Organic amendments stimulate co-precipitation of ferrihydrite and dissolved organic matter in soils

Ferrihydrite-organic matter (OM) complexes in soil play a vital role in preserving soil organic carbon (C), either through adsorption (reaction of OM with existing ferrihydrite) or co-precipitation (neo-formation of ferrihydrite in the presence of OM). However, there is still limited understanding of the effects of long-term fertilization on the pathways for the formation of ferrihydrite-OM in soil. In this study, the dissolved OM (DOM) extracted from a Ferralic Cambisol under 25-year inorganic nitrogen (N), phosphorus (P) and potassium (K) (NPK) or NPK plus organic pig manure (NPKM) was used to synthesize adsorption complexes and co-precipitates. Compared with the adsorption, the co-precipitation of ferrihydrite and DOM resulted in higher C/Fe ratios in the solid products, particularly under NPKM. Analysis of the iron K-edge X-ray absorption near-edge fine structure (XANES) showed that there was a preference for DOM from soil under NPKM treatment to co-precipitate with ferrihydrite, whereas adsorption of ferrihydrite on DOM was more likely with the long-term NPK fertilized soil. Fourier-transform infrared (FTIR) spectra suggested that a higher intensity of Si-O functional groups, clay minerals, and aromatic groups in the DOM from the NPKM treatment played important roles in the co-precipitation process. Desorption of co-precipitated OM was lower than that of adsorbed OM in soils, indicating a higher chemical stability of OM in the co-precipitates. In summary, our results demonstrate that 25-year long-term NPK and NPKM fertilizations have contrasting effects on the formation pathway of ferrihydrite-OM complexes. Organic amendments can stimulate the co-precipitation of DOM and ferrihydrite in soil, leading to greater biological C preservation than the adsorption process, and a further enhancement of long-term SOC protection.