Novel oxalate-carbonate pathways identified in the tropical dry evergreen forest of Tamil Nadu, India

Abstract. The tropical dry evergreen forest (TDEF) is a vital but endangered ecosystem in India, crucial for supporting cultural services, biodiversity, and organic carbon storage. The oxalate-carbonate pathway (OCP) is an understudied process in which plants and oxalotrophic microorganisms convert atmospheric CO2 into calcium carbonate (CaCO3) within plant tissues or tree-adjacent soils. Yet, despite its significance, the OCP has not been studied in the TDEF of India. This study aimed to assess novel OCP systems associated with three TDEF diagnostic species (Diospyros ebenum, Lepisanthes tetraphylla, Sapindus emarginatus) and one local agroforestry species (Artocarpus heterophyllus) in the restored- and primary-TDEF of Tamil Nadu. Surface soil samples (0–10 cm) were collected from an adjacent and control distance away from trees, along with tree biomass samples, and investigated for oxalate production (microscopy and enzymatic assays), oxalotrophic microbial communities (frc gene sequencing), and tree-induced shifts in soil biogeochemistry. Oxalate was detected in all species (4.4 ± 3.2 % dry weight), accompanied by CaCO3 precipitation on biomass. Oxalotrophic microbial communities were dominated by Actinomycetota (86 %), which were also identified in electron micrographs. Soil biogeochemical shifts indicative of active OCPs were also observed, particularly in the hollowed-out trunks of the TDEF trees. However, differences between adjacent and control soils were less pronounced, suggesting that monsoon conditions leached OCP precipitated CaCO3 from the adjacent soils. This research provides the first evidence of active OCPs in Indian TDEF, highlighting a previously unrecognized mechanism for organic and inorganic carbon cycling in this threatened ecosystem.