The assessment methods for Soil Organic Matter (SOM) dynamics are crucial for understanding the soil carbon cycle and predicting changes in soil carbon storage. Existing methods are centered around traditional paradigms, while emerging methods have made progress in theory, measurement, and model construction, complementing each other and advancing the research onSOM dynamics.
Existing and emerging methods for assessing Soil Organic Matter (SOM) dynamics. The existing methods are robust because all three nodes—theory, measurement, and modeling—form a strong bidirectional connection and are well balanced(a). As the understanding of the controls onSOM dynamics grows, recent and ongoing innovations at each node expand theSOM paradigm triangle. However, if the expansion of one node outpaces the integration speed of the connections within the triangle, cracks will form, leading to a lack of applicability and adaptability to the changing environmental conditions(b).
Existing methods: The existing methods for assessingSOM dynamics form a relatively complete paradigm, consisting of three core parts: theory, measurement, and modeling. Theoretically, soil is conceptualized as discrete carbon pools with different turnover times, consideringSOM to be heterogeneous, composed of carbon pools with different decomposition rates. Measurement methods mainly include respiration time course determinations in laboratory incubations, separation ofSOM pools based on physical properties (such as size or density), and radiocarbon analysis, which provide data support for model parameterization. In terms of models, soil profile scale models represented byCENTURY and RothC are widely used, based on first-order kinetics to describe the flow between carbon pools according to pool size and environmental conditions, effectively simulating large-scale patterns ofSOM dynamics to some extent, with adaptability and operability. However, this paradigm also has limitations, as it oversimplifies and overlooks many system-specificities, making it difficult to cope with rapidly changing environmental conditions, such as the complex impacts of climate change on the soil carbon cycle.
Emerging methods
Theoretical developments: New theories emphasize that the persistence ofSOM is an ecosystem property controlled by the balance of microbial decomposition and physicochemical protection processes. The “onion layer” theory describes the self-assembly of organic molecules on mineral surfaces, providing a new perspective for the stabilization of organic matter, while supporting the concept of soil carbon saturation, which states that the efficiency of soil carbon storage decreases as the soil approaches saturation. The microbial accessibility theory points out that the physical accessibility of organic compounds to microbes, rather than their metabolic capacity, is the main factor regulating their utilization rate, but these theories face challenges in experimental validation and model integration due to the difficulty of conducting relevant key measurements and the complex spatial scales of soil processes involved, which are hard to reflect in models.
Measurement technology innovations: Advances in analytical techniques have greatly expanded the ability to characterizeSOM and its influencing factors. Spectroscopic methods such as ESI-FTICR-MS can obtain detailed chemical composition information of extractableSOM, but the sample processing may alter the properties of target compounds; solid-state methods such as 1HNMR spectroscopy, FTIR, etc., can in situ characterize the chemical composition of unextractedSOM, but their connection to mechanistic theories and numerical models is not close enough. Methods based on DNA and RNA can delve into the composition of microbial communities, but the large data volume poses challenges for application in biogeochemical models, and there is a gap between gene detection results and the actual functions of microbes. X-ray tomography technology can achieve non-destructive visualization of soil physical structure, helping to determine the position ofSOM in soil pore structures, but there is still a need to strengthen its association with theories and models, such as quantifyingSOM accessibility and integrating it with pore-scale models.
Model improvements and innovations: Numerical models are continuously evolving, with process modifications allowing models to cover more complex situations. Microbial explicit models explicitly consider the role of microbes inSOM dynamics, while reactive transport models are used to study processes related to soil carbon cycling and stability. Microbial explicit models provide opportunities to explore the theoretical implications of microbial physiology and functional community composition onSOM formation and responses to environmental changes, but they face significant challenges in translating data and theories into models of appropriate scales. Reactive transport models, although they encounter computational and analytical obstacles when describing the complex reactions of organic matter behavior in soils, such as quantifying reaction rate constants and equilibrium distribution coefficients, have the potential to serve as diagnostic and predictive tools forSOM dynamics. Emerging models like Millennial models attempt to more directly linkSOM turnover times with the time scales of physical protection and isolation, transitioning from implicitly incorporating spatial heterogeneity and biological activity to explicitly including relevant factors, but the connections between physical, chemical, and biological parameters in the models and the theories and measurements remain insufficiently close.