Items

• 

  Meta-analysis on carbon sequestration through Conservation Agriculture in Africa

  Africa is the smallest contributor to global greenhouse gas emissions among the continents, but the most vulnerable to the impacts of climate change. The effects will not be limited to a rising average temperature and changing rainfall patterns, but also to increasing severity and frequency in droughts, heat stress and floods. Agriculture is not only impacted upon by climate change but also contributes to global warming. However, not all agricultural systems affect negatively climate change. Conservation Agriculture (CA) is a farming system that promotes continuous no or minimum soil disturbance (i.e. no tillage), maintenance of a permanent soil mulch cover, and diversification of plant species. Through these principles it enhances biodiversity and natural biological processes above and below the ground surface, so contributing to increased water and nutrient use efficiency and productivity, to more resilient cropping systems, and to improved and sustained crop production. Conservation Agriculture is based on the practical application of three interlinked principles along with complementary good agricultural practice. The characteristics of CA make it one of the systems best able to contribute to climate change mitigation by reducing atmospheric greenhouse gas concentration. In this article, the carbon sequestration potential of CA is assessed, both in annual and perennial crops, in the different agro-climatic regions of Africa. In total, the potential estimate of annual carbon sequestration in African agricultural soils through CA amounts to 143 Tg of C per year, that is 524 Tg of CO2 per year. This figure represents about 93 times the current sequestration figure.
• 

  Data for: Long-term tillage, residue management and crop rotation impacts on N2O and CH4 emissions on two contrasting soils in sub-humid Zimbabwe

  These are the raw data of the paper "Long-term tillage, residue management and crop rotation impacts on N2O and CH4 emissions on two contrasting so...
• 

  Belowground C sequestrations response to grazing exclusion in global grasslands: Dynamics and mechanisms

  Globally, grazing exclusion is a widely implemented management strategy for restoring degraded grassland ecosystems and sequestering carbon (C). However, there is limited understanding regarding the temporal responses and underlying factors influencing ecosystem C stocks following grazing exclusion. In this study, we conducted a comprehensive synthesis of data from 199 independent experiments (454 pairwise observations) to analyze responses of plant and soil C stocks to grazing exclusion across four distinct grassland ecosystems (desert, typical, meadow, and alpine) in the globe. We found that rates of change in plant biomass C stocks and soil organic C stocks exponentially or rationally decreased with years since enclosure. Grazing exclusion generally enhanced aboveground biomass C in plants, while its effects on C stocks of belowground biomass and soil were more contingent upon various factors, such as climate, initial levels of C stocks, and grazing exclusion duration. Furthermore, the responses of C stocks of plant biomass and soil to livestock grazing cessation tend to stabilize over time, with equilibrium typically reaching after approximately 40 years, while soil C sequestration responses exhibited a lagged pattern compared to plant biomass C. Our results underscored the effectiveness of grazing exclusion as an effective strategy to enhance C stocks in regions characterized by low C content and non-water limited conditions. We propose that grazing exclusion for 1–5 years was the best restoration time for typical, meadow and alpine grasslands. Given the limited effects of grazing exclusion on soil organic C stocks of desert types, grazing exclusion might not be an effective measure to increase the soil organic C stocks in water-limited areas like desert grasslands.
• 

  Piéger le carbone dans le sol : ce que peut l’agriculture

  On trouve trois fois plus de carbone dans les sols que dans l'atmosphère. À l'heure du dérèglement climatique, les facultés de l'agriculture à stocker plus ou moins de carbone sont scrutées de très près.
• 

  Artificial intelligence and soil carbon modeling demystified: power, potentials, and perils

  The global soil carbon pool has been estimated to exceed the amount of carbon stored in the atmosphere and vegetation, though uncertainties to quantify below-ground carbon and soil carbon fluxes accurately still exist. Modeling soil carbon using artificial intelligence (AI) - machine learning (ML) and deep learning (DL) algorithms has emerged as a powerful force in the carbon science community. These AI soil carbon models have shown improved performance to predict soil organic carbon (SOC) storage, soil respiration (Rs), and other properties of the global carbon cycle when compared to other modeling approaches. AI systems have advanced abilities to optimize fits between inputs (geospatial environmental covariates) and outputs (e.g., SOC or Rs) through advanced pattern recognition, learning algorithms, latent variables, hyperparameters, hyperplanes, weighting factors, or multiple stacked processing (e.g., convolution and pooling). These machine-oriented applications have shifted focus from knowledge discovery and understanding of ecosystem processes, carbon pools and cycling toward data-driven applications that compute digital soil carbon outputs. The purpose of this review paper is to explore the emergence, applications, and progress of AI-ML and AI-DL algorithms to model soil carbon storage and Rs at regional and global scales. A critical discussion of the power, potentials, and perils of AI soil carbon modeling is provided. The paradigm shift toward AI modeling raises questions how we study soil carbon dynamics and what conclusions we draw which impacts carbon science research and education, carbon management, carbon policies, carbon markets and economies, and soil health.
• 

