Objectives The size fraction distribution and spectral characteristics of dissolved organic matter (DOM) released from different sources of organic fertilizers were studied to guide the application of organic fertilizer in agricultural production and the subsequent environmental effect of DOM.

Methods The DOM in the aqueous solution of the different organic fertilizers (i.e., wheat straw biochar, shrimp peptide, seaweed and sheep manure) was classified by ultrafiltration classification technology. The content of each size fraction was determined by total organic carbon (TOC) analyzer, and the spectra were characterized by Fourier transformed infrared spectroscopy (FTIR), ultraviolet visible absorption spectroscopy (UV-Vis) and three-dimensional fluorescence spectroscopy (3D-EEM).

Results In terms of total (<0.7 µm) dissolved organic carbon (DOC), the results were in order of wheat straw biochar (308 mg/kg) < shrimp peptide (1060 mg/kg)<seaweed (1266 mg/kg)<sheep manure (2989 mg/kg). The proportion of DOC in the smallest particle size (<1 kDa) was the highest, except for algae, which accounted for 47%, the other organic fertilizers reached 50% or more. The fluorescence index (FI) and autochthonous index (BIX) of the four organic fertilizers increased with the decrease of DOM size fractions, while the SUVA₂₅₄, SUVA₂₆₀ and humification index (HIX) decreased with the decrease of DOM size fractions. The protein-like fractions of shrimp peptide DOM were high in each size fraction and mainly endogenous DOM, rich in organic matter from autochthonous sources and highly bioavailable. The size fractions of sheep manure DOM were greatly affected by human activities, however, the FI<1.4 for wheat straw biochar DOM in the larger size fraction (>100 kDa) indicated that its DOM in the larger size fraction was mainly exogenous with relatively low contribution from autochthonous production and microbial activities. In addition, although the HIX value of DOM of each size fraction of seaweed, sheep manure and shrimp peptide gradually decreased with the decrease of size fraction, except for the <1 kDa component of shrimp peptide DOM, the degree of humification was still high (HIX>10). While the HIX<4 of the small size fraction DOM of the wheat straw biochar indicated that the small size fraction DOM of the wheat straw biochar had a high content of hydrophobic components and a relatively low degree of humification. The fluorescence fractions and infrared spectra showed that the DOM of the four different organic fertilizers was dominated by humic substances and contained a large number of functional groups such as amino acid N―H, O―H and C―O.

Conclusions Based on the size fractions distribution and spectral characteristics of organic fertilizers, DOM in seaweed, sheep manure, and shrimp peptide was mainly small size fraction, which was highly humified and has low protein content. The biostability of wheat straw biochar DOM was higher than other organic fertilizers, and the bioavailability was lower; therefore, the application of excessive biochar did not promote the microbial degradation and utilization of soil DOM; while the organic fertilizer of shrimp peptide source had the largest contribution of DOM protein-like fraction and high bioavailability. The application of shrimp peptide organic fertilizer may facilitate the degradation and utilization of soil DOM by microorganisms.