Cysteine conversion inhibitor drives humic substances formation by regulating aromatic amino acid metabolism of microbial community

Abstract

Humic substances (HS), formed through the transformation of organic matter, play a critical role in agroforestry ecosystem functions. However, the microbial mechanism underlying sulfur conversion and HS formation remains poorly understood. Therefore, this study applied DL-propargylglycine (PAG), an inhibitor of cysteine lyase, to investigate how PAG addition affected the formation of HS under high and low aeration rates. Results showed that PAG addition significantly increased HS contents by 21.30% under high aeration and by 11.80% under low aeration. Meanwhile, the total sulfur content in HS and the humification degree also were elevated, confirming that sulfur conversion contributed to the HS formation. Variation partitioning analysis further illustrated that sulfate conversion significantly contributed to HS formation under high aeration, whereas cysteine conversion had a significant contribution under low aeration rate, which was related to the differential effect of PAG on sulfur conversion. Furthermore, core microorganisms associated to HS formation mediated the transformation between sulfur and HS. PAG addition elevated the abundance of these microorganisms and increased functional genes related to aromatic amino acid metabolism. Eventually, structural equation models delineated the influencing paths of PAG addition, demonstrating that regulating microbial cysteine cleavage enhanced aromatic amino acid metabolism, thereby promoting HS formation. This offered new insights for improving the resource utilization efficiency of livestock manure and advancing sustainable agricultural development.

Keywords: Cysteine cleavage; Formation theory; Functioning microorganisms; Humic substances; Sulfate reduction.