Depth-dependent effects of Ericoid Mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under Arbuscular Mycorrhizal Trees

Plant mycorrhizal associations influence the accumulation and persistence of soil organic matter and could therefore shape ecosystem biogeochemical responses to global changes that are altering forest composition. For instance, arbuscular mycorrhizal (AM) tree dominance is increasing in temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond positively to canopy disturbances. Yet how shifts in the co-occurrence of trees and shrubs with different mycorrhizal associations will affect soil organic matter pools remains largely unknown. We examine the effects of ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial activity at different depths across gradients of AM versus ectomycorrhizal (EcM) tree dominance in three temperate forest sites. We find that ErM shrubs strongly modulate tree mycorrhizal dominance effects. In surface soils, ErM shrubs increase particulate organic matter accumulation and weaken the positive relationship between soil organic matter s..., We worked in a 3,213-ha second-growth, mixed-hardwood forest in Connecticut, USA (41°57’ N, 72°07’ W). We established 18 10-m radius plots, each containing a pair of 1-m radius subplots (n =36), evenly arrayed across three forest stands that contained areas of both high AM and high EcM tree relative basal area as well as a patchy distribution of the ErM shrub Kalmia latifolia.            
 
Within each of the 18 plots, we established paired 1-m radius subplots with and without K. latifolia in the understory ( “+/- ErM subplot”) within 2 m of the center of the 10-m radius plot. In each 1-m radius subplot, we measured trees ≥1 cm diameter at breast height (DBH; 1.37 m). We also measured DBH of all trees ≥20 cm DBH within 10 m and trees ≥5 cm DBH within 5 m of plot center. We calculated the percentage of EcM tree basal area out of total basal area, scaled to m2 ha-1.
 
In June 2021, we collected and pooled two soil samples for each of three depths within the 36 paired subplots (i.e. 18 +Er..., This dataset is comprised of two tabs in a single excel file. See the \"README.md\" file for information pertaining to the variables measured and analyzed. The \"Data by soil layer\" tab includes all data collected or analyzed at the resolution of the three separate soil depths (i.e. the organic, surface mineral, and subsurface mineral soil layers). The \"Data by subplot\" tab includes all data collected or analyzed at the resolution of the subplot, including the cumulative carbon and nitrogen stocks that are the sum of the three individual soil layers. 
All equivalent soil mass (esm1) stocks were calculated using the the SimpleESM R script (Ferchaud et al. 2023) available at https://github.com/fabienferchaud/SimpleESM and described in the following reference: 
Fabien Ferchaud, Florent Chlebowski, Bruno Mary. SimpleESM: R script to calculate soil organic carbon and nitrogen stocks at Equivalent Soil Mass. Institut National de Recherche pour l’Agrgriculture, l’Alimentation et l’Environnement (...