5 research outputs found
Geochemical Processes Constraining Iron Uptake in Strategy II Fe Acquisition
Phytosiderophores
(PS) are natural chelating agents, exuded by
graminaceous plants (grasses) for the purpose of Fe acquisition (Strategy
II). They can form soluble Fe complexes with soil-Fe that can be readily
taken up. PS are exuded in a diurnal pulse release, and with the start
of PS release a âwindow of iron uptakeâ opens. In the
present study we examined how this window is constrained in time and
concentration by biogeochemical processes. For this purpose, a series
of interaction experiments was done with a calcareous clay soil and
the phytosiderophore 2âČ-deoxymugineic acid (DMA), in which metal and DMA speciation were examined as a
function of time and DMA concentration. Various kinetically and thermodynamically
controlled processes affected the size of the window of Fe uptake.
Adsorption lowered, but did not prevent Fe mobilization by DMA. Microbial
activity depleted DMA from solution, but not on time scales jeopardizing
Strategy II Fe acquisition. Complexation of competing metals played
an important role in constraining the window of Fe uptake, particularly
at environmentally relevant PS concentrations. Our study provides
a conceptual model that takes into account the chemical kinetics involved
with PS-mediated Fe acquisition. The model can help to explain how
success or failure of PS-mediated Fe acquisition depends on environmental
conditions
Retention of phytosiderophores by the soil solid phase â adsorption and desorption
Background and aims: Graminaceous plants exude phytosiderophores (PS) for acquiring Fe. Adsorption of PS and its metal complexes to the soil solid phase reduces the FePS solution concentration and hence Fe uptake. In this study we aimed to quantify adsorption, and to determine to what extent adsorption depends on the complexed metal and on soil properties. Furthermore, we examined if adsorption is a reversible process.
Methods: Adsorption and desorption of PS and metal-PS complexes were examined in batch experiments in which the PS 2âČ-deoxymugineic acid (DMA) and its metal-complexes (FeDMA, CuDMA, NiDMA and ZnDMA) interacted with several calcareous soils.
Results: Adsorption of DMA ligand (0â1000 ÎŒM) and metal-DMA complexes (0â100 ÎŒM) was linear in the concentration range examined. Adsorption varied by a factor â2 depending on the complexed metal and by up to a factor 3.5 depending on the soil. Under field-like conditions (50 % water holding capacity), 50â84 % of the DMA was predicted to be retained to the soil solid phase. Alike adsorption, desorption of metal-DMA complexes is fast (approximate equilibrium within 1 hour). However, only a small fraction of the adsorbed FeDMA (28â35 %) could be desorbed.
Conclusions: Despite this small fraction, the desorbed FeDMA still exceeded the amount in solution, indicating that desorption of FeDMA from soil reactive compounds can be an important process buffering the solution concentration