Organic amendments with low C/N ratios enhanced the deposition of crop root exudates into stable soil organic carbon in a sodic soil

Background and aims

Numerous studies have demonstrated the enhancement effects of organic amendment additions on soil organic carbon (SOC) accumulation in agroecosystems. However, the effects of different organic amendment types on stable SOC formation through belowground inputs remain poorly understood, especially under stress conditions. This study aims to investigate the effects of three organic amendment types, namely lignin- (LDA), humus- (HDA), and vetch-derived (VDA) organic amendments, on the transformation process of ¹³C-rhizodeposits into SOC in sodic soil.

Methods

Sorghum bicolor L. was used in the experiments, and labelled using ¹³C-CO₂ for seven days after 75 days growing in a closed glass chamber.

Results

Our results showed that the nitrogen (N) compounds in the organic amendments accounted for 0%, 6.21%, and 11.37% of the LDA, HDA, and VDA, respectively. Organic amendments with low C/N ratios (HDA and VDA) enhanced the transformation of ¹³C-rhizodeposits into SOC, particularly into mineral-associated carbon (¹³C-MAOC). In addition, HDA and VDA substantially decreased the exchangeable sodium percentage (ESP) and increased the soil nutrient contents (e.g., total N and total phosphorus) compared with LDA, providing more favorable environmental conditions for both the crop and rhizosphere microbial growth. These effects, consequently, enhanced the deposition of the crop root exudates into ¹³C-MAOC in the sodic soil. Furthermore, compared with LDA, HDA and VDA enriched beneficial bacteria (e.g., Bacillaceae and Vermamoebidae) and inhibited pathogenic bacteria (Burkholderiaceae) through potential cross-trophic interactions, promoting crop growth and enhancing the production of root exudate deposition into ¹³C-MAOC.

Conclusion

Organic amendments with low C/N ratios enhanced the conversion of ¹³C-rhizodeposits into ¹³C-MAOC, by providing more favorable envrionmental conditions and enriching beneficial bacteria for plants. Our study provides a novel approach to selecting organic amendments with suitable and effective chemical structures to promote stable SOC formation through belowground inputs, especially under sodic conditions.