Soil organic matter transformation influenced by silver birch (Betula pendula Roth) succession on abandoned from agricultural production sandy soil

A chronosequence approach was applied to evaluate the effects of silver birch succession on soil organic matter (SOM). The studies covered five plots (arable field [AF], fallow [FA] and three birch stands aged 8, 33 and 40 years), and all located in the same complex of Brunic Arenosols in central Poland. The O and A horizons of the soils were sampled and analysed using the standard procedures in soil science. It was found that the birch succession was accompanied by dynamic changes in the stock and quality of the SOM. The transition from AF to FA soils did not significantly change the content or pool of SOM in the A horizon; however, it did modify its fractional composition and internal structure. This was reflected by an increase in the humic acids content and a decrease in the fulvic acid, as well as in the values of the elemental and 13C-NMR analyses. Rapid losses in the SOM content and stock in the A horizon were recorded after entering the silver birch. The soils under the young birches were characterised by lower contents of low-molecular humus fractions, which are more susceptible to microbial degradation, than the FA soils. In addition, the birch succession contributed to a lower internal oxidation of the humic acids and an increase in the content of aliphatic structures in their molecules. The next stages of succession were characterised by gradually increasing SOM contents and stocks, accompanied by the increasing internal oxidation and maturity of the humic acids.