Climatic and Lithogenic Controls on Soil Organic Matter-Mineral Associations

Abstract

Interactions of organic matter (OM) with soil mineral phases strongly affect the storage and dynamics of soil OM as well as other ecosystem processes. This study examined aspects of organo-mineral associations in soils at different scales. First, I assessed the potential controls of climate and parent rock type on organo-mineral associations using two sets of undisturbed tropical forest soils developed on two contrasting rocks along an altitudinal gradient in Borneo, Southeast Asia. Density fractionations showed that OM stored in surface mineral soils partitioned towards plant detritus fraction under cooler climates on both rock types. Thus climate exerted stronger control on soil OM storage and partitioning patterns than parent rock in the study area. The plant detritus associated with soil mineral grains also increased its standing stock under cooler climates, suggesting that abundance of mineral-free detritus and its comminution had stronger control than soil mineralogical factors. Second, gas sorption approaches were applied to the same sets of soils to assess OM associations with soil mineral surfaces. Surface characterization before and after OM removal revealed that, with increasing altitude and OM loading, OM appear to accumulate in globular forms that incidentally encapsulate fine mineral grains, rather than accumulating via sorption onto all mineral surfaces. Similar control of soil OM loading on the organo-mineral arrangements was found in soils of different geographic areas and soil types (n = 33), suggesting much wider generality of this relationship. Third, I examined the importance of hydrous iron oxides (FeOx), a common soil mineral phase known to have strong sorptive capacity, for soil OM storage using a wider range of mineral soils spanning eight soil orders. With a modified selective FeOx dissolution method, I achieved the first quantification of the organic carbon (OC) that can be released from FeOx phases. Iron-bound OC accounted for only minor fractions of total soil OC (mean: 11%, range: 0-37%), indicating limited capacity of FeOx to sorptively store bulk of soil OM. The mass ratios of OC to iron (0C:Fe) of the extracts in some low pH, organic samples (e.g., spodic horizons) implied the presence of organo-iron complexes rather than adsorbed forms