Effects of heavy metals on organic matter decomposition in inundated soils: Microcosm experiment and field examination Osim Enya a, ⁎, Natalie Heaney a , Grace Iniama b , Chuxia Lin a,c a School of Science, Engineering and Environment, University of Salford, Manchester, M5 4WT, United Kingdom b Department of Pure and Applied Chemistry, University of Calabar, Nigeria c Faculty of Science, Engineering and Built Environment, Deakin University (Melbourne Burwood Campus), Burwood, Melbourne, Victoria, Australia HIGHLIGHTS • Inhibition of organic matter decomposi- tion occurred in water-inundated soils. • Arsenic had stronger inhibitory effects, as compared to other heavy metals. • Chromium had weaker inhibitory ef- fects, as compared to other heavy metals. • The same was not clearly observed under field conditions in the study area. • pH, Eh and EC played more important roles in affecting soil carbon dynamics. GRAPHICAL ABSTRACT abstract article info Article history: Received 16 January 2020 Received in revised form 24 March 2020 Accepted 24 March 2020 Available online 25 March 2020 Editor: Filip M.G. Filip M.G.Tack Keywords: Heavy metal Wetland soil Organic matter decomposition Soil carbon storage Soil respiration Microcosm and field investigation were conducted to examine the effects of heavy metals on the decomposition and accumulation of organic carbon in contaminated Mersey estuarine floodplain, northwest England. The re- sults show that inhibition of microbially mediated decomposition of organic matter occurred in the water- inundated soils. However, individual heavy metals had differential effects on the inhibition of soil organic matter decomposition with arsenic and copper being much stronger, as compared to other investigated heavy metals. The weak inhibitory effects of chromium on organic matter decomposition was due to the conversion of highly toxic Cr(VI) to less toxic Cr(III) under reducing conditions. Lead also had a weaker capacity to inhibit organic mat- ter decomposition due to its low solubility. It was surprising that the same phenomenon was not clearly observed during the field examination. The inhibitory effects of heavy metals on soil organic matter decomposition could be curtained under field conditions. pH, Eh and EC played more important roles, as compared to soil-borne heavy metals, in affecting the soil carbon dynamics in the contaminated Mersey estuarine floodplain. Crown Copyright © 2020 Published by Elsevier B.V. All rights reserved. 1. Introduction Soil contamination by heavy metals is widespread (Vig et al., 2003; Wang et al., 2007; Wang et al., 2017; Zwolak et al., 2019). It has been long recognized that the presence of heavy metals could adversely affect microbial activities in soils (Hiroki, 1992; Oorts et al., 2006; Lejon et al., Science of the Total Environment 724 (2020) 138223 ⁎ Corresponding author. E-mail address: o.enya@edu.salford.ac.uk (O. Enya). https://doi.org/10.1016/j.scitotenv.2020.138223 0048-9697/Crown Copyright © 2020 Published by Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv