Effects of intercropping mulch on the content and composition of soil dissolved organic matter in apple orchard on the loess plateau

Dissolved organic matter (DOM) is an important parameter that reflects soil fertility and quality. In this study, the effects of intercropping perennial ryegrass and straw mulch on the content, composition and spectral characteristics of soil DOM in orchards in arid and semi-arid areas were investigated by three-dimensional fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (PARAFAC). Three treatments were applied in a 6-year-old apple orchard on the Loess Plateau: (1) clean tillage (CT); (2) inter-cropping perennial ryegrass mulch (RE); (3) intercropping straw mulch (CS). Soil samples (0-10, 10-20, 20-40, 40-60, 60-80, 80-100 cm) were collected from different treatments. The total soil organic carbon (SOC) and dissolved organic carbon (DOC) content were ranked as CS > RE > CT, and decreased with soil depth. In addition, the vertical distribution of DOC with different soil depth was caused by organic matter input but not leaching. Three humic-like components and two protein-like components were identified by EEM-PARAFAC. Fluorescence spectra showed that humic-like compounds were the dominant fractions in soil DOM in the CS treatment. Fluorescence Index (FI) indicated that the proportion of plant residues and soil organic matter derived DOM ranked as CT > RE > CS and gradually decreased with soil depth. Our results highlight the key role of orchard intercropping mulch in improving the content and compositions of soil DOM through different forms of organic inputs and provide new theoretical support for the evaluation of soil fertility in orchards.