Abstract: Periphytic biofilms are ubiquitous at the soil-water interface in paddy fields with great impact on soil nutrient transformation through various pathways. However, the effects of phototrophic biofilm biomass decomposition on the coupling of dissolved organic matter (DOM), Fe, and phosphorus in paddy soil remains unclear. In this study, a microcosm experiment was carried out by adding different amounts of periphytic biofilms to paddy soil; and the effects of phototrophic biofilm biomass decomposition on paddy soil DOM components, Fe²⁺ content, activity of iron oxide (Fe_o/Fe_d), and phosphorus fractions were investigated. Fourier-transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was used to elucidate the molecular characteristics of DOM. The results show that the decomposition of periphytic biofilms increased DOM content and changed the proportion of different DOM components, of which the relative abundance of tannin-like compounds increased by 1.97%-9.74%. Also, we observed a significant increase in soil Fe²⁺ content and Fe_o/Fe_d ratio as well as enhanced soil reducibility. The content of inorganic phosphorus and especially the Fe-bound P in soil increased, which translated to increased P availability. Phototrophic biofilm biomass decomposition changed soil DOM components and Fe forms, while recalcitrant tannin-like components affected the bioavailability of P by reducing its adsorption to minerals. Principal component analysis (PCA) show that there were great differences in DOC content, the relative abundance of tannin-like compounds, Al-bound P content, Fe²⁺ content, and Fe_o/Fe_d ratio between the phototrophic biofilm decomposition treatments and the control. The correlation results show that there were significantly positive correlations between Fe²⁺ content and Fe-bound P content, Fe_o/Fe_d ratio, and Al-bound P content, and the relative abundance of tannin-like compounds was positively correlated with Fe_o/Fe_d and Al-bound P content. In a word, soil DOM components, Fe forms, and P forms interacted with each other, and both the increase of tannin-like DOM and the transformation of Fe forms had positive effects on the increase in P bioavailability. The results have revealed the effects of periphytic biofilm biomass decomposition on the coupling of DOM, Fe, and P in paddy soil. It also provides theoretical support for the future use of periphytic biofilm to regulate the coupling process of carbon, Fe, and P, and to improve the bioavailability of P in paddy soil.

Key words: dissolved organic matter component, Fe reduction, phosphorus bioavailability, FT-ICR MS