Structure and composition of rhizosphere-soil humic acid and fulvic acid as affected by the land-use change from paddy to upland fields

To meet the growing needs of people, land use change has gradually become the theme of global change. This change makes soil quality and agricultural ecosystem fertility change. Among these, soil organic matter (SOM) is an important component of soil, but the effects of land-use change on SOM are still uncertain. In particular, the composition and functional groups of humic acid (HA) and fulvic acid (FA) in rhizosphere soil of different crops under land use change from paddy to upland fields are still unclear. Therefore, three land-use-change forms [paddy-to-wheat rhizosphere soil (P–W), paddy-to-celery rhizosphere soil (P–C), and paddy-to-grape rhizosphere soil (P–G)] were used. In addition, paddy rhizosphere soil (CK) without a change in land use was selected as a control to evaluate the effects of different land-use forms on HA and FA in rhizosphere soil. The results demonstrated that total organic carbon, permanganate oxidizing carbon, and dissolved organic carbon contents in rhizosphere soils of P–W and P–G sites increased significantly with changes in land use types. The C/H ratio of HA was reduced significantly in P–G, and the O/C and (O + N)/C ratios were altered, but there were no significant differences among the different land uses. Spectral analysis showed that the structure and composition of the functional groups in the rhizosphere soil with different crops were different in HA and FA, as evidenced by differences in ultraviolet spectra, nuclear magnetic resonance spectra, and fluorescence composition. Changes in land use patterns can impact the content, structure, and composition of organic matter in rhizosphere soils. This result helps to understand the ecological functions and environmental benefits of SOM under different land use types for better sustainable use of agricultural soils.