The affinity of iron oxides and hydroxides for phosphorus is thought to contribute to phosphorus limitation to net primary productivity in humid tropical forests on acidic, highly weathered soils. Perennially warm, humid conditions and high biological activity in these soils can result in fluctuating redox potential that in turn leads to considerable iron reduction in the presence of labile carbon and humic substances. We investigated the effects of reducing conditions in combination with the addition of labile carbon substrates (glucose and acetate) and an electron shuttle compound on iron reduction and phosphorus release in a humid tropical forest soil. Glucose or acetate was added to soils as a single dose at the beginning of the experiment, and as pulsed inputs over time, which more closely mimics patterns in labile carbon availability. Iron reduction and phosphorus mobilization were weakly stimulated by a single low level addition of carbon, and the addition of the electron shuttle compound with or without added carbon. Pulsed labile carbon additions produced a significant increase in soil pH, soluble iron, and phosphorus concentrations. Pulsed labile carbon inputs also promoted the precipitation of ferrous hydroxide complexes which could increase the capacity for P sorption, although our results suggest that rates of P solubilization exceeded re-adsorption. Plant and microbial P demand are also likely to serve as an important sinks for released P, limiting the role of P re-adsorption. Our results suggest that reducing conditions coupled with periodic carbon inputs can stimulate iron reduction and a corresponding increase in soil phosphorus mobilization, which may provide a source of phosphorus to plants and microorganisms previously undocumented in these ecosystems.

No abstract

No abstract

Relationships among soil phosphorus distribution, soil organic carbon and biogeochemistry of iron and aluminum were studied along a flooded forest gradient of the Mapire river, Venezuela. Soil samples were collected during the dry season in three zones subjected to different flooding intensity: MAX inundated 8 months per year, MED inundated 5 months per year, and MIN inundated 2 months per year. Total labile phosphorus (resin + bicarbonate extractable fractions) was significantly higher in MIN than in MAX. The longer non-flooding period in MIN probably allowed a higher accumulation of microbial biomass in soils of this zone and consequently a greater release of the bicarbonate organic fraction. The moderately labile phosphorus fraction associated with the chemisorbed phosphorus on amorphous and some crystalline aluminum and iron was significantly lower in MAX than in MIN following the same tendency observed for crystalline iron oxides. This result allowed us to hypothesize that the combined effect of a long flood period and a high soil organic carbon content in the MAX, could be appropriate conditions for microbial reduction of stable forms of iron. The ratio of soil organic carbon to total organic phosphorus decreased from MAX to MIN, indicating higher mineralization of organic phosphorus in MIN. Our results suggests two distinct flood-dependent mechanisms operating for phosphorus release along the gradient. In MAX mineralization process appears to be limited, while microbial mineral dissolution appears to be an important source of phosphorus. In MIN supply of phosphorus is associated with the stability of soil organic matter.

No abstract

No abstract

No abstract

No abstract