Soil organic carbon sourcing variance in the rhizosphere vs. non-rhizosphere of two mycorrhizal tree species

Abstract

Soil organic carbon (SOC) plays a central role in ecosystem carbon sequestration and climate change mitigation, and its stability and dynamics are related to sourcing from microbial vs. plant residues. However, SOC sourcing and its regulating mechanisms remain poorly understood in soil's most bioactive compartment, the rhizosphere, which may differ from non-rhizosphere and under different mycorrhizal tree species. To fill the knowledge gap, here we collect the rhizosphere and non-rhizosphere soils under an arbuscular mycorrhizal (AM; Castanopsis eyrie) vs. an ectomycorrhizal (ECM) tree species (Pinus massoniana) of varied tree diameters (i.e., ages) in the Gutianshan subtropical forest of China. Plant and microbial residual components are quantified by lignin phenols and amino sugars, respectively. Coupled with the measurements of soil, microbial community and plant litter properties, we assess potential mechanisms (i.e., saprotrophic bacteria competition, microbial necromass recycling/reuse, and substrate quality control) influencing the distribution of plant and microbial residues in the rhizosphere vs. non-rhizosphere. We show that lignin phenols are more concentrated in rhizosphere than non-rhizosphere SOC, especially under the ECM trees showing inhibited saprotrophic decomposition induced by competition between ECM fungi and (saprophytic) bacteria. Amino sugars are also more concentrated in the rhizosphere of ECM trees due to ECM fungal contribution, but not under AM trees exhibiting reduced fungal necromass stability partially reflected by low biomass-normalized necromass accumulation coefficients in the rhizosphere. As a result, ratios of amino sugars to lignin phenols are relatively lower in the rhizosphere than non-rhizosphere under AM tree, challenging the presumed microbial dominance in rhizosphere carbon accumulation. These results highlight differences in and controls on rhizosphere SOC sourcing related to different mycorrhizal tree species, providing new information on the mechanisms regulating soil carbon dynamics in root-soil systems.