Drip fertigation with food waste biogas effluent in a humid area is possible but challenging due to increased soil soluble sodium

To assess changes in soil salinity and salinity leaching following rainfall when food waste biogas effluent was used as a fertilizer in a humid region, a field experiment was conducted in 2020–2022 focused on broccoli drip fertigation with different percentages of biogas effluent. Five treatments consisted of 0% (CF), 25% (T25), 50% (T50), 75% (T75), and 100% (T100) biogas effluent blended with fresh water, with the same total N, P, and K. The experiment included a control (CK) that consisted of drip irrigation without fertilizer. The electrical conductivity of soil saturation paste extract (ECe), soluble sodium, sodium adsorption ratio (SAR), cation ratio of structural stability (CROSS), and broccoli yield were monitored. Drip fertigation with biogas effluent (electrical conductivity=19.0 dS/m) increased soil salinity and the ECe rose as the biogas effluent percentage increased, especially in the 0–20 cm soil profile. The T25, T50, T75, and T100 treatment ECe values were 1.2–2.2 times those of the CF treatment. Soluble sodium, SAR, and CROSS increased during the fertilization season. However, compared with CF, 25–100% biogas effluent drip fertigation did not significantly affect broccoli yield. During the post season, the soil salinity was effectively leached by rainwater. After one study year of rotation, there was almost no significant difference in ECe between treatments, but the soluble sodium, SAR, and CROSS values were significantly higher in T100 than in the CF treatment. In other words, when the total applied fresh water amount (including rainfall and irrigation) is more than 45–48 times the biogas effluent application amount (i.e., irrigation with a biogas effluent concentration of ≤75%), it may not significantly increase the soil soluble sodium. In conclusion, drip fertigation with biogas effluent can be used for broccoli fertigation in humid areas, and treatment with≤ 75% biogas effluent is suggested. Combined with the local rainfall depth, a biogas effluent application depth of ≤ 28–30 mm/year may be feasible.