A 200-year snapshot of soil development in pyroclastic deposits derived from the 1815 super explosive eruption of Mount Tambora in Indonesia

Early-stage pedogenic processes and formation rates on completely obliterated volcanic landscapes, such as the
super explosive 1815 Mount Tambora eruption, have not previously been robustly explored. The objectives of
this study were to determine (i) the mineralogical composition of the sand fraction, selected physical and
chemical soil properties and potential nutrient reserves after 200 years of pedogenesis, and (ii) chemical
weathering indices, rate of soil formation, and rates of C and N accretion. Soil formation was examined for five
soil profiles on stable plain/foot slope positions representing the diversity of soils in these landscape positions,
which are important for agricultural production. Results showed that the soil mineralogical composition of the
sand fraction was dominated by easily weatherable minerals (e.g., labradorite and augite volcanic glass) indi
cating high potential nutrient reserves (e.g., Ca2+, Mg2+, K+, P) as confirmed by X-ray fluorescence (XRF) an
alyses. Allophanic material formation was minimal (<2.3%) owing to the preferential accumulation of Al3+into
Al-humic complexes. The low contents of allophanic materials and metal-humus complexes resulted in low P-
retention (17.5–43.4%) within the soil solum, with the highest value in surface horizons (33–43%). Morpho
logical features showed rapid solum (A +B horizons) development of 22 to 107 cm. Shallow soils occurred on
shallow eruption deposits (consisting of pyroclastic flow underlain by pumice), whereas the deepest soils were
found on thick deposit consisting of either trachyandesite pyroclastic or basaltic andesite materials. The chemical
index of alteration (CIA) followed the order of A horizons (48.4 ±4.6) >B horizons (45.4 ±2.4) >C horizons
(43.8 ±4.2) ≈tephra/lava (43.1), indicating accumulation of Al oxides and depletion of base cations in the
upper horizons. Similarly, the base depletion index (BDI) showed a trend of A horizons (1.13 ±0.18) <B hori
zons (1.25 ±0.09) <C horizons (1.34 ±0.20) ≈tephra/lava (1.35), indicating depletion of base cations (Ca2+,
Mg2+, K+, Na+) from the soil surface to C horizons. Based on solum depths (A +B horizons) and 200 years of
post-eruption soil development, the solum formation rate ranged from 1.2 to 5.3 mm yr 1. Appreciable stocks of
SOC (2.3–12.8 kg C m2) and SON (0.21–0.77 kg m2) accumulated over the 200 year period. Eruption materials
from the Mt. Tambora eruption with a precisely known timescale contributed new pedological insights doc
umenting rapid soil formation rates from pyroclastic materials leading to a rapid recovery of soil functions to
support agricultural production.