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Aims Silicon (Si) has beneficial effects in a variety of plant species and environments. Soil and climate affect silica accumulation in given plant species, but their roles on foliar silicification patterns and balance between silica and C-rich biopolymers as structural components is poorly known. Methods We studied silica deposition in situ in sugarcane leaves collected in three tropical environments differing in soil and climate. Plant silica deposits were physically extracted from leaves through wet digestion. Leaves were observed and mapped for Si by ESEMEDX. The C-rich biopolymers in leaves were determined by the Van Soest method. Results Silicon accumulation in the leaves was related to bioavailable Si in soil and plant transpiration. Epidermal silica deposits were either limited to silica cells as dumbbell-shaped phytoliths, or expanded to long and short cells arranged in prominent veins fully silicified, depending on whether the leaf Si concentration was lowest or highest. The size of silica deposits increased with increasing leaf Si through an increasing number of conjoined silicified cells. Leaf ash-free cellulose and Si concentrations were negatively correlated. Conclusions Soil and climate impact markedly the magnitude of foliar silicification, with possibly significant impact on mechanical properties and Si-related plant functions. Environmental conditions further impact the counterbalance between silica and cellulose as leaf structural components via different levels of Si accumulation.

The continental cycle of silicon (Si) relies on the Si soil-to-plant cycle where the primary source of Si for plants is the reserve of soil weatherable lithogenic silicates (LSi). LSi weathering releases dissolved Si (DSi), which may follow four routes: formation of pedogenic silicates (PSi), adsorption on oxide surfaces, leaching to watersheds, uptake by plants that form silica bodies named phytoliths (PhSi). Within plant debris, Si returns to soil where phytoliths dissolve to provide plant available DSi. DSi from soil to plant and back to soil for plant uptake defines the biological Si feedback loop of the terrestrial cycle of Si. In croplands, harvesting and exportation of plant residues disrupt this loop, and aggravate natural soil desilication. Pyrolysis of phytolithic biomass from monocot residues produces Si-rich biochar which, when applied to soil, is a promising and environmentally friendly technique challenging desilication. Our scientific question is how to enhance the biological Si feedback loop in agroecosystems, given the many benefits of Si in plants. In a first step, we set up a soil-plant experimental design controlling PhSi supply through biochar and LSi supply through dosed additions of weatherable LSi minerals to a soil free of them, hence simulating a soil weathering gradient. The contribution of supplied phytoliths to Si accumulation in rice increased from 26% to 80% with increasing soil weathering stage, i.e. decreasing total reserve in bases from TRB 405 to 5 cmolc kg-1. LSi dissolution contributed to 7-14% of allophanic PSi synthesis, but to less than 1% of plant PhSi. Allophane rapidly formed from LSi dissolution at a maximum rate of 0.85 g kg-1 day-1. The increase in rice plant accumulation of Si enhanced photosynthetic activity, water uptake and transpiration, stomatal conductance, plant biomass and grain yield. In a second step, we study the impact of soil properties and processes upon the mobility of Si in the soilplant system. Our resu, (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2019

There is a lack of long-term field approach investigating biochar impact on soil properties and vegetation, particularly in forest ecosystems. Relic charcoal hearths (RCHs), the result of the historical charcoal production in the forests, preserve a charcoal-enriched topsoil horizon, thus representing a suitable proxy for studying the long-term effect of biochar addition to soil. In this study, we analyzed the chemical properties of a soil as impacted by charcoal accumulation in three RCH plots in southern Wallonia (Belgium) compared to the soil outside RCHs. We further evaluated the effects of RCHs soil properties on the growth performances of silver birch and European beech as well as the leaves’ nutrient concentration of the latter. RCHs soil stored much more carbon and nitrogen than the reference ones. Most of the C in RCHs derived from charcoal (70‒94% of total organic carbon), which would correspond to a total input of 342 tons of biochar per hectare in these soils. Such an accumulation of charcoal still affects nutrient status of soil even after 150 years since charcoal hearths abandonment: CEC and K, Ca, Mg, Na, Mn and Zn concentration remained higher in RCHs soil compared to the reference one. In spite of a seemingly higher fertility of RCHs soil, elemental concentrations of European beech leaves grown in RCHs did not show any significant difference compared to the reference plots, except for C and Mn concentration, higher and lower, respectively, in the leaves of European beech trees grown inside than outside RCHs. Overall, RCHs soil chemical properties were not a decisive factor in significantly improving tree growth. On the contrary, tree ring width average values of both tree species was slightly lower in RCH plots, suggesting to better investigate the potential long-term detrimental effect of a large biochar addition to soil on forest trees.

