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Improving estimates of maximal organic carbon stabilization by fine soil particles
Organic carbon (C) associated with fine soil particles (<20 μm) is relatively stable and accounts for a large proportion of total soil organic C (SOC). The soil C saturation concept proposes a maximal amount of SOC that can be stabilized in the fine soil fraction, and the soil C saturation deficit (i.e., the difference between current SOC and the maximal amount) is presumed to affect the capacity, magnitude, and rate of SOC storage. In this study, we argue that predictions using current models underestimate maximal organic C stabilization of fine soil particles due to fundamental limitations of using least-squares linear regression. The objective was to improve predictions of maximal organic C stabilization by using two alternative approaches; one mechanistic, based on organic C loadings, and one statistical, based on boundary line analysis. We collected 342 data points on the organic C content of fine soil particles, fine particle mass proportions in bulk soil, dominant soil mineral types, and land use types from 32 studies. Predictions of maximal organic C stabilization using linear regression models are questionable because of the use of data from soils that may not be saturated in SOC and because of the nature of regression itself, resulting in a high proportion of presumed over-saturated samples. Predictions of maximal organic C stabilization using the organic C loading approach fit the data for soils dominated by 2:1 minerals well, but not soils dominated by 1:1 minerals; suggesting that the use of a single value for specific surface area, and therefore a single organic C loading, to represent a large dataset is problematic. In boundary line analysis, only data representing soils having reached the maximal amount (upper tenth percentile) were used. The boundary line analysis estimate of maximal organic C stabilization (78 ± 4 g C kg−1 fraction) was more than double the estimate by the linear regression approach (33 ± 1 g C kg−1 fraction). These results show that linear regression models do not adequately predict maximal organic C stabilization. Soil properties associated with soil mineralogy, such as specific surface area and organic C loading, should be incorporated to generate more mechanistic models for predicting soil C saturation, but in their absence, statistical models should represent the upper envelope rather than the average value. -
Drivers of the amount of organic carbon protected inside soil aggregates estimated by crushing: A meta-analysis
Given the importance of soil organic carbon (SOC) stocks and their dynamics in the regulation of climate change, understanding the mechanisms of SOC protection from decomposition is crucial. It is recognized that soil aggregates can provide effective protection of organic carbon from microbial decomposition. Currently, there is no systematic method for estimating the amount of protected carbon within aggregates. However, differences between CO2 emissions from incubation of intact versus crushed aggregates have been widely used as a proxy for SOC physical protection within aggregates. There is no global analysis on this type of experiment yet, nor on the drivers of the amount of SOC physically protected in soils. Using a meta-analysis including 165 pairs of observations from 22 studies encompassing a variety of ecosystems, climate and soil types, we investigated the crushing effects on cumulative carbon mineralization from laboratory incubation experiments. The aggregates were initially separated by either wet sieving or dry sieving before dry crushing. Our results indicated that aggregate crushing led on average to +31 % stimulation of carbon mineralization compared with intact aggregates, which represented 0.65 to 1.01 % of total SOC. This result suggests the mineralization of a previously protected pool of labile organic carbon. The linear regression analysis showed that the crushing effect on carbon mineralization depended on soil characteristics (carbon content, clay content and pH) as well as on aggregate size. Crushing aggregates stimulated carbon mineralization relative to control, up to +63 % in large aggregates (>10 mm), +38 % in large macro-aggregates (2–8 mm), +14 % in small macro-aggregates (0.25–2 mm) and +54 % in micro-aggregates (<0.25 mm). Within each aggregate size-class, the crushing effect depended on the crushing intensity. The destruction of aggregates to <0.05 mm size had a greater effect on carbon mineralization (+130–133 %) than the destruction of aggregates to >2 mm (+3 to 40 %), < 2 mm (+58 to 62 %) and < 0.25 mm (+32 to 62 %) sizes regardless of the initial aggregate size. These results suggest that macroaggregates (>0.25 mm) are less protective than microaggregates (<0.25 mm). Our dataset also show that soil physicochemical characteristics and experimental conditions influenced more the amount of protected SOC than land use and management. Contrary to our expectations the crushing effect was not affected by tillage practices nor land use. Standardizing the experimental conditions of aggregate crushing and subsequent incubation is needed to assess and compare the amount of physically protected SOC in diverse soils, and then to better understand the processes and drivers of SOC protection inside aggregates. -