  Calibrating the STICS soil-crop model to explore the impact of agroforestry parklands on millet growth

  Context Agroforestry systems provide critical benefits for food security and climate change mitigation. Yet, they are complex and heteregoneous sytems hard to optimize. The use of process-based crop models provides an opportunity to understand better the interactions between soil, crop, tree and climate and explore the impact of agroforestry on crop growth, for contrasting crop management. Objective The objectives of this study were to i) calibrate the soil-crop STICS model for pearl millet (Pennisetum glaucum) in order to simulate millet potential growth and impact of water and nitrogen limitations on millet growth in open fields and ii) explore the impacts of the parkland tree Faidherbia albida on millet performance for contrasting N fertilizer inputs. Methods We gathered a comprehensive database of 28 agronomically contrasting situations, ranging from near-potential growth to drought- and N-stress, either on-station or in a farmer’s home- or bush-fields. Parameters governing relevant plant and soil processes for grain yield were calibrated in a stepwise procedure. The calibrated model was used to explore the impact on millet growth of the widely reported benefits of Faidherbia albida, namely a minimum reduction in radiation thanks to the peculiar reverse phenology of this tree, improvement of soil water content at the beginning of the growing season and of organic nitrogen in the topsoil. Results Model simulations with the calibration dataset were reasonably accurate for aboveground biomass and grain yield. Normalized Root Mean Square Errors (nRMSE) for these variables were 29% and 26%, respectively; model efficiency (EF) was 0.58 for both. The nRMSE ranged from 33% to 53% for Soil Water Content (SWC), plant N uptake, grain number, and leaf-area index (LAI). Model accuracy was lower with the evaluation dataset. In the virtual experiment, millet yield decreased with incoming solar radiation, but only at levels of shading (e.g. below 80% of the radiation obtained with full sun) that do not occur under Faidherbia. The decline was greater when millet was fertilized. Increasing the initial soil water content did not affect simulated millet growth. Simulated millet aboveground biomass and grain yield increased with higher organic nitrogen contents of the topsoil, by 80% when millet was not fertilizer, but only by 25% when millet was fertilized. Implications This study provides the first set of comprehensively calibrated parameters for applying STICS to pearl millet in open cropland. A virtual experiment with historical climate suggests that the benefits of Faidherbia decrease if farmers intensify crop production by adding more mineral N fertilizer. Hence, precise fertilizer management is recommended in Faidherbia parklands. These results illustrate the benefits of process-based crop modelling for better understanding the functioning of agroforestry systems.
• 

  Recycling crop and livestock co-products on agro-pastoral farms for the agroecological transition: more than 60% potentially recoverable in western Burkina Faso

  Recyclage des co-produits végétaux et animaux dans les exploitations agropastorales pour la transition agroécologique : plus de 60 % potentiellement valorisables dans l'ouest du Burkina Faso Description du sujet. Face à l'augmentation du prix des aliments pour animaux et des engrais de synthèse, les agriculteurs agro-pastoraux d'Afrique de l'Ouest et du Burkina Faso en particulier, tentent d'augmenter l'utilisation des Co-Produits des Cultures (CCP : paille, fanes, tiges) et des Co-Produits des Animaux d'élevage (CPA : fèces) pour nourrir les animaux et fertiliser les champs cultivés, mais ils sont confrontés à des difficultés pour y parvenir. Objectifs. Caractériser à l’échelle de l’exploitation les pratiques de gestion et de recyclage des CCP et CPA et évaluer leur contribution à la couverture des besoins en fourrages et en fumure. Méthode. Données collectées par enquête sur 60 exploitations agro-pastorales de six villages de l’Ouest du Burkina Faso. Pratiques de gestion des CCP et CPA caractérisées par huit variables. Flux de valorisation des CCP et CPA caractérisés à trois niveaux : exploitation, par atelier de cultures et d’élevage, et par type de système de gestion des co-produits. Résultats. Sur les ~21 tonnes de CCP disponibles par exploitation, 23 % sont recyclés en fourrage, et 77 % ne sont pas ou peu valorisés par les exploitations. Sur les 24 tonnes de CPA disponibles par exploitation, 40 % sont récupérés pour être utilisés comme fumier organique, 60 % sont dispersés hors de l'exploitation pendant le pâturage et la mobilité. Le recyclage des co-produits ne couvre que 16 % des besoins des exploitations en fourrage et en fumure organique. Conclusions. Les co-produits sont peu valorisés et leur niveau de récupération dépend de l’équipement agricole, de la main-d'œuvre, et de la mobilité des troupeaux. Face à cette insuffisance, nous développons un outil de bilan/conseil en recyclage des co-produits à l’échelle de l'exploitation pour soutenir les agriculteurs dans la transition agroécologique.
• 

