Rui Jiang
College of Resources and Enviromental Science, Northwest A&F University 相关阅读

Nitrogen (N) export from the agricultural area is regarded as one of the main sources for water pollution in the world. To mitigate the water pollution associated with agricultural N losses, investigate the transport processes and export mechanisms of N from agricultural areas to aquatic systems is crucial. However, the processes and mechanisms of N export are complicated due to a variety of factors, such as land use, topography, hydrological characteristics, and N source, all of which contribute to different spatial and temporal patterns of N export.
The impact of hydrological process on river N export was investigated in stream water during four storm events in 2003 in the Shibetsu watershed. The results showed that particulate N quickly responded to rainfall and peaked before discharge peak during all storm events; while dissolved N peaked just following the peaks of the shallow groundwater table. The M (V) curve, defined as nutrient mass distribution vs. the volume of discharge, showed a “first flush” for all N components. However, particulate N contributed over 80% of fluxes during the first 50% of the discharge; while dissolved N only released 50% of fluxes during the same stage. Thus the significant flush of particulate N was likely to derive from soil erosion and relate to surface runoff; while the slowly export of dissolved N might originate from the near-surface soil layer associated with the rising shallow ground water table, and then flushed with subsurface runoff.
Shibetsu watershed had a coupled land use and topography characteristics which are described as agriculture area with flat topography and forest area with steep slope. To investigate the effect of the coupled characteristics on N export, this study was investigated in three adjacent headwater streams (agriculture-dominated watershed: AW; forest-dominated watershed: FW; and the mixed agriculture-forested watersheds: AFW) in Shibetsu watershed during 2003-2005. The monitoring was conducted from monthly baseflow, over 20 rainfall events and three snowmelt seasons for each watershed. The results showed that higher NO3--N concentrations were observed in the agriculture watershed, lower in the forest watershed, and medium in the mixed watershed. A negative exponential relationship (R2=0.33, P<0.01) was found between the relief ratio (the difference between maximum and minimum elevation of a watershed divided by its maximum length of the river or stream) of the three watersheds and the normalized time by peak discharge time when NO3--N peaked. We observed the NO3--N concentrations peaked before the peak of discharge in the FW for all hydrological events, regardless of the difference in hydrological characteristics. The quick release of NO3--N was attributed to “flushing mechanism”, which was driven by fast response of subsurface flow due to the macropores in forest soil and the steep slope. The AW showed a consistent “prolonged flush” of NO3--N, where NO3--N concentrations peaked after the peak of discharge, which might attribute to the slow occurrence of subsurface flow because of the flat slope and low hydraulic conductivities in the pasture.
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