Does acidification of a soil biofilter compromise its methane-oxidising capacity?

Abstract

A biofilter made using volcanic pumice soil from a landfill in Taupo, New Zealand has been found to mitigate CH4 emissions from New Zealand dairy effluent ponds. However, the biofilter after drying out following almost 5 years of use removed little or no CH4. Furthermore, H2S present in the biogas (from the dairy effluent ponds) had increased the acidity (pH) in the soil biofilter from 5.2 to 3.72 during this 5-year period. In this study, we adjusted the soil moisture to 60 % water-holding capacity (WHC) and investigated the CH4-oxidising capacity of a reconstituted acidic soil biofilter operating at low pH (3.72) and characterised the abundance and diversity of methane-oxidising bacteria (MOB) using quantitative polymerase chain reaction (qPCR) and terminal-restriction fragment length polymorphism (T-RFLP). The acidic soil biofilter achieved a maximum CH4 removal rate of 30.3 g m–3 h–1. Both types I and II MOB communities, along with some uncultured novel MOB strains or species in the biofilter column, were present. Among these, Methylocapsa-like type II methanotrophs were significantly more prominent than the other MOB. Other MOB, Methylococcus (type I), Methylobacter/Methylomonas/Methylosarcina (type I) genera, Methylosinus and Methylocystis (type II), were least abundant. During the 90-day study, the population of Methylocapsa-like MOB increased 4-fold, demonstrating the ability of these soil microorganisms to grow under acidic pH conditions in the biofilter, whereas the populations of type I MOB remained stable, and the populations of type II MOB (except Methylocapsa) decreased. Our results indicated that (i) a soil biofilter can effectively regain efficiency if sufficient moisture levels are maintained, regardless of the soil acidity; (ii) changes in the MOB population did not compromise the capacity of the volcanic pumice soil to oxidise CH4; (iii) the more acidic environment (pH 3.72) tends to favour the growth and activity of acid loving Methylocapsa-like MOB while being detrimental to the growth of Methylobacter/Methylocystis/Methylococcus group of MOB; and (iv) novel species or strains of uncultured Methylomicrobium/Methylosarcina/Methylobacter (type I MOB) could be present in the soil biofilter. This study has revealed the MOB population changes in the biofilter with acidification did not compromise its capacity to oxidise CH4 demonstrating that soil biofilter can operate effectively under acidic conditions.