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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]

Take‐Home Message Climate warming is transforming the Arctic at an unprecedented rate with previously barren and sparsely vegetated landscapes undergoing "greening". We postulate that the observed vegetation changes throughout the Arctic are not only tied to warming, but to changes in soil properties and their impacts on plants and soil microbial communities. A key to understanding extent and patterns of greening of formerly sparsely vegetated Arctic environments will be to unravel the interactions between the biosphere and the role of soil genesis. [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]