Reviews and syntheses: The mechanisms underlying carbon storage in soil

Abstract. Soil organic matter (OM) represents a key C pool for climate regulation but also an essential component for soil functions and services. Scientific research in the 21st century has considerably improved our knowledge of soil organic matter and its dynamics, particularly under the pressure of the global disruption of the carbon cycle. This paper reviews the processes that control C dynamics in soil, the representation of these processes over time, and their dependence on variations in major biotic and abiotic factors. The most recent advanced knowledge gained on soil organic matter includes the following. (1) Most organic matter is composed of small molecules, derived from living organisms, without transformation via additional abiotic organic polymerization; (2) microbial compounds are predominant in the long term; (3) primary belowground production contributes more to organic matter than aboveground inputs; (4) the contribution of less biodegradable compounds to soil organic matter is low in the long term; (5) two major factors determine the soil organic carbon production “yield” from the initial substrates: the yield of carbon used by microorganisms and the association with minerals, particularly poorly crystalline minerals, which stabilize microbial compounds; (6) interactions between plants and microorganisms also regulate the carbon turnover time and therefore carbon stocks; (7) among abiotic and biotic factors that regulate the carbon turnover time, only a few are considered in current modeling approaches (i.e., temperature, soil water content, pH, particle size, and sometimes C and N interactions); and (8) although most models of soil C dynamics assume that the processes involved are linear, there are now many indications of nonlinear soil C dynamics processes linked to soil OM dynamics (e.g., priming). Farming practices, therefore, affect soil C stocks not only through carbon inputs but also via their effect on microbial and organomineral interactions, yet it has still not been possible to properly identify the main mechanisms involved in C loss (or gain). Greater insight into these mechanisms and their interdependencies, hierarchy and sensitivity to agricultural practices could provide future levers of action for C sequestration in soil.

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Reviews and syntheses: The mechanisms underlying carbon storage in soil

10.5194/bg-2020-49 ◽

2020 ◽

Cited By ~ 2

Author(s):

Isabelle Basile-Doelsch ◽

Jérôme Balesdent ◽

Sylvain Pellerin

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Abiotic Factors ◽

Soil C ◽

Carbon Production ◽

C Stocks ◽

Organomineral Interactions ◽

Living Organisms ◽

Major Factors

Abstract. Scientific research in the 21st century has considerably improved our knowledge of soil organic matter and its dynamics, particularly under the pressure of the global disruption of the carbon cycle. This paper reviews the processes that control C dynamics in soil, the representation of these processes over time, and their dependence on variations in major biotic and abiotic factors. The most recent advances in soil organic matter knowledge are: – Most organic matter is composed of small molecules, derived from living organisms, without transformation via additional abiotic organic polymerization. – Microbial compounds are predominant in the long term. – Primary belowground production contributes more to organic matter than aboveground inputs. – Contribution of less biodegradable compounds to soil organic matter is low in the long term. – Two major factors determine the soil organic carbon production yield from the initial substrates: the yield of carbon used by microorganisms and the association with minerals, particularly poorly crystallized minerals, which stabilize microbial compounds. – Interactions between plants and microorganisms and between microbial communities affect or even regulate carbon residence times, and therefore carbon stocks. Farming practices therefore affect soil C stocks not only through carbon inputs but also via their effect on microbial and organomineral interactions.

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No long-term effect of land-use activities on soil carbon dynamics in tropical montane grasslands

10.5194/bg-2017-113 ◽

2017 ◽

Author(s):

Viktoria Oliver ◽

Imma Oliveras ◽

Jose Kala ◽

Rebecca Lever ◽

Yit Arn Teh

Keyword(s):