  Variabilité des stocks de carbone du sol sous et hors houppier dans la zone sylvopastorale du Sénégal

  The pastoral livestock system is criticized for its environmental impact caused by high greenhouse gas emissions per unit of product, and the adverse effects on soil and vegetation. In Senegal, the carbon stock in vegetation has been monitored by the Center for Ecological Monitoring at 24 sites in the Sahel since 1987. However, soil C stock, an essential parameter for the humification process of tree litter and rhizodeposition of grasses, is not taken into account. This study aimed to determine the influence of tree and environmental factors on soil C content. Thus, 15 of the sites monitored in the Center were selected, and 480 soil samples were collected at 0–10 cm and 10–30 cm depth under and outside the tree crown. Total C and N contents were predicted by near-infrared spectroscopy (NIRS), and C and N stocks were calculated from 60 soil samples taken from under and outside the tree crown. A principal component analysis was carried out to identify the factors involved in C stock variation. Thus, in the sylvopastoral area of Senegal, these stocks varied along a north-south gradient; they were positively correlated with rainfall, tree cover, and biomass. They varied between 9.3 and 29.7 Mg C/ha at 10–30 cm. At a finer scale, the tree induced an increase in C and N contents under the crown in the 0–10 cm layer. This study showed that soil C stocks can be used to determine the need for C compensation in Sahel through tree planting.
• 

  Do Cover Crops Impact Labile C More Than Total C? Data Synthesis

  The potential of cover crops (CC) to increase soil organic C (SOC) concentration can be inconsistent, but labile SOC is considered to be more sensitive to management than total SOC. This leads to two questions: Do CCs impact labile SOC more than total SOC? Do CCs increase labile SOC more rapidly than total SOC? This review compares CC impacts on labile and total SOC based on CC studies reporting both parameters up to Dec 31, 2022. Labile and total SOC concentrations were measured in 31 CC study locations. Cover crops increased labile SOC concentration in 58% (18 of 31) and had no effect in 42% (13 of 31) of locations, suggesting CCs do not increase labile SOC in all cases. Within the 18 locations, CCs increased labile SOC without increasing total SOC only in 19% (6 of 31 locations), while in the rest (12 of 31) of locations, CCs increased both labile and total SOC. Thus, CCs increased labile SOC more rapidly than total SOC only in one-fifth of cases. Also, the few studies that monitored changes in labile SOC with time found CCs do not always increase labile more rapidly than total SOC. In the 12 locations where CCs increased both labile and total SOC, CCs increased labile SOC by 54 ± 30% and total SOC by 23 ± 10%, indicating CCs can increase labile SOC by about two times compared with total SOC in some locations. Increased CC biomass production and reduced residue decomposition can increase labile SOC. Overall, CCs increase labile SOC in most cases but may not always increase labile SOC more rapidly than total SOC although more CC studies monitoring changes in SOC pools with time are needed to better understand CC impacts on SOC fractions under different CC management scenarios and climatic conditions.
• 

  Are carbon-storing soils more sensitive to climate change? A laboratory evaluation for agricultural temperate soils

