Leaf litter and fine roots have distinct effects on particulate and mineral‐associated soil organic matter in a tree common garden

Soil organic matter (SOM) is primarily derived from leaf and root inputs, but the relative contributions of each are difficult to study without the use of isotopic tracers. Furthermore, associations between trees and mycorrhizal fungi can influence the production and persistence of SOM. We quantified tree inputs and carbon and nitrogen content of three SOM fractions - free and occluded particulate organic matter (fPOM and oPOM, respectively), and mineral-associated organic matter (MAOM) - in a tree common garden in Maryland, USA, where the trees and soil have distinct isotopic signatures due to prior use of the land as a cornfield. We found that stem basal area was not associated with concentrations of carbon (C) in any SOM pool but that fine root biomass was positively related to the proportion of tree-derived MAOM-C. POM was positively associated with leaf litter carbon : nitrogen ratio (C : N), but tree-derived MAOM was not associated with fine root C : N. Tree mycorrhizal type did not influence the relative importance of leaf and root-derived SOM. Our results indicate that leaf litter and fine roots have distinct roles on POM and MAOM formation, respectively, and that these effects are consistent across tree mycorrhizal associations in early stand development.