Soils are open systems that act as a membrane at Earth’s surface. Water and dissolved
acids are the main materials transferred into soils, whereas water and lithogenic solutes dominate
the output with the net result being depletion of rock forming constituents such as silica and base
cations. Weathering and pedogenesis are biogeochemical processes that are constrained by
thermodynamics, but still maintain considerable flexibility as a result of parallel reaction kinetics
in a spatially heterogeneous matrix. Not surprisingly, many processes are governed by nonlinear
response to changes in environmental variables and/or internal soil properties. In fact, the
chemistry of pedogenesis is characterized by a number of thresholds. Simultaneous acid-base,
ion exchange, redox and mineral transformation reactions interact to determine the direction and
rate of change, but we cannot predict well the environmental conditions that lead to thresholds.
Here we focus on the role of soil as a pH buffer and environmentally driven thresholds
associated with temporal loss of buffering capacity within the soil.
Over time, the reaction of atmospheric acids with soil bases changes the acid neutralizing
capacity of soil; the amount of buffering reaction and its effect on pH depend on the nature of the
reactive species, their relative amounts, and their respective rates of reaction. Ion exchange and
surface complexation reactions consume protons in the short term but long-term buffering
derives from mineral weathering. Soils are well buffered to pH change in the alkaline and acid
regions but far less so in the neutral to slightly acid zones where primary mineral weathering is
the major source of acid consumption. We demonstrate that lithology, climate and landform age
combine in a nonlinear fashion to determine the amount of primary mineral depletion and loss of
buffering capacity. Rainfall and the amount of water available to leach ions from soil are among
the most important features determining mineral weathering, secondary mineral synthesis and
soil chemical properties.
In Hawaii, soil acidification is controlled by climate and lava flow age. Hawaii’s
environment provides the opportunity to compare soil behavior on the constructional surfaces of
lava flows of different ages as influenced by different rainfall regimes defined by location of the
flows relative to the prevailing trade winds. Here we report on four climosequences collected on
lava flows with the following ages: 10 ka, 170 ka, 400 ka, 4100 ka. I will first present soil buffer
results from the 170 ka climosequence sampled on Kohala Mountain and use those data as a
touchstone to compare the climosequence results from younger and older lava flows.
On Kohala Mountain, Hawaii, we found that weathering and soil properties change in a
non-linear fashion with increased rainfall. The lava soils are influenced by a strong rain shadow
with mean annual precipitation (MAP) averaging 160 mm near the coast and rising to > 3000
mm near the summit. The temperature decline from 24 to 15 C with increasing elevation is
matched by a decline in evapotranspiration (ET). A water balance model (monthly precipitation
minus monthly ET) is combined with the porosity in the top meter of soil to provide a leaching
index. The index reaches 1 (total filling of the pore space on an annual basis) at about 1400 mm
MAP. Index values > 1 imply intense leaching conditions because of pore water displacement
and replacement. In these 170 ka soils leaching losses of soluble base cations and Si are nearly
complete at index values > 1, whereas only 60% of Al has been lost. At index values < 1
leaching losses are progressively lower with the lowest rainfall sites having lost 10 to 20% of the
original base cations and Si and none of the Al. At all sites, the secondary clay mineral
assemblage is dominated by metastable non-crystalline weathering products; humid soil profiles
contain very few crystalline minerals whereas the arid profiles contain halloysite, hematite,
gibbsite and small amounts of carbonates. Soil surface exchange properties are strongly
influenced by climatic conditions and show a dramatic threshold in base cation saturation, pH,
and effective cation exchange capacity (ECEC) at leaching index of 1. Soils with leaching index
of < 1 have high base cation saturation, neutral pH and high ECEC, whereas those that are > 1
have lost their buffering capacity leading to low pH and low ECEC.
The non-linear decline in ECEC is irreversible under natural conditions; base cation
depleted soils will remain so even if the climate shifts to drier conditions. In contrast a climate
shift to wetter conditions can drastically modify surface chemical properties existing in the drier
soils as weathering depletes primary minerals, elements are lost to leaching and surface
chemistry is modified. Loss of buffering capacity through leaching leads to a transfer of
biological acidity deeper into the vadose zone or into the aquatic system. The details of this
transfer depend on present and past climate, and the age and erosional stability of landscapes.
The results from Kohala Mountain suggest an age dependency to the location of the
threshold in buffering capacity in rainfall space. We can test this idea by comparing soil
properties under differing rainfall regimes on younger and older lava flows. Whereas there is a
threshold at about 1400 mm of rain on Kohala, the threshold occurs at about 1700 mm of rain on
a 10 ka lava flow in Kona, at about 1000 mm of rainfall on a 400 ka lava flow on Maui, and on a
4,100 ka lava flow on Kauai soils under rainfall regimes as low as 500 mm have lost their acid
buffering capacity to a pH < 4.5. For each climosequence there is a direct relationship between
weathering and leaching losses of base cations and the location of the threshold. The progressive
decline in the rainfall amount at which the threshold occurs is due to the progressive depletion of
primary minerals and loss of base cations from the top meter of the soil. We conclude that over
time buffering of acidity is transferred deeper into the vadose zone and away from the soil
proper. Likely too, more of the hydrologic flow paths will intersect the fluvial system before the
acidity as been neutralized. Thus one consequence of landscape stability is a greater potential for
acid impact on downstream ecosystems.