A new method for estimating carbon dioxide emissions from drained peatland forest soils for the greenhouse gas inventory of Finland
- 1Natural Resources Institute Finland (Luke), Joensuu, Finland
- 2Natural Resources Institute Finland (Luke), Kokkola, Finland
- 3Natural Resources Institute Finland (Luke), Helsinki, Finland
- 4Department of Forest Sciences, University of Helsinki, Helsinki, Finland
- 1Natural Resources Institute Finland (Luke), Joensuu, Finland
- 2Natural Resources Institute Finland (Luke), Kokkola, Finland
- 3Natural Resources Institute Finland (Luke), Helsinki, Finland
- 4Department of Forest Sciences, University of Helsinki, Helsinki, Finland
Abstract. Reporting the greenhouse gas (GHG) emissions from the LULUCF sector in the GHG inventory requires sound methods for estimating both the inputs and outputs of carbon (C) in managed ecosystems. Soil CO2 balance of forests consists of the CO2 released from decomposing soil organic matter (SOM) and the C entering the soil through aboveground and belowground plant litter input. Peatlands drained for forestry release soil C as CO2 because the drainage deepens the oxic peat layer prone to SOM decomposition. IPCC Guidelines provide default CO2 emission factors for different climatic zones and the defaults or locally adapted static emission factors are commonly in use in GHG inventory reporting for drained peatlands. In this paper, we describe a new dynamic method to estimate the CO2 balance of drained peatland forest soils in Finland. Contrary to static emission factors, the annual CO2 release from soil is in our method estimated using empirical regression models driven by time series of tree basal area (BA), derived from the national forest inventories in Finland, time series of air temperature and the drained peatland forest site type. Aboveground and belowground litter input is also estimated using empirical models with newly acquired turnover rates for tree fine roots and BA as a dynamic driver. All major components of litter input from ground vegetation and live, harvested and naturally died trees are included. Our method produces an increasing trend of emissions from 1.4 to 7.9 Mt CO2 for drained peatland forest soils in Finland for the period 1990–2021, with a statistically significant difference between years 1990 and 2021. Across the period 1990–2021, annual emissions are on average 3.4 Mt and −0.3 Mt in southern and northern parts of Finland, respectively. When combined with data of the CO2 sink created by trees, it appears that in 2021 drained peatland forest ecosystems were a source of 2.3 Mt CO2 in southern Finland and a sink of 2.5 Mt CO2 in northern Finland. We compare the emissions produced by the new method with those produced by the old GHGI method of Finland and discuss the strengths and vulnerabilities of our method in comparison to static emission factors.
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Jukka Alm et al.
Status: open (until 12 Feb 2023)
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RC1: 'Comment on egusphere-2022-1424', Anonymous Referee #1, 17 Jan 2023
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Review report, egusphere-2022-1424
The paper addresses a very important and timely research question, i.e. the CO2 balance of drained forested peat soils. The CO2 emissions from drained forested peat soil receives currently a very strong attention in many countries. Accurate and precise methods to estimate the soil CO2 fluxes, as well as the full ecosystem level CO2 fluxes are urgently needed.
The overall approach is statistical modelling with the main data input from the national forest inventory and meteorological data and parametrization based on empirical data on litter production as well as decomposition of different main components from dominating plant functional types.
The main results is that forested drained organic soils in Finland represents source of CO2 and that the total soil CO2 emissions from these soils have increased from 1.4 to 7.9 Mt CO2
for the period 1990–2021. Accounting for the entire ecosystem, i.e. also including photosynthesis and calculated for the whole country, forests growing on drained peatlands were a net sink of 0.2 Mt CO2 in 2021, i.e. close to C neutral.
The paper is an important contribution to a science based ground for accessing the land-atmosphere CO2 balance of forested drained organic soils.
My major concern is that critical validation of the method is missing. The authors have made an extensive comparison with other emission factors. Still I am missing comparisons with direct measurements for a few example systems representing dominating types and climate settings. It is not an easy task, still urgently needed.
Also the current version of the abstract is hard to digest. It currently require that the reader has read the full article before reading the abstract. Please see detailed comments on the abstract.
Detailed comments:
L 15 Discharge C export needs at least to be considered and potential bias if not included must be discussed. Necessarily not in the discussion section
L15 conceptually I agree that the soil C balance is made up by just by above- and below- ground litter input and heterotrophic (saprotrophic) CO2 respiration. AND possibly also discharge C-export. It is though important to clarify why autotrophic root respiration is excluded.
