Responses of the content and spectral characteristics of dissolved organic matter in intercropping soil to drought in northeast China

Background and aims Dissolved organic matter (DOM) has a key role in soil biogeochemistry and is vulnerable to global climate change such as drought stress. Intercropping provides greater bioavailable resources and soil carbon inputs, compared to monoculture. However, little data exist on the DOM quantity and quality of intercropping soil responses to drought. Methods We conducted a two-year manipulated rainfall-reduction field experiment including water (the control and drought), and cropping system (millet/peanut intercropping and corresponding monoculture). After crop harvesting, DOM contents and compositions were determined and characterized using ultraviolet–visible absorbance and fluorescence spectroscopy. Results In 0–20 cm, DOC contents were reduced by 44.3% in intercropping and 23.8% under drought. However, the specific ultraviolet absorbance at 254 nm (SUVA 254 ) and 260 nm (SUVA 260 ) of DOM were increased in intercropping, indicating greater aromatic and hydrophobic DOM in intercropping soil, and the enhanced effect was observed under drought. Four identified DOM components were UVC humic-like (C1), microbial-derived humic-like (C2), unidentified humic-like (C3), and tyrosine-like (C4). In detail, C1, C2, C3, and C4 were all higher in intercropping, especially under drought treatment ( P < 0.05). Humification index (HIX), fluorescence index (FI), and biological index (BIX) indicated mixed DOM sources (terrestrially-derived and microbial-derived), FI and BIX were higher under drought. Conclusion Although drought decreased DOC content, the DOM components in intercropping could be more stable due to their higher aromaticity and hydrophobicity. These findings provide a new theoretical basis to evaluate soil fertility in various agroecosystems and understand how soil DOM responding to drought.