Landscape-level drivers of coarse-heavy associated carbon in variable charge soils

Characterizing soil fractions offers insights into soil organic matter (SOM) structural composition, improving our understanding of SOM stabilization and cycling to inform land management and carbon (C) models. While SOM is commonly partitioned into particulate (POM) and mineral-associated (MAOM) fractions, combined size and density methods isolate a third fraction relevant to SOM dynamics in variable charge soils of the global tropics. The coarse-heavy associated organic matter (CHAOM) fraction (> 1.85 g/cm-3 & > 53 µm) accounts for < 10% of total soil organic C (TOC) in temperate systems. However, in this study we show that CHAOM represents up to 30% of TOC in Hawaiian Andisols and Oxisols and contains dispersion-resistant aggregates (i.e., pseudo-sands). Our survey included 60 sites spanning five soil suborders and three land uses (croplands, pastures, and forests) to investigate the effects of mineralogy and management on POM, MAOM, and CHAOM C fractions in Hawaiian soils. The proportion of C in CHAOM was similar in Andisols (21 ± 2%) and Oxisols (21 ± 2%), which were greater than Mollisol proportions (7 ± 1%). Pastures contained greater CHAOM-C proportions than forests and croplands, exemplifying the influence of management on stable aggregate formation. In Oxisols, LPOM-C proportion decreased (p < 0.001), MAOM-C increased (p <0.01), and CHAOM-C remained consistent to 90 cm, reflecting the indistinct horizonation of highly developed Oxisols and confirming “pseudo-sand” aggregation throughout the deep soil profile. CHAOM-C positively correlated to nitrogen mineralization potential, warranting further investigation into the functional role of CHAOM-C in variable charge soil biogeochemistry.