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The aim of the study: Analysis of carbon stocks in soils and vegetation on the territory of the Pilot Carbon Polygon “Ugra”. Location and time of the study. Kaluga region, Ugra National Park, March–October, 2020. Methodology. The study was carried out on soils of four land use types: i) arable lands, ii) abandoned arable lands under grassland vegetation, iii) young forest vegetation (25–30 years old), and iv) mature mixed forest (75–80 years old). Two-four plots were located in each land use type. Soil samples were taken up to 50 cm by 10 cm layers. The content of organic carbon (Corg), total nitrogen (N), and the C/N ratio were determined. Considering soil density, the Corg and N stocks were calculated in the top 20 and top 50 cm. In the top 20 cm, the particle size distribution, pH(KCl), water holding capacity (WHC), and microbial properties (basal respiration, BR and content of microbial biomass, Cmic) were determined. Based on the forest inventory, the carbon stocks in phytomass and mortmass (dead trees) were estimated. Main results. The Retisols of the Carbon Polygon “Ugra” are characterized by a sandy loamy texture and slightly acidic (arable and abandoned lands) or acidic (forest areas) reaction. Microbial activity in the top 20 cm of soil decreases in the following order: abandoned lands under grassland > young forest stands (25–30 years old) > arable lands > mature mixed forest. The BR variability was determined by 86–90 % by the Cmic content. The highest Corg stock in the 0–50 cm layer was in the arable soils (65.4±4.4 t C/ha), and the lowest Corg stock (41.4±0.4 t C/ha) was observed in the soils under young forest. The C/N ratio in the top 10 cm varied from 8.9 to 17.6 and depended on the composition of plant litter at the soil surface. The main C pool in the forest ecosystems, regardless of their age, was the phytomass of woody plants. This C pool exceeded the total Corg stocks in the 50-cm soil layer by 1.6 and 4 times in young forest and mature mixed forest, respectively. The total C stocks in the mature mixed forest were 3 times larger than in young forest stands. Conclusions. The C stocks and the state of the soils of the Carbon Polygon “Ugra” depend on the current land use type, which in turn determines the composition of plant residues input on and in the soil. This is the reason for the most distinct differences between the studied soils in terms of Corg and N content, C/N ratio, and microbial characteristics. The age of forest stands is a key factor determining the total C stocks in soils and forest biomass.

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Soil respiration (SR) is one of the largest fluxes in the global carbon cycle. The temperature sensitivity of SR (often termed as Q10) is a principal parameter for evaluating the feedback intensity between soil carbon efflux and global warming. The present study aimed to estimate the seasonal and interannual dynamics of the temperature sensitivity of SR based on a long-term 24-year series of measurements in two temperate forest ecosystems in European Russia. The study was conducted in a mature mixed forest with sandy Entic Podzol and in a secondary deciduous forest with loamy Haplic Luvisol. The SR rate was measured continuously from December 1997 to November 2021 at 7–10-day intervals using the closed chamber method. Sandy Entic Podzol demonstrated a higher temperature sensitivity of SR in comparison with loamy Entic Luvisol. The Q10 values for both soils in dry years were 1.3–1.4 times lower than they were in the years with normal levels of humidity. For both types of soil, we observed a significant positive correlation between the Q10 values and wetness indexes. The interannual variability of Q10 values for SR in forest soils was 18%–40% depending on the calculation approach and levels of aridity/humidity over the growing season. The heterogeneous Q10 values should be integrated into SR and C balance models for better predictions.