Analyzing the Transformation of Forest Litter Organic Matter at Different Stages of Secondary Succession of a Middle Taiga Forest Using the IR Fourier Spectrometry Methods

Organic matter decomposition is a key process in the carbon cycle that controls the rate of carbon dioxide emission, carbon accumulation in the soil, and availability of mineral elements for plants. Changes in the forest stand composition during secondary succession lead to changes in the quality of litter, which affects the rate and depth of its transformation. We have analyzed how the chemical structure of the L-horizons of litter changes from October to August at different stages of progressive succession in typical forest ecosystems of middle taiga in Western Siberia using IR Fourier spectrometry and elemental analysis. It is revealed that the structure of organic matter in the L-horizons was most significantly transformed at intermediate stages of succession (in an aspen forest with a dark coniferous second layer), while changes at previous (monodominant aspen forests) and subsequent (mixed and dark coniferous forests) successional stages were less pronounced. These changes include a decrease in the proportion of relatively easily decomposable components (cellulose and carbohydrates) and accumulation of aromatic compounds and polyesters, which are more resistant to decomposition. Aspen forest with the dark coniferous second layer and dark coniferous forest have proven to be objects with the largest discrepancy in terms of changes in the litter elemental composition: the increase in the ratio of total carbon to nitrogen from October to August was minimal in the former and maximal in the latter. This combination of the results of IR Fourier spectrometry and elemental analysis can be explained by differences in the efficiencies of depolymerization of nitrogen-containing compounds in litter. On the whole, the results show that the transformation of litter during decomposition does not always depend solely on its initial quality, even in closely located ecosystems, where physical conditions are nearly identical. The cause of these differences in transformation at different stages of succession may be the functioning of the microbial community.