  A range of agroecological practices allow to increase soil organic carbon (SOC) stocks, which makes a positive impact on climate change mitigation and soil health, but the permanence of this additional SOC storage can be questioned, in particular in a climate change context. Increased temperatures, accentuated evaporation of terrestrial water and increased atmosphere moisture content are anticipated, resulting in more frequent droughts and heavy precipitation events. Understanding the SOC dynamics and assessing the sensitivity of carbon mineralization to these climatic events is necessary to anticipate future carbon losses in terrestrial ecosystems. To this respect, it seems relevant to investigate carbon-storing soils as increased carbon mineralization induced by climate change may limit the carbon storing potential in agricultural soils. Thus, we evaluated the sensitivity of SOC mineralization to increased temperature, decreased soil moisture and drying-rewetting cycles using soils from long-term field experiments. We performed an incubation experiment on topsoil (0–30 cm) samples from temperate luvisols that had been under 20 years under conservation agriculture (CA), organic agriculture (ORG) and conventional agriculture (CON-LC) at the La Cage experiment, and under organic waste products (OWPs) applications in QualiAgro experiment, including biowaste composts (BIOW), residual municipal solid waste composts (MSW), farmyard manure (FYM) and conventional agriculture without organic inputs (CON-QA). Soil samples were incubated in the lab for 3 months under different temperature conditions (20, 28 and 35 °C) or under different moisture conditions (matric potential: pF1.5; pF 2.5 and pF 4.2) or under several dry (pF 4.2)-wet (pF 1.5) cycles (DWC). The results shown that, whatever the agricultural practices, soil moisture regime and temperature significantly affect the SOC mineralization. Overall, the DWC did not stimulate soil carbon mineralization relative to wet controls (pF1.5 and pF2.5). Whatever the soil moisture regime and temperature, specific carbon mineralization was similar between agricultural practices at La Cage, while at QualiAgro, specific carbon mineralization was lower in soils receiving organic waste products (OWPs) compared to the baseline soil. These results suggest a strong carbon stabilization by OWPs in soils as assessed by laboratory incubation experiments. Within each long-term experiment, we observed no significant difference between the carbon-storing soils (CA, ORG, MSW, FYM and BIOW) and their respective baseline soils (CON-LC and CON-QA) in the delta SOC mineralized whatever the soil moisture regime. The Q10 also indicated no significant difference between carbon-storing soils and their respective baseline soils. These results indicate that the SOC mineralization in carbon-storing soils had a similar sensitivity to the soil moisture regime and temperature as the baseline ones. Hence, the implementation of these agroecological practices appears beneficial for climate change mitigation, even in the context of extreme climatic events.
• 

  A global meta-analysis of soil organic carbon in the Anthropocene

  Anthropogenic activities profoundly impact soil organic carbon (SOC), affecting its contribution to ecosystem services such as climate regulation. Here, we conducted a thorough review of the impacts of land-use change, land management, and climate change on SOC. Using second-order meta-analysis, we synthesized findings from 230 first-order meta-analyses comprising over 25,000 primary studies. We show that (i) land conversion for crop production leads to high SOC loss, that can be partially restored through land management practices, particularly by introducing trees and incorporating exogenous carbon in the form of biochar or organic amendments, (ii) land management practices that are implemented in forests generally result in depletion of SOC, and (iii) indirect effects of climate change, such as through wildfires, have a greater impact on SOC than direct climate change effects (e.g., from rising temperatures). The findings of our study provide strong evidence to assist decision-makers in safeguarding SOC stocks and promoting land management practices for SOC restoration. Furthermore, they serve as a crucial research roadmap, identifying areas that require attention to fill the knowledge gaps concerning the factors driving changes in SOC.
• 

  Maximizing soil organic carbon stocks under cover cropping: insights from long-term agricultural experiments in North America

  Cover crops are widely advocated for increasing soil organic carbon (SOC) levels, thereby benefiting soil health improvement and climate change mitigation. Few regional-scale studies have robustly explored SOC stocks under cover cropping, due to limited long-term experiments. We used the unique experimental data from the North American Project to Evaluate Soil Health Measurements conducted in 2019 to address this issue. This study included 19 agricultural research sites with 36 pairs of cover cropping established between 1896 and 2014. Explanatory variables related to site-specific environmental conditions and management practices were collected to identify and prioritize contributing factors that affect SOC stocks with cover crops, by coupling the Boruta algorithm and structural equation modeling. Overall, cover crops significantly (P < 0.05) improved several indicators of soil health, including greater SOC (concentration: +8%; stock: +7%), total nitrogen (+8%), water-stable aggregates (+15%), and potential carbon mineralization (+34%), on average, compared to no cover crop control. Likewise, on average, cover crops sequestered SOC 3.55 Mg C ha-1 (0–15 cm depth), with a sequestration rate of 0.24 Mg C ha-1 yr-1. In addition, we found climate (Hargreaves climatic moisture deficit) was important in explaining the variation of SOC stocks with cover crops, followed by soil properties (e.g., soil clay content). In terms of management practices, cover crop type had a significant positive (0.36) effect on SOC stocks, with non-legumes showing a greater impact, compared to legumes and mixtures. Crop rotational diversity also had a positive (0.28) effect on SOC accumulation. Our findings suggested that integrating non-legume cover crops into diverse crop rotation is likely to be a promising strategy to maximize SOC stocks with cover crops across North America.
• 

  Conservation agriculture increases soil organic carbon stocks but not soil CO2 efflux in two 8-year-old experiments in Zimbabwe

