Managing soil organic matter (SOM) stocks to address global change challenges requires well‐substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management ...practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross‐study comparisons difficult and hindering broad‐scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral‐associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO2 enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well‐supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad‐scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.
Soil organic matter (SOM) is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Particulate (POM) and mineral‐associated (MAOM) organic matter are two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their contrasting properties and responses to global change factors, and propose the POM versus MAOM framework as the way forward to understand and predict broad‐scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.
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.
Soil organic matter (SOM) comprises ~80% of global terrestrial carbon stocks, much of which is stored as mineral‐associated organic matter (MAOM). MAOM is derived from both plant and microbial biomolecules, but the quantitative contributions of each remain uncertain (represented by the mixing of red and blue into purple). There is substantial overlap in the compounds produced by microbes, plant roots and those released through the extracellular decomposition of plant litter, and limitations in our current methods constrain quantification. We review the methods used to quantify plant‐ and microbial‐derived SOM, examining their limitations and key future research priorities towards elucidating the plant and microbial pathways of SOM formation.
To predict the behavior of the terrestrial carbon cycle, it is critical to understand the source, formation pathway, and chemical composition of soil organic matter (SOM). There is emerging consensus ...that slow‐cycling SOM generally consists of relatively low molecular weight organic carbon substrates that enter the mineral soil as dissolved organic matter and associate with mineral surfaces (referred to as “mineral‐associated OM,” or MAOM). However, much debate and contradictory evidence persist around: (a) whether the organic C substrates within the MAOM pool primarily originate from aboveground vs. belowground plant sources and (b) whether C substrates directly sorb to mineral surfaces or undergo microbial transformation prior to their incorporation into MAOM. Here, we attempt to reconcile disparate views on the formation of MAOM by proposing a spatially explicit set of processes that link plant C source with MAOM formation pathway. Specifically, because belowground vs. aboveground sources of plant C enter spatially distinct regions of the mineral soil, we propose that fine‐scale differences in microbial abundance should determine the probability of substrate–microbe vs. substrate–mineral interaction. Thus, formation of MAOM in areas of high microbial density (e.g., the rhizosphere and other microbial hotspots) should primarily occur through an in vivo microbial turnover pathway and favor C substrates that are first biosynthesized with high microbial carbon‐use efficiency prior to incorporation in the MAOM pool. In contrast, in areas of low microbial density (e.g., certain regions of the bulk soil), MAOM formation should primarily occur through the direct sorption of intact or partially oxidized plant compounds to uncolonized mineral surfaces, minimizing the importance of carbon‐use efficiency, and favoring C substrates with strong “sorptive affinity.” Through this framework, we thus describe how the primacy of biotic vs. abiotic controls on MAOM dynamics is not mutually exclusive, but rather spatially dictated. Such an understanding may be integral to more accurately modeling soil organic matter dynamics across different spatial scales.
We posit that how a plant carbon compound forms slow‐cycling mineral‐associated soil organic matter is tied to its point of entry to the mineral soil. In our spatially explicit conceptual model of soil organic matter formation, the microbial formation pathway is more dominant for belowground root inputs entering into the rhizosphere, whereas the direct sorption pathway is more dominant for aboveground dissolved organic matter inputs entering into the bulk soil. Due to these differences, the primacy of biotic vs. abiotic controls on soil organic matter formation will vary at fine spatial scales in soil space.
Understanding how ecosystems store or release carbon is one of ecology's greatest challenges in the 21st century. Organic matter covers a large range of chemical structures and qualities, and it is ...classically represented by pools of different recalcitrance to degradation. The interaction effects of these pools on carbon cycling are still poorly understood and are most often ignored in global-change models. Soil scientists have shown that inputs of labile organic matter frequently tend to increase, and often double, the mineralization of the more recalcitrant organic matter. The recent revival of interest for this phenomenon, named the priming effect, did not cross the frontiers of the disciplines. In particular, the priming effect phenomenon has been almost totally ignored by the scientific communities studying marine and continental aquatic ecosystems. Here we gather several arguments, experimental results, and field observations that strongly support the hypothesis that the priming effect is a general phenomenon that occurs in various terrestrial, freshwater, and marine ecosystems. For example, the increase in recalcitrant organic matter mineralization rate in the presence of labile organic matter ranged from 10% to 500% in six studies on organic matter degradation in aquatic ecosystems. Consequently, the recalcitrant organic matter mineralization rate may largely depend on labile organic matter availability, influencing the CO
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emissions of both aquatic and terrestrial ecosystems. We suggest that (1) recalcitrant organic matter may largely contribute to the CO
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emissions of aquatic ecosystems through the priming effect, and (2) priming effect intensity may be modified by global changes, interacting with eutrophication processes and atmospheric CO
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increases. Finally, we argue that the priming effect acts substantially in the carbon and nutrient cycles in all ecosystems. We outline exciting avenues for research, which could provide new insights on the responses of ecosystems to anthropogenic perturbations and their feedbacks to climatic changes.
