Tropical rainforests play a major role in the global carbon cycle, even though their net primary productivity can be limited or co-limited by availability of cations (rock-derived, positively charged ions). For mature forests, ‘tight’ cation cycling, i.e., without leaching losses, can maintain biomass cation stocks. This mechanism cannot explain how tropical secondary forests regenerating on soils depleted in available cations can accumulate large biomass cation stocks, however. We propose a conceptual framework that integrates tree species traits with soil biogeochemical processes to describe how differences in species traits could result in differential alteration of soil pH, soil cation availability, and cation uptake and retention in plant biomass. We addressed aspects of this framework in a unique setting: a randomized block experiment in which climate, soil, and previous land-use history were similar for four native tree species grown in mono-dominant plantations for 25 years on an Oxisol. The four species differed in multiple traits related to biogeochemistry. Thus, we hypothesized that soil pH would differ among species. Soil pH is a ‘master’ soil biogeochemical driver that affects cation exchange capacity and alters colloid aggregation and dispersion, thereby providing a mechanism for releasing cations that become occluded during pedogenesis. We hypothesized that cation stocks accrued in biomass would differ among species and correlate positively with soil pH. We evaluated effects of several plant traits on cation accrual in biomass.
Initial surface-soil pH was 4.5; after 25 years, pH ranged from 4.08 to 4.85, under the nodulated legume Pentaclethra macroloba and the Al-accumulating Vochysia guatemalensis, respectively. This range corresponds to a five-fold difference in proton concentrations, which is sufficient to cause variability in dispersion/aggregation of organo-mineral colloids. Despite low soil stocks of exchangeable mineral nutrients, accumulation in biomass reached 10, 53, 88, 307, 540, 950, and 1562 kg/ha of Sr, Mn, Fe, Mg, Ca, K, and Al respectively. Cation stocks in biomass were lowest in Pentaclethra, which acidified soil, and highest in Vochysia, which de-acidified soil. Stocks differed among species by 1.7-, 1.9-, 2.8-, 2.9-, 3.1-, 3.5-, and 17.2-fold for iron (Fe), calcium (Ca), potassium (K), manganese (Mn), strontium (Sr), magnesium (Mg), and aluminum (Al), respectively. Driven by plant traits related to biogeochemistry, these plant-soil interactions result in large species-specific differences in cation biomass stocks. Our results indicate that soil pH represents a factor that integrates the effects of multiple plant traits on geochemistry, with clear relevance for cation availability and ecosystem function.