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Sustainable intensification schemes such as integrated soil fertility management (ISFM) are a proposed strategy to close yield gaps, increase soil fertility, and achieve food security in sub-Saharan Africa. Biogeochemical models such as DayCent can assess their potential at larger scales, but these models need to be calibrated to new environments and rigorously tested for accuracy. Here, we present a Bayesian calibration of DayCent, using data from four long-term field experiments in Kenya in a leave-one-site-out cross-validation approach. The experimental treatments consisted of the addition of low- to high-quality organic resources, with and without mineral nitrogen fertilizer. We assessed the potential of DayCent to accurately simulate the key elements of sustainable intensification, including (1) yield, (2) the changes in soil organic carbon (SOC), and (3) the greenhouse gas (GHG) balance of CO 2 and N 2 O combined. Compared to the initial parameters, the cross-validation showed improved DayCent simulations of maize grain yield (with the Nash–Sutcliffe model efficiency (EF) increasing from 0.36 to 0.50) and of SOC stock changes (with EF increasing from 0.36 to 0.55). The simulations of maize yield and those of SOC stock changes also improved by site (with site-specific EF ranging between 0.15 and 0.38 for maize yield and between - 0.9 and 0.58 for SOC stock changes). The four cross-validation-derived posterior parameter distributions (leaving out one site each) were similar in all but one parameter. Together with the model performance for the different sites in cross-validation, this indicated the robustness of the DayCent model parameterization and its reliability for the conditions in Kenya. While DayCent poorly reproduced daily N 2 O emissions (with EF ranging between - 0.44 and - 0.03 by site), cumulative seasonal N 2 O emissions were simulated more accurately (EF ranging between 0.06 and 0.69 by site). The simulated yield-scaled GHG balance was highest in control treatments without N addition (between 0.8 and 1.8 kg CO 2 equivalent per kg grain yield across sites) and was about 30 % to 40 % lower in the treatment that combined the application of mineral N and of manure at a rate of 1.2 t C ha -1 yr -1. In conclusion, our results indicate that DayCent is well suited for estimating the impact of ISFM on maize yield and SOC changes. They also indicate that the trade-off between maize yield and GHG balance is stronger in low-fertility sites and that preventing SOC losses, while difficult to achieve through the addition of external organic resources, is a priority for the sustainable intensification of maize production in Kenya. [ABSTRACT FROM AUTHOR]

Maintaining soil organic matter (SOM) is crucial for healthy and productive agricultural soils and requires understanding at the process level, including the role of SOM protection by soil aggregates and the connection between microbial growth and aggregate formation. We developed the Soil Aggregation through Microbial Mediation (SAMM) model, to represent this important connection. The pools of SAMM are fully measurable, and we calibrated and evaluated it against data from a long-term bare fallow experiment in a tropical sandy soil. This experiment received additions of plant litter of different compositions, which resulted in twice the soil carbon stocks in the best treatment compared to the control (about 8 vs. 4 t C ha -1 in 0–15 cm soil depth) after 25 years. As hypothesized, the SAMM model effectively represented the microbial growth response after the addition of litter and the subsequent formation and later destabilization of aggregates. The low correlations between different calibrated model parameters (r<0.5 for all parameters; r>0.4 for only 4 of 22) showed that SAMM is parsimonious. SAMM was able to capture differences between treatments in soil organic carbon (Nash–Sutcliffe modeling efficiency (EF) of 0.68), microbial nitrogen (EF of 0.24), and litter carbon (EF of 0.80). The amount of carbon within the aggregates (EF of 0.60) and in the free silt and clay fraction (EF of 0.24) was also simulated very well to satisfactorily. Our model results suggested that in spite of the sandy soil, up to 50 % of carbon stocks were stabilized through aggregate protection mechanisms; and that microbial and physical aggregate formation coexists. A version of the SAMM model without aggregate protection (SAMMnoAgg) initially failed to stabilize soil organic carbon (EF decreased to -3.68) and the simulation of microbial nitrogen worsened (EF of 0.13). By recalibrating SAMMnoAgg, it was possible to partially correct for the lack of aggregate protection by reducing the rate of mineral-attached carbon decomposition by about 85 % (EF of 0.68, 0.75, and 0.18 for SOC, litter carbon, and microbial nitrogen, respectively). However, the slightly better evaluation statistics of SAMM (e.g., Akaike information criterion of 5351 vs. 5554) suggest that representing aggregate dynamics in SOM models can be beneficial and necessary to understand the mechanism behind SOM dynamics. Our results indicate that current models without aggregate formation partly compensate for the absence of aggregate protection by lowering the turnover rates of other pools. Thus, they remain suitable options where data on aggregate associated carbon are not available. [ABSTRACT FROM AUTHOR]

