Soil fulvic acid characteristics and proton binding behavior as affected by long-term municipal waste compost amendment under semi-arid environment

The fulvic acids (FAs) isolated from municipal waste compost (MWC) and unamended and MWC-amended soils in a long-term (nine years) field experiment were characterized to evaluate the cumulative and residual effects of amendment used at rates of 20 and 80 t ha− 1 on their chemical, compositional, structural, functional and acid–base properties. With respect to the control soil FA, the MWC-FA had a greater content of N, smaller contents of O and acidic groups, larger affinity for proton binding by the carboxylic-type groups, smaller molecular size, and a limited polycondensation degree. In the residual effect experiment, the application of MWC affected only partially the elemental and functional group composition of soil FA. In the cumulative effect experiment, the repeated application of MWC affected the elemental and acidic functional composition of soil FA by increasing N and S contents and proton affinity of carboxylic-type groups, and decreasing the acidic functional group contents. These effects were more evident in cumulative than in residual MWC-amended soil FAs.