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 CH 4 emissions from New Zealand dairy effluent ponds. However, the biofilter after drying out following almost 5 years of use removed little or no CH 4 . Furthermore, H 2 S 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 CH 4 -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 CH 4 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 CH 4 ; (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 CH 4 demonstrating that soil biofilter can operate effectively under acidic conditions.

Assessing the Performance of Floating Biofilters for Oxidation of Methane from Dairy Effluent Ponds

“…Interestingly, an increasing trend in the abundance of the Methylocapsa ‐like methanotrophs was noticed in the pond biofilters, and this could be related to the ability of these methanotrophs to grow under acidic conditions. This concurs with our previous research finding (Syed et al, 2016b). Although the diversity (Shannon's) increased in the pond biofilters over the study period, the population remained even, which suggested the presence of a stable methanotroph community in the pond biofilters.…”

“…The increase in type I methanotroph abundance was supported by the increase in the Methylobacter group of methanotrophs, but the Methylococcus abundance was very low. The suppressed growth of the Methylococcus population at higher CH 4 concentrations was also evident in our previous studies (Syed et al, 2016a, 2016b). Interestingly, an increasing trend in the abundance of the Methylocapsa ‐like methanotrophs was noticed in the pond biofilters, and this could be related to the ability of these methanotrophs to grow under acidic conditions.…”

“…The CH 4 removal rate from these biofilters could be a function of many factors, including the importance of the surface area. In a previous study, a maximum removal rate of 30.3 g CH 4 m −3 h −1 was achieved with a 58‐L biofilter column (54‐cm height and 0.35‐m diameter or surface area) (Syed et al, 2016b), whereas the floating biofilter with about half of this volume (30 L, 6‐cm depth and 0.5‐m × 1‐m surface area) removed up to 101.5 g CH 4 m −3 h −1 of CH 4 . This indicates that the CH 4 oxidizing capacity of the biofilters can be limited by the surface area (Cohen, 2001), rather than the height or depth of the biofilter.…”

“…Water and nutrients were not added to the biofilters as a treatment, but during extremely dry conditions (once in February and once in October), 500 mL of water was added to keep the biofilter bed moist, since a previous study has shown that volcanic pumice soil at moisture of ≤15% (dry wt.) does not support CH 4 oxidation (Syed et al, 2016b). Before the deployment of the floating biofilters, the volcanic pumice soil, previously stored at 4°C, was exposed to 80% CH 4 (6 g m −3 h −1 ) for ∼1 mo under laboratory conditions to allow methanotroph activation in the soil.…”

“…The PCR was performed in 40‐μL reaction volumes using a thermocycler (MaxyGene). Reaction mixtures were prepared as described in Syed et al (2016b). The PCR products were digested for 4 h at 37°C with 2.5 U of MspI restriction enzyme in 40‐μL reaction volumes (Henneberger et al, 2012).…”