Deep root information “hidden in the dark”: A case study on the 21-m soil profile of Robinia pseudoacacia in the critical zone of the Chinese loess Plateau

Deep roots can uptake deep soil water to support plant growth and mitigate climate change in water-limited ecosystems. Few studies have been conducted on the deep root characteristics in the soil profile as limited field data are available. To explore the relationship between the deep root characteristics, depth, and soil water status, we obtained the distribution pattern of deep roots and soil water content along a 21-m soil profile (n = 22) in a mature Robinia pseudoacacia plantation. The maximum rooting depth, mean root biomass, and mean depths corresponding to 50% and 90% of the total root biomass of 17-year-old R. pseudoacacia trees were >20 m, 0.083 ± 0.029 g·cm⁻², 1.17 ± 1.74, and 6.15 ± 3.98 m, respectively. We established an exponential power model to fit the relationship between the deep root biomass and soil depth, which had a higher goodness-of-fit than the typical exponential model, logistic model, and piecewise linear model. The root morphological characteristics along the 21-m vertical profile rapidly decreased with depth in the 0–1-m layer, and then stabilized, while the specific root length (SRL) and specific root area (SRA) increased with depth. The relationships between the SRL, SRA, and soil water content in the dried soil layers with roots (SWC-DSL) were significantly positive (both p < 0.01), while those between average root diameter density and SWC-DSL were significantly negative (p < 0.001), suggesting that roots increased their length rather than diameter to acquire water resources with relatively low carbon investment. The soil depth and soil water availability co-controlled root biomass and root morphological characteristics. Our findings highlight the importance and complexity of deep roots and their morphological characteristics, which are fundamental for forest models and management to maintain sustainable development between soil water and root water uptake.