Response of the chemical structure of soil organic carbon to modes of maize straw return.

Elucidating the chemical structure of soil organic matter (SOM) is important for accurately evaluating the stability and function of SOM. Aboveground vegetation directly affects the quantity and quality of exogenous organic matter input into the soil through plant residues and root exudates, which in turn affects soil microbial species, community structure, and activity, and ultimately impacts the chemical structure of SOM. In this study, a 13C nuclear magnetic resonance technique was used to analyze the chemical structure characteristics of soil organic carbon (SOC) under various rates of straw returning combined with rotary tillage and under full straw mulching. The results showed that full straw returning with rotary tillage and full straw mulching more effectively increased the SOC content than reduced rate of straw returning (1/2 and 1/3 of full straw) with rotary tillage. The contents of alkyl C and alkoxy C in the functional groups of SOC under various straw returning treatments were increased compared with those under the treatment of maize stubble remaining in soil (CK). Furthermore, the contents of aromatic C and carboxyl C were decreased, which were consistent with the chemical shift changes of SOC. Compared with CK treatment, straw returning decreased the content of aromatic C in the functional groups of SOC, but increased the content of alkoxy C, which could be associated with the change in integral areas of absorption peaks of alkyl C and alkoxy C moving toward left and right, respectively. The content of total SOC was significantly positively (P < 0.05) correlated with that of alkoxy C and significantly negatively (P < 0.01) correlated with that of aromatic C. The molecular structure of SOC tends to be simplified due to the decreasing in refractory C and the increasing in easily decomposed C after straw returning to the field.

Impacts of Long-term Fertilization on the Molecular Structure of Humic Acid and Organic Carbon Content in Soil Aggregates in Black Soil.

Soil aggregates are the basic units of soil structure, and their composition and carbon (C) stability directly affect soil fertility. As cementing agents, humic substances play an important role in the formation and stability of soil aggregates. Long-term fertilization not only changes the structure of humic acid (HA), but also affects the content and stability of organic C in soil aggregates. In this study, based on a long-term fertilization experiment, the relationship between the molecular structure of HA and the stability of organic C in the aggregates was examined. Compared with the non-fertilization control (CK), both the application of organic manure alone (M) and organic manure combined with inorganic fertilizer application (MNPK) increased organic C content in the bulk soil and in HA. In addition, the application of organic manure (M, MNPK) favored the formation of macroaggregates (>0.25 mm) and showed a higher organic C contents of soil aggregates with different sizes than the CK. Moreover, the content of aliphatic C, the ratios of aliphatic C/aromatic C and alkyl C/O-alkyl C was increased with the application of organic fertilizer. A significant negative correlation was observed between aromatic C and organic C content of the aggregates with sizes of >2 mm, 2-0.25 mm, and 0.25-0.053 mm (P < 0.05). The findings indicated that organic fertilization treatments (M and MNPK) increased the aliphatic C content of HA, which favored the increase in the organic C content and stability of the aggregates.

Exploring Soil Factors Determining Composition and Structure of the Bacterial Communities in Saline-Alkali Soils of Songnen Plain.

Songnen Plain is originally one of the three major glasslands in China and has now become one of the three most concentrated distribution areas of sodic-saline soil worldwide. The soil is continuously degraded by natural and anthropogenic processes, which has a negative impact on agricultural production. The investigation of microbial diversity in this degraded ecosystem is fundamental for comprehending biological and ecological processes and harnessing the potential of microbial resources. The Illumina MiSeq sequencing method was practiced to investigate the bacterial diversity and composition in saline-alkali soil. The results from this study show that the change in pH under alkaline conditions was not the major contributor in shaping bacterial community in Songnen Plain. The electrical conductivity (EC) content of soil was the most important driving force for bacterial composition (20.83%), and the second most influencing factor was Na+ content (14.17%). Bacterial communities were clearly separated in accordance with the EC. The dominant bacterial groups were Planctomycetes, Proteobacteria, and Bacteroidetes among the different salinity soil. As the salt concentration increased, the indicators changed from Planctomycetes and Bacteroidetes to Proteobacteria and Firmicutes. Our results suggest that Proteobacteria and Firmicutes were the main indicator species reflecting changes of the main microbial groups and the EC as a key factor drives the composition of the bacterial community under alkaline conditions in saline-alkali soil of Songnen Plain.

Effects of long-term fertilization on soil humic acid composition and structure in Black Soil.

The composition and structure of humic acid (HA) can be affected by fertilization, but the short-term effects are difficult to detect using traditional analysis methods. Using a 35-year long-term experiment in Black Soil, the molecular structure of HA was analyzed with Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance spectroscopy (NMR), and fluorescence spectroscopy. Variation in HA was analyzed after long-term fertilization, including fertilization with manure (M), inorganic N, P and K fertilizer (NPK), manure combined with inorganic N, P, and K fertilizer (MNPK), and a no-fertilizer control (CK). The application of each fertilizer treatment increased crop yields compared with the CK treatment, and the MNPK treatment increased crop yield the most. The ratio of main IR absorption peak of HA at 2,920 cm-1 compared with the peak at 2,850 cm-1 (2920/2850) was higher in the NPK and MNPK treatments compared with the CK treatment. The application of manure (MNPK and M treatments) increased the ratio of hydrogen to carbon (H/C) in HA, and raised the ratio of the main IR absorption peak of HA at 2920 cm-1 to that at 1720 cm-1 (2920/1720). Manure treatments also raised the ratio of aliphatic carbon (C) to aromatic C, alkyl C to alkoxy C and hydrophobic C to hydrophilic C and the fluorescence index (f 450/500), but decreased the degree of aromatization of HA, when compared with the CK treatment. The ratio between each type of C in HA was similar among all the fertilizer treatments, but NPK had a lower ratio of H/C and a lower content of aliphatic C compared with the CK treatment. These results indicated that the molecular structure of HA in Black Soil tends to be aliphatic, simpler, and younger after the application of manure. While the application of inorganic fertilizers increased in the degree of condensation of HA and made HA structure complicated. The application of manure alone or combined with inorganic fertilizers may be an effective way to increase crop yield and improve the structure of soil organic matter.

[Effect of long-term fertilization on microbial community functional diversity in black soil].

In order to study the effects of long-term different fertilization on microbial community functional diversity in arable black. soil, we examined microbial metabolic activities in two soil la- yers (0-20 cm, 20-40 cm) under four treatments (CK, NPK, M, MNPK) from a 35-year continuous fertilization field at the Ministry of Agriculture Key Field Observation Station of Harbin Black Soil Ecology Environment using Biolog-ECO method. The results showed that: in the 0-20 cm soil layer, combined application of organic and inorganic fertilizer(MNPK) increased the rate of soil microbial carbon source utilization and community metabolism richness, diversity and dominance; In the 20-40 cm layer, these indices of the MNPK treatment was lower than that of the NPK treat- ment; while NPK treatment decreased soil microbial community metabolism evenness in both layers. Six groups of carbon sources used by soil microbes of all the treatments were different between the two soil layers, and the difference was significant among all treatments in each soil layer (P < 0.05) , while the variations among treatments were different in the two soil layers. Canonical correspondence analysis (CCA) showed that soil microbial community metabolic function of all the treatments was different between the two soil layers, and there was difference among all treatments in each soil layer, while the influences of soil nutrients on soil microbial community metabolic function of all treatments were similar in each soil layer. It was concluded that long-term different fertilization affected soil microbial community functional diversity in both tillage soil layer and down soil layers, and chemical fertilization alone had a larger influence on the microbial community functional diversity in the 20-40 cm layer.