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

Abstract

<p>Methane (CH<sub>4</sub>) has high global warming potential, and its atmospheric concentration is increasing rapidly at the present rate of 0.3% yr<sup>-1</sup>. 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 CH<sub>4</sub>; however, the complex interactions between microbial processes of CH<sub>4</sub> production and oxidation, and environmental drivers are not well understood. Balance of CH<sub>4</sub> 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 CH<sub>4</sub> depends on the soil type, environmental conditions, species composition, living trees and deadwood, age and health conditions of the tree stand, and their CH<sub>4</sub> balance can vary between seasons and years.</p><p>In this study, we present long-term CH<sub>4</sub> 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 CH<sub>4</sub> 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.</p><p>The results showed that mature pine forest soil was an annual net sink of CH<sub>4</sub>: −21.14 + 0.59 g ha<sup>−‍1</sup> yr<sup>-1 </sup>(mean + SE). No significant difference (p < 0.05) was found between the soil CH<sub>4</sub> uptake and tree age. Methane uptake correlated negatively (r<sup>2 </sup>= 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 CH<sub>4</sub> uptake (−36.93 g ha<sup>−‍1</sup>) was observed in July 2018, the warmest and driest month during the overall period. Even though soil moisture was only weakly correlated (r<sup>2</sup> = 0.15, p < 0.05) with CH<sub>4</sub> uptake, the CH<sub>4</sub> flux was affected by precipitation. As a result of this, it is noticed that CH<sub>4</sub> uptake in the cold and wet conditions decreased with increasing precipitation in winter and increased with warming during warm and dry conditions in summer.</p><p>Concluded, our coniferous pine forest was sequestering CH<sub>4</sub> during the investigated five years. The soil CH<sub>4</sub> uptake could be explained by CH<sub>4</sub> oxidation at optimal temperature in the water-unsaturated surface soil regulating the soil's microbial activity.</p>