Change of soil K, N and P following forest restoration in rock outcrop rich karst area

The link between soil properties and forest restoration is not well understood in soils with a high proportion of bare rock outcrops (ROCs). This study compared the concentration of total potassium (TK) and available potassium (AK), soil organic matter (OM), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), available phosphorus (AP), volumetric proportions of soil (VPS), pH, and bulk density in soil with ROC emergence under a sequence of grass, shrubs, young regenerated forest, and closed forest on karst landforms at Shilin, S.W. China. The aims were to determine the relationship between the changes in soil properties, biomass accumulation, and plant species under the context of ROCs, and to identify the most limiting factor for forest restoration on this specific geology. We found that (1) the concentration of TK and AK, pH, and bulk density decreased, and OM, TN, AN, and TP concentrations slightly increased in 0-20 cm, 21-40 cm, and 41-100 cm soil layers during vegetation restoration from grass to shrubs and then young regenerated forest. Only TP and AK in all soil layers, TN in the 0-20 cm layer, and TK and bulk density in the 0-20 cm and 21-40 cm layers were significantly different between the soils of grass areas and closed forest. (2) The biomass was negatively correlated with TK and AK stocks in the 0-20 cm soil layer and AK stock in the entire 1 m pedon, and positively with TN, TP, and AN stocks in the entire 1 m pedon. The optima of various species groups, which account for a small proportion, corresponded to stock of N, P, K in canonical correspondence analysis. In conclusion, biomass accumulation during forest recovery in the context of ROCs had depleted soil K stocks and caused the reduction of TK and AK concentrations, contrary to the slight enrichment of soil N, and P and improvement of OM, pH and bulk density. Any measure increasing soil K may benefit forest restoration in karst areas.