Vegetation succession can change the function and quality of the soil. Exploring the changes in soil properties during secondary forest restoration is of great significance to promote forest restoration and improve the ecological service function of subtropical ecosystems in South China. In this study, we chose three typical forests in subtropical China as restoration sequences, broadleaf–conifer mixed forest (EF), broad-leaved forest (MF), and old-growth forest (LF), to study the changes in soil physico-chemical and biological properties and the changes of soil comprehensive quality during the secondary succession of subtropical forest. The results showed that the soil physical structure was optimized with the progress of forest succession. The soil bulk density decreased gradually with the progress of forest restoration, which was significantly affected by soil organic carbon (p < 0.01). In LF, the soil moisture increased significantly (p < 0.05), and its value can reach 47.85 ± 1.93%, which is consistent with the change of soil porosity. With the recovery process, soil nutrients gradually accumulated. Except for total phosphorus (TP), there was obvious surface enrichment of soil nutrients. Soil organic carbon (15.43 ± 2.28 g/kg), total nitrogen (1.08 ± 0.12 g/kg), and total phosphorus (0.43 ± 0.03 g/kg) in LF were significantly higher than those in EF (p < 0.05). The soil available nutrients, that is, soil available phosphorus and available potassium decreased significantly in LF (p < 0.05). In LF, more canopy interception weakened the P limitation caused by atmospheric acid deposition, so that the soil C:P (37.68 ± 4.76) and N:P (2.49 ± 0.24) in LF were significantly lower than those in EF (p < 0.05). Affected by TP and moisture, microbial biomass C and microbial biomass N increased significantly in LF, and the mean values were 830.34 ± 30.34 mg/kg and 46.60 ± 2.27 mg/kg, respectively. Further analysis showed that total soil porosity (TSP) and TP (weighted value of 0.61) contributed the most to the final soil quality index (SQI). With the forest restoration, the SQI gradually increased, especially in LF the value of SQI was up to 0.84, which was significantly higher than that in EF and MF (p < 0.05). This result is of great significance to understanding the process of restoration of subtropical forests and improving the management scheme of subtropical secondary forests.

Over the past several decades, vegetation restoration has been carried out extensively in South China. Theoretically, the process of vegetation restoration is usually accompanied by changes in soil properties. However, the effects of vegetation restoration on soil hydraulic properties are poorly documented in humid subtropical China. In this study, we compared soil hydraulic properties across three undisturbed subtropical forests, i.e., Pinus massoniana forest (PF), mixed Pinus massoniana/broad-leaved forest (MF), and monsoon evergreen broad-leaved forest (BF), which represented a vegetation restoration sequence in South China. Our results showed that vegetation restoration decreased the bulk density while increasing the total porosity and the soil organic matter (SOM). The clay content and capillary porosity of soil in the middle- and late-recovery-stage forests were significantly higher than those in the early stage, which was consistent with the soil water-holding capacity. The saturated hydraulic conductivity (KS) values of BF were always significantly higher than those of the other forests. In the whole soil profile, the water-holding capacity and KS in the topsoil (above 30 cm depth) were significantly higher than those in the deep soil for all forests. Further analyses indicated that the SOM was the main factor that affected KS, and the relationship of them could be fitted by a linear equation. Overall, our study revealed vegetation restoration ameliorates soil hydraulic properties in humid subtropical China. And the role of SOM in improving soil hydraulic properties should be emphasized in future forest ecosystem management.

Soil organic carbon (SOC) is a crucial component of the soil carbon pool that regulates fundamental soil properties and water status. In the global context of restoring vegetation, the soil carbon-water coupling relationship has gained attention. In particular, the regulatory mechanism of SOC on soil moisture requires further research. In this study, three typical forests in subtropical China were chosen as restoration sequences to investigate the changes in SOC and soil moisture during subtropical forest restoration and its regulation mechanisms: broadleaf-conifer mixed forest (EF), broad-leaved forest (MF), and old-growth forest (LF). The soil water content (35.71 ± 1.52%), maximum water holding capacity (47.74 ± 1.91%), capillary water holding capacity (43.92 ± 1.43%), and field water holding capacity (41.07 ± 1.65%) in LF were significantly higher than those in EF (p < 0.01). As forest restoration progressed, the amount of litter returning to the soil increased gradually, and the SOC content (0–100 cm) increased from 9.51 ± 1.42 g/kg (EF) to 15.60 ± 2.30 g/kg (LF). The SOC storage increased from 29.49 ± 3.59 to 42.62 ± 5.78 Mg/ha. On one hand, forest restoration led to a change in SOC content, which optimizes the soil structure and enhances soil porosity (path coefficient of 0.537, p < 0.01), further leading to a change in soil water content (path coefficient of 0.940, p < 0.01). On the other hand, the increase in SOC influenced the change in soil nutrient content, i.e., total nitrogen (TN) and total phosphorus (TP) (path coefficient of 0.842, p < 0.01). Changes in SOC and soil nutrients stimulated changes in the stoichiometric ratio, i.e., C:P and N:P (path coefficients of 0.988 and –0.968, respectively, p < 0.01), and the biological activity in soil changed appropriately, which eventually led to a change in soil water content (path coefficient of –0.257, p < 0.01). These results highlight the changes in SOC and soil water content (SWC), as well as the mechanism of SOC controlling SWC as a result of vegetation restoration, which is of tremendous importance for advancing our understanding of the eco-hydrological process of subtropical forest restoration.

Acid rain is a global environmental issue and causes serious adverse impacts on natural ecosystems and human health. The acid rain and its subsequent impacts have been extensively studied in temperate regions and in big cities globally, but the monitoring of acid rain in non-urban areas in subtropical region is still limited. Here, we analyze 18 years of water acidity observations from a non-urban site in western Pearl River Delta region, south China. From 2000 to 2018, annual mean precipitation pH values in the study site showed a marked increase (P<0.0001) from 4.96 in 2000 to 6.88 in 2018. This 18-year dataset of precipitation acidity shows a clear recovery from acid deposition over the annual period, dry seasons and wet seasons. Within a year, precipitation pH was relatively lower in dry seasons (5.46±0.95) than in wet seasons (5.80±0.89) during the whole study period, and the seasonal pattern of precipitation pH was amplified during drier years as a strong dilution effect was detected between precipitation amount and its acidity. The pH of surface runoff water (4.74±0.70) was significantly lower than that of precipitation as a result of the regulating effects of the highly acidified soil and the rich humus on the forest floor. The groundwater pH (6.32±0.63) was significantly higher than the precipitation pH especially during dry seasons. The results of the present study indicated that the groundwater has a strong acid buffer and neutralizing effects and thus plays an important role in supplies of clean water. Long-term changes of water acidity in an intact forested watershed in south China