nutrient cycling

Iron reduction and soil phosphorus solubilization in humid tropical forests soils: the roles of labile carbon pools and an electron shuttle compound

Chac´on, N., Silver, W.L., Dubinsky, E.A. & Cusack, D.F. 2006. Iron
reduction and soil phosphorus solubilization in humid tropical forest
soils: the roles of labile carbon pools and an electron shuttle compound.
Biogeochemistry, 78, 67–84.

Abstract: 
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.

Top-down effects of a terrestrial frog on forest nutrient dynamics

Beard,Karen H.; Vogt, Kristiina A.; Kulmatiski,Andrew 2002. Top-down effects of a terrestrial frog on forest nutrient dynamics.. Oecologia 133 :583 593.

Abstract: 
Many studies have found top-down effects of predators on prey, but few studies have linked top-down effects of vertebrate predators to nutrient cycling rates in terrestrial systems. In this study, large and significant effects of a terrestrial frog, Eleutherodactylus coqui (coqu), were recorded on nutrient concentrations and fluxes in a subtropical wet forest. In a manipulative experiment, coqus at natural densities were contained in or excluded from 1 m3 enclosures for 4 months. Chemistry of leaf wash (throughfall), foliage, and decomposed leaf litter in the enclosures were measured as indicators of coqu effects on nutrient cycling. Coqu exclusion decreased elemental concentrations in leaf washes by 83% for dissolved organic C, 71% for NH4 +, 33% for NO3 –, 60% for dissolved organic N, and between 60 and 100% for Ca, Fe, Mg, Mn, P, K, and Zn. Coqu exclusion had no effect on foliar chemistry of plants transplanted into the enclosures. However, coqu exclusion decreased nutrient availability in decomposing mixed leaf litter by 12% and 14% for K and P, respectively, and increased C:N ratios by 13%. Changes in nutrient concentrations that occurred with coqu exclusion appear to be due to concentrations of nutrients in coqu waste products and population turnover. The results supported our hypothesis that coqus have an observable effect on nutrient dynamics in this forest. We suggest that the primary mechanism through which they have this effect is through the
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