The Hazards of Elevated Sulfide Levels in Water for Agricultural Production

Sulfide is a common reducing inorganic pollutant in water bodies, primarily originating from industrial wastewater discharges from tanneries, printing and dyeing mills, paper mills, and coking plants, as well as from domestic sewage and livestock wastewater. 

In China, the Standards for Irrigation Water Quality (GB 5084—2021) sets clear limits for sulfide (calculated as S²⁻): 1.0 mg/L for paddy crops such as rice, 0.5 mg/L for dryland crops, and 0.2 mg/L for vegetables. When sulfide concentrations in irrigation water exceed these thresholds, agricultural production suffers from multiple, interconnected harms.

Degradation of Soil Physicochemical Properties

Irrigation water with elevated sulfide content first alters the soil's redox status. Sulfide is oxidized chemically or transformed by microbial metabolism into sulfate, a process that consumes substantial oxygen, lowering the soil's redox potential and creating hypoxic conditions. 

Under such oxygen-deprived conditions, metal ions such as iron and manganese are reduced to soluble low-valence forms, potentially exacerbating indirect toxicity to crops. More critically, sulfate ions generated from sulfide oxidation combine with calcium, magnesium, and other cations in the soil, leading to sulfate accumulation under poor drainage conditions—a phenomenon known as secondary salinization. 

Long-term irrigation with high-sulfide water progressively lowers soil pH (acidification), destroys soil colloidal structure, reduces permeability, and intensifies compaction, directly impairing root extension and respiration.

Furthermore, sulfide reacts with heavy metal ions in the soil—such as cadmium, lead, copper, and zinc—to form insoluble metal sulfide precipitates. Although this process temporarily reduces the solubility and bioavailability of heavy metals, changes in soil conditions—such as drainage, sun-drying, or tillage aeration—can reoxidize these metal sulfides, releasing heavy metal ions once again and creating "secondary pollution". This uncertainty poses a significant threat to the long-term safe use of farmland.

Direct Toxicity to Crop Physiological Metabolism

The toxic effects of sulfide on crops are primarily realized through root uptake and subsequent translocation within the plant. Even at extremely low concentrations, hydrogen sulfide inhibits the activity of key respiratory chain enzymes such as cytochrome c oxidase, disrupting normal mitochondrial function and obstructing aerobic respiration in crop roots. 

Affected plants exhibit poor root development, browning root tips, sparse lateral roots, diminished capacity for water and nutrient uptake, and above-ground symptoms including stunting, leaf yellowing, or wilting—similar to nutrient deficiency syndromes.