Predicting the quantity of soil organic carbon (SOC) requires understanding how different factors control the amount of SOC. Land use has a major influence on the function of the soil as a carbon sink, as shown by substantial organic carbon (OC) losses from the soil upon deforestation. However, predicting the degree to which land use change affects the OC content in soils and the depth down to which this occurs requires context-specific information related to, for example, climate, geochemistry, and land use history. In this study, 266 samples from forests and agricultural fields were collected from 94 soil profiles down to 300 cm depth in a subtropical region (Arvorezinha, southern Brazil) to study the impact of land use on the amount of stabilized OC along the soil profile. We found that the stabilized OC content was not affected by land use below a depth of 90 cm. Along the soil profile, the amount of stabilized OC was predominantly controlled by land use and depth in addition to the silt and clay content and aluminium ion concentrations. Below 100 cm , none of the soil profiles reached a concentration of stabilized OC above 50 % of the stabilized OC saturation point (i.e. the maximum OC concentration that can physically be stabilized in these soils). Based on these results, we argue that it is unlikely that deeper soil layers can serve as an OC sink over a timescale relevant to global climate change due to the limited OC input in these deeper layers. Furthermore, we found that the soil weathering degree was not a relevant control on the amount of stabilized OC in our profiles because of the high weathering degree of the studied soils. It is therefore suggested that, while the soil weathering degree might be an effective controlling factor of OC stabilization over a large spatial scale, it is not an informative measure for this process at regional and local scales (with similar climate, bedrock, and weathering history) in highly weathered soils. [ABSTRACT FROM AUTHOR]

The concept of soil organic carbon (SOC) saturation emerged a bit more than 2 decades ago as our mechanistic understanding of SOC stabilization increased. Recently, the further testing of the concept across a wide range of soil types and environments has led some people to challenge the fundamentals of soil C saturation. Here, we argue that, to test this concept, one should pay attention to six fundamental principles or "rights" (R's): the right measures, the right units, the right dispersive energy and application, the right soil type, the right clay type, and the right saturation level. Once we take care of those six rights across studies, we find a maximum of C stabilized by minerals and estimate based on current data available that this maximum stabilization is around 82 ± 4 g C kg -1 silt + clay for 2 : 1-clay-dominated soils while most likely being only around 46 ± 4 g C kg -1 silt + clay for 1 : 1-clay-dominated soils. These estimates can be further improved using more data, especially for different clay types across varying environmental conditions. However, the bigger challenge is a matter of which C sequestration strategies to implement and how to implement them in order to effectively reach this 82/46 g C kg -1 silt + clay in soils across the globe. [ABSTRACT FROM AUTHOR]