Effects of land clearing for agriculture on soil organic carbon stocks in drylands: a meta-analysis
Agricultural activities have been expanding globally with the pressure to provide food security to the earth’s growing population. These agricultural activities have profoundly impacted soil organic carbon (SOC) stocks in global drylands. However, the effects of clearing natural ecosystems for cropland (CNEC) on SOC are uncertain. To improve our understanding of carbon emissions and sequestration under different land uses, it is necessary to characterize the response patterns of SOC stocks to different types of CNEC. We conducted a meta-analysis with mixed-effect model based on 873 paired observations of SOC in croplands and adjacent natural ecosystems from 159 individual studies in global drylands. Our results indicate that CNEC significantly (P < 0.05) affects SOC stocks, resulting from a combination of natural land clearing, cropland management practices (fertilizer application, crop species, cultivation duration) and the significant negative effects of initial SOC stocks. Increases in SOC stocks (in 1m depth) were found in croplands which previously natural land (deserts and shrublands) had low SOC stocks, and the increases were 278.86% (95% confidence interval, 196.43–361.29%) and 45.38% (26.53–62.23%), respectively. In contrast, SOC stocks (in 1m depth) decreased by 24.11% (18.38–29.85%) and 10.70% (1.80–19.59%) in clearing forests and grasslands for cropland, respectively. We also established the general response curves of SOC stocks change to increasing cultivation duration, which is crucial for accurately estimating regional carbon dynamics following CNEC. SOC stocks increased significantly (P < 0.05) with high long-term fertilizer consumption in cleared grasslands with low initial SOC stocks (about 27.2 M g/ha). The results derived from our meta-analysis could be used for refining the estimation of dryland carbon dynamics and developing SOC sequestration strategies to achieve the removal of CO2 from the atmosphere. -
Effects of land clearing for agriculture on soil organic carbon stocks in drylands: a meta-analysis
Agricultural activities have been expanding globally with the pressure to provide food security to the earth’s growing population. These agricultural activities have profoundly impacted soil organic carbon (SOC) stocks in global drylands. However, the effects of clearing natural ecosystems for cropland (CNEC) on SOC are uncertain. To improve our understanding of carbon emissions and sequestration under different land uses, it is necessary to characterize the response patterns of SOC stocks to different types of CNEC. We conducted a meta-analysis with mixed-effect model based on 873 paired observations of SOC in croplands and adjacent natural ecosystems from 159 individual studies in global drylands. Our results indicate that CNEC significantly (P < 0.05) affects SOC stocks, resulting from a combination of natural land clearing, cropland management practices (fertilizer application, crop species, cultivation duration) and the significant negative effects of initial SOC stocks. Increases in SOC stocks (in 1m depth) were found in croplands which previously natural land (deserts and shrublands) had low SOC stocks, and the increases were 278.86% (95% confidence interval, 196.43–361.29%) and 45.38% (26.53–62.23%), respectively. In contrast, SOC stocks (in 1m depth) decreased by 24.11% (18.38–29.85%) and 10.70% (1.80–19.59%) in clearing forests and grasslands for cropland, respectively. We also established the general response curves of SOC stocks change to increasing cultivation duration, which is crucial for accurately estimating regional carbon dynamics following CNEC. SOC stocks increased significantly (P < 0.05) with high long-term fertilizer consumption in cleared grasslands with low initial SOC stocks (about 27.2 M g/ha). The results derived from our meta-analysis could be used for refining the estimation of dryland carbon dynamics and developing SOC sequestration strategies to achieve the removal of CO2 from the atmosphere. -
Long-term tillage, residue management and crop rotation impacts on N2O and CH4 emissions from two contrasting soils in sub-humid Zimbabwe
