Dissolved organic matter (DOM) strongly affects water quality within boreal forest ecosystems. However, how the quality of DOM itself changes spatially is not well understood. In this study, to examine how the diversity of DOM molecules varies in water moving through a boreal forest, the number of DOM molecules in different water samples, i.e., rainwater, throughfall, soil water, groundwater, and stream water was determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in Norway spruce and Scots pine stands in eastern Finland during May and June 2010. The number of molecular compounds identified by FT-ICR MS (molecular diversity) ranged from 865 to 2,194, revealing large DOM molecular diversity in the water samples. Additionally, some of the molecular compounds were shared between different water samples. The DOM molecular diversity linearly correlated with the number of low-biodegradable molecules, such as, lignin-like molecules (lignins), but not with dissolved organic carbon concentration. The number of lignins shared between different sampling locations was larger than that of any other biomolecular class. Our results suggest that low-biodegradable molecules, especially lignins, regulate spatial variations in DOM molecular diversity in boreal forests.

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Identifying nonpoint phosphorus (P) sources in a watershed is essential for addressing cultural eutrophication and for proposing best-management solutions. The oxygen isotope ratio of phosphate (δ18OPO4 ) can shed light on P sources and P cycling in ecosystems. This is the first assessment of the δ18OPO4 distribution in a whole catchment, namely, the Yasu River Watershed in Japan. The observed δ18OPO4 values in the river water varied spatially from 10.3‰ to 17.6‰. To identify P sources in the watershed, we used an isoscape approach involving a multiple-linear-regression model based on land use and lithological types. We constructed two isoscape models, one using data only from the whole watershed and the other using data from the small tributaries. The model results explain 69% and 96% of the spatial variation in the river water δ18OPO4 . The lower R 2 value for the whole watershed model is attributed to the relatively large travel time for P in the main stream of the lower catchment that can result in cumulative biological P recycling. Isoscape maps and a correlation analysis reveal the relative importance of P loading from paddy fields and bedrock. This work demonstrates the utility of δ18OPO4 isoscape models for assessing nonpoint P sources in watershed ecosystems.

Effects of discharge level on the load of dissolved and particulate components of stream nitrogen and phosphorus from a small afforested watershed of Japanese cypress (Chamaecyparis obtusa) Abstract The behavior of dissolved and particulate components of stream nitrogen (N) and phosphorus (P) were measured for 2 years in a small mountainous watershed covered primarily with a plantation forest of Japanese cypress (Chamaecyparis obtusa). The load of dissolved N and P bore a consistent relationship to discharge while the load of particulate N and P varied by up to two orders of magnitude at a given discharge level. Most N was exported in a dissolved form (DN) while most P was exported in a particulate form (PP), which bears similarity to loads from agricultural watersheds. Owing to the different behaviors of DN and PP, changes in the total nitrogen (TN) load were primarily attributed to variations in discharge unlike changes in the total phosphorus (TP) load. High fl ow conditions, resulting from heavy rainfall, displayed PP release significantly larger than expected. The TP load in high fl ow conditions was severely underestimated using a regression equation expressed as a function of discharge, which was based on the weekly sampling data biased toward low fl ow conditions. In addition, the TN load during peak discharges in heavy rain events was underestimated by the regression equation because of unpredictable increases in the particulate component. Our study shows that the particulate component ratio determines whether discharge can explain changes in load regardless of chemical species. The results suggest that plantation forests in rainy regions can be a diffuse source of particulate nutrients depending on soil surface conditions.

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.

Abstract:To investigate the effects of storm flow samplings on the evaluation of DIN and SO 4 2 budgets, we conducted storm flow samplings and discrete interval (weekly) samplings of stream water, throughfall, and stemflow. This investigation lasted for three years and took place in a small forested watershed in western Japan. Annual exports were calculated by the sum of the load (L total )-sum of the discharge (Q total ) relationships obtained by storm flow samplings. These were then compared to calculated ones that were based on LQ relationships obtained by discrete interval sampling. The results of discrete interval sampling showed that the relationships of DIN or SO 4 2 concentration to the discharge amount found in stream water differed from those of storm flow sampling. This was especially notable in years with higher annual precipitation and subsequent higher annual discharges. Also, exports of DIN and SO 4 2 during storm flow periods accounted for 83% and 78% of annual exports on 3-year averages, respectively. These results confirmed that storm flow samplings are essential for estimating annual exports of DIN and SO 4 2 . Annual exports that were calculated based on just discrete interval sampling (46% and 17%, respectively) overestimated DIN and SO 4 2 from discrete interval and storm flow samplings on a 3-year average. These overestimates were pronounced in years of higher discharge, and critically affected the rates of net-retention or export of DIN and SO 4 2 . The relationship of smaller input than output of DIN was reversed if storm flow sampling was included.

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