In sub-Saharan Africa, maize is one of the most important staple crops, but long-term maize cropping with low external inputs has been associated with the loss of soil fertility. While adding high-quality organic resources combined with mineral fertilizer has been proposed to counteract this fertility loss, the long-term effectiveness and interactions with site properties still require more understanding. This study used repeated measurements over time to assess the effect of different quantities and qualities of organic resource addition combined with mineral nitrogen (N) on the change of soil organic carbon (SOC) contents over time (and SOC stocks in the year 2021) in four ongoing long-term experiments in Kenya. These experiments were established with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been conducted for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha -1 yr -1 in the form of Tithonia diversifolia (high quality, fast turnover), Calliandra calothyrsus (high quality, intermediate turnover), Zea mays stover (low quality, fast turnover), sawdust (low quality, slow turnover) and local farmyard manure (variable quality, intermediate turnover). Furthermore, the addition of 240 kg N ha -1 yr -1 as mineral N fertilizer or no fertilizer was the split-plot treatment. At all four sites, a loss of SOC was predominantly observed, likely because the sites had been converted to cropland only a few decades before the start of the experiments. Across sites, the average decline of SOC content over 19 years in the 0 to 15 cm topsoil layer ranged from 42 % to 13 % of the initial SOC content for the control and the farmyard manure treatments at 4 t C ha -1 yr -1 , respectively. Adding Calliandra or Tithonia at 4 t C ha -1 yr -1 limited the loss of SOC contents to about 24 % of initial SOC, while the addition of sawdust, maize stover (in three of the four sites) and sole mineral N addition showed no significant reduction of SOC loss over the control. Site-specific analyses, however, did show that at the site with the lowest initial SOC content (about 6 g kg -1), the addition of 4 t C ha -1 yr -1 farmyard manure or Calliandra with mineral N led to a gain in SOC contents. The other sites lost SOC in all treatments, albeit at site-specific rates. While subsoil SOC stocks in 2021 were little affected by organic resource additions (no difference in three of the four sites), the topsoil SOC stocks corroborated the results obtained from the SOC content measurements (0–15 cm) over time. The relative annual change of SOC contents showed a higher site specificity in farmyard manure, Calliandra and Tithonia treatments than in the control treatment, suggesting that the drivers of site specificity in SOC buildup (soil mineralogy, soil texture, climate) need to be better understood for effective targeting management of organic resources. Farmyard manure showed the highest potential for reducing SOC losses, but the necessary quantities to build SOC are often not realistic for smallholder farmers in Africa. Therefore, additional agronomic interventions such as intercropping, crop rotations or the cultivation of crops with extended root systems are necessary to maintain or increase SOC. [ABSTRACT FROM AUTHOR]

In sub-Saharan Africa, long-term maize cropping with low external inputs has been associated with the loss of soil fertility. While adding high-quality organic resources combined with mineral fertilizer has been proposed to counteract this fertility loss, the long-term effectiveness and interactions with site properties still require more understanding. This study used repeated measurements over time to assess the effect of different quantities and qualities of organic resource addition combined with mineral N on the change of soil organic carbon concentrations (SOC) over time (and SOC stocks in the year 2021) in four ongoing long-term trials in Kenya. These trials were established with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been managed for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha−1 per year in the form of Tithonia diversifolia (high quality, fast turnover), Calliandra calothyrsus (high quality, intermediate turnover), Zea mays stover (low quality, fast turnover), sawdust (low quality, slow turnover) and local farmyard manure (variable quality, intermediate turnover). Furthermore, the addition or absence of 240 kg N ha−1 per year as mineral N fertilizer was the split-plot treatment. At all sites, a loss of SOC, rather than gain, was predominantly observed due to a recent conversion from permanent vegetation to agriculture. The average reduction of SOC concentration over 19 years in the 0 to 15 cm depth ranged from 42 % to 13 % of the initial SOC concentration for the control and the farmyard manure treatments at 4 t C ha−1 yr−1, respectively. Adding Calliandra or Tithonia at 4 t C ha−1 yr−1 limited the loss of SOC concentrations to about 24 % of initial SOC, while the addition of saw dust, maize stover (in 3 of 4 sites) and sole mineral N addition, showed no significant reduction in SOC loss over the control. Site specific analyses, however, did show, that at the site with the lowest initial SOC concentration (about 6 g kg−1), the addition of 4 t C ha−1 yr−1 farmyard manure or Calliandra plus mineral N led to a gain in SOC concentrations. All other sites lost SOC in all treatments, albeit at site specific rates. While subsoil SOC stocks in 2021 were little affected by organic resource additions (no difference in 3 of 4 sites), the topsoil SOC stocks corroborated the results for SOC concentrations. The relative annual change of SOC concentrations showed a higher site specificity in high-quality organic resource treatments than in the control, suggesting that the drivers of site specificity in SOC buildup (mineralogy, climate) need to be better understood for effective targeting of organic resources. Even though farmyard manure showed the most potential for reducing SOC loss, our results clearly show that maintaining SOC with external inputs only is not possible at organic resource rates that are realistic for small scale farmers. Thus, additional agronomic interventions such as intercropping, crop rotations or strong rooting crops may be necessary to maintain or increase SOC. [ABSTRACT FROM AUTHOR]