The respective contribution of conservation agriculture (CA) principles (no-tillage, permanent soil cover/mulch and crop rotations) on greenhouse gas (GHG) emissions is still unclear. This study was conducted at two long-term experimental sites established in 2013 in Zimbabwe, on an abruptic Lixisol at Domboshava Training Center (DTC) and on a xanthic Ferralsol at the University of Zimbabwe Farm (UZF). The purpose of the study was to unravel the individual and combined effects of tillage, mulching and rotation on N2O and CH4 emissions in low nitrogen (N) input maize-based cropping systems (< 60 kg N ha−1) and to compare emissions within maize rows and between maize rows. We hypothesised that integrating no tillage, mulch and cereal-legume rotation would enhance N2O emissions. Six treatments, replicated four times were investigated: conventional tillage, conventional tillage with rotation, no-tillage, no-tillage with mulch, no-tillage with rotation, no-tillage with mulch and rotation. The main crop was maize (Zea mays L.) and treatments with rotation included cowpea (Vigna unguiculate L. Walp.). Gas samples were regularly collected using the static chamber method in the maize row and inter-row spaces during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. Soil moisture and mineral N were measured in the 0–20 cm soil depth. In 2019/20, cumulative total N2O emissions were significantly higher in mulch treatments at DTC, while at UZF N2O emissions were higher with cowpea rotation. Cumulative total N2O emissions ranged from 215 to 496 g N2O-N ha−1 yr−1 and from 226 to 395 g N2O-N ha−1 yr−1, at DTC and UZF, respectively. In 2020/21, N2O emissions were much lower and no differences were found between treatments on both sites (145 to 179 g N2O-N ha−1 yr−1 and 83 to 136 g N2O-N ha−1 yr−1 at DTC and UZF, respectively). A significant relationship was found between soil nitrate and daily N2O emissions. At UZF, highest N2O emissions were observed at a water-filled pore space of 60–70%. There were no significant differences in yield-scaled N2O emissions between treatments at both sites for the two seasons. DTC was a net source of CH4 (694 g CH4-C ha−1 yr−1 on average), while UZF was a net sink of CH4 (−494 g CH4-C ha−1 yr−1 on average). No evidence was found for in situ CH4 production at DTC, and an external source is most likely. Our study indicates that for low N input cropping systems in the sub-humid tropics, N loss through N2O is low. -
How does soil water status influence the fate of soil organic matter? A review of processes across scales
Due to its influence on multiple soil processes, water intervenes in biogeochemical cycles at multiple spatial scales with contrasting effects on soil organic carbon (SOC) dynamics. On all scales, water availability influences biological processes, such as plant growth and (micro-)biological activity, leading to organic matter input, its decomposition and stabilisation. On the other hand, SOC influences soil hydrology via its impact on soil wettability and its structural organisation. Our objectives were to review the mechanisms involved in the complex relationship between water and SOC at different scales and to discuss levers of action to improve its modelling and management. We carried out a systematic review and synthesised the information of 987 articles dealing with SOC sequestration and soil water. At the landscape scale, precipitation levels influence vegetation type and biomass production as well as horizontal and vertical transport, determining SOC stocks and their spatial distribution. At the profile scale, SOC and water both control biological processes including those involved in soil aggregate formation, and organisation of soil porosity. Soil organic matter (SOM) decomposition and stabilisation processes occur at the microscale, where water movement facilitates the co-occurrence of SOM and microorganisms. All these multiscale processes may change the nature and distribution of SOM, leading to promotion or inhibition not only of biogeochemical cycling but also of the water cycle. Taking into account these mutual feedback mechanisms in mechanistic models requires their representation at multiple scales through developing modelling parameters in particular for microbial processes occurring in the pore space. This could greatly reduce modelling uncertainty and improve our understanding of global carbon cycling. Levers of action to improve soil water status and consequently SOC accrual include irrigation, and use of organic amendments. Sustainable agricultural practices should focus on (1) optimising the management of water resources and (2) choosing crop species adapted to various water levels to maintain and foster SOC sequestration, to adapt to climate change and in particular extreme events, such as drought and flooding. -
The nitrogen gap in soil health concepts and fertility measurements
