Impact of N-Saturated Upland Forests on Downstream N Pollution in the Tatara River Basin, Japan

Abstract

This study evaluated whether nitrogen (N) saturated upland forests can degrade downstream water quality in the Tatara River Basin, northern Kyushu, western Japan. Our hypothesis is that elevated atmospheric N deposition degrades downstream water quality in a watershed containing N-saturated forests because a considerable amount of atmospherically deposited N passes into the streams without being retained. Synoptic stream water samplings were conducted at 23 sites across a wide range of land-use categories in the basin over 1 year. A long-term temporal analysis of downstream water quality over the last 30 years was conducted and compared with longterm trends in related factors such as urban/agricultural activity, sewage wastewater treatment, atmospheric N deposition, and forest condition. The results showed that atmospherically deposited N to N-saturated forests can be a large enough nonpoint source of N leaving the watershed to impact downstream water quality. This was highlighted by the reduction in pollutant exports derived from urban/agricultural activities, an increase in atmospheric N deposition, and the maturation of coniferous plantation forests in the past 30 years. These have led to reductions in total phosphorus and organic nitrogen concentrations in downstream water, whereas downstream nitrate (NO 3 -) concentrations increased over the last 30 years. The consequent increase in the downstream N:P ratio indicated P limitation. Reducing the NO 3 -exports from N-saturated upland forests is suggested as a strategy to improve regional downstream NO 3 -pollution, but involves intercontinental-scale action in reducing atmospheric N emissions.

Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin

“…Spearman's rank correlation coefficient (r) was used to examine the relationships between the logarithm NO 3 − concentration and δ 15 N and the relationships between the local urban area (LUA) and LTI at each river to evaluate denitrification in downstream in mixed land use area. All statistical analyses were carried out using an open source statistical programing language of R Version 3.3.3. in urban-or suburban-forested watersheds compared with rural-forested watersheds [10,25,26], and regional-scale spatial patterns of higher atmospheric N deposition were observed in urbanor suburban-forested areas [27][28][29]. In a previous study, river basins included suburban forests, and atmospheric N deposition was moderately high (16 kg N ha −1 year −1 in these areas [30]), which is above the threshold value that could potentially cause N leaching from a forested watershed.…”

“…The population density in this basin was 1316 people km −2 in 2013 (Fukuoka Prefecture, http://www.pref.fukuoka.lg.jp/dataweb/jinko-2014y.html). Upland forests in the Tatara River Basin exhibit N-saturation [10]. The land use of the upstream areas is largely forestry, and the middle and downstream areas are largely agricultural and urban areas (Figure 1b).…”

“…Thus, N-saturated upland forests can also act as an influential NPS, as can agricultural and urban areas [10,11]. Chiwa et al [10] conducted synoptic stream water samplings in the Tatara River Basin, northern Kyushu, Japan, and found that the stream NO 3 − concentrations in the N-saturated upland forest were high compared to those downstream, concluding that the N-saturated upland forests could be a substantial NPS of N downstream.…”

“…Chiwa et al [10] conducted synoptic stream water samplings in the Tatara River Basin, northern Kyushu, Japan, and found that the stream NO 3 − concentrations in the N-saturated upland forest were high compared to those downstream, concluding that the N-saturated upland forests could be a substantial NPS of N downstream. However, neither loading, nor downstream removal processes of N (including assimilation and denitrification) have been clarified.…”

“…We hypothesized that NO 3 − -N from upland N-saturated forest is assimilated or denitrified in downstream areas. The objectives of this study are (1) to evaluate the NPS from upland forest and (2) to identify additional NPS and N removal processes in the downstream area using δ 15 N and δ 18 O values of NO 3 − in the Tatara River Basin where upland forest has been N saturated [10]. This is the first study to show the N removal process in the mixed land use downstream area in a catchment with N-saturated upland forest.…”