  Abstract. Conservation agriculture (CA), combining reduced or no tillage, permanent soil cover and improved rotations, is often promoted as a climate-smart practice. However, our understanding about the impact of CA and its respective three principles on top and sub-soil organic carbon (SOC) stocks and on soil CO₂ efflux in low input cropping systems of sub-Saharan Africa is rather limited. The study was conducted at two long-term experimental sites established in 2013 in Zimbabwe. The soil types were abruptic Lixisols at Domboshava Training Centre (DTC) and xanthic Ferralsol at the University of Zimbabwe farm (UZF). Six treatments, replicated four times were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), NT, no-tillage with mulch (NTM), no-tillage with rotation (NTR), no-tillage with mulch and rotation (NTMR). Maize (Zea mays L.) was the main crop and treatments with rotation included cowpea (Vigna unguiculata L. Walp.). SOC concentration and bulk density were determined for samples taken from the 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75 and 75–100 cm depths. Gas samples were regularly collected using the static chamber method during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. SOC stocks were significantly (p < 0.05) higher under NTM, NTR and NTMR compared to NT and CT in top 5 and 10 cm layers at UZF, while SOC stocks were only significantly higher under NTM and NTMR compared to NT and CT in top 5 cm at DTC. NT alone had a slightly negative impact on top SOC stock. Cumulative SOC stocks were not significantly different between treatments when considering the whole 100 cm soil profile. Regardless of larger organic carbon inputs in mulch treatments, there were no significant differences in CO₂ efflux between treatments, but it was higher in maize rows than in inter-rows as a result of autotrophic respiration from maize roots. Our results show the overarching role of crop residue mulching in CA cropping systems in enhancing SOC storage but that this effect is limited to the topsoil.

• 

  Contribution of roots to soil organic carbon: From growth to decomposition experiment

  Carbon (C) derived from roots, rhizodeposition of living roots and dead root decomposition, plays a critical role in soil organic C (SOC) sequestration. Recent studies suggest that root inputs exert a disproportionate influence on SOC formation, and therefore, it is necessary to test separately the effects during root growth (i.e. rhizodeposition) and dead root decomposition (i.e. belowground litter) undernaturalconditions. A field experiment was carried out in grasslands with three typical plants: Stipa bungeana (St.B), Artemisia sacrorum (Ar.S), and Thymus mongolicus (Th.M) to differentiate the effects of root growth vs decomposition on C inputs by using in-growth soil cores and litter bag methods. For root growth experiment, the SOC content increased (2.5–3.8 g·kg−1) with root biomass, especially in soil under Ar.S. C input increased mineral-associated organic C (MAOC%) from 57% to 65%. The SOC δ13C was consistent with roots δ13C, indicating the roots were the primary source of SOC. The root decomposition experiment showed that the increase in SOC (0.74–2.6 g·kg−1) was highest at 90 days. Root decomposition rate was fast before 45 days, and SOC increased with the MAOC% during this period. After 45 days, particulate organic C % (POC%) was raised and was higher than in the control soil. The increase in SOC under root growth (2.5–3.8 g·kg−1) after one year was greater as compared to the rate under root decomposition (-0.4–0 g·kg−1). It suggested that C released during root growth was more effectively retained in the soil than that caused by dead root decomposition. The rise in MAOC during root growth and decomposition mainly explains the increase in SOC. Our results provided direct field based evidence illustrating the specific contribution of root growth and decomposition to SOC.
• 

  SOIL CARBON CHECK: A TOOL FOR MONITORING AND GUIDING SOIL CARBON SEQUESTRATION IN FARMER FIELDS

  In 2015, 17 Sustainable Development Goals (SDGs) were approved, including SDG13, which addresses actions to increase carbon capture (CO2-C storage) for climate change mitigation. However, no analytical procedures have been defined for quantifying soil organic carbon (SOC) sequestration. This paper presents a rapid tool for guiding farmers and for monitoring SOC sequestration in farmer fields. The tool consists of multiconstituent soil analyses through near-infrared spectroscopy (NIRS) and an SOC mineralization model. The tool provides forecasts of SOC sequestration over time. Soil analyses by NIRS have been calibrated and validated for farmer fields in European countries, China, New Zealand, and Vietnam. Results indicate a high accuracy of determination for SOC (R2 ≥ 0.93), and for inorganic C, soil texture, and soil bulk density. Permanganate oxidizable soil C is used as proxy for active SOC, to detect early management-induced changes in SOC contents, and is also quantified by NIRS (R2 = 0.92). A pedotransfer function is used to convert the results of the soil analyses to SOC sequestration in kg·ha−1 C as well as CO2. In conclusion, the tool allows fast, quantitative, and action-driven monitoring of SOC sequestration in farmer fields, and thereby is an essential tool for monitoring progress of SDG13.
• 

  Responses of soil carbon sequestration to climate-smart agriculture practices: A meta-analysis

