Impact du travail du sol sur les bilans de carbone à long terme : analyse d'essais de longue durée et modélisation

La concentration atmosphérique de CO2 a considérablement augmenté depuis l’ère industrielle et contribue majoritairement au changement climatique. L’agriculture peut jouer un rôle important dans la mitigation des émissions de CO2 à travers des pratiques culturales qui stockent du carbone dans les sols. Le travail du sol est souvent considéré comme une des pratiques culturales qui ont le plus d’impact sur les stocks de C. Cependant, l’effet de la réduction du travail du sol sur la séquestration de carbone reste sujetà controverse. L’objectif principal de ce travail de thèse est étudier l’effet à long terme de différents systèmes de travail du sol sur les stocks de C et leur dynamique. Nous nous sommes appuyés sur 2 essais de long terme conduits à la station expérimentale de Boigneville par Arvalis-Institut du Végétal. Dans l'essai A, initié en 1970, nous avons examiné l’impact de trois systèmes contrastés de travail du sol (FIT = labour, ST = travail superficiel et NT = semis direct) sur l'évolution des stocks de C calculés à masse de sol équivalente. Dans l'essai E, deuxsystèmes de travail du sol (FIT et NT) ont été établis en 1991 et maintenus jusqu’à 2011. En 2005, la moitié des parcelles de chaque système a été convertie: du semis direct vers labour et vice versa. Nous avons conduit une incubation afin d’évaluerla minéralisation du C et le priming effect en fonction du mode de travail du sol (après 40 ans de différentiation) et de la disponibilité en nutriments dans le sol. Enfin, nous avons simulé l’évolution des stocks de C et déterminé les taux de minéralisation effectif et potentiel grâce à une approche de modélisation inverse avec le modèle AMG multicouche. Les résultats montrent que le travail réduit du sol dans l'essai A a permis de séquestrer du C (par rapport au labour) au cours des 28 premières années, mais que cette différence a disparu au cours des 13 années suivantes puisque les stocks mesurés après 1998 n'étaient pas significativement différents. La cinétique de séquestration n'était donc ni linéaire, ni monotone. Dans l’essai E, les alternances de mode de travail du sol n’ont pas engendré de différences significatives de stocks de C et N au bout de 6 ans de conversion. L'étude faite en incubation montre que le taux de minéralisation spécifique dépend du travail de sol et de la couche de sol, alors que le priming effect (mesuré à apport de cellulose fixé) n'en dépend pas. Par contre il dépend de la disponibilité en nutriments. Le modèle AMG multicouche a permis de bien simuler l'évolution des stocks C, C3 et C4 de chacune des 4 couches de sol considérées sur 0-28 cm. Le taux de minéralisation effectif du carbone diminue avec la profondeur dans tous les traitements et est légèrement supérieur dans le labour si l'on considère la profondeur 0-28 cm. Ce travail montre l’intérêt des analyses diachroniques des études à long terme afin de prédire la dynamique de stockage et séquestration du C

Changes in soil carbon and nitrogen following tillage conversion in a long-term experiment in Northern France

International audienceAlthough continuous no-till (NT) is recommended for erosion control and carbon sequestration, it often has a limited duration since farmers alternate between NT and full inversion tillage (FIT) to control weed infestation and avoid soil compaction. In this paper, we evaluate the effect of continuous tillage and tillage conversion of NT to FIT and vice versa on SOC and SON stocks, in a long-term experiment at Boigneville in Northern France. Continuous NT (CNT) and FIT (CFIT) treatments were established in 1991 and maintained until 2011 while half of the plots were converted in 2005: from CNT to new FIT (NFIT) and CFIT to new NT (NNT). Bulk densities and organic C and N contents were determined in 2001 and 2011 down to the old ploughing depth (opd) which was also measured. SOC and SON stocks were calculated at equivalent soil mass by correcting either bulk densities or the opd. Both methods produced very close results and similar conclusions. A typical gradient of SOC and SON concentrations vs depth was observed in CNT as opposed to a rather uniform distribution in CFIT. CNT resulted in SOC concentration in the top soil (0-5 cm) higher by 38% in 2001 and 53% in 2011 compared to CFIT. Conversely, it led to a SOC reduction in the deeper layer (ca. 10-28 cm) by 14% in 2001 and 18% in 2011. The global effect was no significant change in SOC and SON stocks between treatments over the old ploughed layer (4060 t soil ha(-1)) in both years: 43.2 and 45.0 t C ha(-1) in 2001 and 44.7 and 45.8 t C ha(-1) in 2011, in CNT and CFIT, respectively. In 2011, six years after tillage conversion, the stratification of SOC and SON had disappeared in NFIT whereas a new one had appeared in NNT with a smaller gradient than in CNT. SOC or SON stocks over the old ploughed layer did not differ significantly between treatments after 6 years of conversion: SOC stocks were 45.8, 43.2, 44.7 and 43.1 t C ha(-1) in the CFIT, NFIT, CNT and NNT treatments, respectively. Furthermore, SOC stocks below the old ploughed layer (ca. 28-40 cm) were slightly greater in FIT than in NT treatment (10.9 vs 8.7 t C ha(-1)). In this experiment, continuous or conversion tillage did not result in any C sequestration benefit. (c) 2013 Elsevier B.V. All rights reserved

