To establish the Qiannian Xiulin (Millennium Forest of Xiong’an) that closely resembles a natural forest and is stable and self-adjusting, this study utilized soil precipitation and soil moisture data from 32 weather stations and the Qiannian Xiulin Regional Observation Station in Xiong’an New Area, Hebei Province. Machine learning and mathematical analysis methods were employed to investigate the change pattern of soil moisture content in the Qiannian Xiulin. The results indicate that the soil moisture content of the Qiannian Xiulin in Xiong’an is significantly influenced by the monsoon climate and exhibits distinct seasonal fluctuations. Specifically, soil moisture content increases slightly after soil thawing from January to March, gradually decreases from March to June, and reaches the lowest point in June. After June, it shows an upward trend, reaching the maximum in August, and then decreases again after September. The relative humidity of the surface soil at a depth of 0–10 cm in Qiannian Xiulin is positively correlated with precipitation, while the change pattern of relative soil humidity and precipitation in 20–30 cm and 30–50 cm soil layers are not obvious. There are differences in the impact of precipitation intensity on relative soil moisture in different soil layers. Significant changes in soil moisture will only occur when the daily precipitation exceeds 20 mm. The soil moisture content at these weather stations is consistent with the field observation data of Qiannian Xiulin. This study provides scientific soil moisture change rules for artificial planting management of the Qiannian Xiulin.

In order to simulate changes in natural nitrogen deposition and explore the effects of nitrogen addition on soil organic carbon and soil respiration, four treatments (CK (control), N50 (50 kg/(hm²·a) of N), N100 (100 kg/(hm²·a) of N) and N150 (150 kg/(hm²·a) of N)) were conducted in a subtropical evergreen broad-leaved forest in Wuyi mountain, Fujian Province. The results showed that the effect of nitrogen addition on the TOC of the surface soil (0–20 cm) was not significant, and the effect on the content of its different components is different. Compared to the CK, N100 and N150 significantly increased the soil POC content by 110.7% and 147.9% (p₁ = 0.024, p₂ <0.001). The content of soil MAOC tended to decrease with the increase of nitrogen addition, but the difference was not significant. The annual dynamic of soil respiration rate was unimodal distribution, and nitrogen treatments had different effects on soil respiration in different observation time. Based on fitting equations of soil respiration rate and soil temperature, the annual average carbon efflux of soil respiration in CK, N50, N100, and N150 plots from 2018 to 2020 were 1205.31, 1191.56, 1287.56 and 1128.61 g C/m², respectively. Compared to the CK, annual average carbon efflux of soil respiration did not change significantly in N50, significantly increased by 6.82% in N100 (p<0.001), and significantly reduced by 6.8% in N150 (p<0.001), which meaned that N100 promoted annual carbon efflux from soil respiration, while N150 had an inhibitory effect on it.