Iron: A Biogeochemical Engine That Drives Carbon, Nitrogen, and Phosphorus Cycling in Humid Tropical Forest Soils

Abstract:

The abundance of redox active Fe minerals has the potential to alter the storage and loss of C, contribute to gaseous N emissions, and control P retention in upland tropical forest soils. High concentrations of short-range order Fe minerals led to Fe(II) production rates of 26-206 µg g d^-1 under short-term low redox conditions (Chacón et al. 2006, Liptzin and Silver 2009, Dubinsky et al. 2010). Potential C mineralization from Fe(II) reduction was 34-263 g CO₂-C m^-2 y^-1, C losses equivalent to approximately 10-60 % of annual litterfall production in this forest. Decreased acidity during Fe reduction can destabilize soil aggregates and lead to C losses. Iron is rapidly reoxidized during aerobic periods, which can subsequently lead to C stabilization via complexation reactions. Fe oxidation can also stimulate C losses via pH-driven dissolved organic C production and directly via Fenton reactions. In laboratory experiments, rates of CO₂ production were strongly linearly correlated with Fe(II) loss under aerobic conditions, increasing by 0.51 ± 0.02 µg CO₂-C g soil h^-1 respired for each mg of Fe(II) g^-1 soil oxidized or sorbed (Hall and Silver 2013). Iron oxidation has also been linked to dissimilatory NO₃^- reduction to NH₄⁺ leading to N retention in ecosystems. Fe(III) reduction coupled with NH₄⁺ oxidation (Feammox) can lead to N losses as dinitrogen gas (N₂) or nitrous oxide (N₂O), a potent greenhouse gas. Estimates suggest that Feammox resulted in gaseous N losses of 1-4 kg N ha^-1 y^-1 (Yang et al. 2012), rates equivalent to total denitrification in this forest. Oxidized Fe can strongly bind P decreasing it’s availability to plant roots. While this is commonly cited as a potential limitation to net primary production in tropical forests, it also helps to retain P in ecosystems with high rainfall and potential leaching losses. Microbial biomass P availability increased significantly with Fe(II) production, suggesting the P mobilized during Fe(II) reduction was rapidly immobilized into biological pools (Liptzin and Silver 2009). Data suggest that Fe-redox cycling may decrease P limitation to NPP, and help maintain forest nutrient stocks. In summary, our results highlight the biogeochemical significance of Fe cycling in upland soils environments and its important role in the dynamics of humid tropical forests.