Dynamics of soil organic carbon and dissolved organic carbon in Robina pseudoacacia forests

We investigated the variation patterns of organic carbon in soil and soil solution of four selected Robinia pseudoacacia forests aged 10a, 25a, 31a, and 35a, as well as a contrastive tillage site in a similar topography condition in Loess Plateau, China. The purpose was to explore the dynamics of soil organic carbon (SOC) and dissolved organic carbon (DOC) in R. pseudoacacia forests. On average depths of 20, 40, and 60 cm, SOC, active organic carbon (AOC), and DOC gradually increase with increased forest age. After forest restoration, the AOC/SOC ratio and resistant organic carbon/SOC ratio increase, whereas the slow organic carbon/SOC ratio decreases. The soil solutions in the subsoil layer have low DOC: DON ratio and high UV absorption at 280 nm. At 40 and 60 cm, the depth distribution is indicated as special low values for DOC concentration in the C99 site (10a site), as well as for soil water content, SOC, and AOC in the 25a forest site. Our results provide evidence that during forest restoration, SOC does not consistently increase linearly. The change points of different SOC proportions and DOC concentrations at various depths are not same, i.e., asynchronous changes exist.