Soil dissolved organic matter dynamics under dryland restoration: disentangling land use, vegetation, and management controls.

INTRODUCTION: Dryland soils are highly sensitive to land use change and restoration, yet the relative influence of land use and associated vegetation and management conditions on soil dissolved organic matter (DOM) remains poorly understood. METHODS: This study evaluated DOM quantity and quality across five land use types (cropland, orchard, grassland, shrubland and forestland) and the additional differences associated with vegetation and management within specific land use systems. Soils were sampled from sixteen sites and three depths (0-20, 30-50, and 60-80 cm), including naturally restored and artificially planted shrublands, grasslands with taproot and fibrous-root systems, orchards under high-input and low-input management. RESULTS: Dissolved organic carbon (DOC) and DOM optical properties were characterized using ultraviolet-visible absorbance and fluorescence spectroscopy. Overall, DOC content was highest in forestland topsoil (235.22 mg kg-1) and lowest in orchards and cropland (10.16-89.15 mg kg-1), and declined with depth. Humic-like components dominated the fluorescence signal, whereas protein-like components increased in deeper layers. Woody systems showed higher optical signatures consistent with larger apparent molecular size, greater aromaticity, and stronger humification, whereas cropland showed the opposite pattern. Within land use types, the walnut/low-input orchard showed higher DOC and stronger humic-like optical signals than the apple/high-input orchard, whereas grassland root type and shrubland restoration mode showed weaker changes. Multivariate analysis showed that DOC content and DOM composition covaried with surface inputs, fine texture, and biological soil crust development. DISCUSSION: This framework provides a useful basis for interpreting soil carbon responses to restoration and land management in fragile dryland ecosystems.