Driving factors influencing spatiotemporal variation of natural organic chlorine in Shennongjia forest soil

Studying the geochemical behavior of chlorine is the basis of understanding the chlorine cycle in nature. To explore the spatiotemporal distribution of natural organic chlorine (Cl org ), L layer (litter fall), F–H layer (humification zone), topsoil layer (0–20 cm), and deep soil layer (20–40 cm) samples were collected from 18 sampling sites at different altitudes (851–2918 m) in Shennongjia Forest in May, August, and December. Cl org content was analyzed, and the Cl org stocks were calculated. The major factors affecting the distribution of Cl org were explored. The results revealed that the sum of Cl org content in four layers varied from 7.958 to 184.686 mg/kg, and the highest value was observed in August. Cl org accounted for 46%–77% of total chlorine, with the highest mean ratio in soil layer (0–20 cm). Cl org content exhibited the following trend: F–H layer > L layer > topsoil layer (0–20 cm) > deep soil layer (20–40 cm). The seasonal patterns of Cl org in soil layers were different from that in L and F–H layers, which were mainly controlled by the content and humification degree of organic matter. Cl org storage was much higher in soil layers (61–246 kg/ha) than those in F–H layer (1.1–7.1 kg/ha) and in L layer (0.1–0.8 kg/ha) because of the large thickness of the soil layers. Overall, the Cl org content exhibited an increasing trend with altitude, except at an altitude of approximately 1800 m. Cl org content in L and F–H layers varied more obviously with altitude than that in soil layers. When inorganic chlorine (Cl in ) was not a limiting factor for the chlorination process, Cl org content in L and F–H layers was significantly affected by climate and organic matter controlled by altitude, while Cl org content in soil layers was also mediated by metal ions and pH, and soil particle size. This study could provide a scientific basis for assessing the chlorine cycle in nature.