Rôle du biochar centenaire sur les cycles des éléments nutritifs et la stabilité de la matière organique dans des agroécosystèmes tempérés

Natural earth-atmosphere carbon (C) fluxes have been disrupted by anthropogenic activities since the onset of the industrial revolution. Repeated scientific warnings to humanity regarding the consequences of such imbalance have drawn the global population’s attention to the importance of limiting global warming to ensure the sustainability of ecosystems and livelihoods worldwide. Alongside a drastic drawdown of C emissions, carbon dioxide removal practices are needed. Among possible practices, the enhancement of soil organic carbon (SOC) stocks and in particular, the incorporation of charcoal in soils, thus known as biochar, holds great potential as a C sequestration tool and as a soil crop amendment. Number of studies have reported the agronomic benefits of biochar on the short term. However, precisely because of its persistence in soils which generally outlasts research parties, its long-term effects have seldom been addressed. Here, we focus on leftovers from charcoal production in hearths intended to meet pre-industrial fuel demands in Belgium and we use it as a proxy for 200-year-old biochar. Our aim is to address the long-term stability of biochar in soils and its potential co-benefits on biogeochemical functioning of agroecosystems. This manuscript presents our findings through an ascending spatial scale approach that deals with impacts of century-old biochar (CoBC): i) at the micrometer scale by focusing on organo-mineral interactions and the distribution of PyOM in the soil matrix (chapter 3), ii) at the meter scale, by addressing the vertical mobility of PyOM and century-old biochar effects on SOC stocks in the soil profile (chapter 4), iii) at the hectare scale, to determine the role of PyOM on nutrient cycles and crop performance (chapter 5) and finally iv) at the regional scale, where we compare the effects of PyOM on soil microbiology across soils differing in texture and land use (chapter 6). This work brings particular attention to the need for in-situ studies by focusing on a specific, thoroughly equipped observatory set-up in a farmland near Gembloux (Belgium). Three studied soils, defined as Luvisol, containing either century-old biochar (CoBC), recently amended young biochar (YBC) and finally a charcoal-free reference soils (REF) were compared. For the three soils and their three horizons (Ap: 0-30, E=30-60, Bt= 60-90 cm), five field replicates were analyzed and monitored. Yields and the elemental composition of winter wheat and chicory were monitored and analyzed. Soil temperature and volumetric water content as well as meteorological variables were continuously monitored over the study period (2018-2021). Suction cups were set up in the three soil horizons (20 cm, 45 cm, 80 cm) to monitor the pore water composition and its evolution continuously for 2 years. Samples of the topsoil (Ap) and subsoils (E) horizon underwent a soil fractionation protocol based on size and density to separate OM fractions displaying similar stabilization processes: the free light fraction (LF), the occluded LF and the mineral associated heavy fraction (HF). The elemental composition and thermal signature of bulk soil samples and fractions were analyzed to quantify and differentiate PyOM and non-PyOM. We show that CoBC soils store on average an additional 54 ± 25 tC ha-1 when compared to adjacent REF soils in the uppermost meter of soil. This increase in absolute C stocks is particularly marked in the topsoil horizon (Ap, 0-30 cm) but relatively larger in the subsoil (E, 30-60 cm) horizon in comparison to REF. This increase in C stocks is largely due to PyOM, which accounts for 35 ± 13 tC ha-1 (1 m profile), but not only as we provide evidence throughout the manuscript for an increase in non-PyOM accounting for between 6-16% of the additional carbon occurring in CoBC topsoils. The fractionation of soils highlighted the presence of charcoal in all particulate and mineral-associated fractions in both top and subsoil horizons. We show PyOM is strongly bound to mineral fractions and we discuss it as a means to promote PyOM stability in soils. Our results support the assumption that biochar functionalization promotes organo-mineral associations and OM inaccessibility to decomposers. We further provide evidence for a shift in the main C pool of subsoils whereby most PyOM occurs as particulate light fraction in CoBC subsoils as opposed to being mainly mineral-associated in REF soils. PyOM is also further stabilized through vertical migration, however, our analysis of dissolved organic matter (DOM) contents and optical properties did not provide evidence for PyOC dissolution. We suspect bioturbation to be a major process involved in the vertical mobility of PyOM. Our research also investigates the effect of biochar of contrasting ages on nutrient cycling in the soil-water-plant continuum of conventionally cropped agroecosystems. Here we provide field-based evidence that the effects of biochar evolve over its lifetime in soils. Indeed, recently amended biochar had a strong impact mainly on the soil pore water nutrient concentrations, resulting in decreased N-NO3- and K+ leaching out of the profile while increasing P-PO4- concentrations in the topsoil horizon. Century-old PyOM, on the other hand, had little impact on the pore water nutrient concentration but increased soil total N, plant available K+ and Ca2+ but decreased plant available P content. Overall, the presence of biochar in soils decreased the N, K+ and Ca2+ exports under the winter-wheat crop studied but had no significant effect on crop productivity. Overall, the work presented in this manuscript showed that beyond its stability as a metabolically unappealing source of OM for decomposers, CoBC is further stabilized through its interaction with surrounding soil constituents and its transfers to deeper soil horizons. Its persistence vouches for its potential as a carbon dioxide removal tool. Simultaneously, CoBC improves the accessibility of certain nutrients (N, K+, Ca2+) thus highlighting a high potential for improving nutrient use efficiency in agroecosystems.Projet CHA