plasticity are scarce. This study provides insights into compound-specific root exudation in three temperate tree species in two seasons (late spring and late summer) and two soil compartments (forest floor and the top mineral soil), including the effect of soil chemistry. At four sites with differing mean annual temperatures and soil phosphorus levels, root exudates were sampled using an in situ cuvette-based system and analysed by gas chromatography-mass spectrometry. We found seasonally and spatially varying site- and species-specific exudation patterns. While the seasonal pattern was similar among species and sites, with higher exudation rates in late spring, soil compartment-specific exudation depended on species and site. Acer pseudoplatanus tended to exude more into the mineral soil at warmer sites, while Picea abies exuded more in the mineral soil at all sites. Exudation by Fagus sylvatica was independent of the soil compartment. A significant correlation between exchangeable soil cations and specific compounds exuded by F. sylvatica and P. abies was found. Exudation of specific compounds in F. sylvatica increased with the concentration of exchangeable Mg, Al, and Fe, whereas exudation rates in P. abies decreased with most base cations' concentration, while sugar exudation increased with the exchangeable non-base cations Al and Fe. These results demonstrate that root exudation is dynamically adjusted to the species-specific nutritional needs governed by site, season, and soil characteristics.