Abstract:【Objective】 Dissolved organic matter (DOM) is highly sensitive to environmental changes, and its dynamic changes are crucial for understanding regional/global carbon cycling under global change scenarios. However, it is not yet clear how the characteristics of soil DOM molecules change under nitrogen deposition. This study aimed to investigate the response of DOM molecular composition and stability to nitrogen addition. 【Method】 In this study, three nitrogen addition levels (0, 40, and 80 kg·hm–2·a–1) were conducted in a Pinus taiwanensis forest by using urea addition to simulate nitrogen deposition in the field. The effect of short-term (three years) nitrogen addition on the molecular composition of DOM and its stability was investigated using high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). 【Result】 The results of FT-ICR MS analysis revealed that DOM molecules were mainly concentrated in 250-400 Da, and CHO compounds accounted for more than 50% of all compounds. Of the eight types of DOM molecules, lignin-like molecules dominated all soil DOM molecules, followed by tannins and condensed aromatics, with the relative abundance of readily decomposable small molecules (including lipids, proteins, and carbohydrates) being low. There was no statistically significant change in the content and optical properties of DOM under nitrogen addition, but significant changes occurred in the properties and composition of DOM molecules. Compared to high nitrogen treatment, low nitrogen treatment significantly reduced the relative abundance of carbohydrate molecules in DOM by 73.33%. This may be largely attributed to the increase in microbial biomass and hydrolytic enzyme activities. Nitrogen addition did not change the nitrogen-containing compounds in DOM molecules, but reduced the sulfur-containing compounds. Furthermore, the average molecular weight and ratio of double bond equivalent to carbon atom number (DBE/C), modified aromaticity index (AImod), and aromaticity equivalent (Xc) of DOM molecule did not show significant changes under nitrogen addition. However, a significant increase in DBE values was observed under low nitrogen addition, indicating an improvement in the molecular stability of DOM. The improvement of DOM molecular stability may have a potential impact on soil carbon pool stability. Pearsons correlation analysis revealed that DBE values were significantly negatively correlated with small molecule compounds such as carbohydrates and proteins/amino sugars, while the correlation with large molecules such as lignin and condensed aromatics was not significant. Besides, nitrogen addition did not significantly change the difficult-to-decompose molecules such as lignin and condensed aromatic compounds in DOM. This suggests that the molecular stability of DOM under short-term nitrogen addition may depend on the removal of readily decomposable small molecules, such as carbohydrates, rather than the increment of refractory molecules. 【Conclusion】 Collectively, this study provides a new perspective at the molecular level for understanding the behavior of soil DOM under nitrogen deposition, and a reference for understanding the potential relationship between DOM molecules and soil carbon stability.

Abstract:Under a long-term experiment on different fertilization treatments, the contents of water-soluble organic carbon (WSOC) in the soil ranged from 32.1 to 45.5 mg kg-1, following the order of NPK+4500 kg rice straw>NPK+2250 kg rice straw>CK>NPK. The percentages of WSOC to total soil organic carbon (TOC) ranged from 0.15% to 0.19%, following the order of NPK+4500 kg rice straw≈CK>NPK+2250 kg rice straw>NPK. The contents of WSOC had significantly positive correlations with both TOC and soil microbial biomass carbon(SMBC). 13C-NMR results showed that WSOC were mainly com posed of carbohydrates carbon, carboxyl carbon and long-chain aliphatic carbon. Under different fertilization treatments, the coefficients of variation (CV) of WSOC/TOC and SMBC/TOC were lower than those of their absolute contents in soil. Furthermore, the CV of WSOC/TOC was higher than that of SMBC/TOC. The percentage of WSOC to TOC was a good indicator of the quality of soil organic matter under different fertilization treatments.

Abstract:Contents of water soluble organic carbon (WSOC) of 19 soil samples collected from the eastern part of China were determined and the spatial variation in WSOC were analyzed by means of trend surface and semevariagram analysis.It was found that WSOC contents in the studied samples ranged from 0.008 to 0.379mg g-1 C with a median of 0.159mg g-1 C.There is a decreasing tendency of WSOC content from NNE to NNW.Fulvic acid is one of the main sources of WSOC and pH plays an important role in WSOC accumulation in soil.