Effets du travail du sol sur les cycles biogéochimiques de l'azote et du carbone : de la compréhension des mécanismes aux conséquences pour la gestion des pratiques agricoles

Effets du travail du sol sur les cycles biogéochimiques de l'azote et du carbone : de la compréhension des mécanismes aux conséquences pour la gestion des pratiques agricole

Effect of nutrients availability and long-term tillage on priming effect and soil C mineralization

Agricultural management practices including soil tillage exert strong control on soil organic matter(SOM) turnover and its interactions with global C cycle through different mechanisms. One controlmechanism is the priming effect (PE) which consists in stimulating SOM mineralization with the additionof fresh, energetic plant material. In this study, we quantified C mineralization and PE in soils sampled intwo contrasted long-term (40 years) tillage treatments which deeply modified soil properties (e.g.organic C concentration, microbial biomass, pH). We hypothesized that soil tillage might affect theseprocesses through changes in C addition rates, nutrient availability, and long-term variations in SOMcontent and microbial communities. We investigated the relationship between PE intensity, tillage andnutrients availability in soil samples taken in no till (NT) and full inversion tillage (FIT) in two layers (0e5and 15e20 cm). Soils were incubated with or without addition of 13C labeled cellulose and mineralnutrients. Potential C mineralization and primed C were measured during 262 days. Unlabeled soilmicrobial biomass C was determined at the end of the experiment to separate apparent and real primingeffect.Basal cumulative C mineralization in the control soil ranged from 363 to 1490 mg kg 1 soil at day 262.It was strongly correlated with soil organic carbon (SOC) concentration. Specific mineralization rateswere 44.8 and 68.8 g kg 1 SOC in the 0e5 cm layer for the FIT and NT treatments, respectively and werestrongly linked with the particulate organic matter content (r ¼ 0.99***). These results suggest that SOCwas more active in the upper layer of the NT treatment due to the high concentration of readilydecomposable,particulate organic matter. The cellulose was entirely metabolized after 60 days and itskinetics of mineralization was affected neither by tillage, depth nor nutrients. The percentage of celluloseC released as CO2 represented 55e61% of the added cellulose-C at day 262. A positive PE was found in alltreatments and its kinetics was parallel to that of cellulose mineralization. The cumulative PE significantlyvaried with nutrients level but not tillage, ranging from 73 to 78 mg kg 1 under high nutrientslevel and from 116 to 136 mg kg 1 in low nutrients level. No significant differences were found in unlabeledmicrobial biomass C between control and amended soil, suggesting no apparent priming effect.We conclude that the priming was mainly controlled by nutrient availability but not tillage, in spite ofstrong tillage-induced changes in SOC concentration and microbial biomass. Since PE is known to dependon C addition rate, tillage is expected to affect in situ PE through variations in the ratio of fresh carbon tonutrient concentration along the soil profile

The impacts of CENTURY model initialization scenarios on soil organic carbon dynamics simulation in French long-term experiments

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Long-term effect of contrasted tillage and crop management on soil carbon dynamics during 41 years

International audienceAlthough numerous studies have been conducted on the effect of tillage on soil organic carbon (SOC), there is still no consensus on the importance of sequestration which can be expected from reduced tillage. Most studies have used a synchronic approach in fields or long-term experiments which were often poorly characterized with respect to initial conditions. In this paper, we used a diachronic approach to quantify SOC changes in a 41 years experiment comparing no-till (NT), shallow till (ST) and full inversion tillage (FIT) combined with crop managements (residues removal, rotation and catch crops). It included SOC measurements at time 0 and every 4 years, calculations at equivalent soil mass within or below the old ploughed layer. Results show that tillage or crop management had no significant effect on SOC stocks after 41 years both in the old ploughed layer (ca. 0-28 cm) and deeper (ca. 0-58 cm). Tillage had no effect on crop yields and residues. In the reduced tillage treatments (ST and NT), SOC accumulated in the surface layer (0-10 cm), reaching a plateau after 24 years but declined continuously in the lower layer (10-28 cm) at a rate of 0.42-0.44% yr(-1). The difference in SOC stocks (ST or NT minus FIT) over the old ploughed layer followed a non-monotonic pattern over time. Reduced tillage caused a rapid SOC sequestration during the first 4 years which remained more or less constant (mean = 2.17 and 1.31 t ha(-1), resp.) during the next 24 years and disappeared after 28 years. The drop was attributed to the higher water balance recorded during years 24-28. In the reduced tillage treatments, the changes in SOC over time were negatively correlated with the water balance, indicating that sequestration rate was positive in dry periods and negative in wet conditions. This study highlights the interest of diachronic approaches to understand the effect of tillage and its interaction with environmental and management factors

