Synchronous influence of soil amendments on alkylmercury and methane emissions in mercury-contaminated paddy soil

Abstract

Mercury (Hg) alkylation and methane (CH4) emissions pose significant global concerns. Paddy soil, due to its long-term anaerobic conditions and abundant organic matter, is hotspots for soil Hg alkylation and CH4 emissions. However, the relevance between Hg alkylation and CH4 emissions, especially their simultaneous reduction strategies, remains poorly understood. Here, we investigated the effects of biochar (BC), selenium (Se) and rice straw (RS) amendments on Hg alkylation and CH4 emissions in paddy soil, and the accumulation of Hg speciation. Results found that both BC and RS amendments significantly increased the levels of soil organic carbon (SOC) and humification index (HIX). Furthermore, BC decreased the concentrations of Hg(II), methylmercury (MeHg) and ethylmercury (EtHg) by 63.1%, 53.6% and 100% in rice grains. However, RS increased Hg(II) concentration but decreased the total Hg (THg), MeHg and EtHg concentrations in rice grains. Compared to the CK, RS significantly increased CH4 emissions, while BC decreased CH4 emissions, and Se showed no significant difference. Se amendment increased the Hg(II) and EtHg concentrations by 20.3% and 17.0% respectively, and decreased the MeHg concentration in grains by 58.3%. Both BC and RS impacted the abundance of methanogens by enhancing SOC and HIX, subsequently modulating the relevance between Hg alkylation and CH4 emissions. These findings provide insights into the relevance between Hg alkylation and CH4 emissions and propose potential mitigation mechanisms in Hg-contaminated paddy soil.