Structure and biodegradability of dissolved organic matter from Ultisol treated with long-term fertilizations

Abstract

Dissolved organic matter (DOM) is a labile and reactive fraction of soil organic matter. Ultisol usually has low content of organic matter. Fertilization is a normal way in regulating the state of soil organic matter. This study aimed to investigate the effects of different fertilizations on the content, distribution, structure, and biodegradability of DOM from Ultisol. Ultisol treated by no fertilizer (Ck), chemical fertilizer (NPK), the mixture of chemical fertilizer and straw (NPKS), and animal manure (AM) were collected from a long-term fertilization experimental site. Soil DOM was extracted by deionized water with solid to water ratio of 1:5. The concentration of dissolved organic carbon and total soluble nitrogen in DOM was determined by TOC analyzer. The structure of DOM was investigated by fluorescence spectroscopy through the emission spectra, excitation spectra, synchronous-scan spectra, and excitation-emission matrix. Soil DOM was incubated for 21 days after the addition of soil microbial inoculum and nutrient solution, the biodegradability of DOM was estimated by calculating the percentage of dissolved organic carbon lost after 21 days of incubation. The content of dissolved organic carbon in the soil of Ck, NPK, NPKS, and AM was 22.14, 27.05, 36.08, and 56.04 mg/kg, respectively. The distribution of dissolved organic carbon on large macroaggregate increased from 47.0% in Ck to 65.0–71.5% in fertilized treatments but that on small macroaggregate decreased from 35.5% in Ck to 18.3–19.9% in fertilized treatments. Similar peak wavelength and a fluorophore at wavelength pair of 330–340ex/406–422em were found in all examined DOM. However, a sequence of AM > NPKS > NPK > Ck was found in the fluorescence intensity of examined DOM, indicating the simplification of DOM structure induced by fertilization. The biodegradability of DOM from the Ck, NPK, NPKS, and AM treatment after 21 days of incubation was 23.9, 28.7, 34.2, and 42.7%, respectively. Organic and inorganic fertilization increased the content of DOM from Ultisol. Compared to the Ck treatment, the fertilization treatments increased the distribution of dissolved organic carbon on large macroaggregate but reduced its distribution on small macroaggregate. Humic acid-like compounds were the dominant fractions in DOM from all treatments. DOM from Ck and NPK treatments had more complex molecular structure and lower biodegradability than that from NPKS and AM treatments.