Soil processes partly govern the terrestrial cycle of silicon (Si). The biological and physiological functions related to Si deposits in plants are increasingly studied. The understanding of tradeoffs between silicon and cellulose in leaf structure remains unclear and available studies are restricted to rice plants, mainly under controlled conditions. The soil processes underlying this tradeoff are, moreover, not inferred despite the well-known impact on Si bioavailability and soil-to-plant transfer. Here, we study the tradeoff between silica and cellulose in leaves of sugarcane (Saccharum officinarum L.) cropped on three different soils (Nitisol, Andosol, Vertisol). The soil CaCl2 extractable Si concentration, so-called bioavailable Si, was measured after extraction using CaCl2 solution (0.01M) for 6h. The total leaf Si concentration was determined after drying, calcination and further alkaline fusion followed by ash dissolution using concentrated HNO3. Plant silica bodies were physically extracted from sugarcane leaves through wet digestion. The leaves were observed and mapped for Si by ESEM-EDX. Leaf cellulose, hemicellulose and lignin concentrations were determined according to the Van Soest method. As expected, total leaf Si concentration increases with increasing soil CaCl2-extractable Si following the sequence Nitisol

Silicon(Si)inplantsincreasestheirphotosyntheticactivityandtoleranceagainstvariousbioticandabioticstresses. It is thus important to increase the pool of dissolved silicon (DSi) in soils to promote plant Si uptake and form biogenic Si (BSi) deposited as phytolith in plant tissues. The BSi pool may vary according to the soil weathering stage and removal of plant residues since both affect the DSi pool. This can impact the Si biocycling, particularly in agricultural systems due to large phytolith deposition within monocots. Applying biochar (Bi) pyrolyzed from phytolith-rich plants on soils may increase the DSi pool in croplands. Here we explore this possibility through a pot experiment during two successive rice growing periods. We compare the impact of Bi on rice above-ground biomass and Si mineralomass that of Wollastonite-Wo- (CaSiO3), a common Si fertilizer. We produced rice straws through a hydroponic device using Yoshida nutrient solutions, respectively enriched in H4SiO0 4 (40 ppm Si:Si+) and devoid of H4SiO0 4 (0 ppm Si: Si-). The straws were pyrolyzed to obtain biochar materialsexhibiting identical physico-chemical characteristics exceptfor their Si content (51.3g Si kg−1 inSi+Bi vs 0.3 g Si kg−1 in Si-Bi). We further applied them at the rate of 8.5 t ha−1 Bi once in a Cambisol and compared it to Wo (1.8 t ha−1) during two successive growing periods. Si+Bi application led to the most positive impact on biomass and Si mineralomass as well as K, Ca, Mg and P mineralomassinbothricegrowingperiods.Si-Biapplicationdidnotsignificantlyincreasethephytolithcontentin any of the rice organs during any growing period. As expected, Si+Bi and Wo significantly increased the phytolith content of rice plants in the two growing seasons. Thusconvertingphytolith-richricestrawtobiocharandusingitasasoilamendmentcanincreasesoilBSicontent and bioavailable Si, the uptake of which improves plant productivity and Si mineralomass during two successive rice growing periods. Consequently bioc

Phytoliths from biochar thus provide bioavailable Si that increases wheat plant Si uptake, biomass and mineralomass, particularly in the low pH - buffered weathered Nitisol.