Soil nitrogen (N) often limits productivity in agroecosystems, prompting fertilizer applications that increase crop yields but can degrade the environment. Nitrogen's dual role in both productivity and environmental quality should center it in soil health frameworks. We use recent evidence to argue that N availability is an emergent property of the integrated soil biogeochemical system and is strongly influenced by plant traits and their interactions with microbes and minerals. Building upon this, we theorize that the sources of plant and microbial N shift across soil health gradients, from inorganic N dependence in ecologically simple systems with poor soil health to a highly networked supply of organic N in healthy soils; ergo, investments in soil health should increase ecological complexity and the pathways by which plants can access N, leading to more resilient nutrient supplies and yields in a variable climate. However, current N assessment methods derive from a historical emphasis on inorganic N pool sizes and are unable to capture the shifting drivers of N availability across soil health gradients. We highlight the need to better understand the plant-microbial-mineral interactions that regulate bioavailable N as a first step to improving our ability to measure it. We conclude it will be necessary to harness agroecosystem complexity, account for plant and microbial drivers, and carefully integrate external N inputs into soils' internal N network to expand the routes by which N from organic pools can be made bioavailable. By emphasizing N in soil health concepts, we argue that researchers can accelerate advances in N use efficiency and resiliency. -
Management-induced changes in soil organic carbon on global croplands
Abstract. Soil organic carbon (SOC), one of the largest terrestrial carbon (C) stocks on Earth, has been depleted by anthropogenic land cover change and agricultural management. However, the latter has so far not been well represented in global C stock assessments. While SOC models often simulate detailed biochemical processes that lead to the accumulation and decay of SOC, the management decisions driving these biophysical processes are still little investigated at the global scale. Here we develop a spatially explicit data set for agricultural management on cropland, considering crop production levels, residue returning rates, manure application, and the adoption of irrigation and tillage practices. We combine it with a reduced-complexity model based on the Intergovernmental Panel on Climate Change (IPCC) tier 2 method to create a half-degree resolution data set of SOC stocks and SOC stock changes for the first 30 cm of mineral soils. We estimate that, due to arable farming, soils have lost around 34.6 GtC relative to a counterfactual hypothetical natural state in 1975. Within the period 1975–2010, this SOC debt continued to expand by 5 GtC (0.14 GtC yr−1) to around 39.6 GtC. However, accounting for historical management led to 2.1 GtC fewer (0.06 GtC yr−1) emissions than under the assumption of constant management. We also find that management decisions have influenced the historical SOC trajectory most strongly by residue returning, indicating that SOC enhancement by biomass retention may be a promising negative emissions technique. The reduced-complexity SOC model may allow us to simulate management-induced SOC enhancement – also within computationally demanding integrated (land use) assessment modeling.
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Current and emerging methodologies for estimating carbon sequestration in agricultural soils: A review
This review covers the current and emerging analytical methods used in laboratory, field, landscape and regional contexts for measuring soil organic carbon (SOC) sequestration in agricultural soil. Soil depth plays an important role in estimating SOC sequestration. Selecting appropriate sampling design, depth of soil, use of proper analytical methods and base line selection are prerequisites for estimating accurately the soil carbon stocks. Traditional methods of wet digestion and dry combustion (DC) are extensively used for routine laboratory analysis; the latter is considered to be the “gold standard” and superior to the former for routine laboratory analysis. Recent spectroscopic techniques can measure SOC stocks in laboratory and in-situ even up to a deeper depth. Aerial spectroscopy using multispectral and/or hyperspectral sensors located on aircraft, unmanned aerial vehicles (UAVs) or satellite platforms can measure surface soil organic carbon. Although these techniques' current precision is low, the next generation hyperspectral sensor with improved signal noise ratio will further improve the accuracy of prediction. At the ecosystem level, carbon balance can be estimated directly using the eddy-covariance approach and indirectly by employing agricultural life cycle analysis (LCA). These methods have tremendous potential for estimating SOC. Irrespective of old or new approaches, depending on the resources and research needed, they occupy a unique place in soil carbon and climate research. This paper highlights the overview, potential limitations of various scale-dependent techniques for measuring SOC sequestration in agricultural soil. -
Smallholder farmers' perceptions of the natural and anthropogenic drivers of deforestation and forest degradation: a case study of Murehwa, Zimbabwe
