Calcium Supply Modulates Growth, Photosynthesis, and Rhizosphere Soil Carbon and Nitrogen Dynamics in Four Tropical Woody Plant Seedlings

Calcium (Ca) is an essential mineral nutrient for plant growth and reproduction and plays a critical, yet often overlooked, role in soil organic matter dynamics in forest ecosystems. However, the mechanisms by which Ca supply simultaneously regulates plant growth, photosynthesis, and rhizosphere carbon (C) and nitrogen (N) dynamics still remain poorly understood. In this study, growth and photosynthetic responses of four tropical woody plant seedlings (Hibiscus elatus Sw., Heteropterys laurifolia (L.) A. Juss., Adenanthera pavonina L. (a nodulating legume), and Cassia javanica L.) to a gradient of Ca supply ranging from 0 to 20 mmol·L−1, with 4 mmol·L−1 (the standard Hoagland concentration) as the reference 100% were investigated after three months of pot cultivation in a greenhouse. In a parallel experiment using the same Ca supply gradients with plant-free controls, we determined soil C and N fractions in both the rhizosphere (planted pots) and the plant-free control soil. The results of two-way analysis of variance (ANOVA) revealed significant main effects of Ca supply and plant species on all determined parameters (p < 0.001), with significant interactions for most parameters. Most growth and photosynthetic parameters exhibited a unimodal “low- and high-inhibition, optimal-promotion” response to increasing Ca supply, with peaks occurring at 50%–100% of the standard Hoagland concentration. Root length and root-to-shoot (R/S) ratio were highest under Ca deficiency, reflecting a morphological adaptation of plant individuals to Ca scarcity. All soil C and N fractions also followed the same unimodal trends with most maxima at 100% Ca supply, whereas the turnover time of soil labile organic carbon (TSLC) showed a U-shaped curve. Among species, A. pavonina displayed the greatest rhizosphere C and N accumulation across the gradients, coupled with the shortest TSLC (36.2 ± 4.1 d), likely attributable to its symbiotic N-fixation. H. elatus showed the most pronounced photosynthetic capacity and plant biomass, and the highest soil microbial biomass carbon (SMC, 3.22 ± 0.33 mg·g−1) and total soil N (SMN, 22.38 ± 2.43 mg·g−1) under the optimal Ca supply, representing a “C-driven” strategy. H. laurifolia exhibited a rhizosphere priming effect (RPE) at moderate Ca supply (i.e., 50%), while C. javanica was highly sensitive to Ca excess. Our findings demonstrate that an optimal Ca supply (approximately 4 mmol·L−1) simultaneously enhances plant photosynthesis and rhizosphere C and N accumulation by stimulating root C inputs and microbial activity. The legume A. pavonina and the high-photosynthetic H. elatus are promising candidates for further evaluation in field trials on Ca-rich or Ca-poor tropical soils, respectively.