Land Use ◽

Organic Matter ◽

Co2 Fluxes ◽

Soil Co2 ◽

Soil C ◽

C Dynamics ◽

Total Soil ◽

C Stocks ◽

Montane Grasslands

Abstract. Montane tropical soils are a large carbon (C) reservoir, acting as both a source and a sink of CO2. Enhanced CO2 emissions originate, in large part, from the decomposition and losses of soil organic matter (SOM) following anthropogenic disturbances. Therefore, quantitative knowledge of the stabilization and decomposition of SOM is necessary in order to understand, assess and predict the impact of land management in the tropics. In particular, labile SOM is an early and sensitive indicator of how SOM responds to changes in land use and management practices, which could have major implications for long term carbon storage and rising atmospheric CO2 concentrations. The aim of this study was to investigate the impacts of grazing and fire history on soil C dynamics in the Peruvian montane grasslands; an understudied ecosystem, which covers approximately a quarter of the land area in Peru. A combination of density and particle-size fractionation was used to quantify the labile and stable organic matter pools, along with soil CO2 flux and decomposition measurements. Grazing and burning together significantly increased soil CO2 fluxes and decomposition rates and reduced temperature as a driver. Although there was no significant effect of land use on total soil C stocks, the combination of burning and grazing decreased the proportion of C in the free LF, especially at the lower depths (10–20 and 20–30 cm). The free LF in the control soils made 20 % of the bulk soil mass and 30 % of the soil C content compared to the burnt-grazed soils, which had the smallest recovery of free LF (10 %) and significantly lower C content (14 %). The burnt soils had a much higher proportion of C in the occluded LF (12 %) compared to the non-burnt soils (7 %) and there was no significant difference among the treatments in the heavy F (~ 70 %). The synergistic effect of burning and grazing caused changes to the soil C dynamics. CO2 fluxes were increased and the dominant temperature driver was obscured by some other process, such as changes in plant C and N allocation promoting autotrophic respiration. In addition, the free LF was negatively affected when these two anthropogenic activities took place on the same site. Most likely a result of reduced detritus being incorporated into the soil. A positive finding from this study is that the total soil C stocks were not significantly affected and the long term C storage in the occluded LF and heavy F were not negatively impacted. Possibly this is because of low intensity fire, fire-resilient grasses and the grazing pressure is below the threshold to cause severe degradation.

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The effects of burning and grazing on soil carbon dynamics in managed Peruvian tropical montane grasslands

Biogeosciences ◽

10.5194/bg-14-5633-2017 ◽

2017 ◽

Vol 14 (24) ◽

pp. 5633-5646 ◽

Cited By ~ 3

Author(s):

Viktoria Oliver ◽

Imma Oliveras ◽

Jose Kala ◽

Rebecca Lever ◽

Yit Arn Teh

Keyword(s):

Land Use ◽

Organic Matter ◽

Co2 Fluxes ◽

Soil Co2 ◽

Soil C ◽

C Dynamics ◽

Total Soil ◽

C Stocks ◽

Montane Grasslands

Abstract. Montane tropical soils are a large carbon (C) reservoir, acting as both a source and a sink of CO2. Enhanced CO2 emissions originate, in large part, from the decomposition and losses of soil organic matter (SOM) following anthropogenic disturbances. Therefore, quantitative knowledge of the stabilization and decomposition of SOM is necessary in order to understand, assess and predict the impact of land management in the tropics. In particular, labile SOM is an early and sensitive indicator of how SOM responds to changes in land use and management practices, which could have major implications for long-term carbon storage and rising atmospheric CO2 concentrations. The aim of this study was to investigate the impacts of grazing and fire history on soil C dynamics in the Peruvian montane grasslands, an understudied ecosystem, which covers approximately a quarter of the land area in Peru. A density fractionation method was used to quantify the labile and stable organic matter pools, along with soil CO2 flux and decomposition measurements. Grazing and burning together significantly increased soil CO2 fluxes and decomposition rates and reduced temperature as a driver. Although there was no significant effect of land use on total soil C stocks, the combination of burning and grazing decreased the proportion of C in the free light fraction (LF), especially at the lower depths (10–20 and 20–30 cm). In the control soils, 20 % of the material recovered was in the free LF, which contained 30 % of the soil C content. In comparison, the burnt–grazed soil had the smallest recovery of the free LF (10 %) and a significantly lower C content (14 %). The burnt soils had a much higher proportion of C in the occluded LF (12 %) compared to the not-burnt soils (7 %) and there was no significant difference among the treatments in the heavy fraction (F) ( ∼  70 %). The synergistic effect of burning and grazing caused changes to the soil C dynamics. CO2 fluxes were increased and the dominant temperature driver was obscured by some other process, such as changes in plant C and N allocation. In addition, the free LF was reduced when these two anthropogenic activities took place on the same site – most likely a result of reduced detritus being incorporated into the soil. A positive finding from this study is that the total soil C stocks were not significantly affected and the long-term (+10 years) C storage in the occluded LF and heavy F were not negatively impacted. Possibly this is because of low-intensity fire, fire-resilient grasses and because the grazing pressure is below the threshold necessary to cause severe degradation.

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Unifying soil organic matter formation and persistence frameworks: the MEMS model

Biogeosciences ◽

10.5194/bg-16-1225-2019 ◽

2019 ◽

Vol 16 (6) ◽

pp. 1225-1248 ◽

Cited By ~ 22

Author(s):

Andy D. Robertson ◽

Keith Paustian ◽

Stephen Ogle ◽

Matthew D. Wallenstein ◽

Emanuele Lugato ◽

...