  Climate-smart agriculture (CSA) management practices (e.g., conservation tillage, cover crops, and biochar applications) have been widely adopted to enhance soil organic carbon (SOC) sequestration and to reduce greenhouse gas emissions while ensuring crop productivity. However, current measurements regarding the influences of CSA management practices on SOC sequestration diverge widely, making it difficult to derive conclusions about individual and combined CSA management effects and bringing large uncertainties in quantifying the potential of the agricultural sector to mitigate climate change. We conducted a meta-analysis of 3,049 paired measurements from 417 peer-reviewed articles to examine the effects of three common CSA management practices on SOC sequestration as well as the environmental controlling factors. We found that, on average, biochar applications represented the most effective approach for increasing SOC content (39%), followed by cover crops (6%) and conservation tillage (5%). Further analysis suggested that the effects of CSA management practices were more pronounced in areas with relatively warmer climates or lower nitrogen fertilizer inputs. Our meta-analysis demonstrated that, through adopting CSA practices, cropland could be an improved carbon sink. We also highlight the importance of considering local environmental factors (e.g., climate and soil conditions and their combination with other management practices) in identifying appropriate CSA practices for mitigating greenhouse gas emissions while ensuring crop productivity.
• 

  Revisiting IPCC Tier 1 coefficients for soil organic and biomass carbon storage in agroforestry systems

  Agroforestry systems comprise trees and crops, or trees and pastures within the same field. Globally, they cover approximately 1 billion hectares of land and contribute to the livelihoods of over 900 million people. Agroforestry systems have the capacity to sequester large quantities of carbon (C) in both soil and biomass. However, these systems have not yet been fully considered in the approach to C accounting developed by the Intergovernmental Panel on Climate Change, largely due to the high diversity of agroforestry systems and scarcity of relevant data. Our literature review identified a total of 72 scientific, peer-reviewed articles associated with biomass C storage (50) and with soil organic carbon (SOC) (122), containing a total of 542 observations (324 and 218, respectively). Based on a synthesis of the reported observations, we are presenting a set of Tier 1 coefficients for biomass C storage for each of the eight main agroforestry systems identified, including alley cropping, fallows, hedgerows, multistrata, parklands, shaded perennial-crop, silvoarable and silvopastoral systems, disaggregated by climate and region. Using the same agroforestry classification, we are presenting a set of stock change factors (F LU ) and SOC accumulation/loss rates for three main land use changes (LUCs): cropland to agroforestry; forest to agroforestry; and grassland to agroforestry. Globally, the mean SOC stock change factors (± confidence intervals) were estimated to be 1.25 ± 0.04, 0.89 ± 0.07, and 1.19 ± 0.10, for the three main LUCs, respectively. However, these average coefficients hide huge disparities across and within different climates, regions, and types of agroforestry systems, highlighting the necessity to adopt the more disaggregated coefficients provided herein. We encourage national governments to synthesize data from local field experiments to generate country-specific factors for more robust estimation of biomass and SOC storage.
• 

  Limits of conservation agriculture to overcome low crop yields in sub-Saharan Africa

  Conservation agriculture (CA) has become a dominant paradigm in scientific and policy thinking about the sustainable intensification of food production in sub-Saharan Africa. Yet claims that CA leads to increasing crop yields in African smallholder farming systems remain controversial. Through a meta-analysis of 933 observations from 16 different countries in sub-Saharan African studies, we show that average yields under CA are only slightly higher than those of conventional tillage systems (3.7% for six major crop species and 4.0% for maize). Larger yield responses for maize result from mulching and crop rotations/intercropping. When CA principles are implemented concomitantly, maize yield increases by 8.4%. The largest yield benefits from CA occur in combination with low rainfall and herbicides. We conclude that although CA may bring soil conservation benefits, it is not a technology for African smallholder farmers to overcome low crop productivity and food insecurity in the short term.
• 

  Soil carbon sequestration as a climate strategy: what do farmers think? | SpringerLink

• 

  Initial soil organic carbon stocks govern changes in soil carbon: Reality or artifact?

  Changes in soil organic carbon (SOC) storage have the potential to affect global climate; hence identifying environments with a high capacity to gain or lose SOC is of broad interest. Many cross-site studies have found that SOC-poor soils tend to gain or retain carbon more readily than SOC-rich soils. While this pattern may partly reflect reality, here we argue that it can also be created by a pair of statistical artifacts. First, soils that appear SOC-poor purely due to random variation will tend to yield more moderate SOC estimates upon resampling and hence will appear to accrue or retain more SOC than SOC-rich soils. This phenomenon is an example of regression to the mean. Second, normalized metrics of SOC change—such as relative rates and response ratios—will by definition show larger changes in SOC at lower initial SOC levels, even when the absolute change in SOC does not depend on initial SOC. These two artifacts create an exaggerated impression that initial SOC stocks are a major control on SOC dynamics. To address this problem, we recommend applying statistical corrections to eliminate the effect of regression to the mean, and avoiding normalized metrics when testing relationships between SOC change and initial SOC. Careful consideration of these issues in future cross-site studies will support clearer scientific inference that can better inform environmental management.
• 

