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.
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.
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