Abstract: An investigation was made at a double-rice paddy field in the Qiyang Red Soil  Field Experimental Station, Hunan Province, China to study the species and biomass of weeds growing in rice (Oryza sativa L.) growth season after 34-year application of sulfur (SO₄^2--) and chloride (Cl^-)-containing chemical fertilizers under the same application rates of nitrogen (N), phosphorus (P), and potassium (K). Long-term application of Cl^--containing chemical fertilizer resulted in the greatest species number of weeds and the highest biomass of floating weeds and wet weeds, compared with long-term application of SO₄^2-- and Cl^-+SO₄^2--containing chemical fertilizers. In early rice growth season, the biomass of weeds after applying Cl^--containing chemical fertilizer was 51.4% and 17.6% higher than that after applying Cl^-+SO₄^2-- and SO₄^2--containing chemical fertilizers, respectively; in late rice growth season, the increment was 144% and 242%, respectively. More floating weeds were observed after applying Cl^-+SO₄^2-- and SO₄^2--containing chemical fertilizers, but few of them were found after applying Cl^--containing chemical fertilizer. The total dry mass of weeds and the dry mass of wet weeds were positively correlated with soil Cl^- content (r=0.764, P<0.01 and r=0.948, P<0.01, respectively), but negatively correlated with soil SO₄^2--S content (r=0.849, P<0.01 and r=0.641, P<0.05). Soil alkali-hydrolyzable N and available P, under the co-effects of soil SO₄^2--S, Cl^-, and pH, had indirect effects on the total dry mass of weeds. By adopting various fertilization measures to maintain proper soil pH and alkali-hydrolyzable N and available P contents, increase soil SO₄^2--S content, and decrease soil Cl^- content, it could be possible to effectively inhibit the growth of wet weeds and to decrease the total biomass of weeds in double-rice paddy field.

Key words: sulfur-containing chemical fertilizer, chloride-containing chemical fertilizer, long-term fertilization, weed, paddy field