Forests are an integral part of social-ecological systems, which provide economic, cultural and ecosystem services. The natural and socio-economic drivers of deforestation and forest degradation are affecting the sustainability of social-ecological systems. Several measures have been put in place to manage forest ecosystems. Nonetheless, multiple and complex drivers of deforestation and forest degradation have compromised these measures. The study sought to establish smallholder farmers’ perceptions on the multiplicity and complexity of factors to which they attribute deforestation and forest degradation. This is important for the successful formulation of improved forest conservation and management frameworks. The study was carried out with smallholder farmers in three wards of Murehwa District in Mashonaland East Province of Zimbabwe. Using ‘Q’ sort and factor analysis, we find that the smallholder farmers attribute deforestation and forest degradation to climate change, insects and diseases, unavoidable external events, a lack of alternative sources of fuel and the failure of existing institutional arrangements. Under such circumstances, forest governance and management efforts that focus solely on controlling human activities may not bring desired outcomes. Therefore, there is a need to take into account external factors when designing an effective contextual forest management strategy or framework. -
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Chapter Three - Soil carbon accumulation in crop-livestock systems in acid soil savannas of South America: A review
Acid soil savannas of tropical America are a vast resource to expand agricultural production, alleviate the pressure on tropical rainforest and reduce greenhouse gas (GHG) emissions. During the past three decades there have been major changes in land use in the Cerrados of Brazil and to a lesser extent in the Llanos of Colombia. Monocropping and improved pasture grasses were adopted widely to boost crop and animal production. Various types of integrated crop-livestock systems and no-till cropping systems were introduced to not only recuperate degraded pastures but also to sustain crop and livestock productivity. Several studies showed that well-managed pastures based on deep rooted tropical forage grass and legume species could accumulate significant amounts of soil organic carbon (SOC) in deeper soil layers. Among the number of factors that influence SOC accumulation, deep rooting ability of grasses and high root turnover seem to play a major role in accumulation of SOC in deeper soil layers in the form of particulate organic carbon (POC) and mineral associated organic carbon (MAOC). This review provides insights toward some key approaches and management options to increase both POC and MAOC accumulation and particularly MAOC accumulation in deeper soil layers in crop-livestock systems. There are some important gaps in our knowledge, particularly regarding the influence of length of pasture phase on MAOC accumulation in deeper soil layers from crop-livestock systems. Finally, we highlight the importance of land use policies and suggest some future research priorities for consideration to increase benefits from the use of integrated crop-livestock systems in acid soil savannas. -
The efficiency of organic C sequestration in deep soils is enhanced by drier climates
Accurate assessment of organic C sequestration in deep soils is crucial to C management and understand the role of deep-rooted vegetation in the C cycle. Trees in drylands usually develop roots to access deep water resources. Deep soils typically contain large stores of sequestrated C because the microbial activities that decompose C are limited and C turnover time is long. However, we know little about whether root water uptake can benefit organic C sequestration in deep soils and the effect of precipitation on organic C sequestration. To address this, we selected five sites along a precipitation gradient from 422 mm to 606 mm on China’s Loess Plateau, and collected soil samples down to 1000 cm to measure soil organic C (SOC) content and soil water content (SWC) in both apple orchards and arable lands. We found that SOC storage (SOCS) and soil water storage (SWS) of two vegetation types in 0–800 cm soil layers increased significantly with increasing mean annual precipitation (MAP). Apple orchards showed greater SOC sequestration, particularly in deep soils (200–1000 cm), across each precipitation gradient relative to the corresponding arable lands. The ΔSOCS (difference in SOCS between apple orchards and the corresponding arable lands) in deep soils gradually decreased as MAP increased, and ΔSOCS for MAP = 422 mm was almost twice as great as that for MAP = 606 mm. Moreover, the ratio of ΔSOCS/ΔSWS in deep soils significantly increased as MAP decreased in the interval 400–610 mm. This indicates that the efficiency of SOC sequestration in deep soils is enhanced in a drier climate. The findings here indicate that deep soils may contribute greatly to organic C sequestration, and may provide insights into the water-C relationships in deep soils. -
Is it possible to attain the same soil organic matter content in arable agricultural soils as under natural vegetation?
