Baseline Soil Carbon Stock Analysis in an Agroforestry Restoration Site in Kākoʻo ʻŌiwi, Heʻeia, Oʻahu

Abstract

In Hawaiʻi, there is growing interest in restoring biodiverse agroforestry systems for multiple benefits, including native species restoration, food production, cultural connection to place, and other ecosystem services including carbon (C) sequestration. In this study, I collaborated with a community based organization, Kākoʻo ʻŌiwi, an interdisciplinary team at the University of Hawaiʻi at Mānoa, and the Heʻeia National Estuarine Research Reserve to establish a soil C baseline prior to the biocultural restoration of an agroforest designed for cultural, ecological, and economic benefits. Soil C is the most significant terrestrial C pool and has important implications for the broader health of the soil and land. Yet, how restoration affects soil C remains poorly understood, particularly for agroforestry systems. To establish a baseline from which change can be detected over time, we sampled a total of 48 1-meter soil cores (divided into 5, 20-cm increments) from one control and two agroforestry treatment plots using a gridded design to capture spatial heterogeneity. I analyzed data combining three methods to characterize a soil C baseline: 1) variability of C concentration by depth and plot; 2) total soil C using the equivalent soil mass method; and 3) geostatistical analysis and kriging. This baseline will provide the foundation for a long-term dataset of soil C change over space and time with agroforestry restoration. In Hawaiʻi this is the first in-depth case study of change in soil C with agroforestry restoration and the first globally to evaluate change from a novel, invaded forest, to an agroforest.