关键词: 氮磷输出, 连接度, 面源污染控制, 排水沟, 盐城滩涂垦区

Abstract: Ditch corridors not only serve as the nutrient transportation and discharge channel, but also provide retention and purification services for nitrogen and phosphorus removal. Therefore, drainage ditch corridors and their connectivity are closely related to nutrient transportation process critical for non-point source pollution control in agricultural areas. However, a large body of literatures has only quantified the retention and removal of agriculture-driven nitrogen and phosphorus emissions in single drainage ditch. These studies specifically have explored the impacts of different biophysical characteristics (e.g., length, size, sediment, and plant species) of the single drainage ditch on retention and removal of nutrients. Few studies have explicitly addressed the effects of drainage ditch network and connectivity on nutrient transportation and retention from a landscape perspective. This paper built the drainage Ditch Network Connectivity model (DNC) in accordance with the use of “connectivity” in the science of landscape ecology and hydrology for characterizing the degree that nitrogen and phosphorus was expedited or blocked within the drainage ditch network. We applied the DNC model to a typical reclamation area of Yancheng coastal zone, in which the total nitrogen (TN) and total phosphorus (TP) concentrations were measured before and after each rain event in August and October of the year 2015, captured their spatio-temporal dynamics respectively. We further employed the regression model to unravel the relationship of drainage ditch connectivity with nitrogen and phosphorus exports, and ultimately explored the effects of drainage ditch connectivity on diffusion and removal of nitrogen and phosphorus. Our results showed that: 1) TN and TP concentrations showed spatio-temporal variations that October pre-rainfall<August pre-rainfall < October post-rainfall<August post-rainfall, and that TN and TP concentrations increased first and decreased afterwards along the runoff flow direction. 2) TN and TP exports showed an inverted-“U” relationship with the increase of drainage ditch connectivity as revealed from regression models. TN and TP exports reached the inflection point when the drainage ditch connectivity is 0.79 and 0.77, respectively. This implied that improving drainage ditch connectivity could be critical for control of agricultural non-point source pollution due to the important role that ditch connectivity plays in modifying nutrients diffusion and removal processes. The DNC model proposed in this paper can be applied to other agricultural flat reclamation and irrigation areas and provide scientific basis for better control of nitrogen and phosphorus emissions.

Key words: connectivity, drainage ditch, nitrogen and phosphorus export, non-point source pollution control, Yancheng tidal flat reclamation area