Varied Mulching Practices Modulate Soil Hydrothermal-Nutrient Regimes and Root Yield of Indigowoad Root in an Oasis

Water scarcity and plastic film pollution increasingly threaten the sustainable production of Indigowoad Root (Isatis tinctoria L.) in semi-arid regions. However, the mechanisms by which different mulching practices under flat planting regulate soil hydrothermal conditions, nutrient availability, and yield remain poorly understood. The study aimed to provide theoretical and technical support for water-saving and high-yield production of Indigowoad Root in oasis areas. Bare soil without mulching was used as the control (CK), and five mulching treatments were established: biodegradable white plastic film mulching (DW), biodegradable black plastic film mulching (DB), white polyethylene film mulching (PW), black polyethylene film mulching (PB), and straw mulching (SM). The effects of these treatments on soil water storage, soil temperature, soil organic matter, soil nutrient status, leaf area index, dry matter accumulation, and yield of Indigowoad Root were evaluated. The mulching treatments improved soil hydrothermal conditions and nutrient status and promoted plant growth and yield formation in Indigowoad Root. Compared with CK, DB significantly increased soil water storage and soil temperature. It also increased the contents of ammonium nitrogen, nitrate nitrogen, available phosphorus, and available potassium, thereby promoting dry matter accumulation and yield formation. The PLS-PM results indicated that improved soil hydrothermal conditions enhanced soil nutrient availability, thereby promoting plant growth and driving yield formation (increased by 25%). Biodegradable black film mulching (DB) increased Indigowoad Root yield by improving soil hydrothermal conditions, enhancing soil nutrient availability, and promoting plant growth, thereby providing a promising water-saving and yield-enhancing mulching strategy for Indigowoad Root production in the semi-arid Hexi Oasis.