  Combining manure with mineral N fertilizer maintains maize yields: Evidence from four long-term experiments in Kenya

  Context Crop productivity in sub-Saharan Africa cannot be substantially improved without simultaneously addressing short-term crop nutrient demand and long-term soil fertility. Integrated soil fertility management tackles both by the combined application of mineral fertilizers and organic resource inputs but few studies examined its‘ long-term effectiveness. Objective To address this knowledge gap, this study analysed maize yield trends in four long-term (31–37 cropping seasons) field experiments in Kenya with contrasting soil textures and under different climates. Methods All sites had two maize cropping seasons per year, received a base P and K fertilization and tested combinations of organic resource addition (1.2 and 4 t C ha-1 yr-1 ranging from farmyard manure, to high-quality Tithonia diversifolia and Calliandra calothyrsus material to low-quality saw dust), combined with (+N) and without (-N) mineral N fertilizer (120 kg N ha-1 season-1). General maize yield trends across sites and site specific trends were analyzed. Results Across sites, the no-input control experienced significant average maize yield reductions of 50 kg ha-1 yr-1 over the study period. In contrast, the treatment with farmyard manure +N maintained yields at both 1.2 and 4 t C ha-1 yr-1. High initial yields following additions of Tithonia and Calliandra, reduced over time. Assessment by site showed site specificity of maize yields and yield trends. For example, the two climatically favorable sites in western Kenya experienced yield gains with high quality organic resources at 4 t C ha-1 yr-1, leading to yields of up to 8 t ha-1 per season, while sites in central Kenya experienced yield losses, leading to 3.5 t ha-1 per season. Yield site specificity for ± mineral N treatments was stonger than for organic resource treatments, e.g. the clayey site in central Kenya in the end showed no yield differences between ± N, except for the 1.2 t C ha-1 yr-1 farmyard manure treatment. Yet, farmyard manure plus mineral N consistently achieved highest yields of all organic resource treatments at all sites and farmyard manure addition at 1.2 t C ha-1 yr-1 (about 5 t dry matter) was the most N-efficient treatment. Conclusions At realistic application rates, maize yield in integrated soil fertility management is best sustained by a combined application of farmyard manure and mineral N. Implications Mixed crop-livestock systems and a combined manure and mineral N application are key ingredients for sustained productivity of smallholder systems in sub-Saharan Africa.
• 

  Multi-modelling predictions show high uncertainty of required carbon input changes to reach a 4‰ target

  Soils store vast amounts of carbon (C) on land, and increasing soil organic carbon (SOC) stocks in already managed soils such as croplands may be one way to remove C from the atmosphere, thereby limiting subsequent warming. The main objective of this study was to estimate the amount of additional C input needed to annually increase SOC stocks by 4‰ at 16 long-term agricultural experiments in Europe, including exogenous organic matter (EOM) additions. We used an ensemble of six SOC models and ran them under two configurations: (1) with default parametrization and (2) with parameters calibrated site-by-site to fit the evolution of SOC stocks in the control treatments (without EOM). We compared model simulations and analysed the factors generating variability across models. The calibrated ensemble was able to reproduce the SOC stock evolution in the unfertilised control treatments. We found that, on average, the experimental sites needed an additional 1.5 ± 1.2 Mg C ha−1 year−1 to increase SOC stocks by 4‰ per year over 30 years, compared to the C input in the control treatments (multi-model median ± median standard deviation across sites). That is, a 119% increase compared to the control. While mean annual temperature, initial SOC stocks and initial C input had a significant effect on the variability of the predicted C input in the default configuration (i.e., the relative standard deviation of the predicted C input from the mean), only water-related variables (i.e., mean annual precipitation and potential evapotranspiration) explained the divergence between models when calibrated. Our work highlights the challenge of increasing SOC stocks in agriculture and accentuates the need to increasingly lean on multi-model ensembles when predicting SOC stock trends and related processes. To increase the reliability of SOC models under future climate change, we suggest model developers to better constrain the effect of water-related variables on SOC decomposition. Highlights The feasibility of the 4‰ target was studied at 16 long-term agricultural experiments. An ensemble of soil organic carbon models was used to estimate the uncertainty of the predictions. On average across the sites, carbon input had to increase by 119% compared to initial conditions. High uncertainty of the simulations was mainly driven by water-related variables.
• 

  Evaluation of parameterisation approaches for estimating soil hydraulic parameters with HYDRUS-1D in the groundnut basin of Senegal

