Tree growth rate regulate the influence of elevated CO₂ on soil biochemical responses under tropical condition

Tree growth rate can complicate our understandings of plant belowground responses to elevated CO₂ (eCO₂) in tropical ecosystems. We studied the effects of eCO₂ on plant growth parameters, and rhizospheric soil properties including soil organic carbon (SOC), glomalin related soil protein (GRSP), microbial biomass C (C_mic), CO₂ efflux (C_efflux), and microbial extracellular enzyme activities under two tropical tree saplings of fast-growing Tectona grandis (Teak) and slow-growing Butea monosperma (Butea). We exposed these saplings to eCO₂ (∼550 ppm) and ambient CO₂ (aCO₂; ∼395 ppm) in the Indo-Gangetic plain region, and further (after 10 and 46 months) measured the changes in their rhizospheric soil properties. With respect to aCO₂ treatment, eCO₂ significantly increased plant height, stem and shoot weight, and total plant biomass of Teak. However, these plant traits did not considerably differed between eCO₂ and aCO₂ treatments of Butea. The eCO₂ induced greater extent of increase in rhizospheric soil properties including SOC fractions (particulate OC, non-particulate OC and total OC), GRSP fractions (easily extractable- GRSP, difficulty extractable- GRSP and total- GRSP), C_mic, C_efflux and extracellular enzyme activities (phosphatase, dehydrogenase, β-glucosidase and fluorescein diacetate) were observed under Teak compared with Butea. Compared with aCO₂ treatment, eCO₂ slightly reduced soil available N and P under the Teak, but no changes were apparent between eCO₂ and aCO₂ treatments of the Butea. The greater extent of responses from soil variables observed after longer period (46 months) of CO₂ exposure. The multivariate analysis confirmed that eCO₂ treatment with Teak is more responsive compared with other treatments of Teak and Butea. This contrasting rhizospheric soil feedback to eCO₂ between two tropical trees, suggesting fast-growing species will be more responsive to future climate. Such species will have a competitive advantage over coexisting less responsive species (e.g. Butea) under future eCO₂ climate.