Geochemical Resilience of a Ferralsol Subjected to Anoxia and Organic Matter Amendment

Abstract

The spreading of vinasse on soils may favor not only anoxia but also mineral alterations and metal mobilization. Our aims were to record the geochemical changes occurring in a ferralsol subjected to vinasse input and anaerobiosis, and to check for soil resilience after a return to aerobic conditions. Soil slurries were therefore incubated under successive 7 d of aerobic, 0 to 28 d of anaerobic, and 28 d of aerobic conditions, vinasse being supplied at one of three levels when anaerobiosis began. At several dates, the slurry solution was characterized (pH and organic and mineral compounds) and Mn and Fe oxidation states in solids were assessed. Before incubations, about 20% of the Fe in solids was already reduced, whereas almost all Mn was found as Mn(IV). During the first aerobic period, mobilized metals included Pb, Zn, Ni, Cu, and Cr. After vinasse input during the first 14 d of anaerobiosis, the principal biotransformations were fermentation and true acetogenesis; Fe and Mn were mobilized during this period (up to 4.05 and 6.2 mmol L−1, respectively), and most Mn and an unknown but small fraction of Fe in solids were reduced. During the subsequent 14 d of anaerobiosis, only acetoclastic methanogenesis was observed. The return to aerobic conditions led to rapid oxidative immobilization of Fe, desorption of exchangeable Fe(II), and the partial immobilization of other metals (Ca, Mg, K, Na, Mn, Pb, and Ni). Manganese was not oxidized, and there was no return to the initial conditions for Fe and Mn in solids.

Variations in the cation exchange capacity of a ferralsol supplied with vinasse, under changing aeration conditions

“…The rum vinasse exacerbated anaerobic microbial activity (Lahlah et al, 2009) and led to the reductive mobilisation of Fe and Mn, among other metals (Fig. 5a).…”

“…By contrast, Mössbauer spectrometry suggests that Fe reduced in solid phases during the anoxic period could have remained reduced after the return to oxic conditions (Fe oxidation status was only weakly affected by anoxic conditions due to the high soil content in Fe), although reduced Fe in solution was quickly oxidised during the second oxic period. In addition, Mössbauer spectrometry suggested a decrease in the dimensions of Fe solid phases, which may affect their adsorption properties or their coating of clays: Mössbauer maximal absorption decreased throughout the incubation (3.2%, 2.1% and 0.7% at the ends of the first oxic period, the anoxic period, and the second oxic period respectively (Lahlah et al, 2009)). …”

“…Spreading liquid food industry by-products enriches the soil with nutrients and organic matter, but also enhances the risk of anoxia (Gemtos et al, 1999), since it increases soil moisture, stimulates O 2 consumption by supplying organic C (Lahlah et al, 2009) and decreases soil structural stability González, 2005, 2006). It thus increases the risk of anaerobic microbial activity (Stemmler et al, 2004) and can induce reduction of Fe in oxyhydroxide form or contained in the clay crystal structure (FeStr) (Lovley, 1991;Ona-Nguema et al, 2002;Stucki, 1997;Kostka et al, 2002;Berthelin et al, 2006).…”