Journal
GLOBAL CHANGE BIOLOGY
Volume 28, Issue 24, Pages 7167-7185Publisher
WILEY
DOI: 10.1111/gcb.16413
Keywords
amino sugars; biomarkers; microbial-derived organic matter; mineral-associated organic matter; molecular fingerprinting; plant-derived organic matter; soil carbon
Funding
- U.S. Department of Agriculture National Institute of Food and Agriculture [NHAES NH00666]
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To predict and mitigate changes in soil carbon stocks, understanding the factors regulating soil organic matter formation and persistence is crucial. Recent studies have emphasized the dominance of microbial-derived organic matter inputs, but others have challenged this view, suggesting larger contributions from plants. However, methodological limitations introduce uncertainty into quantifying plant- and microbial-derived SOM. It is important to re-evaluate methods, define their limitations, and improve quantification strategies.
Predicting and mitigating changes in soil carbon (C) stocks under global change requires a coherent understanding of the factors regulating soil organic matter (SOM) formation and persistence, including knowledge of the direct sources of SOM (plants vs. microbes). In recent years, conceptual models of SOM formation have emphasized the primacy of microbial-derived organic matter inputs, proposing that microbial physiological traits (e.g., growth efficiency) are dominant controls on SOM quantity. However, recent quantitative studies have challenged this view, suggesting that plants make larger direct contributions to SOM than is currently recognized by this paradigm. In this review, we attempt to reconcile these perspectives by highlighting that variation across estimates of plant- versus microbial-derived SOM may arise in part from methodological limitations. We show that all major methods used to estimate plant versus microbial contributions to SOM have substantial shortcomings, highlighting the uncertainty in our current quantitative estimates. We demonstrate that there is significant overlap in the chemical signatures of compounds produced by microbes, plant roots, and through the extracellular decomposition of plant litter, which introduces uncertainty into the use of common biomarkers for parsing plant- and microbial-derived SOM, especially in the mineral-associated organic matter (MAOM) fraction. Although the studies that we review have contributed to a deeper understanding of microbial contributions to SOM, limitations with current methods constrain quantitative estimates. In light of recent advances, we suggest that now is a critical time to re-evaluate long-standing methods, clearly define their limitations, and develop a strategic plan for improving the quantification of plant- and microbial-derived SOM. From our synthesis, we outline key questions and challenges for future research on the mechanisms of SOM formation and stabilization from plant and microbial pathways.
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