Mangroves are widely recognised as key ecosystems for climate change mitigation as they capture and store significant amounts of sediment organic carbon (SOC). Yet, there is incomplete knowledge on how sources of SOC and their differential preservation vary between mangrove sites in relation to environmental gradients. To address this, sediment depth profiles were sampled from mangrove sites ranging from river-dominated to marine-dominated sites and including old and young mangrove sites in the Guayas delta (Ecuador). The stable carbon isotope ratios (δ13 C) and the elemental composition (OC %, C : N) of sediment profiles, local vegetation (i.e. autochthonous carbon) and externally supplied suspended particulate matter (i.e. allochthonous carbon) were obtained to assess variations in the amount and sources of SOC at different locations throughout the delta. In general, across all sites, we found that increasing SOC contents and stocks are associated with decreasing δ13 C and increasing C : N ratios, indicating that SOC stocks and sources are intrinsically related. The SOC stocks (down to 0.64 m depth profiles) are significantly lower in young mangrove sites (46–55 Mg C ha -1) than in old sites (78–92 Mg C ha -1). The SOC in the young mangrove sites is mainly of allochthonous origin (estimated on average at 79 %), whereas in the old sites there is a slight dominance of autochthonous OC (on average 59 %). Moreover, from river- to marine-dominated sites, a pattern was found of increasing SOC stocks and increasing autochthonous SOC contribution. These observed differences along the two studied gradients are hypothesised to be mainly driven by (1) expected higher sedimentation rates in the river-dominated and lower-elevation younger sites, thereby 'diluting' the SOC content and decreasing the relative autochthonous contribution, and (2) potential differences in preservation of the different SOC sources. Our finding of high contributions of allochthonous SOC, especially in young mangroves, implies that this carbon is not originating from CO 2 sequestration by the mangrove ecosystem itself but is externally supplied from other terrestrial, marine or estuarine ecosystems. We argue that accounting for lower SOC stocks and higher contribution of allochthonous SOC in young and river-dominated mangrove sites, as compared to old and marine-dominated sites, is particularly relevant for designing and valuing nature-based climate mitigation programmes based on mangrove reforestation. [ABSTRACT FROM AUTHOR]

Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. In order to underpin the definition of current limits, this study was set up to identify the pre-industrial background PO4 concentration in surface water of the Scheldt River, a tidal river in Flanders. We used the sedimentary records preserved in tidal marsh sediment cores as an archive for reconstructing historical changes in surface water PO4. For sediment samples at sequential depths below the sediment surface, we dated the time of sediment deposition and analysed the extractable sediment P. The resulting time series of sediment P was linked to the time series of measured surface water- PO4 concentrations (data 1967–present). By combining those datasets, the sorption characteristics of the sediment could be described using a Langmuir-type sorption model. The calibrated sorption model allowed us to estimate a pre-industrial background surface water PO4 levels, based on deeper sediment P that stabilized at concentrations smaller than the modern. In three out of the four cores, the sediment P peaked around 1980, coinciding with the surface water PO4. The estimated pre-industrial (∼1800) background PO4 concentration in the Scheldt River water was 62 [57; 66 (95 % CI)] µgPO4-PL-1. That concentration exceeds the previously estimated natural background values in Flanders (15–35 µgTPL-1) and is about half of the prevailing limit in the Scheldt River (120 µgPO4-PL-1). In the 1930s, river water concentrations were estimated at 140 [128; 148] µgPO4-PL-1 , already exceeding the current limit. The method developed here proved useful for reconstructing historical background PO4 concentrations of a lowland tidal river. A similar approach can apply to other lowland tidal rivers to provide a scientific basis for local catchment-specific PO4 backgrounds. [ABSTRACT FROM AUTHOR]