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Model Performance ◽

Soil C ◽

Biogeochemical Models ◽

Novel Approach ◽

C Stocks ◽

Microbial Efficiency ◽

Organic Matter Fractions ◽

Soil Organic Matter Formation

Abstract. Soil organic matter (SOM) dynamics in ecosystem-scale biogeochemical models have traditionally been simulated as immeasurable fluxes between conceptually defined pools. This greatly limits how empirical data can be used to improve model performance and reduce the uncertainty associated with their predictions of carbon (C) cycling. Recent advances in our understanding of the biogeochemical processes that govern SOM formation and persistence demand a new mathematical model with a structure built around key mechanisms and biogeochemically relevant pools. Here, we present one approach that aims to address this need. Our new model (MEMS v1.0) is developed from the Microbial Efficiency-Matrix Stabilization framework, which emphasizes the importance of linking the chemistry of organic matter inputs with efficiency of microbial processing and ultimately with the soil mineral matrix, when studying SOM formation and stabilization. Building on this framework, MEMS v1.0 is also capable of simulating the concept of C saturation and represents decomposition processes and mechanisms of physico-chemical stabilization to define SOM formation into four primary fractions. After describing the model in detail, we optimize four key parameters identified through a variance-based sensitivity analysis. Optimization employed soil fractionation data from 154 sites with diverse environmental conditions, directly equating mineral-associated organic matter and particulate organic matter fractions with corresponding model pools. Finally, model performance was evaluated using total topsoil (0–20 cm) C data from 8192 forest and grassland sites across Europe. Despite the relative simplicity of the model, it was able to accurately capture general trends in soil C stocks across extensive gradients of temperature, precipitation, annual C inputs and soil texture. The novel approach that MEMS v1.0 takes to simulate SOM dynamics has the potential to improve our forecasts of how soils respond to management and environmental perturbation. Ensuring these forecasts are accurate is key to effectively informing policy that can address the sustainability of ecosystem services and help mitigate climate change.

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Regulation of priming effect by soil organic matter stability over a broad geographic scale

Nature Communications ◽

10.1038/s41467-019-13119-z ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 20

Author(s):

Leiyi Chen ◽

Li Liu ◽

Shuqi Qin ◽

Guibiao Yang ◽

Kai Fang ◽

...

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Priming Effect ◽

Basal Respiration ◽

Soil Conditions ◽

The Tibetan Plateau ◽

Geographic Scale ◽

Soil C ◽

Chemical Structures ◽

C Dynamics

Abstract The modification of soil organic matter (SOM) decomposition by plant carbon (C) input (priming effect) represents a critical biogeochemical process that controls soil C dynamics. However, the patterns and drivers of the priming effect remain hidden, especially over broad geographic scales under various climate and soil conditions. By combining systematic field and laboratory analyses based on multiple analytical and statistical approaches, we explore the determinants of priming intensity along a 2200 km grassland transect on the Tibetan Plateau. Our results show that SOM stability characterized by chemical recalcitrance and physico-chemical protection explains more variance in the priming effect than plant, soil and microbial properties. High priming intensity (up to 137% of basal respiration) is associated with complex SOM chemical structures and low mineral-organic associations. The dependence of priming effect on SOM stabilization mechanisms should be considered in Earth System Models to accurately predict soil C dynamics under changing environments.

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Spatial patterns in soil organic matter dynamics are shaped by mycorrhizosphere interactions in a treeline forest

Plant and Soil ◽

10.1007/s11104-019-04398-y ◽

2019 ◽

Vol 447 (1-2) ◽

pp. 521-535

Author(s):

Nina L. Friggens ◽

Thomas J. Aspray ◽

Thomas C. Parker ◽

Jens-Arne Subke ◽

Philip A. Wookey

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Spatial Scales ◽

Mountain Birch ◽

Organic C ◽

Soil C ◽

Soil Organic Matter Dynamics ◽

C Stocks ◽

Organic Matter Dynamics ◽

Individual Trees

Abstract Aims In the Swedish sub-Arctic, mountain birch (Betula pubescens ssp. czerepanovii) forests mediate rapid soil C cycling relative to adjacent tundra heaths, but little is known about the role of individual trees within forests. Here we investigate the spatial extent over which trees influence soil processes. Methods We measured respiration, soil C stocks, root and mycorrhizal productivity and fungi:bacteria ratios at fine spatial scales along 3 m transects extending radially from mountain birch trees in a sub-Arctic ecotone forest. Root and mycorrhizal productivity was quantified using in-growth techniques and fungi:bacteria ratios were determined by qPCR. Results Neither respiration, nor root and mycorrhizal production, varied along transects. Fungi:bacteria ratios, soil organic C stocks and standing litter declined with increasing distance from trees. Conclusions As 3 m is half the average size of forest gaps, these findings suggest that forest soil environments are efficiently explored by roots and associated mycorrhizal networks of B. pubescens. Individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected. However, overall rates of soil C accumulation do vary with distance from trees, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration.

