Effects of salinity on iron-organic carbon binding in the rhizosphere of Kandelia obovata: Insights from root exudate analysis

• Published in: The Science of the total environment 2024-12, Vol.955, p.177214, Article 177214
• Main Authors: Lei, Ying, Bi, Yuxin, Dong, Xinhan, Li, Hongcheng, Gao, Xiaoqing, Li, Xiuzhen, Yan, Zhongzheng
• Format: Article
• Language: English
• Subjects: Carbon - metabolism; Fe redox cycling bacteria; Ferrihydrite; Iron - chemistry; Iron - metabolism; Mangrove; Organic carbon; Plant Exudates - chemistry; Plant Roots - metabolism; Rhizophoraceae - physiology; Rhizosphere; Root exudates; Salinity; Soil - chemistry; Wetlands
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.177214

Iron (Fe) oxides in wetland soils are crucial for stabilizing soil organic carbon (SOC) by forming stable Fe-OC complexes, thus protecting SOC from microbial breakdown and aiding its preservation. This study delves into the response of Fe (hydr-)oxides to salt stress, a relatively unexplored area, by examining Kandelia obovata, a key mangrove species. Through controlled climate chamber experiments, we investigated how salt stress affects the interactions between Fe (hydr-)oxides and SOC in root exudates (REs) and rhizosphere soils. Our results demonstrate that salinity at 30 ppt significantly increases the release of sugars, amino acids, inorganic nutrients (NH4+, NO3−), and phosphorus in K. obovata's REs, while reducing crystalline and amorphous Fe (hydr-)oxides and increasing complexed Fe (hydr-)oxide levels, thereby reducing their crystallinity in rhizosphere soils. Importantly, at elevated salinity (30 ppt), the Fe-OC bond in the rhizosphere shows greater stability, indicating enhanced resilience to salt stress compared to bulk soil. Salt stress also raises the carbon to nitrogen (C/N) ratio in REs. Testing artificial REs (AREs) with different C/N ratios showed that Fe (hydr-)oxide content decreases at C/N ratios of 10 and 30 compared to the control, whereas Fe-OC content increases with higher C/N ratios. Introduction of AREs with a C/N ratio of 20 significantly affected rhizosphere crystalline Fe (hydr-)oxide and Fe-OC content, highlighting AREs' impact on the binding of Fe (hydr-) oxides and OC. The presence of soil microorganisms was critical for the binding of Fe (hydr-) oxides and OC, as sterilized soil exhibited significantly lower levels of Fe (hydr-) oxides and Fe-OC compared to unsterilized soil. This research reveals that under salt stress, mangrove plants play a crucial role in stabilizing rhizosphere SOC by influencing Fe (hydr-) oxide crystallinity and promoting the formation of stable Fe-OC complexes, highlighting the complex interactions between plant REs, salt stress, and soil minerals. [Display omitted] •Salinity boosts K. obovata root exudates and raises their C/N ratio.•Salt stress reduces rhizosphere's crystalline and amorphous Fe (hydr-) oxides content.•Rhizosphere Fe-OC binding shows more resilience to salt stress than bulk soil.•Adding AREs at C/N 20 significantly lowered rhizosphere FeDH and Fe-OC contents.•Fe-OC significantly increases with C/N ratios rise.