Fjölrit RALA - 15.06.2004, Page 123

Acid neutralizing capacity of Andosols:
Effects of weathering stage and sulfur storage
T. Delfosse, P. Delmelle and B. Delvaux
Soil Science Unit, Université catholique de Louvain, Belgium
Soil acidification remains an important environmental issue because it alters soil and water
properties, and thus potentially modifies current processes in whole ecosystems. The capacity
of a soil to neutralize acid inputs depends mainly on reactions involving mineral weathering
and ion exchange, therefore the acid neutralizing capacity of the soil solid phase (ANCs) has
been successfully used to evaluate the degree of soil acidification. However, despite their
agronomical importance, the response of Andosols to increased atmospheric acid deposition
has been poorly investigated.
Here, we report on the effects of (natural and experimental) elevated acid and sulfur
depositions on ANCs in two distinct Andosols transects located downwind from Masaya
volcano (Nicaragua), one of the world’s strongest sources of SO2. The first transect comprises
Eutric Andosols rich in allophanic constituents, and the second involves Vitric Andosols rich
in volcanic glass.
Prolonged acid inputs have led to a general pH decrease and a reduction in exchangeable
base cation concentrations. However, the ANCs was not significantly affected by the
volcanogenic acid inputs. Non-exchangeable (mineral reserve) and exchangeable cations, total
contents of sulfur and phosphorus dictate most of the ANCs variation. In the Vitric Andosols,
mineral reserves contributed up to 97 % to these four additive pools, whereas the
exchangeable cations accounted for 1-4 %. In the Eutric Andosols, the contribution of mineral
reserves was comparatively smaller (71-92 %), but the content of exchangeable cations was
larger (1-20 %), whereas the contribution of soil sulfur was substantial (1-15 %). Acid
leaching column experiments indicate preferential A1 and Si lixiviation in Vitric Andosols,
and preferential base cation hxiviation concomitantly with anion accumulation in Eutric
Andosols. These results indicate that the main process involved in neutralization of acid
inputs is mineral weathering in Vitric Andosols, and cation and anion exchange in Eutric
Andosols.
Despite higher ANCs of Vitric compared to Eutric Andosols, soil pH was smaller in Vitric
than in Eutric Andosols because the reactions involved in the regulation of volcanic acid flux
are kinetically different, ion exchange being much faster than mineral weathering. This
observation emphasizes the importance of the acid-buffering capacity compared to mineral
weathering of soils to resist rapidly to acid deposition.
Prolonged addition of volcanogenic S increased the inorganic sulfate content of the
Andosols up to 5.5 g S kg"1, one of world’s largest soil S content. Inorganic sulfate was stored
mainly in the clay fraction, and composed of three pools: adsorbed on short-range ordered
minerals, occluded SO42 in ferrihydrite, and precipitated in Alx(0H)y(S04)z mineral. The fate
of inorganic sulfate in soils may thus have two conflicting effects: SO42" retention contributes
to acid-buffering through OH" release or H+ consumption, but also results in ANCS depletion
because it stores SO3, the release of which would lead ultimately to sulfuric acid production.
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