In the upper part of the solum of mineral soils, soil organic and mineral constituents co-evolve through pedogenesis, that in turn impacts the transformation and stabilization of soil organic matter (SOM). Here, we assess the reciprocal interactions between soil minerals, SOM and the broad composition of microbial populations in a 530-year chronosequence of podzolic soils. Five pedons, derived from beach sand, are studied. From young to old soils, net acidification parallels mineral dissolution and the formation of eluvial and illuvial horizons. Organo-mineral associations (OMA) accumulate in the illuvial B horizon of the older soils (330–530 years). Apart from contributing to SOM stabilization and protection, organo-mineral compounds progressively fill up interparticle voids. The subsequent loss of porosity leads to horizon induration, decrease of hydraulic conductivity, which promote redoximorphic processes. While recalcitrant SOM is preserved in the topsoil of the old soils, the largest quantity of protected SOM occurs in the indurated, temporalily waterlogged B horizons, through both the OMA accumulation and inhibition of microbial decomposition. SOM protection is thus both time- and horizon-specific. The microbiota also evolve along the chronosequence. Fungi dominate in all horizons of the younger soils and in the topsoil of the older soils, while bacteria prevail in the cemented B horizons of older soils. This shift in microbial community composition is due to the interdependent co-evolution of SOM and minerals during pedogenesis. Our results call for considering the microenvironment and parameters inherent to decomposer microorganisms to understand SOM protection processes in soils.

Banana is one of the main crops in Burundi, Rwanda and South Kivu (DRC), as in the whole of the African Great Lakes Region. Its cultural potential amounts to 60-70 T ha-1 yr-1. Real crop yields deviate from this potential, but in an extremely variable way. The cropping conditions vary as much, in terms of climates, soils and technical routes. By diagnostic survey, we have quantified the variability of banana crop yields and investigated the influence of climate, soil and management practices in fourteen sites in Burundi, Rwanda and South Kivu. Up to seven times higher than those reported in national statistics, yields range from 13 to 37 T ha-1 yr-1. Their variability is due to very marked differences in water and nutrients availability, as well as in management practices. Water constraint and mineral deficiencies, including potassium, phosphorus, and magnesium, are major limiting factors to crop yields. The diagnostic survey also reveals that some practices can be used to correct the environment, particularly non-tillage with mulching and intercropping. The contribution of soil to crop performance is assessed at five sites. Their altitude varies from 880 to nearly 1700 m, the average annual precipitation from 900 to 1600 mm, while the Total Mineral Reserve (TRB) ranges from 10 to 160 cmolc kg-1. The latter is decisive in determining the real yield RY in the absence of fertilizer inputs. Indeed, in these conditions, RY (T ha-1 yr-1) is a relevant indicator of the soil weathering degree (/TRB, cmolc kg-1): RY = 0 (Acrisol/TRB = 10), 1.5 (Ferralsol/44-53), 4.3 (Nitisol/40), 16.6 (Cambisol protoandique/112-115), 25.0 (Fluvisol/146-165). Water stress and mineral deficiencies, including potassium, phosphorus, and magnesium, are major limiting factors to crop yields. The diagnostic survey also reveals that some practices can be used to correct the environment, particularly non-tillage with mulching and intercropping. The banana-bean system with no tillage and mulching is, (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2018

