Soil Changes in Model Tropical Ecosystems: Effects of Stand Longevity Outweigh Plant Diversity and Tree Species Identity in a Fertile Volcanic Soil

Plant or community longevity can strongly influence soil fertility, yet it is seldom among the functional traits considered in studies of biodiversity and ecosystem functioning. For 11 years we tracked the influences of plant longevity, life-form richness, and tree species identity on 12 soil chemical properties in model ecosystems on an allophanic Andisol in the humid lowlands of Costa Rica. The design employed three levels of plant longevity: 1 year and 4 years (trees cut without biomass removal and replanted to same species), and uncut; two levels of life-form diversity (tree alone, or tree plus palm plus giant perennial herb); and three eudicot, non-nitrogen (N)-fixing tree species. The site's Andisol proved remarkably resistant to treatment-induced loss of fertility. Although the magnitude of changes was low, most properties declined during the early phases of plant growth, then stabilized or increased. The greatest declines occurred in stands of shortest life span, where organic matter inputs were low and leaching rates were high. In contrast, massive depositions of organic matter every 4 years sustained or augmented surface-soil cation concentrations, pH, organic carbon (SOC), and extractable phosphorus (P). An increase in diversity from one life form to three led to more SOC and calcium (Ca), whereas potassium (K) decreased due to a species effect: high K uptake by the giant herb. The most notable tree-species effects concerned P: It increased under the species that had the highest litterfall and may facilitate apatite weathering; it decreased under the species of highest tissue-N concentrations. Through its effects on soil exposure and organic matter returns, plant longevity exerted greater influence on more soil properties than either diversity or species identity.