Tree species identity influences the vertical distribution of labile and recalcitrant

22 In terrestrial environments, soil organic matter (SOM) is the largest organic carbon 23 (C) pool. The quantity and quality of organic carbon in soils can be affected by 24 vegetation through influencing the inputs and outputs of SOM. We examined how 25 storage and quality of C in SOM were affected by vegetation under grass cover or 26 single and a polyculture plot of Betula pendula, Alnus glutinosa and Fagus sylvatica. 27 An acid hydrolysis approach was used to quantify three SOM fractions differing in 28 biodegradability. Tree species identity and stand composition had no significant effect 29 on the total amount of C stored in different SOM fractions to a depth of one meter. 30 However, when examining individual SOM fractions in the upper layers of the soil 31 profile, significantly more C was stored in the putatively more labile fractions 1 and 2 32 under F. sylvatica and A. glutinosa, respectively. In deeper soil layers, the highest 33 storage of recalcitrant organic C was found under the tree polyculture. The vertical 34 distribution of these three soil organic C pools was compared to C inputs via 35 decomposed leaf litter. Our data indicated that in the tree species polyculture, 36 combining litter inputs of multiple species can have a positive impact on the 37 accumulation of acid resistant recalcitrant C in deep soil layers in 4 years. This C 38 fraction has the greatest potential for long-term sequestration. 39