Seasonal dynamics of δ(13) C of C-rich fractions from Picea abies (Norway spruce) and Fagus sylvatica (European beech) fine roots.

The (13/12) C ratio in plant roots is likely dynamic depending on root function (storage versus uptake), but to date, little is known about the effect of season and root order (an indicator of root function) on the isotopic composition of C-rich fractions in roots. To address this, we monitored the stable isotopic composition of one evergreen (Picea abies) and one deciduous (Fagus sylvatica), tree species' roots by measuring δ(13) C of bulk, respired and labile C, and starch from first/second and third/fourth order roots during spring and fall root production periods. In both species, root order differences in δ(13) C were observed in bulk organic matter, labile, and respired C fractions. Beech exhibited distinct seasonal trends in δ(13) C of respired C, while spruce did not. In fall, first/second order beech roots were significantly depleted in (13) C, whereas spruce roots were enriched compared to higher order roots. Species variation in δ (13) C of respired C may be partially explained by seasonal shifts from enriched to depleted C substrates in deciduous beech roots. Regardless of species identity, differences in stable C isotopic composition of at least two root order groupings (first/second, third/fourth) were apparent, and should hereafter be separated in belowground C-supply-chain inquiry.