Tree species effects on soil organic matter dynamics in a common-garden study

We studied the influence of fourteen tree species on soil carbon and nitrogen (N) dynamics in a common garden of replicated monocultures of angiosperm and gymnosperm, broadleaf and needleleaf species in southwestern Poland.  We hypothesized that species would influence soil organic matter (SOM) decomposition primarily via biogeochemical recalcitrance, with species having lignin-rich tissues retarding rates of SOM decomposition.  Additionally, since prior work demonstrated substantial divergence in foliar and soil base cation concentrations and soil pH among species, we hypothesized that species would influence chemical stabilization of SOM via cation bridging to mineral surfaces in the A horizon.  Our hypotheses were only partially supported: SOM decomposition and microbial biomass were unrelated to plant tissue lignin concentrations, but in the mineral horizon, were significantly negatively related to the percentage of the cation exchange complex occupied by polyvalent acidic cations (Al and Fe), likely because these cations stabilize SOM via cation bridging and flocculation and potentially because of toxic effects of Al on decomposers.  Soil N dynamics, in contrast, were related to root N concentrations, with species having high-N roots exhibiting faster rates of net N mineralization and nitrification.  Our results indicate that tree species significantly influence SOM dynamics, even in the mineral soil horizons.  To our knowledge the influence of tree species on SOM decomposition via cation biogeochemistry has not been demonstrated previously, but could be important in other poorly buffered systems dominated by tree species that differ in cation nutrition or that are influenced by acidic deposition.