Mass windthrow leads to a sharp increase in the stocks of woody detritus in forest ecosystems. Although highly relevant in connection with the tasks of carbon regulation of the economy, contributions of wood detritus to element cycles are poorly studied. The aim of this study was to estimate the dynamics of carbon, nitrogen, calcium, potassium, magnesium, and phosphorus in downed trunks and humus soil horizon of Podzols and Arenosols, 14 years after a mass windthrow in a mesic broad-leaved forest located in the Kaluzhskie Zaseki State Nature Reserve (Russia, Kaluga Region). At the 2006 windthrow area (53 degrees 30'N, 35 degrees 35'E), deadwood stocks were evaluated on transects in 2010 and 2020. In 2020, 167 samples at 5 decomposition stages were collected from 49 fallen logs and 24 standing live trees of seven species: Acer platanoides, Betula pendula, Fraxinus excelsior, Populus tremula, Quercus robur, Tilia cordata, Ulmus glabra, and Picea abies. Sixty-two paired soil samples were collected from the upper 5 cm of soil beneath the overlying trunks of trees of different species and next to the trunks, and 10 soil samples were collected in the forest surrounding the windthrow area. The volume of deadwood decreased by 1.5 times and the stocks of elements in deadwood decreased by 1.7-2.5 times (for N, Mg and K) and 2.7-3.5 times (for C, P and Ca) over 14 years after the windthrow (See Table 3). There was an increase in the proportion of element stocks contained in the deadwood of ring-porous species (Quercus, Fraxinus, and Ulmus), against a decrease in their proportion in the deadwood of diffuse-porous species (Populus, Betula, Tilia, and Acer). The state of soils with respect to the analysed elements was assessed as stable, with a significant decrease in the C/N ratio from 14 in the surrounding forest to 12 in the windthrow area (See Table 4). Total stocks of elements both in deadwood and soil 14 years after a windthrow decreased not so much as their stock in deadwood: C, Ca and K by 2 times, N and Mg by 10 and 20%, respectively. Before the windthrow, the tree trunks had contained about 80% of total C stock and 20% of total N stock; after 14 years, these values decreased to 60% and 15% in lying trunks (See Fig. 3). In general, 14 years after the windthrow, the stocks of total N, mobile forms of Ca and Mg were higher in soil than in deadwood, and deadwood remained the main store of C, P and K. Our study has shown that windthrows (including the mass ones) are important elements in the natural dynamics of forest ecosystems. A joint study of the processes of deadwood decomposition and the dynamics of soil characteristics, changes in the content and stocks of elements in wood, which pass into mobile forms of the soil stock in the process of wood decay, allows us to assess the quality of forests performing their ecosystem functions, such as regulating and supporting. The difference in the rate of loss of carbon and other elements by diffuse-porous and ring-porous tree species must be taken into account in forest management aiming to regulate carbon in economy planting of ring-porous species and preservation of their deadwood contribute to a relatively long-term deposition of carbon.
