Fast-decaying plant litter enhances soil carbon in temperate forests but not through microbial physiological traits

The study addresses how plant inputs are converted into stable mineral-associated soil organic carbon (SOC) and whether microbial physiological traits mediate this process. Contemporary theory (the necromass stabilization hypothesis) posits that higher litter quality should enhance microbial growth, carbon use efficiency (CUE), and turnover, thereby increasing microbial necromass production and its stabilization on mineral surfaces, leading to SOC accumulation. However, alternative mechanisms—direct sorption of plant compounds, stabilization of microbial extracellular products, priming-induced SOC losses, and overriding soil abiotic controls—could decouple microbial traits from SOC accumulation. The researchers test whether microbial growth rate (MGR), CUE, and microbial biomass turnover rate (MTR) predict mineral-associated SOC and whether these traits link litter decomposition to SOC stabilization across experimental microcosms and natural temperate forest gradients.

Prior work shows mineral-associated SOC comprises a large, slow-cycling pool often derived from microbial products stabilized by minerals. Isotope tracing studies frequently report that high-quality, fast-decomposing inputs transfer more rapidly and efficiently into mineral-associated SOC, interpreted as support for necromass-driven stabilization. Microbially explicit models (e.g., MEMS, MIMICS) and first-order decay models embed assumptions that microbial growth and efficiency enhance SOC stabilization. Yet empirical tests linking community-level physiological traits to mineral-associated SOC are limited and often from controlled microcosms or single-site observations. Additional pathways—direct sorption of plant-derived dissolved and oxidized compounds, stabilization of microbial extracellular polymeric substances, and priming that accelerates SOC decomposition—may offset or bypass necromass pathways. Soil texture, metal oxides (Fe, Al), pH, and other abiotic properties can dominate SOC retention, potentially overriding microbial physiological effects.

The study used two complementary approaches. 1) Microcosm experiment: Sixteen temperate tree leaf litters spanning a broad chemical quality gradient were mixed into a 13C-enriched agricultural soil (Chalmers silty clay loam; pH 6.7; C:N ~12) and incubated for up to 185 days at controlled moisture and temperature. Litter decomposition was tracked via CO2 efflux rates and 13C isotopic partitioning to separate litter- versus soil-derived respiration over time. Mineral-associated versus particulate SOC pools were isolated via size fractionation (≤53 µm considered mineral-associated) at day 30 (early) and day 185 (intermediate), and litter-derived C in each fraction was quantified using 13C mixing models. Microbial physiological traits (MGR, CUE, MTR) were measured at days 15 and 100 using 18O-H2O incorporation into DNA to quantify growth, combined with respiration to derive CUE, and microbial biomass C via chloroform fumigation-extraction to derive turnover. Litter quality and microbial trait indices were generated using principal components analyses. Path analyses tested direct and indirect (via microbial traits) effects of litter quality on mineral-associated SOC formation efficiency and amounts, and on soil-derived SOC losses. 2) Field study across six eastern US temperate forests (Harvard Forest, Lilly-Dickey Woods, Smithsonian Conservation Biology Institute, Smithsonian Environmental Research Center, Tyson Research Center, Wabikon Lake Forest): Fifty-four 20×20 m plots were established along arbuscular vs ectomycorrhizal tree dominance gradients to generate variation in litter quality and soil properties. Surface mineral soils (0–5 cm) were sampled for microbial traits (18O-DNA method for MGR, CUE, MTR), microbial biomass C, mineral-associated SOC (size fractionation), and microbial necromass via amino sugars (glucosamine and muramic acid; converted to fungal and bacterial necromass C). Soil physicochemical properties (pH, texture from 5–15 cm depth, oxalate-extractable Fe and Al as indicators of poorly crystalline oxides), net N mineralization, dissolved organic C, and fine root biomass were measured. Plot-level litter quality indices were computed from species-weighted traits (soluble fraction, acid-unhydrolyzable residue, lignocellulose index, C:N). Linear mixed models with site as a random effect assessed predictors of microbial traits, mineral-associated SOC (total and proportion of total C), and necromass pools; predictors were standardized, and model assumptions checked; highly collinear variables were excluded (retained Feox over Aloxx).

