Biological and mineralogical controls over cycling of low molecular weight organic compounds along a soil chronosequence

Abstract

Low molecular weight organic compounds (LMWOC) represent a small but critical component of soil organic matter (SOM) for microbial growth and metabolism. The fate of these compounds is largely under microbial control, yet outside the cell, intrinsic soil properties can also significantly influence their turnover and retention. Using a chronosequence representing 1200 ka of pedogenic development, we compared physicochemical vs biological controls on the turnover and retention of fast-cycling carbon (C), e.g. glucose (GLU) and p-hydroxybenzoic acid (PHBA). Along the chronosequence, we observed mineralogical gradients whereby amorphous constituents were greatest in intermediate-aged sites, while older sites demonstrated soils with more ordered and less reactive mineralogy. Soil microbial community composition varied along the soil chronosequence and we observed reductions in total biomass and fungal biomass from younger to older sites, but this did not affect the turnover of LMWOC. Microbial utilization of LMWOC was substrate- and soil-dependent; amorphous Fe and Al oxides reduced the respiration of PHBA but respiration from glucose remained less affected. Variation in soil mineralogy did not significantly alter recovery of PHBA within microbial biomass or fungal vs. bacterial biomarkers, suggesting that reduced respiration of the phenolic resulted from direct mineral interaction with ionizable functional groups rather than changes to microbial allocation of PHBA. We conclude patterns of soil carbon storage observed across chronosequences are moderated by mineralogical effects on microbial access to LMWOC, independent of variation in microbial community composition.