Clearing natural vegetation to establish arable agriculture (cropland) almost invariably causes a loss of soil organic carbon (SOC). Is it possible to restore soil that continues in arable agriculture to the pre-clearance SOC level through modified management practices? To address this question we reviewed evidence from long-term experiments at Rothamsted Research, UK, Bad Lauchstädt, Germany, Sanborn Field, USA and Brazil and both experiments and surveys of farmers’ fields in Ethiopia, Australia, Zimbabwe, UK and Chile. In most cases SOC content in soil under arable cropping was in the range 38–67% of pre-clearance values. Returning crop residues, adding manures or including periods of pasture within arable rotations increased this, often to 60–70% of initial values. Under tropical climatic conditions SOC loss after clearance was particularly rapid, e.g. a loss of >50% in less than 10 years in smallholder farmers’ fields in Zimbabwe. If larger yielding crops were grown, using fertilizers, and maize stover returned instead of being grazed by cattle, the loss was reduced. An important exception to the general trend of SOC loss after clearance was clearing Cerrado vegetation on highly weathered acidic soils in Brazil and conversion to cropping with maize and soybean. Other exceptions were unrealistically large annual applications of manure and including long periods of pasture in a highly SOC-retentive volcanic soil. Also, introducing irrigated agriculture in a low rainfall region can increase SOC beyond the natural value due to increased plant biomass production. For reasons of sustainability and soil health it is important to maintain SOC as high as practically possible in arable soils, but we conclude that in the vast majority of situations it is unrealistic to expect to maintain pre-clearance values. To maintain global SOC stocks at we consider it is more important to reduce current rates of land clearance and sustainably produce necessary food on existing agricultural land. -
Advancing the mechanistic understanding of the priming effect on soil organic matter mineralisation
The priming effect (PE) is a key mechanism contributing to the carbon balance of the soil ecosystem. Almost 100 years of research since its discovery in 1926 have led to a rich body of scientific publications to identify the drivers and mechanisms involved. A few review articles have summarised the acquired knowledge; the last major one was published in 2010. Since then, knowledge on the soil microbial communities involved in PE and in PE + C sequestration mechanisms has been considerably renewed. This article reviews current knowledge on soil PE to state to what extent new insights may improve our ability to understand and predict the evolution of soil C stocks. We propose a framework to unify the different concepts and terms that have emerged from the international scientific community on this topic, report recent discoveries and identify key research needs. Seventy per cent of the studies on the soil PE were published in the last 10 years, illustrating a renewed interest for PE, probably linked to the increased concern about the importance of soil carbon for climate change and food security issues. Among all the drivers and mechanisms proposed along with the different studies to explain PE, some are named differently but actually refer to the same object. This overall introduces ‘artificial’ complexity for the mechanistic understanding of PE, and we propose a common, shared terminology. Despite the remaining knowledge gaps, consistent progress has been achieved to decipher the abiotic mechanisms underlying PE, together with the role of enzymes and the identity of the microbial actors involved. However, including PE into mechanistic models of SOM dynamics remains challenging as long as the mechanisms are not fully understood. In the meantime, empirical alternatives are available that reproduce observations accurately when calibration is robust. Based on the current state of knowledge, we propose different scenarios depicting to what extent PE may impact ecosystem services under climate change conditions. Read the free Plain Language Summary for this article on the Journal blog. -
Do agrosystems change soil carbon and nutrient stocks in a semiarid environment?
Ecological processes, such as net primary production, root system development, organic matter mineralization, nutrient removal and fertilizer application interfere in gains and losses of C and nutrients (N, P, K, Ca and Mg) in soils. Herein, we studied how five rainfed livestock and four irrigated agricultural systems affected soil C and nutrient stocks in a semi-arid environment. Soil concentrations, stocks, gains and losses of the nine land-uses were compared to those of the preserved native deciduous forest (Caatinga) along the top 1 m soil layer. Open Caatinga used as pasture, gliricidia and leucaena fields maintained the stocks of most nutrients. The shallower roots of buffel grass and prickly pear led to C (7 and 18%) and N (7 and 20%, respectively) losses, and P, Ca and Mg accumulations in the deeper layers. Irrigated crops reduced soil C and N stocks. C losses in irrigated maize and beans fields (23%) were lower than in rainfed fields in the region, while those in mango fields were large (70 and 66%). Fertilization in beans and grapes increased soil P and K stocks. Knowledge of stock changes allows proper system management to reduce the negative impacts of land-use change and promote sustainable production. -
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COMIFER JT MOS 2021 7 Avril_S Recous_Couplage des cycles du carbone et de l'azote
