Does P-deficiency fertilization alter chemical compositions of fulvic acids? Insights from long-term field studies on two contrasting soils: A Fluvisol and an Anthrosol

Abstract Structural changes in fulvic acid (FA) in response to fertilization strategies, especially P-deficiency fertilization, have critical implications for soil carbon dynamics. This study examined how long-term fertilization influenced chemical compositions of soil FA and its humification degree. Advanced solid-state 13 C nuclear magnetic resonance techniques were used to evaluate structural changes of FAs from two soils (a Fluvisol and an Anthrosol) under four fertilizer treatments: organic fertilizer combined with or without NPK fertilizer (OF and NPKOF), deficient in phosphorus only (NK) and Control. The greater aromaticity, hydrophobicity and alkyl/O-alkyl (A/O-A) ratios of FAs were observed in the Anthrosol than in the Fluvisol. The Anthrosol FAs contained more alkyl and aromatic C but fewer O-alkyls (mostly being protonated) than did the Fluvisol FAs. Both FAs comprised some nonprotonated aromatics, which decreased with applications of organic and NK fertilizers. This trend was less obvious for the Anthrosol than for the Fluvisol. Organic fertilizer treatments (OF and NPKOF) in the Fluvisol increased the A/O-A ratio and aromaticity compared with P-deficient soils (NK and Control), indicating organic fertilization promoted the humification degree of its FA. The NK treatment in the Fluvisol increased H/C ratio of FA. Compared with the Fluvisol FAs, the Anthrosol FAs displayed similar chemical natures, implying their formation followed similar processes with a similar humification degree. The findings suggest that long-term fertilization differently influenced the FAs from the Fluvisol and Anthrosol. Clearly, the Anthrosol FAs shared more common features among the treatments compared with the Fluvisol FAs, whereas P-deficiency rendered the Fluvisol FAs simpler.