Zinc is an essential micronutrient that is ultimately released during mineral weathering. In soils, organic matter plays a key role in influencing Zn partitioning and therefore on Zn biogeochemical cycling. Soil organic matter is partitioned between carbon that is more readily available for decomposition by microorganisms, and more stable carbon transiently preserved from decomposition. The role of the stable pool of soil organic matter on Zn biogeochemical cycling remains poorly understood. The pool of stable carbon is controlled by combination with mineral constituents or is material that is intrinsically resistant to decomposition. The Zn stable isotopes are fractionated by interactions between Zn and soil mineral and organic constituents. This study reports the Zn isotope composition of five Icelandic soil profiles derived from the same parent basalt and characterized by contrasting degrees of weathering and organic matter content (δ66Zn = + 0.10 ± 0.05 to + 0.35 ± 0.02‰), the distribution of reactive mineral constituents available to form associations with soil organic matter, and the amount of stable organic carbon. Throughout these soils, the δ66Zn isotope variations are little influenced by mineral constituents, but rather by soil organic matter content. These data suggest that a combination of organic matter accumulation and Zn loss by leaching is required to explain the observed decrease in Zn concentration in soils and lighter soil δ66Zn with increasing organic carbon content. These results suggest that the presence of stable organic carbon in soils provides a pool of light Zn, attributed to the Zn isotope signature of organic matter partially preserved from decomposition. Crucially, this stable organic carbon pool may also contribute to the formation of the light Zn isotope sink reported in organic-rich marine sediments, a key output required to explain the oceanic mass balance of Zn isotopes., SCOPUS: ar.j, info:eu-repo/semantics/published

Black carbon (BC) plays an important role in terrestrial carbon storage. Nevertheless, the effect of cultivation on long term dynamics of BC in soil has been poorly addressed. To fill this gap, we studied the chemical properties of charcoal particles extracted from preindustrial kilns in Wallonia, Belgium, along a chronosequence of land use change from forest to agricultural soil, up to 200 years of cultivation. Preindustrial charcoal samples were compared with charcoal subjected to short term ageing in a currently active kiln. Cultivation increased the association of charcoal with soil minerals, which is favored by deprotonation of carboxylic acids under liming, thereby enhancing the reactivity of charcoal toward mineral surfaces. The large specific surface area of charcoal, related to its porosity, promotes the precipitation of 2:1 phyllosilicates and CaCO3. Both ageing and cultivation decreased the resistance of charcoal to dichromate oxidation, related to an increase in the H/C of charcoal. Differential scanning calorimetry revealed the presence of three fractions of distinct thermal stability. Saturation of carboxylate groups with Ca2+ under liming decreased the thermal stability of the O-rich, less thermally stable fraction of charcoal. This fraction decreased over time of cultivation, leading to a relative accumulation of the thermally most stable fraction of charcoal. This might result from the preferential loss of the O-rich fraction or the slowdown of charcoal from oxidation via association with minerals. Our results highlight the idea that land use significantly affects the properties of BC through the modification of soil conditions, which might influence the kinetics of BC loss from soil.

Black carbon (BC), the solid residue of the incomplete combustion of biomass and fossil fuels, is pervasive in soil and contributes to critical soil functions. Therefore, soil amendment with artificial BC (biochar) is more and more regarded as a means to mitigate greenhouse gas emissions while improving sustainably soil fertility. Nevertheless, the long-term fate of BC in soil is still poorly understood. In Wallonia, Belgium, intensive in situ charcoal production affected a main fraction of the forested area in the late 18th century. The topsoil of pre-industrial charcoal kiln sites is largely enriched with charcoal residues, which offers the opportunity to evaluate the long-term effect of BC on temperate soil properties. We sampled the soil of 40 pre-industrial charcoal kiln sites from forest and cropland. Soil properties were compared to that of adjacent reference soils, unaffected by charcoal production, and to soil characteristics at a currently active kiln site to trace the evolution of soil properties over time. We also studied the properties of charcoal particles extracted from soil along a chronosequence of land use change from forest to cropland. Dynamic thermal analysis and chemical oxidation were used to investigate the stability of aged charcoal particles and to discriminate between charcoal and uncharred soil organic matter (SOM) in the soil of charcoal kiln sites. The content of charcoal and uncharred SOM was related to the availability of nutrients, acidity, cation exchange capacity and microbial activity, biomass and community structure of soil. Overall, we highlighted that the features of charcoal and its effect on soil properties evolve over time and depend on soil conditions. BC is very persistent in soil, therefore, long-term implications of soil amendment with biochar should be further addressed before allowing its large-scale application., (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2017

