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 13 C-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 (CK br ) and two pig manure (PM) applications (O 1 and O 2 ) at annual rates of 0.9 t ha À1 and 1.8 t ha À1 of organic carbon, respectively; and (ii) paddy soil, zero-treatment (CK pad ), pig manure (O p ) and rice straw (O r ) at annual rates of 2.62 t ha À1 and 1.43 t ha À1 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 (CK c ), and three pig manure applications at rates of 30 g kg À1 (C 1 ), 50 g kg À1 (C 2 ) and 70 g kg À1 (C 3 ), respectively. The total incubation was 180 days. The results indicated that number-average molecular weights (M n ), total acidity, aromaticity, excitation maximum wavelength (l Exmax ), and the heat ratio of the high to moderate temperature exothermic regions (H 3 /H 2 ) of the HA all decreased after OM application. The degree of activation (AD), the absorption intensity ratio of 2920 cm À1 to 1720 cm À1 in infrared spectra (IR 2920/1720 ), alkyl C and O-alkyl C of the HA increased. The HA structure tended to become simpler and more aliphatic.

Because of its insolubility, heterogeneity and structural complexity, humin is the least understood among the three fractions of soil humic substances. This research aimed to evaluate the long-term effect of combined nitrogen and phosphorus (NP) fertilizer addition on the chemical structure of humin under maize (Zea mays L.) monoculture in a Typic Hapludoll of northeast China. Soil samples were collected 12 and 25 years after the initiation of the fertilizer treatment. Soil humin was isolated using NaOH-Na 4 P 2 O 7 extraction to remove humic and fulvic acids, which was followed by HF-HCl treatment to remove most of the inorganic minerals. Solid-state 13 C cross-polarization magic angle spinning nuclear magnetic resonance ( 13 C CPMAS NMR) spectroscopy was used to characterize the chemical structure of the humin isolates. Results showed that the organic carbon (C) content of humin increased after NP fertilizer addition, compared with a no-fertilizer (CK) treatment. 13 C CPMAS NMR indicated that O-alkyl C and aromatic C of humin decreased, while alkyl C and the ratios of alkyl C/O-alkyl C, aliphatic C/aromatic C and hydrophobic C/hydrophilic C all increased in the NP fertilizer treatment. The long-term application of NP fertilizer changed the molecular structure of soil humin to be more alkyl and hydrophobic, and was thus beneficial to the sequestration and stability of organic C in soil.

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A 22-year (1990–2012) field experiment was conducted to assess the effects of different fertilization and cropping regimes on the quantitative changes of total N and organic N forms in the topsoil (0–20 cm) and subsoil(20–40 cm) of a Mollisol in Northeast China. This study included nine fertilizer treatments on maize monoculture [no fertilizer (CK), N, NP, NK, PK, NPK, NPK combined with maize straw (NPKS), NPK combined with pig manure (NPKM), and 1.5’ the rate of NPKM (1.5NPKM)] and one fertilizer treatment with maize-maize-soybean rotation (NPKMR). Compared with the CK treatment, the application of mineral fertilizers alone or in combination with straw generally had no significant effect on the contents of total N and organic N forms, whereas the combined application of mineral fertilizers with manure significantly increased their contents. Manure levels and cropping regimes had no significant effect on the total N content in the topsoil. By contrast, the hydrolysable unknown N content significantly increased with the increasing manure levels, and the amino sugar N content was significantly lower in rotation than in monoculture treatment. Our results imply that manure application integrated with continuous maize cropping can be considered as an optimized strategy for improving soil fertility.