The Plant Community Assembly Mechanism And Its Effect On Groundwater Purification Of Wetlands In Irrigated Agricultural Land In Greeley, Colorado State, USA

Wetlands in arid irrigated agricultural area are of special characteristics featured in receiving surface runoff and agricultural drainage water from farmland and showing clear seasonal hydrological dynamics. This kind of wetlands are playing an important role in treating the non-point pollutant from farmland especially the nitrate nitrogen. The results of this research will provide data accumulations and theoretical guidance towards an optimized management practice on this type of wetlands in purpose of modifying plant community characteristics and ultimately enhancing the ecosystem services.This research selected 6 typical wetlands in arid irrigated agricultural area as the study sites. The community investigation was conducted along with the measurement of the relevant environmental factors such as groundwater table, groundwater nitrate nitrogen content, soil bulk density, soil organic matter content, root ratio, rock ratio. The data were analyzed through statistical methods such as Nonmetric Multidimensional Scaling (NMDS), Generalized Linear Model analysis, RLQ analysis and Structural Equation Model. The key environmental factors that determined the assembly of plants community were identified, and the link between the plant community characteristics and the groundwater purification was established. The results were as following:(1) Environmental factors that significantly determined the assembly of the plant community included groundwater table, soil bulk density and soil organic matter content. The community diversity was affected by average groundwater depth, rock ratio, litter layer mass and groundwater electric conductivity. There was a significant correlation between groundwater table and plot above-ground biomass.(2) The plant functional traits showed response to the environmental gradients. At intra-species level, the leaf traits of Typha angustifolia and Schoenoplectus pungens both showed a significant response to the groundwater table gradient. The functional traits similarity index and species association index were positively correlated when inter-species association was positive indicating that the environmental sieve effect played an important role in the process of community assembly. The trait-based approach indicated that functional traits such as dry leaf mass, specific leaf area, growth form and species origin (native/non-native) responded to the integrated environmental gradient. The hydrological condition of groundwater was an important environmental gradient that impacted on the plant functional traits.(3) Community productivity and species richness were positively correlated. Species richness had significant effects on the removal of groundwater nitrate nitrogen content. The coupling relationship existed between environmental factors, plant community characteristics and groundwater purification.In conclusion, environmental factors had direct effects on the plant community characteristics. The plant functional traits showed response to environmental gradients. The plant community characteristics determined the ability of nitrate nitrogen removal from the groundwater. Therefore, optimized management practice with focus on the control of environmental factors especially the groundwater table could modify the plant community characteristics and ultimately enhance the wetlands groundwater purification in this arid irrigated agricultural area.