Mineral‐associated soil organic matter (MAOM) is the largest, slowest cycling pool of carbon (C) in the terrestrial biosphere. MAOM is primarily derived from plant and microbial sources, yet the ...relative contributions of these two sources to MAOM remain unresolved. Resolving this issue is essential for managing and modeling soil carbon responses to environmental change. Microbial biomarkers, particularly amino sugars, are the primary method used to estimate microbial versus plant contributions to MAOM, despite systematic biases associated with these estimates. There is a clear need for independent lines of evidence to help determine the relative importance of plant versus microbial contributions to MAOM. Here, we synthesized 288 datasets of C/N ratios for MAOM, particulate organic matter (POM), and microbial biomass across the soils of forests, grasslands, and croplands. Microbial biomass is the source of microbial residues that form MAOM, whereas the POM pool is the direct precursor of plant residues that form MAOM. We then used a stoichiometric approach—based on two‐pool, isotope‐mixing models—to estimate the proportional contribution of plant residue (POM) versus microbial sources to the MAOM pool. Depending on the assumptions underlying our approach, microbial inputs accounted for between 34% and 47% of the MAOM pool, whereas plant residues contributed 53%–66%. Our results therefore challenge the existing hypothesis that microbial contributions are the dominant constituents of MAOM. We conclude that biogeochemical theory and models should account for multiple pathways of MAOM formation, and that multiple independent lines of evidence are required to resolve where and when plant versus microbial contributions are dominant in MAOM formation.
The stoichiometry provides a new independent source of data for evaluating the relative contributions of plant and microbial inputs to mineral‐associated soil organic matter formation (MAOM). In our global databases, MAOM C/N ratios are greater than microbial C/N ratios and lower than POM C/N ratios across all ecosystem types. This suggest that MAOM contains both microbial and plant residues. Using fractional abundance of N N/(C + N) in POM and microbial biomass as end‐members in the two‐pool mixing model, we found that microbial inputs accounted for 34%–47% of the MAOM pool, whereas plant residues contributed 53%–66%. Our results challenge the increasingly popular view that microbial contributions are the dominant component of MAOM.
Soil organic matter (SOM) is a fundamental resource to humanity for the many ecosystem services it provides. Increasing its stocks can significantly contribute to climate change mitigation and the ...sustainability of agricultural production. Elucidating the mechanisms and drivers of the formation of the main components of SOM, particulate (POM) and mineral associated (MAOM) organic matter, from the decomposition of plant inputs is therefore critical to inform management and policy designed to promote SOM regeneration. We designed a two-tiered laboratory incubation experiment using 13C and 15N labeled plant material to investigate the effects of the physical nature (i.e., structural versus soluble) of plant inputs as well as their chemical composition on (1) the pathways of SOM formation, (2) the soil microbial community and chemical diversity, and (3) their interaction on the stabilization efficiency of litter-derived C in POM and MAOM, in a topsoil and a subsoil. We found that: i) the physical nature of the plant input (structural vs soluble) drove both the pathways and efficiencies of SOM formation; ii) POM formation from the decomposition of structural residues increased in efficiency the more decomposed were the residues, and linearly with soil microbial and chemical diversity, the latter only for subsoil; ii) more input-derived C and N were retained in subsoil because of both higher stabilization in MAOM and POM, and slower residue decay. Our results also confirm the importance of direct sorption of soluble inputs to silt- and clay-sized minerals for the formation of MAOM in bulk soils. Taken together these finding suggest that the highest potential for SOM accrual is in subsoils characterized by higher C saturation deficit, from the separate addition of decomposed residues and soluble plant inputs.
•Plant soluble vs structural inputs drive pathways and efficiencies of SOM formation.•MAOM forms most efficiently from soluble plant inputs, likely by direct sorption.•POM forms most efficiently from highly decomposed plant residues.•Soil microbial and chemical diversity promote POM but not MAOM formation.•POM and MAOM form most efficiently in subsoil due to lower decay and C saturation.
•After six months litter C was found in all soil fractions.•Similar amounts of litter C in the mineral fraction at six months and five years.•Sand content and precipitation best predict litter C ...formed in mineral fraction.•We found a positive feedback between new litter-derived SOM and soil C content.
Understanding the mechanisms controlling the formation and persistence of soil organic matter (SOM) is important for managing soil health and sustainable food production. The formation of SOM and the degree to which it is protected from decomposition are important for determining the long-term persistence of SOM. We used soils collected in a 13C-labelled litter decomposition study established at agricultural sites in Canada to understand the formation and persistence of newly-formed SOM. The ten agricultural sites spanned a wide range of soil carbon contents, texture, and climatic conditions. We fractionated the soil to isolate water extractable organic matter (WEOM), free light POM (fPOM), sand-sized and occluded particulate organic matter (oPOM), and silt and clay sized particles, referred to as mineral-associated organic matter (MAOM). Quantitative isotope tracing was used to determine the litter-derived C in all fractions. We performed these analyses early (six months after incubation) and later (five years after incubation) in the decomposition process to evaluate factors that control the formation and persistence of POM and MAOM. After six months litter-derived C was found in all fractions, but after five years it had declined in all fractions except the MAOM. Formation of MAOM was related to high mean annual precipitation and low sand content, whereas occluded POM formation was related to high soil C content. Persistence of MAOM and POM during the incubation were associated with low soil temperature and high soil C content. There was no consistent indication that formation of MAOM occurred from the decomposition of POM, suggesting that MAOM and POM are formed by two separate pathways. This has important implications for SOC models, which assume that plant-derived C passes through a sequence of pools, becoming more stable along the way.
