Water and Salt Transport under Subsurface Application of Concentrated Organic Liquid Fertilizer: Effects of Dripper Depth and Barrier Configuration

High-concentration organic liquid fertilizers, derived from the anaerobic digestion of human excreta and kitchen waste, provide a sustainable alternative to chemical fertilizers, promoting nutrient recycling and reducing environmental impacts. However, their application in agricultural systems faces challenges such as uneven nutrient distribution, high viscosity, and the risk of clogging irrigation systems. This study addressed these challenges by developing a novel fertilization system that couples drippers with physical barriers for subsurface application of high-concentration liquid fertilizer. Experimental investigations and HYDRUS-2D modeling indicated that increasing dripper burial depth enhances vertical water and salt migration, improving moisture distribution by 2.2%–6.7%. The inclusion of physical barriers at 5–15 cm induces lateral diffusion, improving moisture uniformity by 3.8%–6.0% and reducing vertical salt accumulation. The optimized configuration (dripper depth of 5 cm, and barrier depth of 10–15 cm) ensures uniform nutrient delivery. The system has been demonstrated in over 100 rural courtyards across several provinces in China, recycling nutrients from household waste to fields, improving fertilizer uniformity and vegetable nutrient uptake, and reducing leaching risks—providing a sustainable sanitation–agriculture integration pathway.