Ecosystem development of coastal forest on planting bases: the role of initial soil properties

Restoring coastal forests damaged by the 2011 Great East Japan Earthquake involved constructing planting bases using transported sandy soil materials. However, these planting bases are often unsuitable for planted black pines, likely due to low availability of plant-available nutrients. Under such conditions, litterfall supplied by planted trees is expected to be a primary driver of nutrient input through decomposition and subsequent accumulation in soils. This study aimed to quantitatively evaluate ecosystem development and nutrient accumulation associated with planted tree growth and litter inputs, and to elucidate the initial conditions influencing the development of vegetation–soil feedbacks within planting bases. Three sites exhibiting contrasting growth performances were selected at a coastal forest restoration site in Natori City, Miyagi Prefecture, where soil physico-chemical properties, annual litterfall inputs, and nitrogen stocks in the litter layer and mineral soil were analyzed. The results indicate that soil physical and chemical constraints of planting bases strongly influence the direction of vegetation–soil feedbacks. Sites with lower soil hardness, better aeration, and less severe acidity promoted a ‘positive feedback’ loop, supporting root development and stable litter retention, which led to litter layer formation and nitrogen accumulation in the soil. Nitrogen stocks in the litter layer were 9.23, 4.06, and 0.56 g N m−2 at the high-, moderate-, and low-growth sites, respectively, and total N stocks in the litter layer and 0–10 cm mineral soil decreased in the same order, from 78.10 to 61.73 and 37.02 g N m−2. Conversely, the site with poor growth was characterized by severe soil compaction, poor aeration and drainage, and strong acidity. Despite substantial litterfall inputs, this site failed to retain litter or accumulate nitrogen, likely because surface runoff prevented litter layer formation. This resulted in a ‘negative feedback’ loop that maintained a nutrient-poor state. Although planted trees can function as ecosystem engineers by supplying nutrients via litterfall, this potential depends critically on appropriate initial soil physico-chemical conditions and subsequent litter retention.