- In microcosms, higher litter quality increased microbial growth, CUE, and turnover (microbial trait index positively related to litter quality; intermediate stage R2 = 0.38; P = 0.01). Traits were better predicted by litter carbon quality than nitrogen content. - Microbial physiological traits did not positively predict litter-derived mineral-associated SOC at early (30 d) or intermediate (185 d) stages (no relationship; P > 0.96). - Path analysis showed that litter quality affected mineral-associated SOC formation directly, not via microbial traits. Direct path coefficients (litter quality to litter-derived mineral-associated SOC) were strongly positive (early: 0.87***; intermediate: 0.68***), whereas microbial traits to mineral-associated SOC were negative (early: −0.32**; intermediate: −0.43*). Indirect effects via microbial traits were negative (early: −0.12; intermediate: −0.26***). Total effects of litter quality remained positive (early: 0.75***; intermediate: 0.41**). - The positive effect of litter quality on mineral-associated SOC formation likely reflects direct sorption of plant-derived dissolved and oxidized compounds and/or stabilization of microbial extracellular products, not necromass production. - Evidence of priming: microbial traits were negatively related to soil-derived mineral-associated SOC and particulate SOC; litter quality indirectly reduced soil-derived mineral-associated SOC via increased microbial traits (path analysis; negative indirect effects). Cumulative soil-derived respiration increased with a litter nitrogen axis (early: R2 = 0.59, P < 0.01; intermediate: R2 = 0.32, P = 0.02). - In the field, litter quality was positively associated with mineral-associated SOC, but microbial traits did not mediate this relationship. Microbial traits were unrelated to total mineral-associated SOC and were negatively related to the proportion of soil C stored as mineral-associated SOC (standardized coefficient ~ −0.54**, Fig. 4a). - Field microbial traits were better predicted by soil C:N (positive up to C:N ~20) than by litter quality, indicating stoichiometric controls on microbial physiology across sites. - Microbial necromass pool size did not correlate with microbial traits, suggesting production does not predict retention. Total necromass had a weak positive relationship with Feox and a negative relationship with clay; fungal necromass decreased with clay, whereas bacterial necromass increased with Feox, indicating stronger mineral stabilization of bacterial products. - Abiotic properties strongly predicted mineral-associated SOC: clay content increased the proportion of mineral-associated SOC, and oxalate-extractable Fe was the strongest predictor of total mineral-associated SOC. - Fine root biomass was associated with higher microbial biomass and necromass yet negatively associated with mineral-associated SOC, consistent with root-driven priming in surface soils. - Overall, across experiments and field sites, microbial growth, efficiency, and turnover were negatively, not positively, related to mineral-associated SOC formation or storage, challenging the generality of the necromass stabilization hypothesis in temperate forest surface soils.

The findings indicate that while higher-quality plant litter enhances mineral-associated SOC, this enhancement is not mediated by increased microbial growth, efficiency, or turnover. Instead, direct stabilization pathways (sorption of plant-derived dissolved and oxidized compounds and stabilization of microbial extracellular products) and priming-induced SOC losses dominate the net outcome. In microcosms, litter-induced stimulation of microbial traits increased microbial biomass and activity, which in turn promoted decomposition of both newly formed and pre-existing mineral-associated SOC, offsetting potential gains from necromass formation. In the field, abiotic factors (clay, Fe oxides) and root influences, along with the origin of necromass (bacterial vs fungal), decoupled necromass production from mineral-associated SOC accumulation. Thus, the necromass stabilization hypothesis alone is insufficient to explain mineral-associated SOC dynamics in temperate forest surface soils. The results refine understanding of SOC formation by emphasizing context-dependent balances among direct sorption, extracellular product stabilization, necromass origin and stabilization likelihood, priming intensity, and mineralogical controls.

This study shows that fast-decaying litter increases mineral-associated SOC in temperate forests primarily via direct stabilization pathways rather than through enhanced microbial growth, efficiency, or turnover leading to necromass accumulation. Microbial physiological traits were negatively related to mineral-associated SOC formation or storage across both controlled microcosms and six temperate forests, indicating that priming and abiotic controls can outweigh necromass-driven SOC gains. The stabilization potential differs between bacterial and fungal necromass, and mineralogy (e.g., Fe oxides, clay) is a dominant control on mineral-associated SOC. Future research should: (1) quantify the relative contributions of direct plant compound sorption and microbial extracellular products to mineral-associated SOC; (2) resolve how necromass origin and chemistry govern stabilization across soils and depths; (3) assess root- and mycorrhiza-mediated SOC formation and priming in situ, especially in subsoils; and (4) expand cross-system comparisons (e.g., croplands, grasslands) where microbial traits may more strongly align with mineral-associated SOC.

- The microcosm system used a single 13C-enriched agricultural soil, which may limit generalization to other soil types and mineralogies. - Fractionation targeted size classes; direct quantification of specific sorption mechanisms and microbial extracellular products was not performed. - Surface soils and aboveground litter inputs were emphasized; deeper horizons and belowground inputs may behave differently. - Field analyses were observational across six forests; while mixed models accounted for site effects, unmeasured covariates may influence relationships. - Amino sugar-based necromass estimates are proxies and may not capture all necromass forms or turnover dynamics. - Temporal resolution of field measurements was limited to a single sampling period, potentially missing seasonal dynamics.