Seasonal dynamics of methane emission from soils of wet forests: А case study of a mixed forest in the Moscow region

Methane (CH₄) the second most potent greenhouse gas in terms of contribution to global warming. Natural sources of methane includes wetlands and other freshwater ecosystems, oceans, natural gas seeps, biomass burning, and termites. However, the contribution of other natural sources should not be underestimated. One such potential source is wet forests, i.e., forests with soils under conditions of constant or temporary waterlogging. Unlike peatlands, forest ecosystems exhibit greater diversity and variability in terms of physico-chemical (e.g., nutrient availability, acidity, redox conditions) and hydrological factors (including periodic flooding and drainage), complicating their study. The magnitude of methane emissions from this source remains uncertain, but fluxes from wet forest soils may be significant. The aim of this study is to assess wet forests as potential methane sources, considering the seasonal variability of observed fluxes (case study of a mixed forest in the Moscow region).

Measurements were conducted from 2019 to 2022 at a wet forest in the Ruzsky District of Moscow Oblast, near the settlement of Dorokhovo. The study site (55°34' N, 36°23' E) is located 67 km west of Moscow's city boundary. The soils of the study site are Umbry-Gleyic Albeluvisols (soddy-podzolic gleyic soils) with silty clay loam texture. The vegetation is presented by a mixed forest dominated by Alnus glutinosa, Quercus robur, Acer platanoides, Asarum europaeum, and Mercurialis perennis. The long-term mean annual air temperature and precipitation for the study site are 5.8°C and 688 mm, respectively.

Twelve field campaigns were carried out in different seasons. Summer campaigns: 5–24 August 2019, 5–25 July 2020, 10–11 and 29–30 August 2021. Autumn campaigns: 24–25 October 2020, 9–10 October and 6 November 2021. Winter campaigns: 9 January and 26 February 2022. Spring campaigns: 8–9 March 2022 and 2–4 May 2022. Methane fluxes were measured using the static chamber method. At each measurement point, 2 to 4 chambers were deployed, with 2 to 19 flux measurements taken per chamber over a 24-hour period, which were treated as replicates. Four gas samples were taken in syringes during each of 9-60 minute flux measurement. Methane concentration in the gas samples was determined by gas chromatography. In 2022, gas concentration inside of the chamber was measured directly using a portable infrared gas analyzer LI-7810 (LI-COR, USA). Additionally, soil temperature and moisture, pH and electrical conductivity of soil water, as well as water table level (WTL) were measured.

Seven points were chosen on a transect from the point Sw1_1 with the average WTL of 31 cm above the soil surface to the point Sw1_7 with the average WTL of 11 cm below the soil surface. The median methane flux at Sw1_1 and Sw1_3, points with the best drainage on the transect, was close to zero, while the maximum flux exceeded 1 mgC × m^-2 × h^-1. At the downslope point Sw1_5, the mean WTL was 15 cm below the surface. Unlike the upslope points, no methane consumption was observed here; the median emission was 0.5 mgC × m^-2 × h^-1, with a maximum of 6.8 mgC × m^-2 × h^-1. At the further downslope point Sw1_2 (mean WTL = 0 cm), the median emission was comparable to that at point Sw1_5, but the maximum emission reached 20 mgC × m^-2 × h^-1. Finally, at points Sw1_4, Sw1_6, and Sw1_7, where the WTL was between 5 and 11 cm above the soil surface, the median flux ranged from 1.4 to 4 mgC × m^‑2 × h^-1, with maximum from 13 to 18 mgC × m^-2 × h^-1. A relatively strong correlation was found exclusively between the median methane flux and WTL (R² = 0.63). For all other investigated factors, the coefficients of determination did not exceed R² = 0.27. Furthermore, the raw data (prior to median calculation) showed no significant regression dependence with any of the factors. Our results correspond to the published data on methane emissions from wet forests in the temperate climatic zone and the southern taiga forests of Western Siberia. Similar median emission values were also observed in a tropical forest in the Congo River basin, although the maximum emission values there were several times higher.

Therefore, our findings indicate that wet forests in the Moscow region can be a source of atmospheric CH₄. Because of the cold seasons of the Moscow region (and, more broadly, the European part of Russia) is relatively warm, methane emissions during the autumn-winter-spring period likely make a significant contribution to the annual flux. Future research should focus on: (1) more precise mapping of wet forest coverage, (2) investigating the mechanism of methane transport by trees and plants in wet forests, (3) studying the spatial variability of methane fluxes across different types of wet forests, (4) quantifying the relative contribution of diffusive and advective methane transport in mineral soils, and (5) understanding the functioning of methanogenic communities under relatively limited (compared to peatlands) availability of carbon sources.