Soil Depth Can Modify the Contribution of Root System Architecture to the Root Decomposition Rate

Abstract

Aims: Changes in root system architecture (RSA) and soil depth affect the root decomposition rate. However, due to soil opacity, many variables of RSA have not been well studied or even measured. Methods: To investigate the effects of soil depth and the characteristics of RSA on the root decomposition rate, soil samples (Soil cores were collected in October 2020 from Cunninghamia lanceolata and Pinus taeda plantations, which were 40 years old) were obtained using a soil auger and had a diameter of 10 cm and a length of 60 cm. Samples were taken from six different soil depths, ranging from 0 to 60 cm with a 10 cm interval between each depth. The RSA in the in-situ soil cores was analyzed using computed tomography scans and Avizo. Results: Root volume and the number of root throats were significantly higher at the 0–10 cm soil depth than at the 10–60 cm soil depth, but root length was significantly lower at the 50–60 cm soil depth (p < 0.05). Structural equation modeling showed that different stand types influenced root biomass and thus the root decomposition rate directly or indirectly through the characteristics of the stand types. RSA, i.e., root thickness and breadth, affected root biomass indirectly and then affected the root decomposition rate. Root biomass contributed the most to the root decomposition rate in the Cunninghamia lanceolata (20.19%) and Pinus taeda (32.26%) plantations. The contribution of the RSA variables to the root decomposition rate exceeded 50% at the 20–30 cm and 40–50 cm soil depths. Conclusions: Our findings suggested that the influence of the RSA variables on the root decomposition rate varies with soil depth. This deserves more consideration in our future studies on root decomposition and RSA.