Long-term soil methane uptake in a mature pine forest in Soontaga Forest Station, Estonia

Methane (CH₄) has high global warming potential, and its atmospheric concentration is increasing rapidly at the present rate of 0.3% yr^-1. Forest ecosystems cover a large part of the biosphere and play a significant role in climate change. Upland forest soils are considered as important terrestrial sinks for atmospheric CH₄; however, the complex interactions between microbial processes of CH₄ production and oxidation, and environmental drivers are not well understood. Balance of CH₄ in the forest ecosystems depends on two main natural processes, i.e., anaerobic methanogenesis and aerobic methanotrophy, driven by multiple environmental factors. A forest ecosystem's ability to exchange CH₄ depends on the soil type, environmental conditions, species composition, living trees and deadwood, age and health conditions of the tree stand, and their CH₄ balance can vary between seasons and years.

In this study, we present long-term CH₄ fluxes (from 2015 to 2019) in a 60-200-year-old coniferous forest site of Scots pine (Pinus sylvestris) grown on loose sandy soil in Soontaga research station (58°01'N 26°04'E) in Estonia. The fluxes of CH₄ were measured every two weeks, using a manual static soil chamber (n = 6) and gas chromatography method. Air temperature, precipitation and humidity, and soil moisture and temperature (10 cm depth) were measured continuously. The average annual temperature and precipitation recorded were 7.3 + 1.0 °C and 54.3 + 3.9 mm, respectively.

The results showed that mature pine forest soil was an annual net sink of CH₄: −21.14 + 0.59 g ha^−‍1 yr^-1 (mean + SE). No significant difference (p < 0.05) was found between the soil CH₄ uptake and tree age. Methane uptake correlated negatively (r² = 0.61, p < 0.05) with soil temperature and showed similar seasonal dynamics being highest during the vegetation period (Apr-Oct) and lowest during the non-vegetation period (Nov-Mar). The highest CH₄ uptake (−36.93 g ha^−‍1) was observed in July 2018, the warmest and driest month during the overall period. Even though soil moisture was only weakly correlated (r² = 0.15, p < 0.05) with CH₄ uptake, the CH₄ flux was affected by precipitation. As a result of this, it is noticed that CH₄ uptake in the cold and wet conditions decreased with increasing precipitation in winter and increased with warming during warm and dry conditions in summer.

Concluded, our coniferous pine forest was sequestering CH₄ during the investigated five years. The soil CH₄ uptake could be explained by CH₄ oxidation at optimal temperature in the water-unsaturated surface soil regulating the soil's microbial activity.