poster - International Conference of Agricultural Engineering

Finding Location for Nitrate Sources along Kitakami River, Japan
Using the Natural Abundance of Nitrogen Isotope
Abstract
Kosuke Noborio 1* , Chitoshi Mizota 2 , Koji Harashina 2 , Mitsuomi Orisaka 3
1 School of Agriculture, Meiji University, Kawasaki, 214-8571 Japan
2 Faculty of Agriculture, Iwate University, Morioka, 020-8550 Japan
3 Iwate Agricultural Research Center, Kitakami, 024-0003 Japan
*Corresponding author. E-mail: noboriok@isc.meiji.ac.jp
It is well known that natural abundance of nitrogen isotope, δ 15 N, in nitrate varies depending
on the sources of nitrate. Nitrate concentration with δ 15 N was determined by sampling water
bimonthly at ten locations along the Kitakami River in Japan. We examined if a various land
use such as forest, rice paddy fields, and upland fields in a watershed affect river water quality,
such as nitrate concentration, and the feasibility of locating a source of nitrate. A mixing
model successfully estimated NO 3 concentration or δ 15 N values of the main stream based on
those values obtained at tributaries only when there was no major denitrification.
Key words: nitrate, δ 15 N, watershed, animal manure, chemical fertilizer
1. Introduction
River water quality not only affects people’s lives living alongside the river but sometimes also
attributes to hypoxia, damaging fishing industries, in a bay where river water runs in. A
proper management of a watershed may need to consider living and industrial environments
involving land-use in an entire watershed. Changes in water quality in river may somehow
be detectable using nitrate concentrations monitored at monitoring stations. It is, however,
quite difficult to find contaminant sources in a watershed using concentration data only
because nitrate is easily diluted water.
Coastal eutrophication, often resulting in hypoxia, is a major environmental problem
worldwide. Relationships between hypoxia and river inputs, and between hypoxia and
increased nitrate-nitrogen, NO 3 , loadings in particular were known before 1990, and additional
research in the early 1990s accumulated compelling evidence of these relationships
(Rabalais et al., 2002). Mitsch et al. (2005) proposed using their modelling results that
creating 22,000 km 2 wetland, which was 65 times larger area than the net gain of wetlands in
the entire US, in the Mississippi River Basin would remove 40% of NO 3 discharged into the
Gulf of Mexico from the river basin. Their proposal, however, seems to be apparently
unrealistic.
Rice paddy fields in the monsoon Asia may act like the wetland in the Mississippi River Basin.
Using a 1,500 m 2 rice paddy field, Takamura et al. (1977) found that surface runoff from the
rice paddy field mainly contained ammonium-nitrogen, NH 4 , but little NO 3 . Tabuchi et al.
(2005) reported, indeed, that NO 3 concentration decreased by half during water run for about
25 m in an experimental rice paddy field in Japan. Although research on NO 3 behavior in a
small scale of rice paddy fields has been conducted, research on a relationship between NO 3
in river water and land use, specifically for rice paddy fields, in a watershed scale has been
taken little attention.
For upland fields in Hokkaido, northernmost Japan, Tabuchi et al. (1995) and Hatano (2005)
reported that there was a linear relationship between NO 3 concentration in river water and
grassland percentage in a watershed. Although the slope of the linear relationship varied
one watershed to another, the linear relationship remained for all the watersheds that they
examined. However, they have not conducted research on seasonal variation of these linear