Volcanic ash soils are generally recognized as soils with excellent and stable physical properties. Here we characterized the porosity and water properties of volcanic ash Andosols and Nitisols from Guadeloupe in contrasting banana systems: (1) perennial crop without mechanization, (2) mechanized and regularly replanted crop. Desiccation from1 kPa to1550 kPa moisture tension leads to significant shrinkage in the Andosol, representing a 50% reduction of the void space. The clayey Nitisol exhibited limited shrinkage. Soil clods from the mechanized plots had a significantly smaller macroporosity than that from perennial plots. The soil hydraulic conductivity was also drastically reduced in the compacted layers of the mechanized plots. However, Nitisols appeared to be less affected than Andosols. Laboratory compression tests showed that both soils were susceptible to compaction at soil moisture close to field capacity.The shrinkage properties of the Andosol were due to microaggregation of non-crystalline components upon drying.The relative stability of the macroporosity in the Nitisol was probably related to the presence of stable microaggregates made of halloysite and iron oxide.Two major processes promote soil structure degradation in the Andosol under mechanized banana cropping, surface desiccation and soil compaction.They are both induced by repeated tillage after clearing.

Aims Supplying phytolith-rich biochar in agrosystems increases soil pH, CEC and nutrient availability, adding to the impact of Si uptake on plant growth. Here we studied this specific impact as influenced by soil properties, and assessed the role of phytoliths to provide plant available Si. Methods We used a young Cambisol and a highly weathered, poorly buffered, desilicated Nitisol. The biochars were produced from rice plants respectively enriched (Si+) and depleted (Si-) in Si. They had identical pH and nutrient contents, but largely differed in Si content (51.3 g Si kg −1 in Si + vs 0.3 g Si kg −1 in Si-). We compared their effects to that of wollastonite (CaSiO 3) on the biomass and mineralomass of wheat plants in a soil:solution:plant device. The contents of soil bioavailable Si and biogenic Si were assessed through an original CaCl 2 kinetic extraction and the DeMaster Na 2 CO 3 alkaline dissolution, respectively. Results The DeMaster technique dissolved Si from phytolith as well as from wollastonite. The soil buffering capacity (cmol c kg −1) was 31 in the Cambisol and 0.2 in the Nitisol. An identical supply of phytolithic biochar increased pH from 4.5 to 4.8 in the Cambisol, and from 4.8 to 7.4 in NI. It further increased the content of bioavailable Si (from 55 to 97 mg kg −1 in the Cambisol, and 36 to 209 mg kg −1 in the Nitisol), as well as plant Si uptake, biomass and Si mineralomass. That increase was largest in the Nitisol. Conclusions The DeMaster technique did not specifically quantify the phytolith pool. This pool was the main source of plant available Si in both the Cambisol and Nitisol amended with phytolithic biochar. At identical phytolithic Si supply, however, soil pH and soil buffering capacity controlled the transfer of Si in the soil-plant system, which was largest in the poorly buffered Nitisol. The effect of phytolithic biochar on Si bioavailability was depending on soil constituents and properties, and thus on soil type.

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Silicon (Si) is beneficial to plants since it increases photosynthetic efficiency, and alleviates biotic and abiotic stresses. In the most highly weathered and desilicated soils, plant phytoliths make up the reservoir of bioavailable Si. The regular removal of crop residues, however, substantially decreases this pool. Si supply may therefore be required to sustain continuous cropping. Available Si fertilizers are costly and usually poor in soluble Si. Biochar produced from the pyrolysis of phytolith‐rich biomass is thus a promising alternative Si source for plants. Taking into account the challenges of increasing food demand and environmental concerns, we evaluate the global potential of biochar produced from major crop residues and manures in terms of phytogenic Si (PhSi) supply. Crop residues contribute to 80% of the global production of biomass dry matter (8,201 Tg/year) of which 3,137 Tg/year are potentially available after pyrolysis, giving a potential application rate of 1.7 T ha−1 year−1 for highly weathered soils in the tropics. The potential PhSi supply from crop biochar amounts to 102 Tg Si/year. On its own, rice straws produce 57.7 Tg PhSi/year, accounting for 56.6% of the potential annual PhSi production. The Si release from crop biochar depends on inter altere feedstock type, pyrolysis temperature, soil pH, and buffer capacity. Furthermore, the amplitude of plant Si uptake and mineralomass depends on plant species, soil properties, and processes. These factors interact and can exert a decisive influence on the effectiveness of phytolithic biochar in releasing Si into highly weathered soils. We conclude that the use of phytolithic biochar as a Si fertilizer offers undeniable potential to mitigate desilication and to enhance Si ecological services due to soil weathering and biomass removal. This potential must be explored, as well as the conditions for using biochar in the field.

A B S TRACT:Six clays from volcanic ash soils at different stages of weathering differ in their relative halloysite content with respect to kaolinite and several surface properties, namely CEC, and exchange selectivity for K +. These three parameters are related to each other in that they all decrease with increasing soil weathering stage. XRD data show that the hydrated 1 : 1 layer-silicates in these clays combine with smectite to form interstratified H/Sm clay minerals. In these mixed-layers, the content and layer charge of smectitic units decrease as the relative halloysite content in the clay decreases. These clays thus depict a weathering sequence that is parallel to the weathering sequence of the soils from which they originate. It is also shown that the smectites in the H/Sm minerals have the distinctive composition and ESR spectrum of Fe-rich 2:1 clay minerals belonging to the beidellite-nontronite series. The information obtained explains why these clays have high CEC and distinct affinities for K + . It is hoped that this study will help to clarify the controversy concerning the CEC and related surface properties attributed to hydrated halloysite.