From volcanic ash to abundant earth: understanding Andisol organic matter dynamics in relation to soil health on Hawai'i Island

To date, research on the role of organic matter dynamics in maintaining the health of (sub)tropical Andisols (i.e., volcanic ash-derived soils) is limited. High concentrations of poorly and noncrystalline minerals in these soils favor greater soil organic matter (SOM) accumulation than in phyllosilicate-dominant soils, yet SOM abundance and composition vary across volcanic landscapes. In this study, we measured the effects of moisture regime and current land use on soil health and SOM physical fractions and identified the carbon (C) and nitrogen (N) fractions that best predicted soil health scores in Andisols. We collected soil samples across humid (Udands) and dry (Ustands) moisture regimes and three land uses (croplands, pastures, forests) on Hawai'i Island. We measured nine dynamic soil properties and integrated them into a soil health score using a structural equation model. Then, we quantified the C and N contents of SOM physical fractions, including light particulate organic matter (LPOM), coarse heavy associated organic matter (CHAOM), and mineral associated organic matter (MAOM). We found that pastures and Udand forests scored highest in soil health while Ustand croplands scored lowest. Pastures contained greater proportions (% of total element) and contents (mg/g soil) of C and N in the CHAOM fraction, suggesting differences in CHAOM composition across ecosystems. All three physical fractions collectively explained 81% of soil health score variation, with MAOM-C explaining substantially more variation than LPOM-N and CHAOM-N. Our framework, which links soil C and N fractions to dynamic soil health properties, holistically captures the unique attributes of (sub)tropical Andisols rich in poorly and noncrystalline minerals.