Mineral control of carbon storage in Andisols
a case study in Hawai’i and applications

Margaret S. Torn

Andisols are characterized by high organic matter content and the abundance of meta-stable minerals such as allophane and imogolite. Soil minerals are known to stabilize soil organic carbon, but how specific types of minerals contribute to spatial and temporal variation in the quantity and turnover of long residence-time organic carbon is not well known.  Although Andisols are not widespread, they may provide important insights into mechanisms of long-term stabilization in soils. The volcanic soils of Hawaii provide a natural laboratory for studying the influence of soil minerals on carbon storage and turnover. We evaluated the interactions between soil mineralogy and soil organic carbon using radiocarbon analysis along two natural gradients, of soil-age and of climate, on the Hawaiian Islands. The two gradients provided systematic variation in mineral composition while keeping other factors that influence soil carbon relatively  constant. During the first ~150,000 y of soil development, the volcanic parent material weathered to meta-stable, non-crystalline minerals; thereafter, the amount of non-crystalline minerals declined and more stable crystalline minerals accumulated. Soil organic content varied in parallel, accumulating to a maximum at 150,000 y, and then decreasing by 50% at a 4.1 million-year site. The accumulation and subsequent loss of organic content were largely driven by changes in millennial-cycling, mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. The specific processes of stabilization and not known, but preliminary ¹³C data are consistent with hypotheses that allophane selectively protects plant and microbial lipids.

An indirect effect of Andisol development and mineralogy is strong gradients in availability of phosphorus and other nutrients. We investigated the influence of nutrients and mineralogy on decomposition of shallow soil organic matter along the same age-gradient of rainforest sites in Hawaii, in which nutrient availability varies systematically. Turnover times of slow-pool SOM ranged from 6 to 20 years in the O horizon, and were roughly two times slower in the A horizon. Turnover times at the different sites varied by two- or three-fold even in soils with the same climate, vegetation community, and, in the case of the O horizon, soil texture. Turnover was correlated with soil fertility, as indicated by plant and soil nutrient levels, which was ultimately controlled by soil mineral development. Thus, both rapidly-cycling and long-term soil organic carbon may be influenced by Andisol development. 
We conclude that soil mineralogy in Andisols is an important factor determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere-ecosystem carbon fluxes during long-term soil development. While the influence of non-crystalline (amorphous, or meta-stable) minerals on long-term carbon storage is most easily observed in Andisols, these minerals likely also contribute to carbon storage in soils developing on common parent materials (such as granite, siltstone, sandstone, and basalt) that contain feldspar, olivine, pyroxene, or glass. Comparing mineral control of carbon storage in two non-andisol chronosequences with the mineral relationships derived in Hawaii shows that the influence of certain minerals may be generalizable. Understanding the influence of specific mineral groups allows the insights gained from studying Andisols to be applied to other soil development sequences that are more common globally.