Long-term soil methane uptake in a mature pine forest in Soontaga Forest Station, Estonia
Authors: Sardar Ali Muhammad Kamil , Mander Ülo , Schindler Thomas , Machacova Katerina , Rannik Kaire , Uri Veiko , Soosaar Kaido 
Published: March 2021

&lt;p&gt;Methane (CH&lt;sub&gt;4&lt;/sub&gt;) has high global warming potential, and its atmospheric concentration is increasing rapidly at the present rate of 0.3% yr&lt;sup&gt;-1&lt;/sup&gt;. 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&lt;sub&gt;4&lt;/sub&gt;; however, the complex interactions between microbial processes of CH&lt;sub&gt;4&lt;/sub&gt; production and oxidation, and environmental drivers are not well understood. Balance of CH&lt;sub&gt;4&lt;/sub&gt; 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&lt;sub&gt;4&lt;/sub&gt; depends on the soil type, environmental conditions, species composition, living trees and deadwood, age and health conditions of the tree stand, and their CH&lt;sub&gt;4&lt;/sub&gt; balance can vary between seasons and years.&lt;/p&gt;&lt;p&gt;In this study, we present long-term CH&lt;sub&gt;4&lt;/sub&gt; 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&amp;#176;01'N 26&amp;#176;04'E) in Estonia. The fluxes of CH&lt;sub&gt;4&lt;/sub&gt; 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 &amp;#176;C and 54.3 + 3.9 mm, respectively.&lt;/p&gt;&lt;p&gt;The results showed that mature pine forest soil was an annual net sink of CH&lt;sub&gt;4&lt;/sub&gt;: &amp;#8722;21.14 + 0.59 g ha&lt;sup&gt;&amp;#8722;&amp;#8205;1&lt;/sup&gt; yr&lt;sup&gt;-1 &lt;/sup&gt;(mean + SE). No significant difference (p&amp;#160;&lt;&amp;#160;0.05) was found between the soil CH&lt;sub&gt;4&lt;/sub&gt; uptake and tree age. Methane uptake correlated negatively (r&lt;sup&gt;2&amp;#160;&lt;/sup&gt;= 0.61, p&amp;#160;&lt;&amp;#160;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&lt;sub&gt;4&lt;/sub&gt; uptake (&amp;#8722;36.93 g ha&lt;sup&gt;&amp;#8722;&amp;#8205;1&lt;/sup&gt;) was observed in July 2018, the warmest and driest month during the overall period. Even though soil moisture was only weakly correlated (r&lt;sup&gt;2&lt;/sup&gt;&amp;#160;=&amp;#160;0.15, p&amp;#160;&lt;&amp;#160;0.05) with CH&lt;sub&gt;4&lt;/sub&gt; uptake, the CH&lt;sub&gt;4&lt;/sub&gt; flux was affected by precipitation. As a result of this, it is noticed that CH&lt;sub&gt;4&lt;/sub&gt; uptake in the cold and wet conditions decreased with increasing precipitation in winter and increased with warming during warm and dry conditions in summer.&lt;/p&gt;&lt;p&gt;Concluded, our coniferous pine forest was sequestering CH&lt;sub&gt;4&lt;/sub&gt; during the investigated five years. The soil CH&lt;sub&gt;4&lt;/sub&gt; uptake could be explained by CH&lt;sub&gt;4&lt;/sub&gt; oxidation at optimal temperature in the water-unsaturated surface soil regulating the soil's microbial activity.&lt;/p&gt;