Soil hardness regulates the root penetration by trees planted on anthropogenic growing bases in coastal forests in Japan: new endeavors to reforest the coastal disaster prevention forests with high resilience for tsunami

Purpose To restore the disaster resilience of coastal forests in tsunami-prone areas, growing bases have been artificially constructed with embanking before afforestation to increase the effective soil depth and allow deeper root system development. The aim of the present study was to describe and record the soil features of these anthropogenic growing bases and root development within them to provide guidance on their construction and comprehensively evaluate their potential for disaster resilience. Materials and methods Soil surveys including root excavation were conducted on anthropogenic growing bases in coastal forests including seaside parks in the Kanto and Tohoku regions of Japan. All of the soils assessed were classified as “Spolic (Subaquatic)” or “Urbic” Technosols because they contained artefacts with large amounts of industrial soil materials in their profiles. Soil samples were taken from each horizon in the soil profiles using 400-ml and/or 100-ml cylinders, and the undisturbed cylinder samples were then analyzed to determine the bulk density, water permeability, and porosity of the soils at each site. The vertical distribution of soil hardness in the anthropogenic growing bases was also measured using a dynamic cone penetrometer. Results and discussion Some of the growing bases were generally soft with high water permeability, indicating that soil improvements such as deep tillage were quite effective for building suitable growing bases. Notably, some vestiges of pedogenic processes would be partly recognized throughout the soil profile of an old growing base after construction. However, some embankments had serious issues in the growing base soil, including thick and continuous planar consolidated pan layers with quite low permeability and vestiges of strong gleyzation in their profiles. These poor soil conditions inhibited plant growth, particularly of the below ground roots, and remained in the growing bases for a long time. Conclusions The study indicated quantitatively that (1) the physical properties of anthropogenic soils constructed using heavy machinery could be maintained for several decades, and (2) compaction should be avoided during soil preparation to avoid soil hardening, water stagnation, and hindrance of root penetration. Although these findings unfortunately lack quantitative consideration of the requirements for stabilizing embankments, they reinforce awareness of the difficulty of constructing anthropogenic soils to foster the growth of trees and provide sufficient strength for embanked foundations. This topic requires further study.