the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Mar 2025
Organic soil carbon balance in drained and undrained hemiboreal forests
Abstract. Drainage of organic soils is associated with increasing soil carbon (C) efflux, which is typically linked to losses in soil C stock. In previous studies, drained organic forest soils have been reported as both C sinks and sources depending on, e.g., soil nutrient and moisture regime. However, most of the earlier research was done in boreal region, and both the magnitude of C efflux and the impact of soil moisture regime on soil C stock are likely to vary across different climatic conditions and ecosystems, depending further on vegetation. A two-year study was conducted in hemiboreal forest stands with nutrient-rich organic soil (including current and former peatlands) and a range of dominant tree species (black alder, birch, Norway spruce, Scots pine) in the Baltic states (Estonia, Latvia, Lithuania). In this study, we analysed the C balance of organic soil in drained (19) and undrained (7) sites. To assess the C balance, soil respiration was measured along with evaluation of C influx into the soil through aboveground and belowground litter. To characterize the sites and factors influencing the C fluxes, we analysed soil temperature, water table level, physical and chemical parameters of soil and soil water. On average, no changes in soil C stocks (0.45±0.50 t C ha⁻¹ year⁻¹) were observed in drained sites dominated by black alder, birch, or Norway spruce, while drained Scots pine sites showed soil C removals with a mean rate of 2.77±0.36 t C ha⁻¹ year⁻¹. In undrained birch- and spruce-dominated sites, soil functioned as mean C sink at 1.33±0.72 t C ha⁻¹ year⁻¹, while the undrained black alder stands showed an uncertain C balance of 1.12±2.47 t C ha⁻¹ year⁻¹. The variability in C balances were influenced by the nutrient-rich soil exhibiting a wide range of nutrient conditions and organic matter quality. Thus, indicating that soil macronutrient concentrations and pH can determine whether the soil functions as a C source or sink.
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RC1: 'Comment on egusphere-2025-1032', Anonymous Referee #1, 09 Apr 2025
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This manuscript addresses the issue of the extent to which peatland soils change their C stock in response to drainage. This is a somewhat contentious issue because of its importance in developing reliable estimates of the C budget of entities, such as political states, in countries where peatland drainage, for forestry and other uses form an important part of the estimation, such as the use of Emission Factors. Contentious because the literature shows very variable results on C balance because of the variability in environment (e.g. climate), extent of drainage (lowering of water table), organic soil thickness, properties of the soil, duration of drainage, vegetation cover and methods employed. The value of the manuscript is in its adoption of similar methods across a wide range of sites, driven by the variability above, involving an area in which peatland drainage for forestry is significant, and including three countries.
The methods involve a variety of processes leading to C import and export of the soil, so a large number of measurements, and estimations, need to be made. Here, the same methods were applied (more or less) across 26 sites, of which 7 were undrained and 19 were drained and over 2 years. This follows a literature review in 2023 (Jauhianen et al. Biogeosciences 20, 4819–4839; the Reference is incomplete on p. 21) for greenhouse gas fluxes in boreal and cool temperate regions with the sites in this manuscript being in the hemi-boreal region of Estonia, Latvia and Lithuania. The distinction of hemi-boreal is a bit confusing with other terms, such as cool temperate, Cool Temperate Mist climate region etc. and could be clarified (says on line 50 ‘between the temperate and boreal zones’). It appears that there was no attempt (or success) to include a comparison of drained and undrained sites, based on the latitude and longitude data in Table S1, though there appear to be two pairs in Latvia (Fig. 1). Please clarify.
The results are combined into a series of graphs and tables (many in Supplementary Information) for individual measurements which are combined into estimates of the annual soil C balance combination which forms the focus of the initial part of the Discussion. The authors recognize there is great variability with some unexpected patterns emerging, though the limited replication of site types means that categorization of type is unwarranted (lines 536 and following). There is also a recognition that the period of drainage at the sites is not clearly known, but could be many decades, so that the early effect of drainage may be muted in the results that were obtained. There is a detailed discussion of the merits and limitations of the methods employed to contribute to the estimate of the soil C balance.
The conclusions of the study are in an assessment of the overall C balance of several forested peatlands, drained or undrained, within a fairly narrow range and that a variety of properties, ranging from environmental to tree type, can influence the results. In the C balance, the expectation is that this measure (C tons balance etc.) converts into CO2. This maybe the case, but what about other C forms in the C cycle? Methane would play an insignificant role in the C balance for most of the sites, given the low water table in most sites, including the undrained ones: probably up to 0.05 t C/ha/yr in the wetter sites and maybe CH4 uptake in the drier sites. Loss of dissolved organic carbon (DOC) would result from leaching of the soil, and may account for up to 0.10 t C/ha/yr additional loss, but also small to most of the soil C balance estimates that have been made.
The manuscript started with a comment on the use of Emission Factors by the IPCC and states, though no EF values were given. If the objective of the study, beyond the science of the forested systems, was to contribute to a better estimate of the variability and magnitude of EF, it would be useful to see how the authors think these study would contribute to that objective. What ‘better’ estimate of EF could have been made using the results assembled in the manuscript, with a lot of good, hard work over two years and standardized methods, compared to the ‘guesswork’ of the past?
Specific comments:
24 It seems that the estimated changes in C do not involve + and – signs. Such as soil C removal from drained Scots pine sites was 2.77 units while C sink occurred in undrained black alder sites there was an average sink of 1.33 units. Throughout the manuscript could ‘loss’ estimates be given a negative sign (e.g. -2.77 +/- 0.36 units) and ‘gain’ estimates be given a positive sign (1.33 +/- 0.72 units. The graphs showing the ‘C balance’ (Fig 9 and 10) include negative values, please be consistent. The notation used in Figures also varies: for example Fig 8 has ‘Carbon flux’ and 9 and 10 have ‘C balance’ with the same units and meaning. Please standardize.
15 the boreal region
35 why not use ‘faster’ rather than ‘higher’ to describe a rate?
46 One of the studies was on a drained peatland used for horticultural crop production, so is not representative of the types used in the EF estimates.
83 Jauhianen et al. was incompletely cited in the References.
136 data were (plural)
164 how ‘small’ was insignificant?
460 Basal area had the strongest correlation with C balance, yet in Fig. 10, the R2 of 0.14 was the smallest in the 6 graphs, several with p values < 0.01. Please check. It would be good to include the slope of the regression to indicate how much change in C balance was created by a change in the independent variable. For example, a reduction in pH from 6 to 2.5 (!) would result in a C balance (gain) of about 3 units. An increase in bGV of 0.1 to 3.1 units would result in a C balance of -1 to 4 units.
Citation: https://doi.org/10.5194/egusphere-2025-1032-RC1
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