Effect of organic matter applications on ¹³C-NMR spectra of humic acids of soil

Much attention has been paid to improving soil fertility with organic matter (OM) application, which not only deals with organic wastes and protects the environment, but also maintains soil fertility and increases crop yields. Much research has also been on the effects of OM applications on the soil's physical, chemical and biological properties, but relatively less attention has been spent on humic substance (HS). In order to clarify the mechanism of improving soil fertility by applying OM, we analysed the changes of structural characteristics of humic acid (HA) from OM applications to soils by ¹³C-NMR, chemical analysis, thermal analysis, optical properties, IR and fluorescence spectroscopy. Samples of a brown soil (Paleudalf) and a paddy soil (an anthropogenic soil originating from Udifluvents) were collected from the plough layer of the long-term field experiment at Shenyang Agricultural University and Liaoning Provincial Alkali-Saline Soil Institute, respectively. Both field experiments included three treatments each: (i) brown soil, zero-treatment (CKbr) and two pig manure (PM) applications (O₁ and O₂) at annual rates of 0.9 t ha⁻¹ and 1.8 t ha⁻¹ of organic carbon, respectively; and (ii) paddy soil, zero-treatment (CKpad), pig manure (Op) and rice straw (Or) at annual rates of 2.62 t ha⁻¹ and 1.43 t ha⁻¹ of organic carbon, respectively. An incubation experiment was also carried out to test the field experiment on the brown soil, namely four treatments: zero-treatment (CKc), and three pig manure applications at rates of 30 g kg⁻¹ (C₁), 50 g kg⁻¹ (C₂) and 70 g kg⁻¹ (C₃), respectively. The total incubation was 180 days. The results indicated that number-average molecular weights (Mn), total acidity, aromaticity, excitation maximum wavelength (λExmax), and the heat ratio of the high to moderate temperature exothermic regions (H₃/H₂) of the HA all decreased after OM application. The degree of activation (AD), the absorption intensity ratio of 2920 cm⁻¹ to 1720 cm⁻¹ in infrared spectra (IR₂₉₂₀/₁₇₂₀), alkyl C and O-alkyl C of the HA increased. The HA structure tended to become simpler and more aliphatic.