C storage in soils : dynamics of organic matter in interaction with soil nanophases

Maintenir le taux de matières organiques des sols - et donc de C - est un enjeu pour la sécurité alimentaire et le climat. La stabilisation du C des sols est principalement assurée par l’association entre matières organiques et phases minérales. Cette association peut être réalisée par un mécanisme quantitativement important de coprécipitation qui lie les composés organiques avec des éléments issus de l’altération minérale (principalement Fe, Si, Al). Les coprécipités, ayant des éléments inorganiques amorphes, sont nommées nanoCLICs (Nanosized Coprecipitates of inorganic oLIgomers with organiCs). La caractérisation des mécanismes contrôlant la dynamique de formation, le maintien ou la déstabilisation des nanoCLICs est incomplète à ce jour. Ainsi, l’objectif de cette thèse est de caractériser ces mécanismes dans les situations de stockage et de déstockage de C, en couplant des approches de chimiométrie, isotopie, microscopie électronique, spectroscopie X et respirométrie. Dans le cas d’une transition forêt-culture d’un andosol, les nanoCLICs sont déstructurés, expliquant une fraction importante de la perte en C. A l’inverse, en situation de gain de C, l’apport de fertilisants organiques ne semble pas promouvoir la coprécipitation de nanoCLICs. L’imagerie aux échelles fines montrent que la nature et la structure des nanoCLICs est caractérisée par un mélange aux proportions variables de C, Al, Si (et Fe dans une moindre mesure) jusqu’aux résolutions de 10 nm. Enfin, les nanoCLICs mis en interaction avec le vivant montrent que le C y est majoritairement protégé de la minéralisation microbienne, mais la simulation d’injection d’exsudats racinaires tend à le déstabiliser.Maintaining organic matter content in soils - and hence C content - is essential to global food security and to mitigating the effects of climate change. Soil C stabilization is mainly provided by the association between organic matter and mineral phases. This association can be achieved by a quantitatively important mechanism of co-precipitation that binds organic compounds with elements derived from mineral weathering (mainly Fe, Si, Al). The coprecipitates, having amorphous inorganic elements, are named nanoCLICs (Nanosized Coprecipitates of inorganic oLIgomers with organiCs). The characterization of the mechanisms controlling the dynamics of formation, maintenance, or destabilization of nanoCLICs is incomplete to date. Thus, the objective of this thesis is to characterize these mechanisms in a context of C storage and destabilization, by coupling chemometrics, isotopy, electron microscopy, X-ray spectroscopy and respirometry approaches. In the case of a forest-to-crop transition in an andosol, the nanoCLICs are disrupted, explaining a significant part of the C loss. Conversely, in a C gain situation, organic fertilizer input does not appear to promote coprecipitation of nanoCLICs. Fine scale imaging shows that the nature and structure of nanoCLICs is characterized by a mixture of variable proportions of C, Al, Si (and Fe to a lesser extent) down to 10 nm resolutions. Finally, nanoCLICs interacting with living organisms show that C is mostly protected from microbial mineralization, but the simulation of root exudates additions tends to destabilize it

Can stable isotopes quantify soil carbon build-up from organic fertilizers?

International audienceCan stable isotopes quantify soil carbon build-up from organic fertilizers? Application of organic fertilizers (OF) can supply carbon (C) to the soil in crop fields. OF-derived C (OF-C) is often estimated using the differential method that can be biased due to indirect effects of OF on soil C. This study tested three methods to quantify OF-C: (i) the widespread differential method, (ii) the synchronic isotopic method comparing plots with and without OF and (iii) the asynchronic isotopic method mimicking a trial without a control plot. These methods were implemented on an Arenosol and an Andosol supplied during 13 years with slurry or compost. The results highlighted the relevance of using the isotopic synchronic method, which focuses on the direct effect of OFs on the soil organic matter (without bias of vegetation change) and considers control soil's evolution. Considering limit of resolution in the Arenosol, the method was suitable after 4 years of fertilisation for an initial δ 13 C difference of 7.5 ‰ between OF and soil, and after 9 years for a difference of 3.5 ‰. A sensitivity analysis showed that particular attention must be paid to OF-δ 13 C variability to guarantee the method validity. The method proved to be suitable to study the factors controlling the OF-C fate in tropical soils

Quantification of soil C inputs from organic fertilizers in tropical long-term field experiments: potential of stable carbon isotopes

