Abstract: Wetlands play a key role in soil organic carbon (SOC) cycling. Most previous studies have focused on the changes of SOC content, whereas the responses of SOC content and its components to the changes of wetland environment remain less understood. Based on midinfrared spectroscopy, we investigated the impacts of wetland degradation and restoration on SOC content and infrared C compounds at 0-100 cm depth and examined the relationships between SOC content and SOC compounds in the Yellow River Delta. Restored wetlands had higher SOC content than degraded ones, especially in the upper layer soils of 0-40 cm, which was likely ascribed to higher soil moisture, salinity, and nutrient conditions. Higher soil water and nitrogen (N) availability and lower salinity increased photosynthetic CO₂ assimilation (represented by lower leaf ¹³C) and C fixation of Tamarix chinensis, resulting in an increase in SOC content in restored wetlands. Compared with degraded wetlands, restored wetlands had higher relative abundance of carbohydrates (1050 cm^-1) but lower relative abundance of aromatic C compounds (1630 cm^-1). The lower litter C/N ratio and greater photosynthetic C product content in restored wetlands explained the variations in carbohydrates and aromatic C compounds through inputting more low molecular substances originated from litter and root exudates. In addition, SOC content had a positive correlation with carbohydrates and a negative correlation with aromatic C compounds, indicating that increased carbohydrates are beneficial to enhancing SOC content of restored wetlands. The regulations of plant and soil variables in SOC and their feedbacks to changing wetlands suggest that sustainable wetland managements (e.g., wetland restoration or conservation) help sequester more C in soils, with implications for carbon neutrality.

Key words: carbon sequestration, carbon neutrality, mid-infrared spectroscopy, wetland conservation, Yellow River Delta.