  Soil hydraulic parameters (SHPs) required as inputs for numerical models are scarce in Sahelian regions. Instead, they are estimated using pedotransfer functions (PTFs), but their ability to simulate soil water dynamics has not been evaluated. This study aims to parameterize SHPs with seven different PTFs and inverse modelling to examine their ability to simulate water fluxes in Senegal’s Groundnut basin. We used four years of field measurements of soil water content (SWC) and actual evapotranspiration (ETa) under pearl millet and groundnut crop rotation for model evaluation. Inverse modelling for SWC (root mean square error [RMSE] ≤ 0.015 cm3 cm−3) and ETa (RMSE ≤ 0.62 mm d−1) yielded the best model performance compared to PTFs (0.024–0.175 cm3 cm−3 and 0.68–0.96 mm d−1, respectively). Where field measurements are lacking for inverse estimation, three of the seven tested PTFs yielded good modelling results and could be used as a parsimonious approach for cultivated Sahelian soils.
• 

  Soil Organic Carbon Under Conservation Agriculture In Mediterranean And Humid Subtropical Climates: Global Meta-Analysis

  Conservation agriculture (CA) is an agronomic system based on minimum soil disturbance (no-tillage, NT), permanent soil cover and species diversification. The effects of NT on soil organic carbon (SOC) changes have been widely studied, showing somewhat inconsistent conclusions, especially in relation to the Mediterranean and humid subtropical climates. These areas are highly vulnerable and predicted climate change is expected to accentuate desertification and, for these reasons, there is a need for clear agricultural guidelines to preserve or increment SOC. We quantitively summarized the results of 47 studies all around the world in these climates investigating the sources of variation in SOC responses to CA, such as soil characteristics, agricultural management, climate and geography. Within the climatic area considered, the overall effect of CA on SOC accumulation in the plough layer (0-0.3 m) was 12% greater in comparison to conventional agriculture. On average this result corresponds to a carbon increase of 0.48 Mg C ha-1 year-1. However, the effect was variable depending on the SOC content under conventional agriculture: it was 20% in soils which had ≤ 40 Mg C ha-1, while it was only 7% in soils that had > 40 Mg C ha-1. We proved that 10 years of CA impact the most on soil with SOC ≤ 40 Mg C ha-1. For soils with less than 40 Mg C ha-1, increasing the proportion of crops with bigger residue biomasses in a CA rotation was a solution to increase SOC. The effect of CA on SOC depended on clay content only with more than 40 Mg C ha-1 and become null with a SOC/clay index of 3.2. Annual rainfall (ranged between 331-1850 mm yr-1) and geography had specific effects on SOC depending on its content under conventional agriculture. In conclusion, SOC increments due to CA application can be achieved especially in agricultural soils with less than 40 Mg C ha-1 and located in the middle latitudes or in the dry conditions of Mediterranean and humid subtropical climates. This article is protected by copyright. All rights reserved.
• 

  How far does the tree affect the crop in agroforestry? New spatial analysis methods in a Faidherbia parkland

  The trees in agroforestry plots create spatial heterogeneity of high interest for adaptation, mitigation, and the provision of ecosystem services. But to what distance, exactly, from the tree? We tested a novel approach, based upon geostatistics and Unmanned Aerial Vehicle (UAV) sensing, to infer the distance at which a single agroforestry tree affects the surrounding under-crop, to map yield, litter (i.e. stover) and compute crop-partial Land Equivalent Ratio (LERcp) at the whole-plot level. In an agro-silvo-pastoral parkland of semi-arid western Africa dominated by the multi-purpose tree Faidherbia albida, we harvested the pearl-millet under-crop at the whole-plot scale (ca. 1 ha) and also in subplot transects, at three distances from the trunks. We observed that the yield was three times higher below the tree crown (135.6 g m−2) than at a distance of five tree-crown radii from the trunk (47.7 g m−2). Through geostatistical analysis of multi-spectral, centimetric-resolution images obtained from an UAV overflight of the entire plot, we determined that the ‘Range’ parameter of the semi-variogram (assumed to be the distance of influence of the trees on the Normalized difference vegetation index (NDVI)) was 17 m. We correlated the yield (r2 = 0.41; RRMSE = 48 %) and litter production (r2 = 0.46; RRMSE = 35 %) in subplots with NDVI, and generated yield and litter maps at the whole-plot scale. The measured whole-plot yield (0.73 t ha-1) differed from the one estimated via the UAV mapping by only 20 %, thereby validating the overall approach. The litter was estimated similarly at 1.05 tC ha-1 yr-1 and mapped. Using a geostatistical proxy for the sole crop, LERcp was estimated 1.16, despite the low tree density. This new method to handle heterogeneity in agroforestry systems is a first application. We also propose strategies for extension to the landscape level.