International audience<p>Soil C stocks can be increased by spreading organic fertilizer (OF) in crop fields. OF-derived C (OF-C) is usually estimated according to the differential with and without OF inputs [1-4]. But OF applications may boost crop production or induce initial-C mineralization due to an indirect effect (e.g. priming) [5-6]. Therefore, crop derived C and native-C (before plot testing) may change during the experiment link to OF additions. Thus, the differential method might not be the suitable one for quantifying OF-C. In this study, we used stable <sup>13</sup>C isotopes to avoid such OF-C estimation biases and compared two isotopic methods to the differential method. Both isotopic methods were set up with synchronous controls (e.g. soil δ<sup>13</sup>C signature compared to plot with and without OF inputs) and diachronous control (e.g. soil δ<sup>13</sup>C signature compared to the soil at the beginning of the experiment). In order to assess the all three methods, this study was implemented on an Arenosol and an Andosol with a 13-year history of compost or slurry amendment. The differential and synchronic isotopic methods gave similar OF-C estimations for the Arenosol, while for the Andosol both isotopic methods estimated twofold higher OF-C levels compared to the differential method. Changes in crop-C production or priming as a result of OF applications might explain this gap. Moreover, the control isotopic signature (without OF) slightly changed due to crop-C integrated during the experiment. Which is why the isotopic synchronic method was the most suitable compared to diachronic isotopic method. According to this method, OF-C retention was OF-nature dependent (21% for compost, 8% for slurry), and soil type and climate dependent (42% compost retention in the Andosol and 21% in the Arenosol), highlighting the recent carbon input retention capacity of Andosols. This method is also relevant to quantify the priming effect in field trials, in our case it was not possible due to the δ<sup>13</sup>C evolution of the soil without OF input.</p><p>[1] Y. Lou et al., CATENA, vol. 87, no 3, p. 386‑390, déc. 2011, doi: 10.1016/j.catena.2011.07.006.</p><p>[2] L. Paetsch et al., Agriculture, Ecosystems & Environment, vol. 223, p. 211‑222, mai 2016, doi:10.1016/j.agee.2016.03.008.</p><p>[3] H. Liu et al., Agriculture, Ecosystems & Environment, vol. 265, p. 320‑330, oct. 2018, doi: 10.1016/j.agee.2018.06.032.</p><p>[4] F. Liang et al., Geoderma, vol. 337, p. 853‑862, mars 2019, doi: 10.1016/j.geoderma.2018.10.033.</p><p>5] Y. Kuzyakov et al., Soil Biology and Biochemistry, vol. 32, no 11, p. 1485‑1498, oct. 2000, doi:10.1016/S0038-0717(00)00084-5.</p><p>[6] S. Fontaine et al., Soil Biology and Biochemistry, vol. 35, no 6, p. 837‑843, juin 2003, doi: 10.1016/S0038-0717(03)00123-8.</p&gt

Nanoscale chemical imaging of organo-mineral fractions of an andosol (La Martinique, France)

International audienc

Nanoscale chemical imaging of organo-mineral fractions of an andosol (La Martinique, France)

International audienc

Nanoscale chemical imaging of soil organo-mineral associations.

International audience<p>Organo-mineral associations drive organic matter (OM) stabilization in soils, but mechanisms controlling their dynamics are still not fully known at micro and nanoscale. Adsorption of OM on minerals’ surfaces is a prevalent viewpoint of OM stabilization processes (Kleber et al., 2007), but Basile-Doelsch et al., (2015) suggested that mineral alteration generating amorphous nanophases and cationic oligomers on minerals’ surfaces is also a driver of OM stabilization through coprecipitation processes. Lab experiments which mimic these processes showed that the nanosized co-precipitates (Nanosized Coprecipitates of inorganic oLIgomers with organiCs: nanoCLICs) are made of inorganic Fe, Al, Si oligomers associated with organic molecules (Tamrat et al., 2019). Andosols are known to have a high OM-stabilization capacity, mostly attributed to associations of OM with nanominerals (imogolite, allophane, proto-imogolite) (Basile-Doelsch et al., 2007; Levard et al., 2012). In the present study, we investigated the presence of nanoCLICs in Andosol fractions from La Martinique (French West Indies). We used Transmission Electron Microscopy (TEM, FEI Tecnai Osiris 200kV) coupled with 4 EDX detectors and EELS to semi-quantify and map major elements. TEM analyzed zones of interest ranged from 5 µm to 10 nm with pixel size from 500 to 1 nm. Few crystallized minerals, particulate OM and amorphous thin fibers that could not be definitively attributed to imogolite nanotubes were observed. However, we mainly observed totally amorphous phases to electron diffraction. Al, Si, C, Fe and O were the main component of the latter amorphous phases. Al, Si and Fe were systematically associated to C even at a size resolution down to 1 nm (semi-quantifications ranged from 11 to 41% of C, 4 to 7% of Fe, 34 to 36% of Al and 22 to 46% of Si). Similar high-resolution images were obtained for the andosol organo-mineral associations and the synthetic nanoCLICs. At the working TEM resolution, the nanoCLICs model proposed by Tamrat et al., (2019) is consistent with the structures observed on the andosol. Based on these results, the majority of C appears to be in nanoCLICs form in these Andosol fractions and confirms the hypothesis puts forward by Basile Doelsch et al., (2015).</p><p>Basile Doelsch et al., 2007. Mineral control of carbon pools in a volcanic soil horizon. Geoderma, 137 (3-4), 477-489. ISSN 0016-706.</p><p>Basile-Doelsch et al., 2015. Are Interactions between Organic Compounds and Nanoscale Weathering Minerals the Key Drivers of Carbon Storage in Soils? Environ. Sci. Technol. 49, 3997–3998.</p><p>Kleber et al., 2007. A Conceptual Model of Organo-Mineral Interactions in Soils: Self-Assembly of Organic Molecular Fragments into Zonal Structures on Mineral Surfaces. Biogeochemistry 85, nᵒ 1 (1 août 2007): 9‑24.</p><p>Levard et al., 2012. « Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils ». Geoderma 183‑184 (1 août 2012): 100‑108. </p><p>Tamrat et al., 2019. « Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands ». Geochimica et Cosmochimica Acta, 10 juin 2019. </p&gt