La Journée Technique organisée par le COMIFER le 7 avril 2021 a pour ambition de présenter un état des connaissances et des exemples concrets sur les différentes thématiques concernant les matières organiques dans les sols agricoles. Pour cela, l’animation et les interventions seront portées par des représentants de la Recherche francophone (AgroParisTech, ENS, FIBL, HEPIA, INRAE, UCL), des instituts techniques (ARVALIS, IFV, Terres Inovia), des organismes de transfert (Agro-Transfert, RITTMO), des laboratoires d’analyses (Auréa AgroSciences, CAPINOV, Celesta-Lab, LDAR, GEMAS), des conseillers terrain (Chambres d’Agriculture d’Alsace et du Rhône, SATEGE du Nord Pas-de-Calais), ainsi que l’ADEME et la DGAL/MAA. La matinée posera les bases d’une bonne compréhension des matières organiques dans les sols agricoles. La séquence introductive définira les matières organiques, leurs rôles et leur évolution, ainsi que le lien étroit avec le cycle de l’azote. -
COMIFER JT MOS 2021 7 Avril_A Duparque_H Clivot_F Ferchaud: Modélisation de l'évolution du C organiq
La Journée Technique organisée par le COMIFER le 7 avril 2021 a pour ambition de présenter un état des connaissances et des exemples concrets sur les différentes thématiques concernant les matières organiques dans les sols agricoles. Pour cela, l’animation et les interventions seront portées par des représentants de la Recherche francophone (AgroParisTech, ENS, FIBL, HEPIA, INRAE, UCL), des instituts techniques (ARVALIS, IFV, Terres Inovia), des organismes de transfert (Agro-Transfert, RITTMO), des laboratoires d’analyses (Auréa AgroSciences, CAPINOV, Celesta-Lab, LDAR, GEMAS), des conseillers terrain (Chambres d’Agriculture d’Alsace et du Rhône, SATEGE du Nord Pas-de-Calais), ainsi que l’ADEME et la DGAL/MAA. La matinée posera les bases d’une bonne compréhension des matières organiques dans les sols agricoles. La séquence introductive définira les matières organiques, leurs rôles et leur évolution, ainsi que le lien étroit avec le cycle de l’azote. La deuxième séquence abordera les méthodes de mesure de ces matières organiques des sols (MOS), l’état des lieux du statut organique des sols français et luxembourgeois, et les interprétations possibles de ces niveaux de MOS, en discutant de la pertinence d’informations complémentaires issues de la caractérisation biologique des sols. La fin de la matinée portera sur la dynamique des MOS à moyen et long terme, avec présentation d’outils disponibles pour prédire cette évolution et illustration de l’impact des pratiques agricoles, à l’échelle de la parcelle mais également du territoire français. -
A well-established fact: rapid mineralization of organic inputs is an important factor for soil carbon sequestration
We have read with interest an opinion paper recently published in the European Journal of Soil Science (Berthelin et al., 2022). This paper presents some interesting considerations, at least one of which is already well known to soil scientists working on soil organic carbon (SOC), i.e. a large portion (80-90%) of fresh carbon inputs to soil is subject to rapid mineralization. The short-term mineralization kinetics of organic inputs are well-known and accounted for in soil organic matter models. Thus, clearly, the long-term predictions based on these models do not overlook short-term mineralization. We point out that many agronomic practices can significantly contribute to SOC sequestration. If conducted responsibly whilst fully recognizing the caveats, SOC sequestration can lead to a win-win situation where agriculture can both contribute to the mitigation of climate change and adapt to it, whilst at the same time delivering other co-benefits such as reduced soil erosion and enhanced biodiversity. -
The Science and Semantics of “Soil Organic Matter Stabilization”
It is a shared goal of the scientific community to improve model projections of soil organic carbon dynamics such that the carbon cycle is represented as accurately as possible in global land models. For several decades, those pursuing this objective have operated under the assumption that “the longer soil carbon persists, the more stable we regard it as being.” While this understanding was helpful in facilitating conversations on the topic, it has created the notion of the existence of a chemically and physically definable organic phase in soil with the ability to “resist” decomposition. Here we argue that “stability” is a conceptual and not a physically existing state of organic matter. To this end, we analyze the semantics of “persistence” and “stability” to point out that (i) they are independent concepts and not interchangeable terms and (ii) that “persistence” is not an automatic consequence of “stability.” Through extensive revision of the literature, we show that slow-cycling carbon does not do so because it has the material property of being “stable,” rather, it persists because it is not being decomposed. It follows that the notion of soil organic matter stabilization is a flawed concept that distracts from the actual causes for slow carbon cycling. We suggest that the question to be asked in future research should no longer be “Why is soil carbon stable?.” Rather, the question should be: What are the constraints that prevent the decomposer community from processing soil carbon to their full metabolic potential? -
Soil carbon sequestration for climate change mitigation: Mineralization kinetics of organic inputs as an overlooked limitation
