Diversity of leaf litter leachates from temperate forest trees and its consequences for soil microbial activity

Leaching of water-soluble compounds is a dominant process during the first stages of litter decomposition, providing the microorganisms in the underlying soil with an important source of labile carbon and nutrients. Leachate composition (quantity and quality) can vary considerably among different plant species, but its consequences for soil microbially-driven processes remains largely unexplored. Here, we evaluated the differences in leachate quantity and quality from freshly fallen leaf litter of widely distributed coniferous and deciduous broadleaf tree species of European temperate forests, and their effects on soil microbial responses in a microcosm experiment under controlled conditions. Leachates of broadleaf litter contained higher amounts of carbon and nitrogen available for microbes, but with substantially higher aromaticity than leachates from coniferous litter. A one-time leachate addition to soils immediately increased soil microbial respiration with longer lasting effects of deciduous broadleaf compared to coniferous litter leachates leading to a microbial community with an apparently more efficient use of carbon. When leachates of different species were mixed, the observed microbial responses differed in some cases from that expected based on soils to which leachates from single species were added. These non-additive effects were partly explained by the functional dissimilarity of leachate traits, suggesting complementary resources for microorganisms when leachates of different species are available. Our data show that species-specific litter-derived leachates of varying quantity and quality and their mixtures distinctly affect soil microorganisms. In forest ecosystems with recurrent leaf litter inputs from the same species, such leachate effects may determine soil processes also in the longer term, controlling biogeochemical cycling to an important degree.