Land use change (deforestation) has several negative consequences for the soil system. It is known to increase erosion rates, which affect the distribution of elements in soils. In this context, the crucial nutrient Si has received little attention, especially in a tropical context. Therefore, we studied the effect of land conversion and erosion intensity on the biogenic silica pools in a subtropical soil in the south of Brazil. Biogenic silica (BSi) was determined using a novel alkaline continuous extraction where Si = Al ratios of the fractions extracted are used to distinguish BSi and other soluble fractions: Si = Al > 5 for the biogenic AlkExSi (alkalineextractable Si) and Si = Al < 5 for the non-biogenic AlkExSi. Our study shows that deforestation can rapidly (< 50 years) deplete the biogenic AlkExSi pool in soils depending on the slope of the study site (10–53 %), with faster depletion in steeper sites. We show that higher erosion in steeper sites implies increased accumulation of biogenic Si in deposition zones near the bottom of the slope, where rapid burial can cause removal of BSi from biologically active zones. Our study highlights the interaction of erosion strength and land use for BSi redistribution and depletion in a soil toposequence, with implications for basin-scale Si cycling.

Incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions, which has implications for Fe export from soils. This study uses Fe isotopes to assess the processes controlling Fe export in a range of Icelandic soils including peat soils derived from the same parent basalt, where Fe isotope variations principally reflect differences in weathering and drainage. In poorly weathered, well-drained soils (non-peat soils), the limited Fe isotope fractionation in soil solutions relative to the bulk soil (D57Fesolution-soil = 0.11 ± 0.12‰) is attributed to proton-promoted mineral dissolution. In the more weathered poorly drained soils (peat soils), the soil solutions are usually lighter than the bulk soil (D57Fesolution-soil = 0.41 ± 0.32‰), which indicates that Fe has been mobilised by reductive mineral dissolution and/or ligand-controlled dissolution. The results highlight the presence of Fe-organic complexes in solution in anoxic conditions. An additional constraint on soil weathering is provided by Si isotopes. The Si isotope composition of the soil solutions relative to the soil (D30Sisolution-soil = 0.92 ± 0.26‰) generally reflects the incorporation of light Si isotopes in secondary aluminosilicates. Under anoxic conditions in peat soils, the largest Si isotope fractionation in soil solutions relative to the bulk soil is observed (D30Sisolution-soil = 1.63 ± 0.40‰) and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation. Si supersaturation in solution with respect to amorphous silica is reached upon freezing when Al availability to form aluminosilicates is limited by the affinity of Al for metal–organic complexes. Therefore, the precipitation of amorphous silica in peat soils indirectly supports the formation of metal–organic complexes in poorly drained soils. These observations highlight that in a scenario of decreasing soil drainage with warming high latitude pea

Black carbon (BC) plays an important role in terrestrial carbon storage. Nevertheless, the effect of cultivation on long term dynamics of BC in soil has been poorly addressed. To fill this gap, we studied the chemical properties of charcoal particles extracted from preindustrial kilns in Wallonia, Belgium, along a chronosequence of land use change from forest to agricultural soil, up to 200 years of cultivation. Preindustrial charcoal samples were compared with charcoal subjected to short term ageing in a currently active kiln. Cultivation increased the association of charcoal with soil minerals, which is favored by deprotonation of carboxylic acids under liming, thereby enhancing the reactivity of charcoal toward mineral surfaces. The large specific surface area of charcoal, related to its porosity, promotes the precipitation of 2:1 phyllosilicates and CaCO3. Both ageing and cultivation decreased the resistance of charcoal to dichromate oxidation, related to an increase in the H/C of charcoal. Differential scanning calorimetry revealed the presence of three fractions of distinct thermal stability. Saturation of carboxylate groups with Ca2+ under liming decreased the thermal stability of the O-rich, less thermally stable fraction of charcoal. This fraction decreased over time of cultivation, leading to a relative accumulation of the thermally most stable fraction of charcoal. This might result from the preferential loss of the O-rich fraction or the slowdown of charcoal from oxidation via association with minerals. Our results highlight the idea that land use significantly affects the properties of BC through the modification of soil conditions, which might influence the kinetics of BC loss from soil.