Mangroves are widely recognised as key ecosystems for climate change mitigation as they capture and store significant amounts of sediment organic carbon (SOC). Yet, there is incomplete knowledge on how sources of SOC and their differential preservation vary between mangrove sites in relation to environmental gradients. To address this, sediment depth profiles were sampled from mangrove sites ranging from river-dominated to marine-dominated sites and including old and young mangrove sites, in the Guayas delta (Ecuador). The stable carbon isotope ratios (δ13C) and the elemental composition (OC%, C : N) of sediment profiles, local vegetation (i.e., autochthonous carbon) and externally-supplied suspended particulate matter (i.e., allochthonous carbon) were obtained to assess variations in the amount and sources of SOC at different locations throughout the delta. In general, across all sites, we found increasing SOC contents and stocks are associated with decreasing δ13C and increasing C / N ratios, indicating that SOC stocks and sources are intrinsically related. The SOC stocks (down to 0.64 m deep profiles) are significantly lower in young mangrove sites (46-55 Mg C ha-1) than in old sites (78-92 Mg C ha-1). The SOC in the young mangrove sites is mainly of allochthonous origin (estimated on average at 79 %) whereas in the old sites there is a slight dominance of autochthonous OC (on average 59 %). Moreover, from river- to marine-dominated sites, a pattern was found of increasing SOC stocks and increasing autochthonous SOC contribution. These observed differences along the two studied gradients are hypothesized to be mainly driven by (1) expected higher sedimentation rates in the river-dominated and lower-elevation younger sites, thereby "diluting" the SOC content and decreasing the relative autochthonous contribution; and (2) potential differences in preservation of the different SOC sources. Our finding of high contributions of allochthonous SOC, especially in young mangroves, implies that this carbon is not originating from CO2 sequestration by the mangrove ecosystem itself, but is externally supplied from other terrestrial, marine or estuarine ecosystems. We argue that accounting for lower SOC stocks and higher contribution of allochthonous SOC in young and river-dominated mangrove sites, as compared to old and marine-dominant sites, is particularly relevant for designing and valuing nature-based climate mitigation programs based on mangrove reforestation. [ABSTRACT FROM AUTHOR]

Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. In order to underpin the definition of currents limits, this study was set up to identify the pre-industrial background PO4 concentration in surface water of the Scheldt river, a tidal river in Flanders. We used the sedimentary records preserved in tidal marsh sediment cores as an archive for reconstructing historical changes in surface water PO4. For sediment samples at different depths below the sediment surface, we dated the time of sediment deposition and analysed the extractable sediment-P. The resulting time series of sediment-P was linked to time series of measured surface water PO4 concentrations (data 1967-present). By combining the sediment-P and water-PO4 data, the sorption characteristics of the sediment could be described. Those sorption characteristics allowed us to estimate a pre-industrial background surface water PO4 levels, based on deeper sediment-P that stabilised at concentrations smaller than the modern. In three out of the four cores, the sediment-P peaked around 1980, coinciding with the peak in surface water PO4. The estimated pre-industrial (~1800) background PO4-concentration in the Scheldt river water was 62 [57; 66 (95%CI)] µg PO4-P/L. That concentration exceeds the previously estimated natural background values for lakes in Flanders (15-35 µg TP/L) and is about half of the prevailing limit in the Scheldt river (120 µg PO4-P/L). In the 1930s, river water concentrations were estimated at 140 [128; 148] µg PO4-P/L, already exceeding the current limit. The method developed here proved useful for reconstructing historical, background PO4 concentrations of a lowland tidal river. A similar approach can apply to other lowland tidal rivers to provide a scientific basis for local, catchment specific PO4 backgrounds. [ABSTRACT FROM AUTHOR]

Over the past decades, average global wheat yields have increased by about 250 %, mainly due to the cultivation of high-yielding wheat cultivars. This selection process not only affected aboveground parts of plants, but in some cases also reduced root biomass, with potentially large consequences for the amount of organic carbon (OC) transferred to the soil. To study the effect of wheat breeding for high-yielding cultivars on subsoil OC dynamics, two old and two new wheat cultivars from the Swiss wheat breeding program were grown for one growing season in 1.5 m deep lysimeters and pulse labeled with 13CO2 to quantify the amount of assimilated carbon that was transferred belowground and can potentially be stabilized in the soil. The results show that although the old wheat cultivars with higher root biomass transferred more assimilated carbon belowground compared to more recent cultivars, no significant differences in net rhizodeposition were found between the different cultivars. As a consequence, the long-term effect of wheat cultivar selection on soil organic carbon (SOC) stocks will depend on the amount of root biomass that is stabilized in the soil. Our results suggest that the process of wheat selection for high-yielding cultivars resulted in lower amounts of belowground carbon translocation, with potentially important effects on SOC stocks. Further research is necessary to quantify the long-term importance of this effect. [ABSTRACT FROM AUTHOR]

