Linking absorptive roots and their functional traits with rhizosphere priming of tree species

Woody plant roots can be classified into absorptive roots and transport roots based on root functions, order and traits. While there is an emerging view that living roots actively affect soil organic carbon (SOC) decomposition via the rhizosphere priming effect (RPE), the linkages of the RPE with C allocation to absorptive roots (relative to total roots) and their functional traits across soils are virtually unknown. Here, we investigated the RPE by growing a tree species (Chinese fir, Cunninghamia lanceolata) in three isotopically-distinct C-4 soil types with different soil properties such as C/nitrogen (N) ratio and texture, and by growing three tree species (Chinese fir, larch (Larix kaempferi) and ash (Fraxinus mandshurica)) with wide variations in root functional traits in one of the C-4 soils. We classified living roots into absorptive roots (first and second orders) and transport roots (third and higher orders) and then quantified their C allocation (relative to total roots) and morphological and chemical traits associated with economic construction, rhizodeposition and resource acquisition. We found that the RPE of Chinese fir across the three soils decreased with an increase in soil C/N ratio. This result conflicted with the N mining hypothesis and suggests that soil C stabilization mechanisms associated with clay minerals may play an important role. Further, significant differences in the RPE among tree species were largely accounted for by the C allocation to absorptive roots. Moreover, there was a significantly negative relationship between specific surface area of absorptive roots and the specific RPE (per unit biomass of absorptive roots) among tree species, suggesting that absorptive root traits shaping the extent of the rhizosphere may regulate the RPE. Taken together, our results provide evidence that absorptive roots play a predominant role in causing the RPE. These findings present an important step toward improving our capability to predict plant effects on SOC decomposition through linking the RPE to absorptive root functional traits.