Effect of drainage and drain-blocking on the molecular and microbial composition of blanket bog peat

Abstract

Over the last century researchers have acquired substantial evidence that water table regime is a fundamental determinant of peatland health. Peatland health refers to the ability of a peatland to perform essential ecosystem functions for humanity, including but not limited to, carbon sequestration, water filtration, flooding prevention and sustenance of high biodiversity. Undrained peatlands with a high, stable water table level perform these services better than drained peatlands. The question is, why?

The molecular and microbial composition of peat determines the physical (e.g. bulk density) and biological (e.g. enzyme activity) properties of peat which are responsible for ecosystem functioning. Peatland drainage changes the molecular and microbial composition of peat. This results in peatlands that no longer perform the essential ecosystem functions listed above. The consequences for humanity are serious: carbon release driving global warming, flooding, fresh water pollution and loss of biodiversity.

The primary aim of this work is to develop a better deterministic understanding of how peatland drainage changes the molecular and microbial characteristics of peat, and whether restoration attempts through drain-blocking have been successful in reverting these changes. To this end, peat cores were extracted from drained, undrained, and drain-blocked regions of four UK blanket bogs and analysed using a range of complementary chemical and biological techniques. The drain-blocked regions were restored between six and nineteen years prior to sampling.

Analysis of water-logger data confirmed that the water table level was deeper below the peat surface in drained regions of bogs than undrained bogs. Drain-blocking showed limited success at raising the water table to undrained levels. In situ field recordings made at the time of sampling showed that drainage caused profound shifts in vegetation cover, from Sphagnum mosses to drier-tolerant vascular plants such as heaths and sedges. In three out of four blanket bogs there was a recovery of Sphagnum in drain-blocked peat. The lack of recovery in one bog is likely to have resulted from the initial severity of drainage.

Physiochemical and enzyme assays undertaken using low-resolution methods found that drained peat had higher bulk density, carbon percentage, phenol content and hydrolase activity and lower pH and oxygen percentage. In general, drain-blocked peat showed physicochemical values closer to undrained peat with the exception of bulk density and carbon content. The extent of physiochemical recovery was not clearly linked to time elapsed since drain-blocking.

An increase in phenol content due to drainage was also observed by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis of peat porewaters. FT-ICR MS analysis showed a notable increase in the diversity of oxy-aromatic phytochemical compounds in drained pore waters which caused the entire molecular pool to increase in mean aromaticity. Drain-blocked peat had mean aromaticity values in between undrained and drain-blocked peat, however the effect of drain-blocking, although not clearly linked to time, was highly dependent on site.

Finally, metataxonomic analysis of bacteria and archaea using high-throughput sequencing of the 16S rRNA gene from DNA extracts found that undrained peat had a distinct microbial community composition compared to drain-blocked and drained peat. Furthermore, there was a high degree of similarity between DNA and RNA communities in the site with the longest time since drain-blocking which indicated stable community composition over the recent past (DNA is representative of both living and dead organisms, whereas RNA is only representative of only living organisms). Together, these results indicated that drain-blocked communities were stuck in a stable drained-like state that species from undrained communities found hard to re-colonise.

Overall, drainage caused fundamental changes to peat at every level of analysis. In general, differences between undrained and drained peat were most evident in peat sampled from 20 to 50 cm compared to surface peat. The effects of drain-blocking varied by site. On the whole, drain-blocked peat showed more undrained-like characteristics in the field and low-resolution datasets and more drained-like characteristics in the high-resolution data. There was a notable lack of change in the microbial community of drained-block peat. This work demonstrates that drain-blocking blanket bogs is partially successful in changing to the molecular composition of peat back to an undrained state, but at the time-scales studied was not successful at changing the microbial composition of peat.