Association of Organic Carbon With Reactive Iron Oxides Driven by Soil pH at the Global Scale

Association of organic carbon (C) with iron (Fe) minerals is one important mechanism for long-term terrestrial C storage. Yet, specific edaphic variables that directly contribute to Fe-associated C across diverse soil types are still unclear. Through analyzing soils from the National Ecological Observatory Network (NEON) and other published data, here we show that soil pH primarily controls Fe-associated C across the globe. Fe-associated C in most soils ranged from 0 to 20 g kg(-1) soil, with a strong increase from pH 4.2 to 3.5, but a small change in soils with pH > 4.2. A microcosm experiment further showed that raising soil pH by liming reduced the formation of Fe-associated C in an acidic Oxisol. Together, these findings demonstrate the dominant role of soil pH in controlling the abundance of Fe-associated C.