Long-term Effects of Drainage and Rewetting on the Degradation and Preservation of Peat Organic Matter in Warm Climate
Abstract. Peatlands cover about 3 % of the earth's land surface, while storing about 20 % of the total global soil organic carbon. These carbon stocks are largely at risk as many peatlands have deteriorated since the Industrial Revolution due to conversion to agricultural land by drainage. Globally, peatland drainage is responsible for over 3.5 % of anthropogenic greenhouse gas emissions. About 75 % of these emissions originate from warm climate regions. Mitigation of these emissions can be achieved by rewetting degraded peatlands. This study focuses on a warm-climate peatland that has been cultivated for the past ~70 years (Hula Valley, Israel). The historic marsh was drained in 1957 for agricultural use and underwent a hydrological restoration project for elevating and stabilizing groundwater table since 1994. This land management history resulted in a sedimentary peat column that can be divided into three distinct sub-sections: drained, rewetted and pristine peat. This setting enables studying the drainage and rewetting effects on soil organic matter (SOM) degradation and preservation under warm climates. For this purpose, five sediment cores, 4 m long each, were excavated from cropland located over the historic marsh area. Locations were chosen to match previous studies on this site. Each soil profile was characterized using Rock-Eval® thermal analysis of the organic matter, and short-term soil aerobic respiration experiments. Integration of these results with historic SOM content data and with SOM modelling was used to explore the long-term process and rate of degradation. We found that the mean SOM content in the top one meter of the soil profile declined from 68 ± 4 wt.% to 21 ± 2 wt.% over the past 66 years, excluding compaction effect. In comparison to the drained section, the rewetted and pristine sub-sections has a mean SOM of 33 ± 2 wt.% and 64 ± 2 wt.%, respectively. A peak in pyrite concentration beneath the recent water table-level, was observed in most profiles, indicating anaerobic conditions and sulfur recycling. Rock-Eval® thermal analysis demonstrated that during decomposition, the residual SOM became more oxidized and contained a lower proportion of thermally labile SOM, with a significant difference found between drained and rewetted peat. These results imply that the raising of the water table (~30 years ago) effectively helped preserving organic matter compared to the drained section. Long-term SOM field data were integrated and studied using an SOM decomposition model and by incorporating respiration fluxes. The resulting trends highlighted that the first few decades of exposure are highly significant for the fate of the carbon stock, leading to substantial CO2 emissions. These emissions were lower by 60–85 % after 70 years. Furthermore, our results suggest that currently, approximately 13–21 wt.% of SOM persists as resistant organic matter in the degraded peat.
General comments:
This paper presents an interesting work conducted on a peatland in Israel. The place was drained to be used for cultivation, then rewetted when problems (fires, erosion, etc.) began to arise. The authors sampled five 4-meter cores in the deepest part of the peatland, and identified three different parts: drained, rewetted, and pristine peat. They conducted Rock-Eval thermal analyses to characterise the thermal stability, stoichiometry and properties of the peat. The results show that rewetting the peatland clearly helped reduce SOM loss, although the rewetted part does not come back to its anterior, pristine state.
My main concern regarding this work was whether it is robust to use Rock-Eval to analyse peat; we know that Rock-Eval shows some limits with highly organic compounds (e.g. litter), which most probably apply to the case of peat. This problem has been addressed in Supplement: the authors took the precaution to investigate this question with LOI procedure. I would still be very cautious when applying Rock-Eval to such organic soil, however on this specific case the results are clear and consistent. This paper is a nice addition to the peatlands knowledge.
Specific comments:
L135: does that mean the WTL was at surface level before drainage? Do we have information on this? (I understand 'old' information is scarce.)
L198: an illustration, similar as Fig.1 in Cécillon et al. (2018), could be useful to visualize how you cut/sum your signals.
L210: why consider only the CO2 emitted during pyrolysis, and not CO? Same, why disregard oxygen emitted as both during oxidation? More generally, you chose to follow Behar et al. (2001)’s definition of oxygen index, taking CO2-carbon into account, rather than more recent definition focusing on oxygen only (with stoichiometric correction as in Cécillon et al., 2018; Saenger et al., 2013; Delahaie et al., 2023). Did you consider both definitions before choosing Behar’s?
L245: the explanation for why 22°C and 33°C could figure above, L231, so that we immediately understand why you chose these.
L249: I think the equations should be rewritten formally so as to only contain variables, not mixed up with units. Describe the variables and their units above or below.
L261: the explanation as to why start at -30 cm and not above could be there instead of L277.
L263: why isn’t there a rewetted section in the cores A, C, and D? Perhaps I missed the explanation, but I don’t understand why the rewetting seems to not have 'worked' everywhere.
L288: usually, when talking about 'persistent', it is good to precise which duration you are referring to, as it cannot be forever: does it persist for decades? Centuries, millennia?
L356: there is debate on the significance of TpkS2, as the peak is not always related to the quantity of hydrocarbons evolved during the whole process (you can have a very short peak at the beginning while most of the matter evolves later). Did you have a look at other indices, such as T90_HC_PYR, the temperature at which 90% of hydrocarbons have evolved (as described in Cécillon et al. (2018) for instance)?
L459: sentence unclear; perhaps the subscript disappeared, it would make more sense with it…
L485: also, the priming effect would probably not extend much under the root depth, which you excluded by starting at -30 cm.
L493: did you consider radiocarbon analyses?
L526: the PARTYSOC model has some limitations, even in its v2 form. In particular, soils with a high SOM should not be treated with this model, as it has never been trained nor tested on such data.
Technical corrections:
General: check for grammar, non-verbal sentences, etc.
General: the abbreviation for gram is g, not gr.
L313: 'purple' as in the caption, not light pink. Maybe homogenise the color with other figures…
References: