. Kumpiene, 2015) where it was shown that 328 soil saturation increased As solubility by the way of reduction of Fe oxides together with more efficient microbial 329 reduction of As(V) Increased As(III) concentration in soil solution previously observed in the saturated level H4 330 of the mesocosm (Thouin et al., 2017) could be related to increased As(V)-reducing microorganism concentrations 331 in the saturated zones, The stimulation of microbial As(V)-reducing activity induced by soil saturation may have 332 negatively impacted the overall As(III) oxidation rate in water-saturated levels, but could not explain a decreased 333, 2009.

. Bachate, For example, high concentrations of organic substrates 335 may inhibit this activity (Challan-Belval et al, 334 OM may also affect microbial As(III) oxidizing activity, pp.2016-336, 2009.

. Yamamura, The As(III) oxidation 337 rate results from the overall activity of all microorganisms involved in As speciation, because microbial As(III) 338 oxidation and As(V) reduction can occur simultaneously, even under aerobic conditions, stimulate aerobic As(V)-reducing activity of soil microorganisms, 2009.

D. and H. /. Oi, components analysis (PCA) was built (Fig.6), including variables that describe OM quantities and qualities (TOC, microbial As transformation parameters (As(III)-oxidation rate, concentrations of As(III)- 342 oxidizing microorganisms and As(V)-reducing microorganisms), and the variables related to the water phase of 343, p.340

. The, As speciation and mobility in the interstitial water ([As], [As(III), 2017.

. Fig, PCA using biogeochemical parameters of soil samples (black) with geochemical parameters of soil solutions 347 (from Thouin et al., 2017) as supplementary data (blue). a: Correlation circle showing variable relationships, p.348

. Yamamura, The concentration of As(V)-reducing 360 microorganisms and the C/N ratio were correlated (p = 0.59) and were the primary contributors to F2. The 361 previously mentioned positive effect of soil saturation on As(V)-reducing microorganisms is expressed by the 362 upward translation of samples H3 and H4 between T0 and T4 along the F2 axis (Fig.6.b), Principal component F2 accounts for 23% of total variability Bioavailable OM may 363 stimulate the As(V)-reducing activity of soil microorganisms under aerobic conditions, p.364, 2009.

. Finally, the quantity of As(V)-reducing microorganisms was correlated with As(III) concentrations in soil 370 interstitial water (p = 0.76) suggesting that microbial As-reducing activity was promoted by soil saturation, p.371

. Drewniak, 2008) but without the high 375 concentration in sulfur species, and (2) the fact that in spite of water saturation and addition or OM, the redox 376 potential did not reach very low values observed a decrease of redox potential down to 0 mV 377 in the porewater of their contaminated surface soil after 20 days of flooding with artificial river water, with a 378 simultaneous strong reduction of As(V), whereas in our experiment, the redox potential always remained higher 379 than +100 mV As a fact, most of the reported studies about redox dynamic were performed 380 with soils or sediments less contaminated than the " Place-à-Gaz " material: 300 ppm As in a polluted floodplain 381 soil 375 ppm As in an artificially polluted soil 63 ppm As in a 382 contaminated sediment (Moon et al., 2017) compared with 100 000 ppm As in our soil. Thus, the present 383 experiment provides information about microbial As transformation in a biogeochemical environment that was 384 not previously explored in variable redox conditions. Thouin et al. (2016) found that elevated toxic element 385 concentrations (Cu, Pb, Zn, As) seemed to have exerted a selective pressure on the microbial communities, with 386 higher As(III)-oxidizing rates and lower microbial activity involved in carbon mineralization of the microbial 387 populations from the most polluted zone of the site. Thus, phenomena linked to high concentrations of metals and 388 arsenic may have attenuated the decrease of redox potential linked to oxygen consumption in the saturated level, 389 while OM was provided, compared with less polluted environments The increasing amount of OM and its quality therefore promoted the growth of 392, ) the extreme level of metals and As 374 concentrations, that resemble those found in mining wastes Up to now, all As(III)-oxidizing bacteria 390 isolated from soils were heterotrophs or facultative autotrophs, 2007.

. Lescure, The opposite trend of overall As(III) oxidizing rate with increasing As(III)-oxidizing 395 microorganisms concentration (p = -0.50) and with the quality and quantity of OM seems to confirm the inhibiting 396 effect of OM on the specific activity of As(III)?oxidizing microbes. However, globally, As(III) oxidizing 397 microorganisms remained active in presence of OM, even in the saturated zones of the mesocosm, 2016.

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