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

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.