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.

[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.