Increasing the pH of tropical peat can enhance methane production and methanogenic growth under anoxic conditions

Abstract

Methane (CH₄), the second-most-potent greenhouse gas, is produced by methanogens under anaerobic conditions. Tropical peat swamp forests, which are widely distributed in Southeast Asia, have recently been subjected to large-scale deforestation and conversion to agricultural land. The pH of low-pH (acidic) tropical peat soils on agricultural lands is generally controlled to promote plant growth. Unintended waterlogged conditions in agricultural lands during the wet season can create an anaerobic environment in which methanogens are active. In this study, surface peat soil collected from a tropical peat swamp forest in Central Kalimantan, Indonesia, was examined to clarify the effect of increasing pH on CH₄ production. Soil samples were prepared at varying pH (3.4, 4.7, 6.6, and 7.7) by adding calcium hydroxide and incubated under anoxic conditions. Soil remained under dark and cool conditions for years before incubation. CH₄ production increased with incubation time at all pH levels. Overall, soils at higher pH showed significantly greater CH₄ production compared with peat at pH 3.4 (control) at most measurement points during a 76-day incubation. Analysis of the phylogenetic composition of archaeal and bacterial communities in peat after a 76-day incubation showed that the relative abundance of Euryarchaeota (Methanobacteria + Methanomicrobia), organisms that produce CH₄, increased from pH 3.4 (∼0%) to pH 4.7 (5.3 %), pH 6.6 (8.7 %), and pH 7.7 (8.7 %). This suggests that increasing the pH of tropical peat enhances CH₄ production by increasing the proportion of methanogens. It should also be noted that the anaerobic microbe populations in tropical peat can survive and recover after long-term cold storage.