Sub-tropic degraded red soil restoration: Is soil organic carbon build-up limited by nutrients supply

The long-term Forest Restoration Experimental Project (FREP) was established in 1991 on a subtropical, barren, degraded, red soil sparsely vegetated grassland in Taihe County, Jianxi province, China. The objective of the FREP was to evaluate the effects of restoration on ecological functions through afforestation by using various local species. This information will provide guidance for future restorations at severely deteriorated landscapes, which are very common in southern China. In this study, we selected five restoration forests: Chinese sweetgum (Liquidamber formosana), schima (Schima superb), masson’s pine (Pinus massoniana), slash pine (Pinus elliottii), Chinese sweetgum × slash pine mixtures, and one experimental control (grassland with low coverage of annual and perennial plants). These were selected to evaluate the differences in soil organic carbon (TC), soil nitrogen including total soil nitrogen (TN) and available nitrogen (Av-N), soil phosphorus including total phosphorus (TP) and available phosphorus (Av-P), and their molar ratios (C:N, C:P, and N:P). A similar assessment was also conducted on the species functional groups (coniferous forest, broad-leaved forest, and mixed-species forest) based on groupings of studied species. Furthermore, we evaluated the relationships between TC and soil N and P, and their stoichiometry to explore the biochemical mechanisms of soil organic matter buildup. Finally, we explored the recovery trajectory of TC and TN in the FREP’s evergreen broad-leaved forest by comparing it with local evergreen climax ecosystems.Over the 19-year study period, restoration significantly improved the TC and soil N and P compared to the control sites, but there were no significant differences in the TC and soil N and P among the restoration functional groups and among forest stand types. The TN and C:P ratios were closely related to soil organic carbon contents suggesting that they were good predicator of soil organic carbon. The overall data clearly demonstrated that the restoration through local species and nutrient cycling concentrating C, N, and P in topsoil. The recover trajectory suggests that the evergreen forest (schima) in FREP is still in the early developmental stages, and its projected rate of TC and TN growth is much slower than the average growth rate in the region. This case study clearly demonstrated that although the recovery is at its early stage with a very slow process, the active restoration can enhance soil carbon sequestration, nutrients availability, and the capability of soil organic carbon sequestration is regulated by the soil nutrients supply.

► Restoration improved TC and soil nutrients (N & P) at FREP study site 19 years after establishment.
► No differences in TC and soil nutrients among functional groups and among forest stand types.
► The TN and C:P ratio were related to TC indicating their biochemical relationships among them.
► The recovery trajectory of TC and TN at FREP sites were in their early successional stage.