Effects of nutrient addition on the composition and chemical characteristics of soil dissolved organic matter in a desert steppe in northern China

Dissolved organic matter (DOM) plays a critical role in ecosystem function and productivity, particularly in carbon (C) cycling in grassland ecosystems. However, changes in the structural complexity of DOM in a desert steppe following long-term treatment with nitrogen (N) and phosphorus (P) remain unclear; this limits our understanding of the nutrient-related soil C cycle in a desert steppe. In the present study, soil experiments were conducted in the 0-10 cm soil layer of a desert steppe in northern China from 2017 to 2021, and four treatments were established: P, N, N + P (NP), and no nutrient addition (CK). The content and chemical composition of soil DOM were determined by ultraviolet-visible absorbance, fluorescence, and Fourier transform infrared spectroscopy. Compared to CK treatment, nutrient addition increased soil DOM content by 2.86%-53.84%. NP addition increased the average molecular weight, aromaticity, and humification degree of soil DOM. The DOM source was attributed to the combination of foreign and local sources. Fluorescent components of the DOM samples were mainly proteins and humic acids; the humic acid content decreased after N addition and increased after P and NP addition. Nutrient availability and pH were the key factors affecting the changes in the source and average molecular weight of the DOM, respectively. The soil organic matter content was significantly positively correlated with the humification index (r = 0.96). These results imply that nutrient addition accelerates the accumulation of DOM and influences its structural complexity; this potentially benefits soil C sequestration in a desert steppe.