SOIL CARBON IN HAWAIIAN RANGELANDS: EVALUATION OF EXTENT, CONTROLS, AND OPTIONS FOR MANAGEMENT

Abstract

Rangelands are a land use type found extensively throughout both the globe and the Hawaiian Islands. These lands provide ecosystem services such as soil carbon sequestration, which can aid in slowing climate change, enhance soil physical and chemical qualities, and potentially provide diversified income for farmers. However, questions remain regarding the extent of sequestered carbon in rangeland soils, as well as the mechanisms controlling the variability of these stocks, especially in Hawaii. The majority of rangelands in Hawaii are situated on Andisols, a soil order known for high soil carbon content due to in part to unique mineralogy. Therefore, my dissertation research sought to intensively sample representative Hawaiian rangelands to identify baseline soil carbon stock, determine a minimum sampling density required for accurate estimates of landscape-scale variability, compare methods of spatial analysis, and to identify environmental factors that affect soil carbon stock distribution. Further, selective dissolution of mineral components of rangeland soils were carried out to relate mineralogy to soil carbon concentration. Finally, I investigated different methods of vegetation management in a silvopastoral production system in a replicated field trial to discern the effect of practice on soil carbon and sward characteristics.High levels of soil carbon stock were found throughout actively grazed rangeland, indicating the importance of these soils in contributing to the regulation of Earth’s climate. Carbon stocks and concentrations varied with environmental as well as mineralogical factors, presenting valuable potential co-variables for consideration in future sampling collections. Integration of livestock in koa (Acacia koa) production systems achieved some understory vegetation management goals, but did not significantly change in soil carbon concentration after one year, highlighting the requirement of long-term studies for soil carbon research. After baseline carbon characteristics are determined, repeated, long-term sampling will be required from different rangeland production systems to identify the effect of management on soil carbon resources. These land use types are important in providing food and ecosystem services to the human population, and a more complete understanding and quantification of their baseline soil qualities, as well as the factors driving those characteristics, will aid in future management decisions that seek to maximize both agricultural productivity and ecosystem health.