Shifting distributions of trees with arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations are driving changes in soil carbon (C) stocks across temperate forests. However, little is known about how shifts in soil C stock relate to changes in soil organic matter (SOM) composition. The aim of this study was to describe the variation in SOM composition in soils formed under AM and ECM trees. Soils were collected from three temperate forest sites with paired AM and ECM vegetation (Harvard Forest, MA; Lilly Dickey Woods, IN; Wabikon Lake, WI). Soils were separated into particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions. Bulk soils and OM fractions were analyzed using 13C cross-polarization magnetic angle spinning nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. Integrated peaks from the NMR spectra and total carbon and nitrogen concentrations were inputs for a molecular mixing model that predicts six compound classes: charcoal, carbonyl, carbohydrates, lignin, protein, and aliphatic C. A principle component analysis statistical approach was used to determine the most significant drivers of SOM composition across bulk soils, POM, and MAOM. We hypothesized that site differences would be the largest driver of SOM composition compared with AM/ECM vegetation and OM fractions.
Results/Conclusions
Across all sampling sites in bulk soil, POM, and MAOM fractions, mycorrhizal type was the most significant driver for SOM composition, based on the PCA analysis. This is both critical and surprising, because previous work in SOM dynamics would suggest that site-specific differences would be more significant factors in the development of SOM composition across both large and small spatial scales. The bulk soils in this study had consistent differences between paired AM vs ECM sites, with ECM soils having a larger proportion of alkyl peaks (integrated area from 0-45 ppm) and AM soils having larger proportions of O-alkyl (60-95 ppm) and amide/carboxyl peaks (165-215 ppm) in the NMR spectra. The alkyl:O-alkyl ratio, a proxy for the degree of microbial processing of the OM, was greater in AM soils relative to ECM soils. Across all sites, the POM fraction had less decomposed SOM than the MAOM fraction, as indicated by the alkyl:O-alkyl ratio and EPR spectra. This work agrees with previous work which indicates that differences in soil properties are derived from differences in litter quality and decomposition potential in sites with AM vs ECM vegetation.