Over the last few years, the question of whether soil carbon sequestration could contribute significantly to climate change mitigation has been the object of numerous debates. All of these debates so far appear to have entirely overlooked a crucial aspect of the question. It concerns the short-term mineralization kinetics of fresh organic matter added to soils, which is occasionally alluded to in the literature, but is almost always subsumed in a broader modelling context. In the present article, we first summarise what is currently known about the kinetics of mineralization of plant residues added to soils, and about its modelling in the long run. We then argue that in the short run, this microbially-mediated process has important practical consequences that cannot be ignored. Specifically, since at least 90% of plant residues added to soils to increase their carbon content over the long term are mineralized relatively rapidly and are released as CO2 to the atmosphere, farmers would have to apply to their fields 10 times more organic carbon annually than what they would eventually expect to sequester. Over time, because of a well-known sink saturation effect, the multiplier may even rise significantly above 10, up to a point when no net carbon sequestration takes place any longer. The requirement to add many times more carbon than what one aims to sequester makes it practically impossible to add sufficient amounts of crop residues to soils to have a lasting, non-negligible effect on climate change. Nevertheless, there is no doubt that raising the organic matter content of soils is desirable for other reasons, in particular guaranteeing that soils will be able to keep fulfilling essential functions and services in spite of fast-changing environmental conditions. Highlights Attempts to promote soil carbon sequestration to mitigate climate change have so far ignored the short-term effects of the mineralization of plant residues added to soils. Only about 10%, at most, of added plan residues remain in soils after mineralization by soil organisms. To have a significant effect on climate change, farmers would need to add impractically large amounts of plant residues, requiring unrealistic nitrogen inputs. Therefore, rather than as a mitigation strategy, farmers should aim to increase the carbon content of soils to make them resilient to climate change. -
Conservation agriculture practices drive maize yield by regulating soil nutrient availability, arbuscular mycorrhizas, and plant nutrient uptake
Conservation agriculture (CA) can sustainably increase crop productivity through improved soil chemical, physical, and biological properties, among others. However, the implementation of all its three main components (i.e., no-tillage, organic soil cover/mulch, and crop diversification) in southern Africa is often challenging, resulting in variable yield responses. Disentangling the contributions of CA practices is necessary to understand the drivers of maize grain yield within the region. Here we analysed two 6-year long component omission experiments, one at a sandy soil location and the other at a clay soil location. In these two experiments, soil chemical parameters, total plant nutrient uptake, rate of crop residue decomposition, and arbuscular mycorrhizal fungi (AMF) colonization of maize roots were assessed. Soil chemical properties only differed across systems at the sandy soil location with the mulched systems under no-tillage (NT) resulting in increased soil organic carbon levels, total nitrogen, and soil available phosphorus as compared to conventional tillage with no mulch or rotation (CT). Conventional tillage-based systems resulted in fastest decomposition of maize residues, while systems with NT and rotation resulted in highest AM fungal root colonization rate of maize at the clay soil location. Total plant N uptake was almost 2-fold higher in tilled and no-tilled systems with both mulch (M) and rotations (R) (i.e., NT+M+R and CT+M+R) as compared to CT. Structural equation modeling was used to disentangle the links between cropping systems, soil chemical and biological properties, plant nutrient uptake, and maize grain yield. Cropping systems had direct and indirect influences on yield at both locations. At both locations, cropping systems influenced yield via plant N uptake, with the NT+M+R and CT+M+R systems having more beneficial effects compared to other systems, as shown by their higher path coefficients. In conclusion, we recommend a more holistic approach to cropping system assessment that includes a higher number of abiotic and biotic determinants. This would allow for a more rigorous evaluation of the drivers of yield and increase our understanding of the effects and performance of practices under the prevailing agro-ecological conditions. -
What is at Stake in Deliberative Inquiry? A Review About a Deliberative Practice
Despite the growing interest in deliberative and dialogue models the research literature lacks investigations of the underlying assumptions of deliberative methods. Starting from the current popularity as well as the broad use of the method of deliberative inquiry -one example of such a deliberative method- this article aims to identify approaches and underlying assumptions of deliberative inquiry. Therefor a systematic literature review of empirical research, of descriptions of practical deliberative procedures and of theoretical research of deliberative inquiry is used. This review demonstrates that the method of deliberative inquiry is elaborated and used within different contexts with a corresponding range of rationales: From (1) a procedure to tackle curriculum questions through (2) a way of investigating and agreeing upon policy actions to (3) collaboratively researching issues. By describing the three approaches and by investigating the assumptions of deliberative inquiry within each approach, we demonstrate a range of rationales behind this method. Despite the distinctions, the primary goal of all manifestations of deliberative inquiry is similar: to contemplate a practical problem in a systemic and collaborative way, to weigh arguments for possible solutions and to make (even temporarily) a decision. This article concludes with future research perspectives.