Impact of clay structural iron reduction at field scale: assessment and challenges.
Fabienne Favre
HES-SO, HEFr, Switzerland
Rice fields in the middle valley of the Senegal River are predominantly cultivated on vertisols, soil rich in swelling clays, by continuous flooding over a period of three months. Water saturates the soil during this time imposing drastic changes in geochemical conditions, for example, oxygen partial pressure is depleted over the cultivation period. The soil contains 57.4 % clay of which 70 % is swelling clay. The total iron content is 87.2 cmol kg-1 of this 52.5 % is structural iron and 47.5 % is contained in iron oxides. The smectite fraction is of beidellite type according to the alkyalmmonium chains sorption characterisation and its calculated theoretical formula is (Si3.658 Al0.342) (Al1.265 Fe0.601 Mg0.155) O10 (OH)2 Ca0.145 Na0.016 K0.131.
In situ monitoring of the soil shows a drastic Eh drop from 0.6 to -0.1 V during the cultivation period while pH increases slightly from 5.1 to 5.7.
We observed in situ a large CEC increase over the flooding period. As the CEC increased, so did the Fe(II)Str/FeTotStr ratio, however, the goethite quantity diminished as revealed by Mossbauer spectroscopy and TEM images.
This behaviour was confirmed in samples incubated in a N2 atmosphere in the laboratory. Reduction of Fe(III)Str and dissolution of positively charged goethite particles both contributed to the large CEC increase. The layer charge increase was mostly balanced by NH4+ and Fe2+ as exchangeable cations in addition to the common Ca2+, Mg2+, K+, Na+ species encountered in the oxidized soil.
The contribution of both mechanisms to the CEC increase is discussed in light of dissolution kinetics and modelling of the Fe(II)Str to CEC relationship. This shows that it is necessary to take into account the soil particle arrangement, instead of just the thermodynamics of single species, to predict the reduction of iron species in this complex system, defined by the nature of particles, their size and their arrangement. The reversibility of the mechanisms is discussed using the comparison of samples cultivated 22 times with uncultivated controls.
Our results established the occurrence of Fe(III)Str reduction and CEC increase, one of its concomitant effects, in the field as well as the necessity to consider the soil particles’ complexity in their nature and arrangement to predict the evolution of the soil’s chemical properties upon reduction.
