Dissolution of iron oxyhydroxide in the rhizosphere of various crop species

The root-induced dissolution of a synthetic goethite was studied in the rhizosphere of four plants species that had been previously supplied with and without adequate amounts of Fe. Plants were grown with a cropping device which enabled an easy collection of whole plants (roots included) and rhizosphere material. For this purpose, roots developed as a planar mat on top of a mesh which separated them from a thin layer of a goethite-quartz mixture. The rates of dissolution of goethite deduced from the uptake of Fe achieved by the plants were compared with the rates of dissolution obtained From a batch experiment with acidic solutions. The results showed that goethite was significantly dissolved by the three strategy I species, more so by rape and pea than by white lupin. The sole strategy II species studied (maize) exhibited the least Fe uptake and the occurrence of any dissolution of goethite remained unsignificant in this case. The amounts of Fe taken up by the plants never exceeded the initial amount of amorphous Fe contained in the goethite (oxalate extractable Fe, Fe-ox). However, Fe-ox increased in the rhizosphere of pea and rape. These results suggest that these species which took up the largest amounts of Fe indeed dissolved some crystalline goethite. Conversely, maize seemed to rely mostly on the dissolution of amorphous Fe contained as trace amounts in the goethite. Considering the low rates of dissolution measured in the batch experiment at the pH close to the rhizosphere pH found for the various species studied, proton excretion by roots could contribute only a small proportion of root-induced dissolution of goethite. The contribution of other mechanisms is discussed.