Reducing Conditions, Reactive Metals, and Their Iinteractions Can Explain Spatial Patterns of Surface Soil Carbon in a Humid Tropical Forest

22 Humid tropical forests support large stocks of surface soil carbon (C) that exhibit high 23 spatial variability over scales of meters to landscapes (km). Reactive minerals and organo-metal 24 complexes are known to contribute to C accumulation in these ecosystems, although potential 25 interactions with environmental factors such as oxygen (O2) availability have received much less 26 attention. Reducing conditions can potentially contribute to C accumulation, yet anaerobic 27 metabolic processes such as iron (Fe) reduction can also drive substantial C losses. We tested 28 whether these factors could explain variation in soil C (0 – 10 and 10 – 20 cm depths) over 29 multiple spatial scales in the Luquillo Experimental Forest, Puerto Rico, using reduced iron 30 (Fe(II)) concentrations as an index of reducing conditions across sites differing in vegetation, 31 topographic position, and/or climate. Fine root biomass and Fe(II) were the best overall 32 correlates of site (n = 6) mean C concentrations and stocks from 0 to 20 cm depth (r = 0.99 and 33 0.98, respectively). Litterfall decreased as reducing conditions, total and dead fine root biomass, 34 and soil C increased among sites, suggesting that decomposition rates rather than C inputs 35 regulated soil C content at the landscape scale. Strong relationships between Fe(II) and dead fine 36 root biomass suggest that reducing conditions suppressed particulate organic matter 37 decomposition. The optimal mixed-effects regression model for individual soil samples (n = 149) 38 showed that aluminum (Al) and Fe in citrate/ascorbate and oxalate extractions, Fe(II), fine root 39 biomass, and interactions between Fe(II) and Al explained most of the variation in C 40 concentrations (pseudo R2 = 0.82). The optimal model of C stocks was similar but did not 41 include fine root biomass (pseudo R2 = 0.62). In these models, soil C concentrations and stocks 42