Subsoil organo-mineral associations under contrasting climate conditions

Organo mineral associations intermediated by Fe and Al are considered one of the most important mechanisms for soil organic carbon (SOC) stabilization. However, since Fe and Al are normally mentioned together as stabilizing agents, we still lack knowledge about their relative role. In addition, this stabilization mechanism can be profoundly affected by climate differences, but the magnitude of this influence whether as a direct effect or an indirect consequence due to changes in soil mineralogy is not yet fully understood. In this study, we evaluated a series of subsoil samples throughout a climate gradient (1800–2400 mm precipitation year^-1 and 15–24º C) on Kohala Mountain, Hawaii to understand the impact of climate differences on organic matter protection. We have used a combined approach of analyses at the bulk soil and microscale using NanoSIMS. At the bulk soil scale, we have observed a concurrent decline of subsoil Fe, Al (i.e., dithionite citrate and ammonium oxalate extractions) and SOC above a precipitation level of 2000 to 2200 mm year^-1_. This decline co-occurred with more reduced forms of Fe s (evaluated by Fe K-edge XANES) and declines in carboxyl-C (evaluated by CP-MAS ¹³C NMR). We found significant positive correlations between SOC with Fe and Al in the bulk soil throughout the gradient, and we could discern the relative role of Fe and Al in promoting organo-mineral associations in contrasting climate conditions (e.g., ~1800 and ~2300 mm year^-1) using NanoSIMS. While Fe contributed to approximately 40% of the microscale organo-mineral associations in the lower precipitation site (assessed by co-localizations with OM segments), this contribution at the higher rainfall regime was only 5%. In contrast, the contribution of Al was approximately the same in both rainfall levels (approximately 30%). This fact indicates that Al may be more important than Fe in stabilizing SOC especially under high precipitation levels. The normalized CN:C ratio was higher when associated with Fe and Al especially in the high precipitation level, which demonstrates the importance of Fe and Al in stabilizing N-rich organic matter. Here we demonstrate that spatial relationships between Fe and Al with SOC at the microscale display a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Thus, they are of great importance to understand the impact of climatic differences on SOC sequestration in organo-mineral associations.