Citation: Kõlli, R.; Kauer, K.; Tõnutare, T.; Lutter, R. Ecosystem Carbon Stocks and Their Annual Sequestration Rate in Mature Forest Stands on the Mineral Soils of Estonia. Forests 2022, 13, 784. https://doi.org/10.3390/ f13050784 Academic Editors: Anna Zavarzina, Irina N. Kurganova, Yakov Kuzyakov, Francisco Matus, Agustin Merino and Wenhua Xiang Received: 19 April 2022 Accepted: 16 May 2022 Published: 18 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Ecosystem Carbon Stocks and Their Annual Sequestration Rate in Mature Forest Stands on the Mineral Soils of Estonia Raimo Kõlli 1 , Karin Kauer 1 ,Tõnu Tõnutare 1 and Reimo Lutter 2, * 1 Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 5, 51006 Tartu, Estonia; raimo.kolli@emu.ee (R.K.); karin.kauer@emu.ee (K.K.); tonu.tonutare@emu.ee (T.T.) 2 Institute of Forestry and Engineering, Estonian University of Life Sciences, Kreutzwaldi 5, 51006 Tartu, Estonia * Correspondence: reimo.lutter@emu.ee Abstract: Mature forest ecosystems are the most considerable reservoir of organic carbon (OC) among terrestrial ecosystems. The effect of soil type on aboveground OC stocks and their annual increases (AI) of overstorey tree, understorey tree and ground vegetation layers in Estonian forest phytocoenoses with mature stands on mineral soils were studied. The study enfolds nine mineral soil groups, which are characterized by their phytocoenoses composition, soil cover properties and tree stands’ taxation data. An assemblage of soil and plant cover or plant–soil system is the main focus point in explaining causal and quantitative sides of ecosystems functioning. Surface densities of OC stocks in aboveground phytomass of forests varied significantly in the range of 52–100 Mg OC ha -1 . High AI or productivity (4.8–5.5 Mg OC ha -1 year -1 ) is a characteristic of forest ecosystems formed on leached, eluviated and pseudopodzolic soils. Forest ecosystem ground vegetation, which is an important ecological indicator, fulfils vacant ecological niches with herbs and/or mosses (up to 0.50 Mg OC ha -1 ). The variation of ecosystem OC stocks and their AI by soil type should be taken into account in regional OC stocks and its annual increase estimations. Keywords: forest ecosystem; phytocoenosis; soil cover; organic carbon stock; phytomass; annual increase 1. Introduction Forest ecosystems (as assemblages of phytocoenoses with functioning soil covers) are the most important sink and reservoir of organic carbon (OC) among terrestrial ecosys- tems [1–3]. Significant OC stocks of temperate and boreal forests are located in aboveground overstorey tree phytomass, whereas the storage of OC in superficial soil layers (mostly in humipedon) may be independent of soil type and it can be both lower or higher than that of the aboveground OC storage [1,3,4]. Soil cover composition and functioning are the main drivers of ecosystem net productivity [5]. Forest ecosystems on mineral soils have a relatively higher capacity to allocate and store OC as an aboveground pool, while in contrast, OC in organic soils is stored mainly in soil cover and to a lesser extent in phytomass [2,6,7]. The local variation of mineral soil properties can be very different, which influences substantially the accumulation of OC in phytomass and its annual production. Soil cover, being the most important component of terrestrial ecosystems [2], is in relatively diverse hemi-boreal conditions. When managing and assessing soil covers, their properties and quality diversities should be taken into account [3,7]. Dependent of soil forming conditions, soils of various genetic type may be presented, each with different moisture conditions, skeletal fraction fine earth texture, organic matter sequestration (or capture, removal accumulation) into the soil and other [8]. Soil cover properties depend not only on the OC sequestration capacity in the soil, but also on the turnover rate between soils and plants [5,9]. Since ancient times, land use of various regions was arranged in perfect harmony with the soil type properties of an area [10]. As a result of this, the existing soil covers have been divided into arable, forest and grasslands, among which different kinds of rangelands may be presented. To the last group belong soils which are unsuitable Forests 2022, 13, 784. https://doi.org/10.3390/f13050784 https://www.mdpi.com/journal/forests