Effect of water content on composition of water hyacinth Eichhornia crassipes (Mart.) Solms residue and greenhouse gas emission

Although, residue of water hyacinth, Eichhornia crassipes (Mart.) Solms, is better compost because it has a smaller volume and a higher dry matter content after being extruded, not much research exists on it. To determine suitable compost water content of water hyacinth, an experiment that focused mainly on the effects of water on compost and environment factors was conducted. Static compost piles were set at 65%, 70%, 75%, and 80% water content through controlling mixing proportion of water hyacinth residue and rice straw. The temperature, pH, carbon, nitrogen and greenhouse gas of the composts were determined. The results showed that pH, humic acid (after composting for 7 d) and fulvic acid contents of the compost were not significantly affected (P > 0.05) by water content. However, temperature, Kjeldahl-N, soluble carbon, humic acid (after composting for 50 d), nitrate-N and ammonium-N were significantly affected (P< 0.05) by water content of the compost. The 75% water content treatment was especially attractive in temperature-rising of the composts and reached the highest temperature of 53.4 °C just after 6 d. On the 50th d, the Kjeldahl-N, nitrate-N and ammonium-N of the 75% water content treatment were higher (P < 0.05) than those of 65% and 70% water content treatments. Compared with at the 7th d under the 75% water content treatment, at the 50th d, Kjeldahl-N content decreased by 21.1% (the highest change), nitrate-N increased by 434% (the highest change), and ammonium-N decreased by 14.1% (the lowest change). Although no significant differences in CH₄ emission fluxes among the water content treatments were noted, differences in N₂O and CO₂ emission fluxes were pronounced. N₂O emission flux in 80% water content treatment was 3.9~23.1 times higher than in other treatments. CO₂ flux in 75% water content treatment was 1.9~2.5 times higher than in other treatments. The study therefore suggested that using water hyacinth residue and rice straw compost at 75% water content was the best in terms of compost efficiency and quality better in the term of environment.