Carbon and Nitrogen Stocks and Microbial Activity of the Humus Horizon of Loamy Soils after Mass Windthrow in the Broadleaved Forest of the Kaluzhskie Zaseki Nature Reserve

The contributions of windthrows and coarse woody debris (deadwood) to soil organic matter dynamics are controversial and poorly studied. Windthrows are a natural disturbance, the frequency of which is predicted to increase because of global climate change. This paper assesses the impact of a mass windthrow, namely, deadwood and gaps in the forest canopy, on the total C and N contents and stocks, as well as on the microbial activity of the soils developed from cover loesslike loam in a multispecies mesophytic broadleaved forest. The soil, which is a combination of soddy podzolic and gray soils (Retisols and Luvisols according to the WRB classification) was studied in a mass windthrow area 15 years after the catastrophic event. Soil was sampled from the top 5-cm layer of the A horizon in three biotopes: (1) under a fallen trunk; (2) 50–70 cm from the trunk in a deadwood-free area; and (3) in the background forest surrounding the windthrow site. A series of one-way ANOVAs and the pairwise Games–Howell test were used to assess the effects of tree species and three types of biotopes on the C and N contents and stocks, C/N ratio, microbial characteristics, pH, soil moisture content, and soil bulk density. The contents and stocks of C and N, soil microbial activity, and moisture were the highest in the mass windthrow area free of fallen trunks. The estimates for the soils under fallen trunks were mostly similar to those of the background forest. Our study shows that the gaps in forest canopy and coarse woody debris following a mass windthrow have significant effects on the characteristics of loamy soil.