Dynamics of carbon loss from an Arenosol by a forest to vineyard land use change on a centennial scale

International audienceFew studies have focused on Arenosols with regard to soil carbon dynamics despite the fact that they represent 7 % of the world's soils and are present in key areas where food security is a major issue (e.g., in Sahelian regions). As for other soil types, land use changes (from forest or grassland to cropland) lead to a loss of substantial soil organic carbon (SOC) stocks and have a lasting impact on the SOC turnover. Here we quantified long-term variations in carbon stocks and their dynamics in a 80 cm deep Mediterranean Arenosol that had undergone a forest-to-vineyard land use change over a 100 years ago. Paired sites of adjacent plots combined with carbon and nitrogen quantification and natural radiocarbon (14 C) abundance analyses revealed a C stock of 53 t ha −1 in the 0-30 cm forest soil horizon, which was reduced to 3 t ha −1 after long-term grape cultivation. Total organic carbon in the vineyard was dramatically low, with around 1 g C kg −1 , and there was no vertical gradient as a function of depth. 14 C showed that deep plowing (50 cm) in the vineyard plot redistributed the remaining carbon both vertically and horizontally. This remaining carbon was old (compared to that of the forest), which had a C : N ratio characteristic of microbial organic matter and was probably stabilized within organomineral associations. Despite the drastic degradation of the organic matter (OM) pool in this Arenosol, this soil would have a high carbon storage potential if agricultural practices, such as grassing or organic amendment applications, were to be implemented within the framework of the 4 per 1000 initiative

The distribution of Silicon in soil is influenced by termite bioturbation in South Indian forest soils

International audienceSi is one of the most abundant element on earth and an abundant literature shows its beneficial effects on plant growth and resistance. We here question the influence of termites, as key soil bioturbators, on the distribution of Si in a tropical soil. The abundance and forms of Si in termite mounds build by Odontotermes obesus (TM) or in the soil eroded from TM but redistributed on the ground surface (EROD) were compared to those measured in the 0–5 (Ctrl0-5) and 70–120 cm soil layers (Ctrl70-120). Although termites use the soil from Ctrl70-120 for building their mounds, we found that TM and EROD had intermediate soil physical, chemical and mineralogical properties between Ctrl0-5 and Ctrl70-120. Clay content was not significantly different between soil materials. However, the lower variability measured in TM than in the soil suggested that termites used soil layers with higher amounts of clay fraction and with a preference especially for layers enriched in 2:1 clay minerals (smectite) most likely because they provide better physical properties in terms of plasticity and water retention than kaolinite. Finally, phytoliths and bioavailable Si (SiCC) contents were increased in TM in comparison with Ctrl70-120, suggesting an incorporation of phytoliths in termite construction through their saliva and/or an increasing availability of SiCC from the minerals. In conclusion, this study highlights how termites, through their feeding and building activities, impact Si distribution in tropical soils

Structure and Chemical Composition of Soil C-Rich Al–Si–Fe Coprecipitates at Nanometer Scale

International audienceSoil carbon stabilization is mainly driven by organo-mineral interactions. Coprecipitates, of organic matter with short-range order minerals, detected through indirect chemical extraction methods, are increasingly recognized as key carbon sequestration phases. Yet the atomic structure of these coprecipitates is still rather conceptual. We used transmission electron microscopy imaging combined with EDX (energy dispersive x-ray) and EELS (electron energy loss spectroscopy) chemical mappings, which enabled direct nanoscale characterization of coprecipitates from Andosols. A comparison with reference synthetic coprecipitates showed that the natural coprecipitates were structured by an amorphous Al, Si and Fe inorganic skeleton associated with C, and were therefore even less organized than short-range order minerals usually described. These amorphous type of coprecipitates resembled previously conceptualized nanosized coprecipitates of inorganic oligomers with organics (nanoCLICs) with heterogeneous elemental proportions (of C, Al, Si and Fe) at nanoscale. These results mark a new step in the high-resolution imaging of organo-mineral associations, while shedding further light on the mechanisms that control carbon stabilization in soil, and more broadly in aquatic colloid, sediment and extraterrestrial samples