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Soil organic matter in rainfed cropping systems of the Australian cereal belt

Soil Research ◽

10.1071/sr99042 ◽

2001 ◽

Vol 39 (3) ◽

pp. 435 ◽

Cited By ~ 139

Author(s):

R. C. Dalal ◽

K. Y. Chan

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Management Practices ◽

Performance Indicator ◽

Annual Rainfall ◽

Soil Productivity ◽

Organic C ◽

Soil C ◽

Eastern Australia

The Australian cereal belt stretches as an arc from north-eastern Australia to south-western Australia (24˚S–40˚S and 125˚E–147˚E), with mean annual temperatures from 14˚C (temperate) to 26˚C (subtropical), and with annual rainfall ranging from 250 mm to 1500 mm. The predominant soil types of the cereal belt include Chromosols, Kandosols, Sodosols, and Vertosols, with significant areas of Ferrosols, Kurosols, Podosols, and Dermosols, covering approximately 20 Mha of arable cropping and 21 Mha of ley pastures. Cultivation and cropping has led to a substantial loss of soil organic matter (SOM) from the Australian cereal belt; the long-term SOM loss often exceeds 60% from the top 0–0.1 m depth after 50 years of cereal cropping. Loss of labile components of SOM such as sand-size or particulate SOM, microbial biomass, and mineralisable nitrogen has been even higher, thus resulting in greater loss in soil productivity than that assessed from the loss of total SOM alone. Since SOM is heterogeneous in nature, the significance and functions of its various components are ambiguous. It is essential that the relationship between levels of total SOM or its identif iable components and the most affected soil properties be established and then quantif ied before the concentrations or amounts of SOM and/or its components can be used as a performance indicator. There is also a need for experimentally verifiable soil organic C pools in modelling the dynamics and management of SOM. Furthermore, the interaction of environmental pollutants added to soil, soil microbial biodiversity, and SOM is poorly understood and therefore requires further study. Biophysically appropriate and cost-effective management practices for cereal cropping lands are required for restoring and maintaining organic matter for sustainable agriculture and restoration of degraded lands. The additional benefit of SOM restoration will be an increase in the long-term greenhouse C sink, which has the potentialto reduce greenhouse emissions by about 50 Mt CO2 equivalents/year over a 20-year period, although current improved agricultural practices can only sequester an estimated 23% of the potential soil C sink.

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Long-term steady state <sup>13</sup>C labelling to investigate soil carbon turnover in grasslands

Biogeosciences ◽

10.5194/bg-4-385-2007 ◽

2007 ◽

Vol 4 (3) ◽

pp. 385-394 ◽

Cited By ~ 15

Author(s):

K. Klumpp ◽

J. F. Soussana ◽

R. Falcimagne

Keyword(s):

