Adsorption characteristics of dissolved organic carbon by soils under different fertilization models

Objective We analyzed the adsorption characteristics of dissolved organic carbon (DOC) by soils under long-term fertilization. We aimed to understand the mechanism of long-term manure mineralization in the cultivation of fertile soils.

Method The treatments included four fertilization regimes selected from the long-term experiment on brown soil and red soil. The treatments were no fertilization (CK), organic fertilizer only (M), NPK fertilizer only (NPK), and manure combined with NPK fertilizer (NPKM). The tested DOC was extracted from fresh pig manure with a concentration of 2400 mg/L, and a series of DOC solutions were made by diluting it for the Langmuir isotherm adsorption experiment. Three-dimensional fluorescence spectroscopy was used to analyze the DOC structure and relative volume fluorescence intensity before and after the adsorption by the soils. The main soil factors affecting the adsorption of DOC were analyzed.

Results The maximum adsorption capacity of DOC was 12.81 g/kg in red soil and 10.82 g/kg in brown soil. The adsorption capacity of the two soils treated with NPKM was higher than in NPK. The fresh DOC was composed of tyrosine (region I), tryptophan-like protein (region II), and soluble microbial metabolites (region IV). Parallel factor analysis showed two fluorescent components in the adsorption of DOC by the soil amended with fresh pig manure. These included protein-like substances with higher molecular weight (tyrosine-like and tryptophan-like) and soluble microbial metabolites (C1 component), and tyrosine-like protein substances with a smaller molecular weight with a lower degree of polymerization and soluble microbial metabolites (C2 component). The most adsorbed components of fresh DOC by soil were tyrosine-like and tryptophan-like. The adsorption capacity of the C1 component was M≈NPKM>NPK>CK and NPK>M≈NPKM>CK for the C2 component. The largest adsorption ( Q_max ) was positively correlated with free iron (Fe_d), complex iron (Fe_p), and soil organic matter; positively correlated with humification index (HIX) and C1 component content of DOC ( P < 0.01), and negatively correlated with fluorescence index (FI) (P < 0.01 ) .

Conclusions Fresh DOC contains a relatively high tyrosine-like protein, tryptophan-like protein, and soluble microbial metabolites, with simple structures and a low degree of humification. DOC structures and soil physical and chemical properties affecte the adsorption of DOC by soil. The high content of SOM, Fe_d, and Fe_p in soil leads to intensive adsorption of DOC. Long-term manure application increases the adsorption capacities of DOC components with a high degree of humification. Long-term application of chemical fertilizers increases the adsorption capacities of DOC components with a low degree of humification.