Dissolved organic matter (DOM) plays an essential role in many environmental processes, particularly in soil ecosystems. In the present study, ultraviolet-visible (UV-Vis) spectroscopy and parallel factor analysis (PARAFAC) of three-dimensional fluorescence excitation-emission matrices (3D-EEMs) were used to characterize DOM extracted from various agricultural soils across four climate regions of China. The maximum (86.01 mg L) and minimum (17.39 mg L) dissolved organic carbon (DOC) concentrations were found in soils from Jiangsu and Yunnan, respectively. Specific UV-Vis absorption at 254 nm (SUVA) for soil DOM from the temperate continental climate (TCC) region was higher than that of soil DOM in other climate regions. Three fluorescence components including UVC humic-like substances (excitation peak at 400 nm, emission peak at 525 nm), UVA humic-like substances (250(330)/430 nm), and tyrosine-like materials (220(275)/320 nm) were identified in soil DOM using PARAFAC analysis. However, there were no significant differences in the distributions of these three components for soil DOM from different climate regions. Positive correlations were found among the humification index (HIX), fluorescence index (FI), and autochthonous index (BIX). Our results demonstrate that EEMs-PARAFAC could be a feasible approach for characterizing DOM in agricultural soils from different crop systems and can be used to further study complex DOM in agricultural environments.

Studies to evaluate the effects on nitrification of 19 trace elements showed that all these elements inhibited nitrification of NH4+‐N added to soils. Results showed that the relative effectiveness of the trace elements in inhibition of nitrification depends on the soils. When the trace elements were compared to using 5 µmoles/g of soil, Ag(I), Hg(II), Cd(II), Ni(II), As(III), Cr(III), B(III), Al(III), Se(IV), and Mo(VI) were the most effective inhibitors (average inhibition >50%), and Mn(II) and Pb(II) the least effective (average inhibition <25%) inhibitors. The average inhibition by the other elements studied [Co(II), Cu(II), Sn(II), Fe(II), Zn(II), Fe(III), V(IV), and W(VI)] ranged from 33% with W(VI) to 49% with Fe(III), Silver(I), Ni(II), Co(II), Zn(II), Mn(II), Pb(II), As(III), B(III), Fe(III), As(V), Mo(VI), and W(VI) inhibited Nitrobacter, causing accumulation of NO2−‐N in one of the soils used.

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Photochemical and biological degradation of dissolved organic carbon (DOC) and their interactions jointly contribute to the carbon dioxide released from surface waters in permafrost regions. However, the mechanisms that govern the coupled photochemical and biological degradation of DOC are still poorly understood in thermokarst lakes. Here, by combining Fourier transform ion cyclotron resonance mass spectrometry and microbial high-throughput sequencing, we conducted a sunlight and microbial degradation experiment using water samples collected from 10 thermokarst lakes along a 1100-km permafrost transect. We demonstrate that the enhancement of sunlight on DOC biodegradation is not associated with the low molecular weight aliphatics produced by sunlight, but driven by the photo-produced aromatics. This aromatic compound-driven acceleration of biodegradation may be attributed to the potential high abilities of the microbes to decompose complex compounds in thermokarst lakes. These findings highlight the importance of aromatics in regulating the sunlight effects on DOC biodegradation in permafrost-affected lakes.