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Purpose: There is increased recognition of the importance of mangroves worldwide, with efforts being made to sustainably manage these ecosystems for forestry and fishery use. Although successive monitoring of mangrove growth after planting has been conducted in some afforestation stands, measurements of soil environmental changes accompanying plant growth have not been made in most stands. In this study, we observed the interactive relationship between the underground root biomass of mangrove, Rhizophora stylosa, and soil chemical properties at an afforestation stand on Tarawa atoll, Kiribati. Methods: We first estimated underground root biomass in the stand. Next, we measured the concentrations of dissolved phosphorus, nitrogen, and other ions (Br−, Ca2+, K+, Na+, Cl−, and SO42−) in soil pore water, as well as the isotopic ratio of leaf carbon and nitrogen in mangrove patches of different ages. Results: Estimated underground root biomass was positively related with phosphate and nitrate concentrations in soil pore water, indicating the formation of a rhizosphere environment. Leaf δ15N analysis suggested that the discrimination of nitrogen isotopes during nitrification and/or uptake of NH4+ and NO3− occurs in accordance with plant growth. Differences in salt stress among the patches were reflected in leaf δ13C, suggesting it would be a good indicator of the physiological response of mangrove plants to salinity. Conclusions: Our findings revealed the changes that occur on a yearly basis in the chemical properties of mangrove leaves and soil pore water after mangrove plantation. These data help to improve our understanding of environmental succession during the formation of mangrove ecosystems. [ABSTRACT FROM AUTHOR]