CATION EXCHANGE CAPACITY OF TROPICAL SOIL CLAYS AS A FUNCTION OF TIME AND PRECIPITATION

Cation exchange capacity (CEC) of the clay fraction of three soil chronosequences in the humid tropics (Pacific coast of Costa Rica) decreases exponentially with age from ~70 cmol_c/kg in < 2 ka soils to values ranging from 1 to 20 cmol_c/kg in > 35 ka soils. The decrease in CEC is associated with the temporal transformation of pedogenic smectite (dominant mineral in < 2 ka soils) to interstratified kaolinite-smectite (K-S), halloysite and kaolinite in early Holocene to late Pleistocene soils, a transition which is driven by the intense leaching of base cations and Si from soils in the humid tropics. Superimposed on the temporal control on CEC is regional variation in mean annual precipitation (MAP) — weathering reactions in an Inceptisol to Oxisol sequence on the Osa Peninsula (5000 mm/year MAP) were found to occur at a rate at least three times greater than in an Inceptisol to Alfisol sequence on the less rainy Nicoya Peninsula (2200 mm /year MAP). Both sequences are characterized by very young (< 2 ka) smectite-dominated clay fractions with CEC in the range of 60 – 80 cmol_c/kg. Over time, rainier Osa Peninsula soil clays attain very low CEC of 1 – 2 cmol_c/kg after only 35 ka of pedogenesis, whereas drier Nicoya Peninsula soil clays reach a CEC in the range 11 -16 cmol_c/kg after 125 ka. XRD and TEM-AEM analysis shows that 35 ka Osa soils are dominated by pedogenic kaolinite whereas Nicoya soils are a mix of interstratified K-S, halloysite and kaolinite — the mineralogical and CEC differences are due to less-intense leaching at Nicoya and slower transformation of smectite layers to kaolinite. The persistence of expandable layers in K-S and halloysite with higher CEC than kaolinite facilitates higher CEC. On a methodological note, CEC was determined for samples and reference clays using 1 N NH₄OAc extraction and cation analysis by ICP-OES, and this method is accurate for smectite- and kaolinite-dominated soils provided that no carbonate minerals are present. TEM-AEM analysis of Ca-saturated clays produce CEC values that agree well with literature values for smectite-dominated samples, but overestimate CEC for kaolinite-dominated samples, perhaps due to issues related to sample preparation. Results of this study have implications for nutrient cycling in humid tropical clay soils and also for accurate determination of CEC in clay-rich soils.