Tree mycorrhizal type predicts within‐site variability in the storage and distribution of soil organic matter

Abstract

Forest soils store large amounts of carbon (C) and nitrogen (N), yet how predicted shifts in forest composition will impact long-term C and N persistence remains poorly understood. A recent hypothesis predicts that soils under trees associated with arbuscular mycorrhizas (AM) store less C than soils dominated by trees associated with ectomycorrhizas (ECM), due to slower decomposition in ECM-dominated forests. However, an incipient hypothesis predicts that systems with rapid decomposition-e.g. most AM-dominated forests-enhance soil organic matter (SOM) stabilization by accelerating the production of microbial residues. To address these contrasting predictions, we quantified soil C and N to 1 m depth across gradients of ECM-dominance in three temperate forests. By focusing on sites where AM- and ECM-plants co-occur, our analysis controls for climatic factors that covary with mycorrhizal dominance across broad scales. We found that while ECM stands contain more SOM in topsoil, AM stands contain more SOM when subsoil to 1 m depth is included. Biomarkers and soil fractionations reveal that these patterns are driven by an accumulation of microbial residues in AM-dominated soils. Collectively, our results support emerging theory on SOM formation, demonstrate the importance of subsurface soils in mediating plant effects on soil C and N, and indicate that shifts in the mycorrhizal composition of temperate forests may alter the stabilization of SOM.

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

“…These results were recently extrapolated to all terrestrial biomes, further suggesting that AM‐dominated forest ecosystems store more carbon in aboveground biomass, whereas EcM‐dominated systems store more carbon in soil (Soudzilovskaia et al ., ). However, three recent North American temperate forest studies revealed no overall mycorrhizal‐type effect (Zhu et al ., ), opposite trends in deep soil (Craig et al ., ), or an interplay between forest type, mycorrhizal type and soil depth (Jo et al ., ).…”

“…Acidification dramatically reduces the abundance of earthworms (Phillips & Fahey, 2006) that play critical roles in soil aeration, litter fragmentation and transportation into deeper soil horizons. This certainly contributes to the development of deep litter layers in many EcM-dominated ecosystems and may explain relatively low rates of C sequestration in mineral soil (Craig et al, 2018;Zhu et al, 2018).…”

Constraining Carbon and Nutrient Flows in Soil With Ecological Stoichiometry

“…In the rhizosphere, sugars and amino acids exuded by roots can release SOM from mineral associations, allowing for microbial processing and turnover of previously adsorbed compounds (Keiluweit et al, 2015). However, the high-quality substrates exuded into the rhizosphere also promote the production of large quantities of microbial necromass, which can be readily converted into stable MAOM and aggregates (Knicker, 2011;Cotrufo et al, 2013;Schrumpf et al, 2013;Craig et al, 2018). The balance of decomposition and formation of MAOM in the rhizosphere appears to produce a net increase in stable MAOM, likely because root exudates provide plenty of C and nutrients for sorption .…”

Relationship Between Belowground Carbon Allocation and Nitrogen Uptake in Saplings Varies by Plant Mycorrhizal Type

“…We collected all soil from Lilly-Dickie Woods in Brown County, Indiana, USA (39 • 14 N, 86 • 12 W; altitude 290 m). For detailed description of the soil and microbial community properties of these soils see Cheeke et al (2017) and Craig et al (2018). Within this forest, we selected from two stands with either >90% basal area of AM-associated or ECMassociated tree species.…”