Response of wetlands to agricultural drainage with herbicide: Tolerance and purification

Background/Question/Methods: Regional agricultural non-point pollution and wetlands degradation for lack of water are two major ecological and environmental problems in dryland area of the world. Understanding the effect of herbicide residues from agricultural drainage on wetland system and its dissipation process are crucial to utilize the water resources of farmland drainage to restore wetland and to purify drainage by engineering wetlands. Based on the field investigation in irrigated paddy lands and salt marshes in the Western Songnen Plain, China, in-situ and laboratory simulated experiments were executed with sexual and asexual reproduction of reeds (Phragmites Australis) to study the resistance and purification mechanisms of reed marsh to Bensulfuron Methyl (BSM) under salt stress. The sorption of BSM in soil was investigated by the sorption isotherms from sorption experiments with pure minerals and humus. Special designed rhizobox with millimeter separating compartments was used to investigate rhizosphere effects on the dissipation of BSM. Hydroponics experiments elucidated the effect of BSM on reed exudate under salt stress.

Results/Conclusions: The results indicate that the application of sulfonylurea herbicides in the paddy field could cause herbicides residues (1.68~3.77μg/L). The removal rate of BSM (0.05mg/kg~0.50mg/kg) is over 65% by salt reed marhes (Na⁺ 25mmol/l) after 90 days, and the efficiency is higher at the lower BSM concentration. The response of reed seeds were confined by the membrane permeability, the seed germination rate, germination uniformity, germination rate and vigor all were inhibited by salt stress (Na⁺ 50mmol/l), while the BSM (0.05mg/l~0.50mg/l) could alleviate this inhibition at low BSM concentration; the influence on reed buds were little. The inhibition on soil microbial biomass carbon and nitrogen were＞20%, and BSM could improve the invertase activity though it was not obvious. In general, the entire reed marsh ecosystem showed tolerance to BSM under salt stress. The dissipation rate of BSM was closely related to the distance from the reed root, BSM followed the order: near-rhizosphere ＞root compartment＞far-rhizosphere soil zones. The most dissipation of near-rhizosphere of BSM was at about 2mm from root zone rather than dense root system with most exudate of root zone. The induction of BSM and salt stress strengthened the secretion of total organic carbon (TOC), total sugar and the amino acid from reed root, and the induction function enhanced gradually along with the increasing BSM concentration. Comprehensive ecological impact of herbicide, nitrogen and phosphorus on wetlands should be analyzed in depth.