16-17 Reformulate. Peatlands drained for forestry release CO2 even if the WT is not change. The change in WT due to drainage and forest ET may change the soil CO2 flux but also non-drained peatlands release soil CO2.
L 20 here it is absolutely necessary that you clarify that the CO2 flux you model emanate from saprotrotrophic CO2 production only. Not stating this explicitly will confuse many readers.
L 24 do not understand. How is “harvested” trees included?
L25 what area is the CO2 emission representing? Is it total or per unit area? I would very much prefer first presenting per unit area, e.g. m-2 or ha-1 and then areal totals. Currently it is very confusing.
L25 “1.4 to 7.9 Mt CO2” You must add time unit, i.e. yr-1.
L27 is this totals for northern and southern Finland or what?`
L28 what about the forest floor PFT´s contribution to CO2 uptake. It can be substantial. If it is not in your data it must me clearly stated that its contribution is so small that it can be neglected, which I really doubt.
L25-30 this result section is very confusing. I suggest presenting both unit area based estimates (also adding the time unit (yr-1)) and areal totals
L49 In the abstract you state an annual drained peatland soil CO2 flux during 2021 of 7.9 Mt and in the introduction state 3.8 for 2020. Thus you need to be specific in the introduction and clarify that according to method xx the annual peatland soil CO2 during 2020 was 3.8 Mt
L63-63 does these references really refer to saprotrophic CO2 flux, NOT including ANY autotrophic respiration. It is very important that you make this very clear. As “soil CO2 flux” normally includes also the autotrophic root respiration I think it is very important that you make it very clear in all of the text what you actually include.
L80 “…. by the old method” You must add reference after this statement
L 87 C mass input
L 91 think the sentence “Negative values denote net removal of CO2 from the atmosphere” is confusing. While a forested peatland represent a net removal from the atmosphere depends on the entire system (ecosystem) and not just the soil.
I suggest that you instead use something like “Negative values denote net increase of soil C ….. and also suggests that the reference to the atmosphere only is valid when considering the entire ecosystem, not just the soil system.
L117 “The areas and proportions of FTYPEs of all drained peatland forests remaining forest in southern and northern Finland,”
!Something is missing in phrase in italics above
L258 ??? “uncertainty less than 100 %;” what does this mean, 2% or 98% or what? Reformulate
L256 – 264 would very much prefer to have data first presented related to unit area, e.g. ha and then as areal totals. Just having national or regional totals makes it impossible to relate to quantitative data from other sources.
L296 give reference to “Yasso07 modelling”
L 320-321 for the autotrophic CO2 sink strength you must include also the forest floor vegetation component. If not including you must at least do a sensitivity analysis on how not including that term affects the results.
L 330-331 the increase in annual temperature is NOT relevant. It is only changes in temperatures above zero (simplifying but much better than referring to annual averages) that actually affects the production or decomposition. If winter time temperatures are -10 or -4 does not affect either litter production or decomposition. Please refer to only seasonally relevant temperatures. Also differentiate between direct temperature effects and e.g. changes in growing season lengths.
L 332 how can the temporal increase in soil CO2 flux be 8.1 when you in the result section state a change from 1.4 to 7.9 Mt over the studied time period?
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RC2: 'Comment on egusphere-2022-1424', Anonymous Referee #2, 01 Feb 2023
reply
This paper developed a new method to estimate soil CO2 emissions based on empirical data and models for SOM decomposition and litter production from drained peatland forest in Finland. There are some merits for this study, which also provide new results can be utilized in IPCC. However, I am not convinced by the predicted data at the current stage. The major concerns are lacking validation of the calculated soil CO2 emissions. The yearly time-scale is also not promising. Second, the authors claimed that water table depth is the main factor that controls decomposition in drained wetlands. So why not predicting water table depth and then calculate soil CO2 emissions?
Some technical comments:
- The abstract is hard to understand at the current stage. The authors should improve it.
- Line (L) 13, explain the meaning of LULUCF.
- L30, explain GHGI.
- Lines 36-50, the logic of these two paragraphs are a bit of confusion. What do you want to say?
- 2.1, CO2 should be CO2.
- The space between value and % can be deleted.
- What are the units of equations 1 and 2? What’s the difference between carbon balance and CO2 balance?
- Figures and Tables should be shown in order.
- The authors used annual temperature, precipitation, and other climatic data to calculate soil CO2 That would cause large discrepancy between the calculated and actual data. I would suggest the authors calculate the daily data, at least monthly data.
- “Old calculation method” is not a good name, which can be revised to the name of this method.
- Results, how can you convince the readers if your data do not have any validation?
Jukka Alm et al.
Jukka Alm et al.
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