Organic Matter ◽

Steady State ◽

Soil Organic Matter ◽

Soil Carbon ◽

Residence Time ◽

Particulate Organic Matter ◽

Mean Residence Time ◽

Carbon Turnover ◽

13C Labelling

Abstract. We have set up a facility allowing steady state 13CO2 labeling of short stature vegetation (12 m2) for several years. 13C labelling is obtained by scrubbing the CO2 from outdoors air with a self-regenerating molecular sieve and by replacing it with 13C depleted (−34.7±0.03‰) fossil-fuel derived CO2 The facility, which comprises 16 replicate mesocosms, allows to trace the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied to the study of soil organic carbon turnover in temperate grasslands. We tested the hypothesis that a low disturbance by grazing and cutting of the grassland increases the mean residence time of carbon in coarse (>0.2 mm) soil organic fractions. Grassland monoliths (0.5×0.5×0.4 m) were sampled from high and low disturbance treatments in a long-term (14 yrs) grazing experiment and were placed during two years in the mesocosms. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow with air at mean CO2 concentration of 425 µmol mol−1 and δ13C of −21.5±0.27‰. Fully labelled mature grass leaves reached a δ13C of −40.8 (±0.93) and −42.2‰ (±0.60) in the low and high disturbance treatments, respectively, indicating a mean 13C labelling intensity of 12.7‰ compared to unlabelled control grass leaves. After two years, the delta 13C value of total soil organic matter above 0.2 mm was reduced in average by 7.8‰ in the labelled monoliths compared to controls. The isotope mass balance technique was used to calculate for the top (0–10 cm) soil the fraction of 13C labelled carbon in the soil organic matter above 0.2 mm (i.e. roots, rhizomes and particulate organic matter). A first order exponential decay model fitted to the unlabelled C in this fraction shows an increase in mean residence time from 22 to 31 months at low compared to high disturbance. A slower decay of roots, rhizomes and particulate organic matter above 0.2 mm is therefore likely to contribute to the observed increased in soil carbon sequestration in grassland monoliths exposed to low disturbance.

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Three long-term trials end with a quasi-equilibrium between soil C, N, and pH: an implication for C sequestration

Soil Research ◽

10.1071/sr12185 ◽

2012 ◽

Vol 50 (7) ◽

pp. 527 ◽

Cited By ~ 19

Author(s):

Mark Conyers ◽

Philip Newton ◽

Jason Condon ◽

Graeme Poile ◽

Pauline Mele ◽

...

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Chemical Properties ◽

C Sequestration ◽

Soil Chemical Properties ◽

Quasi Equilibrium ◽

Soil C ◽

Eastern Australia ◽

N Fertility

The aim of this study was to assess the long-term changes in some key soil chemical properties at the completion of three long-term trials in south-eastern Australia and the relationship between those soil properties. From a soil organic matter perspective, the build-up of carbon (%C) requires an accumulation of nitrogen (%N), and the build-up of %C and %N fertility comes at the cost of soil acidity. Rotation, tillage, and stubble practices combine to alter the quantity, quality (C : N), and the depth distribution of organic matter in a soil, but the three soil chemical properties reported here seem to also be in quasi-equilibrium at the three long-term sites. The consequence is that if the build-up of soil organic matter leads to soil acidification, then the maintenance of agricultural production will require liming. The emission of CO2 when limestone reacts with soil acids, plus the C cost of limestone application, will negate a proportion of the gains from C sequestration as organic matter in soil. Such cautionary information was doubtless unforeseen when these three long-term trials were initiated.

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Testing for soil carbon saturation behavior in agricultural soils receiving long-term manure amendments

Canadian Journal of Soil Science ◽

10.4141/cjss2013-012 ◽

2014 ◽

Vol 94 (3) ◽

pp. 281-294 ◽

Cited By ~ 18

Author(s):

W. Feng ◽

M. Xu ◽

M. Fan ◽

S. S. Malhi ◽

J. J. Schoenau ◽

...

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Soil Carbon ◽

Field Experiments ◽

Agricultural Soils ◽

Agricultural Field ◽

Soil C ◽

Carbon Saturation ◽

Manure Amendments

Feng, W., Xu, M., Fan, M., Malhi, S. S., Schoenau, J. J., Six, J. and Plante, A. F. 2014. Testing for soil carbon saturation behavior in agricultural soils receiving long-term manure amendments. Can. J. Soil Sci. 94: 281–294. Agricultural soils are typically depleted in soil organic matter compared with their undisturbed counterparts, thus reducing their fertility. Organic amendments, particularly manures, provide the opportunity to restore soil organic matter stocks, improve soil fertility and potentially sequester atmospheric carbon (C). The application of the soil C saturation theory can help identify soils with large C storage potentials. The goal of this study was to test whether soil C saturation can be observed in various soil types in agricultural ecosystems receiving long-term manure amendments. Seven long-term agricultural field experiments from China and Canada were selected for this study. Manure amendments increased C concentrations in bulk soil, particulate organic matter+sand, and silt+clay fractions in all the experiments. The increase in C concentrations of silt+clay did not fit the asymptotic regression as a function of C inputs better than the linear regression, indicating that silt+clay did not exhibit C saturation behavior. However, 44% of calculated C loading values for silt+clay were greater than the presumed maximal C loading, suggesting that this maximum may be greater than 1 mg C m−2 for many soils. The influences of soil mineral surface properties on C concentrations of silt+clay fractions were site specific. Fine soil particles did not exhibit C saturation behavior likely because current C inputs were insufficient to fill the large C saturation deficits of intensely cultivated soils, suggesting these soils may continue to act as sinks for atmospheric C.

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