Humid tropical forests contain some of the largest soil C stocks on Earth, much of which likely originates from similarly large root biomass stocks. The influence of rooting depth profiles on soil C profiles is of particular interest in the context of long-term, stable subsoil C storage. However, climatic and edaphic drivers of variation in rooting depth profiles across tropical forest gradients, and relationships among vertical profiles of nutrients, roots, and soil C, are poorly understood for this biome. To address this, we measured the depth distributions of fine roots in relation to nutrient and organic C profiles in the soils of 43 lowland humid tropical forests in central Panama spanning rainfall and soil gradients, including morphological root traits for a subset of sites.
Results/Conclusions
The rooting depth profiles, characterized by fitted β parameters, varied markedly across sites, and were best predicted by soil order, pH, and total extractable potassium (K+) to 1 m depths, with the three most acidic (pH < 4) and K-poor soils (5 – 20 g-K/m2) having 76 ± 5 % of root biomass in the upper 10 cm of the profile, contrasting with only 41 ± 9 % of root biomass in the upper 10 cm in the three most neutral (pH 6 – 6.8), K-rich soils (50 – 300 g-K/m2). Soil C profiles had an opposite depth-response to fertility, with the most base-cation-poor soils having larger proportions of soil C below 50 cm depths (34 ± 8 %, n = 3) relative to base-cation-rich soils (9 ± 2 %, n = 3). Nutrient depth distributions generally did not correspond to either root biomass or soil C stocks through the profiles, with fertile soils having relatively large proportions of nutrients below 50 cm depths, and more even nutrient depth distributions in infertile soils. These results suggest that changes in surface root biomass, as may occur with global change, could affect deep soil C storage, particularly in strongly weathered, infertile tropical soils that are characteristically C-rich.