Effects of carbon additions on iron reduction and phosphorus availability in a humid tropical forest soil

Abstract

In the highly weathered soils of humid tropical forests, iron (Fe) plays a key role in ecosystem biogeochemical cycling through its interactions with carbon (C) and phosphorus (P). We used a laboratory study to explore the role of C quantity and quality in Fe reduction and associated P mobilization in tropical forest soils. Soils were incubated under an ambient atmosphere headspace (room air) with multiple levels of leaf litter leachate or acetate additions. Net Fe reduction occurred in all the treatments and at every time point. The more complex mixture of organic compounds in leaf litter leachate stimulated Fe reduction as much acetate, an easily fermentable C source. At the end of the experiment, Fe reduction was generally greater with higher C additions than in the low C additions and controls. The microbial biomass P had increased significantly suggesting rapid microbial uptake of P liberated from Fe. This occurred without increases in the available (NaHCO 3) P pool. The immobilization of P by microbes during the incubation provides a P conservation mechanism in these soils with fluctuating redox potential, and may ultimately stimulate more C cycling in these highly productive ecosystems. Iron cycling appears to be an important source of P for the biota and can contribute significantly to C oxidation in upland tropical forest soils.