Absorptive and transport roots of two tree species respond differently to soil salinity along soil depth

Salinization is one of the factors seriously affecting the stability of forest ecosystems in coastal areas worldwide, which limits root growth and has adverse effects on forest development. Fine roots of woody plants can be divided into absorptive and transport roots; however, their responses to salinization along soil layers and the potential adaptation mechanisms remain unclear. Herein, absorptive and transport roots of Styphnolobium japonicum (L.) Schott and Robinia pseudoacacia L. in Yellow River Delta were collected in different soil layers, and their functional trait variations were analyzed, respectively. Absorptive roots exhibited greater respiration rates (RR), specific root length and area (SRL and SRA), nitrogen content, but lower root diameter (RD), tissue density (RTD), and dry matter contents than transport roots. Generally, compared with non-salinized roots, salinized absorptive roots had higher RR, lower SRL and SRA, while transport roots were relatively conservative. Robinia pseudoacacia was more sensitive to soil salinization than S. japonicum. Additionally, with the increase of soil depth in both sites, greater responses of SRL, RR, and nitrogen content were found in absorptive roots, whereas larger changes of RD and RTD in transport roots. These results highlight that the acquisitive strategy of absorptive roots, rather than the conservative strategy of transport roots, facilitates resource acquisition in responding to soil salinity. Our findings also imply the importance of applying root functional-based classification in the ecological progress for understanding the root resource foraging strategies.