The effect of drought on dissolved organic carbon (DOC) release from peatland soil and vegetation sources

Abstract

Abstract. Drought conditions are expected to increase in frequency and severity as the climate changes, representing a threat to carbon sequestered in peat soils. Downstream water treatment works are also at risk of regulatory compliance failures and higher treatment costs due to the increase in riverine dissolved organic carbon (DOC) often observed after droughts. More frequent droughts may also shift dominant vegetation in peatlands from Sphagnum moss to more drought-tolerant species. This paper examines the impact of drought on the production and treatability of DOC from four vegetation litters (Calluna vulgaris, Juncus effusus, Molinia caerulea and Sphagnum spp.) and a peat soil. We found that mild droughts caused a 39.6 % increase in DOC production from peat and that peat DOC that had been exposed to oxygen was harder to remove by conventional water treatment processes (coagulation/flocculation). Drought had no effect on the amount of DOC production from vegetation litters; however large variation was observed between typical peatland species (Sphagnum and Calluna) and drought-tolerant grassland species (Juncus and Molinia), with the latter producing more DOC per unit weight. This would therefore suggest the increase in riverine DOC often observed postdrought is due entirely to soil microbial processes and DOC solubility rather than litter layer effects. Long-term shifts in species diversity may, therefore, be the most important impact of drought on litter layer DOC flux, whereas pulses related to drought may be observed in peat soils and are likely to become more common in the future. These results provide evidence in support of catchment management which increases the resilience of peat soils to drought, such as ditch blocking to raise water tables.

CO<sub>2</sub> and CH<sub>4</sub> budgets and global warming potential modifications in <i>Sphagnum</i>-dominated peat mesocosms invaded by <i>Molinia caerulea</i>

“…The gaseous C balance shows that both plant communities act as C sinks, with storage of 30 g C m −2 yr −1 in Sphagnum plots and 223 g C m −2 yr −1 in Sphagnum + Molinia plots. These results contrast with the assumption mentioned in the Introduction that vascular plants could lead to a decrease in C sequestration (Strakova et al, 2011). Nevertheless, the C-sink function of Molinia-dominated peat mesocosms can be questioned in view of the biomass production of Molinia caerulea.…”

“…are expected to cause a plant community shift in peatlands, with an increase in vascular 4086 F. Leroy et al: CO 2 and CH 4 budgets and global warming potential modifications plants (especially graminoids) to the detriment of Sphagnum species (Berendse et al, 2001;Buttler et al, 2015;Dieleman et al, 2015). Vascular plant invasion could lead to a faster decomposition of peat OM due to a change in litter quality as a substrate for decomposers, thereby decreasing C sequestration (Strakova et al, 2011). Furthermore, OM already stored in deep peat may be subject to increased decomposition through the stimulating effect of rhizospheric C input (Girkin et al, 2018).…”

“…If these losses are not compensated by increased gross primary productivity, peatlands could shift from a sink to a source of C and could increase greenhouse gas emissions, mainly carbon dioxide (CO 2 ) and methane (CH 4 ). Vascular plant invasion in peatlands has mostly been studied through a change in decomposition rates (Moore et al, 2007; and modification in decomposer activities (Krab et al, 2013;Strakova et al, 2011). Some studies have paid attention to CH 4 emissions with and without the presence of Carex or Eriophorum (Noyce et al, 2014;Green and Baird, 2012;Greenup et al, 2000) and to CO 2 fluxes with different plant community compositions (Neff and Hooper, 2002;Ward et al, 2013).…”

“…The aim of this study was to investigate how an invading graminoid species, Molinia caerulea, can affect the greenhouse gas C budget (GGCB) of a Sphagnum-dominated peatland. Molinia caerulea encroachment is a well-acknowledged problem in Europe linked to anthropogenic pressures such as nutrient deposition and management practices, but studies of the effects on peatland ecosystem are still limited (Ritson et al, 2017;Berendse et al, 2001;Chambers et al, 1999). Here, CO 2 fluxes and CH 4 emissions were regularly measured in mesocosms entirely covers by Sphagnum rubellum with or without Molinia caerulea during 14 months and were related to biotic and abiotic factors to estimate the annual C budget.…”

Sensitivity of peatland litter decomposition to changes in temperature and rainfall

“…Samples may have been exposed to more extreme drying conditions than under field conditions as moisture levels were not regulated by living vegetation and/or underlying water table, and the lack of a peat substrate beneath the samples from the vegetation groups could also have increased the loss of water and so negated any rainfall treatment effect. Ritson et al (2017) found that any exposure to oxygen (during laboratory experiments) increased DOC from peat and changed the quality of the C.…”