Impact of Subsurface Drainage and Biochar Amendment on the Coastal Soil–Plant System: A Case Study in Alfalfa Cultivation on Saline–Alkaline Soil

Coastal saline–alkaline soils are characterized by high salinity and poor permeability. Subsurface drainage and biochar amendment are both practical methods, and their combination may overcome the limitations of individual measures and achieve rapid desalination and soil improvement. To evaluate the impact of different subsurface drainage spacing and biochar amendment on soil properties and crop yield, the salt-tolerant plant “alfalfa” was used as the main material. We designed four drainage spacing treatments (0 m (CK), 6 m (S1), 12 m (S2), and 18 m (S3)) and three biochar amendment rates (5 t ha−1 (C1), 10 t ha−1 (C2), 15 t ha−1 (C3)). Soil physical indicators, salinity, and alfalfa yield are measured. The results showed that smaller drainage spacing and higher biochar amendment rates were beneficial for soil improvement, including bulk density, porosity, and field capacity. The experimental treatments affected the entire soil profile (0–80 cm), with subsurface drainage showing a greater impact on reducing salinity than biochar amendment. The S1 treatment had the most significant yield-increasing effect compared to other spacings. The increase in the biochar amendment rate promoted alfalfa yield, particularly for the first harvest. Overall, the results indicated that the drainage spacing of 6 m and the biochar amendment of 15 t ha−1 were most beneficial in improving soil properties in the plow layer and promoting alfalfa yield in saline–alkaline soils.