The effects of afforestation tree species mixing on soil organic carbon stock, nutrients accumulation, and understory vegetation diversity on reclaimed coastal lands in Eastern China

Establishing mixed-species plantations on the reclaimed coastal lands has become a new paradigm in forest management in China, because it increases the provision of ecosystem services. However, the mixing effects of trees with different identities remain largely understudied. In subtropical China, plantations have been historically dominated by monocultures, the conversion of which to mixed plantations are now required considering the need of sustainable forestry development. In this study, we evaluated the soil carbon sequestration, nutrient accumulation, and understory regeneration of three coastal plantations with different compositions established 12 years ago, on a reclaimed area in Eastern China. Plots on two scales (30 m × 20 m for trees and 1 m × 1 m for understory vegetation) were set up to conduct vegetation inventory and soil sampling to compare growth parameters, soil physical and chemical properties, and understory composition. We found that coniferous-mixed forest (CMF) had significantly higher soil available water capacity; both CMF and broadleaved-coniferous mixed forest (B-CMF) had significantly higher soil capillary porosity and soil moisture content at saturation; broadleaved mixed forest (BMF) had a greater soil permeability coefficient. Among soil chemical properties, we observed that CMF had higher soil organic carbon stock and cation exchange capacity in both 0–20 cm and 20–40 cm soil layers. CMF and B-CMF had higher hydrolysable nitrogen and total potassium in both soil layers; BMF had higher available phosphorus contents. Moreover, B-CMF supported better canopy tree growth and higher understory plant species diversity. Therefore, our results showed that the introduction of broad-leaved trees into the coniferous-dominated plantations established on the reclaimed coastal lands produced significant effects on soil properties and understory species diversity. Thus, our study may facilitate the development of new afforestation regimes to mitigate the potential impact of increasing environmental changes by improving our understanding of species composition effects on coastal forest ecosystem functioning.