Plant–microbe interactions in the rhizosphere shape carbon and nitrogen cycling in soil organic matter (SOM). However, there is conflicting evidence on whether these interactions lead to a net loss ...or increase of SOM. In part, this conflict is driven by uncertainty in how living roots and microbes alter SOM formation or loss in the field. To address these uncertainties, we traced the fate of isotopically labelled litter into SOM using root and fungal ingrowth cores incubated in a Miscanthus x giganteus field. Roots stimulated litter decomposition, but balanced this loss by transferring carbon into aggregate associated SOM. Further, roots selectively mobilized nitrogen from litter without additional carbon release. Overall, our findings suggest that roots mine litter nitrogen and protect soil carbon.
Root and microbial processes in the rhizosphere shape soil organic matter (SOM) cycling, but conflicting rhizosphere processes can both enhance and reduce SOM retention. To address the resulting uncertainty, we followed litter decomposition and SOM formation as a function of root ingrowth and soil nutrient availability in a Miscanthus x giganteus field. We observed that roots transferred litter carbon into a more protected SOM pool and selectively mobilized nitrogen without additional carbon release, suggesting that roots efficiently mine nitrogen and protect soil carbon.
Soil organic matter (SOM) is the largest actively cycling reservoir of terrestrial carbon (C), and the majority of SOM in Earth's mineral soils (~65%) is mineral‐associated organic matter (MAOM). ...Thus, the formation and fate of MAOM can exert substantial influence on the global C cycle. To predict future changes to Earth's climate, it is critical to mechanistically understand the processes by which MAOM is formed and decomposed, and to accurately represent this process‐based understanding in biogeochemical and Earth system models.
In this review, we use a trait‐based framework to synthesize the interacting roles of plants, soil micro‐organisms, and the mineral matrix in regulating MAOM formation and decomposition. Our proposed framework differentiates between plant and microbial traits that influence total OM inputs to the soil (‘feedstock traits’) versus traits that influence the proportion of OM inputs that are ultimately incorporated into MAOM (‘MAOM formation traits’). We discuss how these feedstock and MAOM formation traits may be altered by warming, altered precipitation and elevated carbon dioxide.
At a planetary scale, these feedstock and MAOM formation traits help shape the distribution of MAOM across Earth's biomes, and modulate biome‐specific responses of MAOM to climate change. We leverage a global synthesis of MAOM measurements to provide estimates of the total amount of MAOM‐C globally (~840–1540 Pg C; 34%–51% of total terrestrial organic C), and its distribution across Earth's biomes. We show that MAOM‐C concentration is highest in temperate forests and grasslands, and lowest in shrublands and savannas. Grasslands and croplands have the highest proportion of soil organic carbon (SOC) in the MAOM fraction (i.e. the MAOM‐C:SOC ratio), while boreal forests and tundra have the lowest MAOM‐C:SOC ratio. Drawing on our trait framework, we then review experimental data and posit the effects of climate change on MAOM pools in different biomes.
We conclude by discussing how MAOM is integrated into soil C models, and how feedstock and MAOM formation traits may be included in these models. We also summarize the projected fate of MAOM under climate change scenarios (Representative Concentration Pathways 4.5 and 8.5) and discuss key model uncertainties.
Read the free Plain Language Summary for this article on the Journal blog.
Read the free Plain Language Summary for this article on the Journal blog.
Pyrogenic organic matter (PyOM) is considered an important soil carbon (C) sink. However, there are evidences that its addition to soil may induce a priming effect (PE) thus influencing its C ...abatement potential. The direction, the size and the mechanisms responsible for PyOM induced PE are far from being understood. We collected approximately 650 data points from 18 studies to analyse the characteristics of the PE induced by PyOM. The database was divided between the PE induced on the native soil organic matter and on fresh organic matter. Most of the studies were short‐term incubation therefore the projections of findings on the long term may be critical. Our findings indicate that over 1 year PyOM induces an average positive PE of 0.3 mg C g−1 soil on native soil organic matter and a PE of approximately the same size but opposite direction on fresh organic matter. We studied the correlation of PE with several properties of soil, of the added PyOM, and time after PyOM addition. We found that PyOM primes positively the native soil organic matter in the first 20 days while negative PE appears in a later stage. Negative PE was correlated with the soil C content. PyOM characterized by a low C content induced a higher positive PE on native soil organic carbon. No correlation was found between the factors record in our database and the PE induced on the fresh organic matter. We reviewed the mechanisms proposed in literature to explain PE and discussed them based on findings from our meta‐analysis. We believe that the presence of a labile fraction in PyOM may trigger the activity of soil microorganisms on the short term and therefore induce a positive PE, while on the long term PyOM may induce a negative PE by promoting physical protection mechanisms.
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