Absorptive rather than transport root decomposition drives soil carbon sequestration: A case study of Platycladus orientalis and Quercus variabilis

Root litter inputs regulate soil organic carbon stocks and turnover, which is increasingly being understood through a functional trait-based approach. Fine roots of plants are classified into absorptive and transport functional modules. However, the relative contribution of absorptive and transport roots to soil carbon sequestration and the mechanisms involved are still unclear. We conducted a 13-week container incubation experiment using soil from a C4 crop (maize) to investigate the impact of absorptive and transport root litter from Platycladus orientalis and Quercus variabilis on the turnover of new versus old soil carbon at the initial stage of root decomposition. We also measured root chemical properties, soil microbial biomass, and enzyme activity to understand how the two root functional modules regulate soil carbon sequestration differently. Absorptive roots decomposed slower than transport roots due to higher nitrogen content, a larger acid-unhydrolyzable fraction (AUF), and a lower concentration of non-structural carbohydrate (TNC) concentrations. Root litter input reduced the total soil carbon loss and increased soil carbon sequestration, with absorptive root input being more effective than transport roots. More root-derived new carbon was formed by absorptive root addition than by transport roots, likely due to the greater content of recalcitrant compounds in absorptive roots contributing to the formation of particulate organic matter. Old soil carbon loss was stimulated by the addition of transport roots and reduced by the addition of absorptive roots, suggesting a positive priming effect induced by transport roots and a negative priming effect induced by absorptive roots. Furthermore, old soil carbon loss induced by transport roots was significantly positively correlated to the presence of labile substrates and soil enzyme activities. Our results suggest that absorptive roots drive greater soil carbon sequestration than do transport roots. This occurs through more gain of new carbon and less loss of old soil carbon, emphasizing the necessity of distinguishing different root functional modules in predicting the soil carbon cycle.