The impact of climate on the biogeochemical functioning of volcanic soils

Abstract

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. Along an arid to humid climosequence on Kohala Mountain, Hawaii, we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall. The lavas 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. A temperature decline from 24 to 15 °C with increasing elevation is matched by lower potential evapotranspiration (ET). A water balance model (monthly precipitation minus monthly ET) defines three broad climate zones along the sampling transect: an arid zone with moisture deficit in every month, an intermediate zone with moisture deficit during low-rainfall summer months and moisture surplus during high-rainfall winter months, and a humid zone with moisture surplus during every month. The annualized water balance can be ratioed with the integrated porosity of 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 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 10–20% of the original base cations and Si and none of the Al. At all sites, the secondary clay mineral assemblage is dominated by metastable noncrystalline 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 influenced strongly by climatic conditions and show a dramatic threshold in base cation saturation, pH and effective cation exchange capacity (ECEC) at leaching index of 1 (1400 mm MAP). Soils with leaching index of <1 have high base cation saturation, near-neutral pH and high ECEC. At MAP >1400 mm, soil buffering capacity has been totally exhausted leading to low pH and low ECEC. The nonlinear 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. There is a time-dimension implied in climate gradient studies; soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall. Loss of buffering capacity means that biological acidity will move more deeply 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.