Modelling crop management effects on soil organic C stocks and pools dynamics using CENTURY

International audienceSoils constitute the major reservoir of organic carbon storing around 2500 Pg C in the top two meters which correspond to approximately more than three times the amount of C stored in the atmosphere (Jobbágy and Jackson, 2000; Tarnocai et al., 2009) and six times the amount of C stored in terrestrial vegetation (Prentice et al., 2001). Consequently, small changes in soil organic carbon (SOC) stocks will greatly affect the global ecosystem carbon cycling and potentially the global climate (Davidson and Janssens, 2006; Heimann and Reichstein, 2008). Beside its importance in the global terrestrial C cycle, SOC is also a key component for sustainable productivity in agro-ecosystems. Improving agricultural practices represents a win-win strategy that has the potential to enhance soil fertility and sequester C (Lal, 2004). However, there still uncertainties about agricultural practices impacts on SOC stocks dynamics and controversial and contradictory results in the literature are common (Luo et al., 2010; Virto et al., 2011). These uncertainties are mainly attributed to the lack of continuous SOC monitoring in long-term experiments, the diversity of climatic conditions (Powlson et al., 2014), and the antagonistic effects of some practices such as fertilization or irrigation. Modeling represents a valuable tool to simulate the spatial and temporal SOC stocks dynamics in response to the fast changes in policies, agriculture practices and their complex interaction with current and projected future climatic conditions. The main objectives of this work were: to analyze the interaction between crop management and climatic conditions in the long-term on SOC dynamics as simulated by theCentury model (Parton et al., 1987) across France and to quantify GHG emissions and SOC stocks at different scales in order to apply the Tier 3 methodology for the GHG inventories. We examined more than 20 sites with different Long-term experiments, ranging from 10 to 40 years, across the France. Different agricultural practices were studied such as tillage treatments, crop rotation, organic amendment, mineral fertilization, etc. We used the well-validated Century model which has been widely used all over the world to simulate SOC dynamics in agricultural systems. Model parameters calibration through inverse modeling using PEST (Doherty, 2010) was applied to estimate some of the parameter values. First results showed that the model gave satisfactory results for the SOC stocks dynamics over the layer 0-20 cm. Model parameters calibration improved the fit for to crop productivity and SOC. An important result emerged from this work emphasis that agricultural practices that maximize C input are more effective strategies for SOC sequestration than those that limit SOC mineralization. Further results will be given in the poster

Uncertainty functions of modelled soil organic carbon changes in response to crop management derived from a French long term experiments dataset

The land use, land-use change and forestry (LULUCF) activities and crop management (CM) in Europe could be an important carbon sink through soil organic carbon (SOC) sequestration. Recently, the (EU decision 529/2013) requires European Union's member states to assess modalities to include greenhouse gas (GHG) emissions and removals resulting from activities relating to LULUCF and CM into the Union's (GHG) emissions reduction commitment and their national inventories reports (NIR). Tier 1, the commonly used method to estimate emissions for NIR, provides a framework for measuring SOC stocks changes. However, estimations have high uncertainty, especially in response to crop management at regional and specific national contexts. Understanding and quantifying this uncertainty with accurate confidence interval is crucial for reliably reporting and support decision-making and policies that aims to mitigate greenhouse gases through soil C storage. Here, we used the Tier 3 method, consisting of process-based modelling, to address the issue of uncertainty quantification at national scale in France. Specifically, we used 20 Long-term croplands experiments (LTE) in France with more than 100 treatments taking into account different agricultural practices such as tillage, organic amendment, inorganic fertilization, cover crops, etc. These LTE were carefully selected because they are well characterized with periodic SOC stocks monitoring overtime and covered a wide range of pedo-climatic conditions. We applied linear mixed effect model to statistically model, as a function of soil, climate and cropping system characteristics, the uncertainty resulting from applying this Tier 3 approach. The model was fitted on the dataset yielded by comparing the simulated (with the Century model V 4.5) to the observed SOC changes on the LTE at hand. This mixed effect model will then be used to derive uncertainty related to the simulation of SOC stocks changes of the French Soil Monitoring Network (FSMN) where only one measurement is done in 16 Km regular grid. These simulations on the grid will be in turn used for NIR. Preliminary results suggest that the model do not adequately simulate SOC stocks levels but succeeds at capturing SOC changes due to management, despite the fact that the model does not explicitly simulate some management such as tillage. This is probably due to inappropriate model parametrization especially for crops and thus Cinput in the French context and/or model initialization

Uncertainty assessment of the soil carbon balance in the context of the 4 per 1000 aspirational target: A case study for France

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Safety and Efficacy of Disease Modifying Therapies After Switching from Natalizumab: A MENACTRIMS Observational Study

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