Tidal marshes are sedimentary environments and are among the most productive ecosystems on Earth. As a consequence they have the potential to reduce atmospheric greenhouse gas concentrations by sequestering organic carbon (OC). In the past decades, most research on soil organic carbon (SOC) storage in marsh environments has focused on salt marshes, leaving carbon dynamics in brackish and freshwater marshes largely understudied and neglecting the diversity among tidal marshes. We therefore conducted an extensive sampling campaign to quantify and characterize SOC stock in marshes along a salinity gradient in the Scheldt estuary (Belgium and the Netherlands). We find that SOC stocks vary significantly along the estuary, from 46 in freshwater marshes to 10 kgOCm-2 in salt marshes. Our data also show that most existing studies underestimate total SOC stocks due to shallow soil sampling, which also influences reported patterns in OC storage along estuaries. In all sampled tidal marsh sediments the SOC concentration is more or less constant from a certain depth downward. However, this concentration decreases with increasing salinity, indicating that the amount of stable SOC decreases from the upper estuary towards the coast. Although the net primary production of macrophytes differs along the estuary, our data suggest that the differences in OC storage are caused mainly by variations in suspended sediment concentration and stable particulate OC (POC) content in the water along the estuary. The fraction of terrestrial suspended sediments and POC that is transported downstream of the maximum turbidity zone is very limited, contributing to smaller amounts of long-term OC sequestration in brackish and salt marsh sediments. In addition, high rates of sediment deposition on freshwater tidal marshes in the maximum turbidity zone promote efficient burial of OC in these marsh sediments. [ABSTRACT FROM AUTHOR]

Tidal marshes are sedimentary environments that are among the most productive ecosystems on earth. As a consequence tidal marshes, and vegetated coastal ecosystems in general, have the potential to reduce atmospheric greenhouse gas concentrations as they efficiently sequester soil organic carbon (SOC). In the past decades, most research has focused on salt marshes, leaving carbon dynamics in brackish- and freshwater marshes largely understudied and neglecting the diversity among tidal marshes. Moreover, most existing studies underestimate total organic carbon (OC) stocks due to shallow soil sampling, which also influences reported patterns in OC storage along estuaries. We find that SOC stocks vary significantly along the salinity gradient of a temperate estuary (Scheldt estuary, Belgium and The Netherlands), from 46 kg OC m-2 in freshwater marshes to 10 kg OC m-2 in saltmarshes. In all tidal marsh sediments the OC concentration has a constant value from a certain depth below the surface downward. However, this concentration decreases with increasing salinity, indicating that the amount of stabile SOC decreases from the upper estuary towards the coast. Although net primary production of macrophytes differs along the estuary, our data suggest that these differences in OC storage are caused mainly by variations in suspended sediment concentration and stable particulate OC (POC) content in the water along the estuary. The fraction of suspended sediments and POC that is transported downstream the maximum turbidity zone is very limited, contributing to smaller amounts of long term OC sequestration in brackish- and saltmarsh sediments. In addition, high rates of sediment deposition on freshwater tidal marshes in the maximum turbidity zone promote efficient burial of OC in these marsh sediments. [ABSTRACT FROM AUTHOR]

Over the past decades, our understanding of soil organic carbon (SOC) cycling has evolved to one where its persistence is mainly due to physical and chemical protection mechanisms, while soil microbes are now acknowledged as being the main precursor of stabilized OC. In line with this understanding, microbial-driven SOC models that explicitly account for these mechanisms have been developed during the past years. While 14C has been widely used to calibrate and validate SOC models, stable carbon isotopes (δ13C) have to date not been used for the calibration of a mechanistic SOC model. The change in these isotopic ratios is, however, caused by fundamentally different mechanisms: the Δ14C value of SOC provides an indication of how long C has been in the soil, while the δ13C value provides information about the intensity with which this C has been processed by soil microorganisms. If the δ13C value of SOC can be mechanistically simulated, this might improve confidence in model calibration and the modeled residence time of SOC substantially.Therefore, we developed a microbial-driven SOC model that simulates depth profiles of total SOC, δ13C and Δ14C. The novelty of this model is that it separates SOC processes in the rhizosphere (with fast OC cycling due to high C inputs and limited protection mechanisms) and the mineral soil (with slow OC cycling due to the protection by organo-mineral interactions). Furthermore, the physical protection of OC in aggregates is simulated while microbial residues are stabilized on mineral surfaces. The simulated depth profiles of δ13C reflect the increasing contribution of microbial residues to SOC with depth, while this isotopic signal is influenced by heterotrophic CO2 assimilation by microorganisms. The model has been used to simulate depth profiles of OC, δ13C and Δ14C of stable landscape locations under forest in western Europe. [ABSTRACT FROM AUTHOR]