Nitrogen and phosphorus are considered the most important limiting elements in terrestrial and aquatic ecosystems. however, very few studies have focused on which is from forested streams, a bridge between these two systems. To fill this gap, we examined the concentrations of dissolved N and P in storm waters from forested watersheds of five regions in Japan, to characterize nutrient limitation and its potential controlling factors. First, dissolved N and P concentrations and the N : P ratio on forested streams were higher during storm events relative to baseflow conditions. Second, significantly higher dissolved inorganic N concentrations were found in storm waters from evergreen coniferous forest streams than those from deciduous broadleaf forest streams in Aichi, Kochi, Mie, Nagano, and with the exception of Tokyo. Finally, almost all the N : P ratios in the storm water were generally higher than 34, implying that the storm water should be P-limited, especially for Tokyo.
Nitrogen (N) and phosphorus (P) are considered the most important limiting elements for vegetation in terrestrial ecosystems, especially for the forested headwater watersheds, where there is no direct application of fertilizer and the soils are commonly considered to be infertile [
The N : P ratio has gained worldwide acceptance as an indicator of biological growth and nutrient cycling and has been successfully used in several studies of both aquatic and terrestrial areas [
In 2003, Turner et al. [
We chose a set of headwater streams located throughout five regions in Japan (Aichi, Kochi, Mie, Nagano, and Tokyo), which were characterized by different climatic, geological, and topographical conditions, matching as closely as possible the following watershed criteria: (1) the presence of first- or second-order streams, (2) the watersheds were entirely forested, and (3) a lack of obvious lakes or wetlands in the watershed. All of the streams were fast flowing and the streambeds were composed of boulders and large cobbles with little accumulation of fine sediment. Within each region we selected two to six streams, for which the vegetation (including evergreen conifer (EC) plantations, and natural deciduous broadleaf forest (DB)) was representative of the region and shared similar geological environments. The topographic, meteorological, and vegetation details of each sampling site are summarized in supplementary material see Table 1 and Figure 1 in Supplementary Material available online at doi: 10.1100/2012/257392).
The Aichi sites were located in the Aichi Research Forest of The University of Tokyo, east of Inuyama in Aichi Prefecture. The watershed was composed mainly of Neogene sediments and had an undulating topography. The Kochi sites were located in the Tsuzuragawa watershed, which is part of the east tributary of the Shimanto River, southeast of Tashouchou in Kochi Prefecture. The terrain comprised somewhat steep and incised hillsides. Sandstone and pelitic rocks were dominant in these areas. The Mie sites were located in Taikichiou, Mie Prefecture, and were characterized by generally steep slopes. The prevalent rock was gneiss, with a typical brown forest soil cover. The Nagano sites were located in the Terasawayama Education and Research Forest of Shinshu University in Ina, Nagano Prefecture. The Tanazawagawa, a small tributary of the Tenryu River, discharges from this area. As in Mie, steep slopes are a common feature of this area. The watersheds were underlain by granite. The Tokyo sites were located in the Joubanzawa watershed, a tributary of the Arakawa River, and near the headwaters of the Narikigawa in Tokyo. Steep, incised hillsides are also common in this area. The watershed is underlain by sandstone, pelitic rocks, and chert.
We performed our sampling regime from June 2004 to July 2005 during 10 storm events (not every storm event occurred in every region, and ensured that samples were taken from both the EC and DB streams during the same event and in the same region). Samples were collected in 500 mL clean polyethylene bottles during rainfall storm events using autosamplers (Teledyne Isco Inc., USA, model 6712) which were activated automatically when the stream water level increased. Discrete samples were usually collected half-hourly, or sometimes hourly or two-hourly during the falling limb of the hydrograph. All samples were shipped by refrigerated express to our laboratory and placed in a cool store until analysis. Next, the samples were filtered through precombusted (at 450°C for 3 hours) and preweighed glass fiber filters (0.45
Descriptive statistics were first conducted to investigate the nutrient concentrations for the complete dataset, and to make comparisons with baseflow conditions. Then we investigated the correlation between the water quality indicators. We grouped the data by region, and performed a one-way analysis of variance (ANOVA) to examine the regional distribution of N and P constituents and the N : P ratio. The Turkey multiple comparison and F-test were used to identify significant differences. We also conducted ANOVA analyses to investigate the differences between the two vegetation types, that is, the EC and DB forests and the factors influencing the concentration of N and P and the N : P ratio. Finally, nutrient limitations and during storm flow conditions were discussed.
Table
Summary of forested runoff nutrient concentrations (
Condition | Value | DTP | DTN | DIP | DIN | DOP | DON | N : P Ratio |
---|---|---|---|---|---|---|---|---|
Storm | Min | 0.02 | 1.36 | 0.02 | 0.18 | 0.02 | 0.43 | 7 |
Max | 2.29 | 226.36 | 2.26 | 210.72 | 1.45 | 416 | 2567 | |
Mean | 0.33 | 51.62 | 0.12 | 45.95 | 0.21 | 17.23 | 236 | |
SD | 0.34 | 56.32 | 0.21 | 55.91 | 0.21 | 32.53 | 279 | |
Baseflow* | Min | 0.03 | 0.75 | 0.02 | 0.60 | 0.02 | 0.18 | 7 |
Max | 1.23 | 112 | 1.03 | 106 | 0.41 | 14.1 | 1858 | |
Mean | 0.16 | 24.6 | 0.13 | 21.5 | 0.09 | 3.59 | 165 | |
SD | 0.21 | 25.2 | 0.17 | 23.6 | 0.07 | 2.52 | 198 |
N : P ratios are defined as DTN : DTP molar ratios; molar N:
The correlation analysis revealed that there were strong positive correlations between any two parameters, with the exception of the correlation between DON and DOP (
Relationships between the different forms of N and P.
DTP | DTN | DIP | DIN | DOP | DON | |
DTP | <0.001 | <0.001 | <0.001 | <0.001 | <0.001 | |
DTN | 0.466** | <0.001 | <0.001 | <0.001 | <0.001 | |
DIP | 0.798** | 0.374** | <0.001 | <0.001 | <0.001 | |
DIN | 0.427** | 0.992** | 0.360** | 0.062 | <0.001 | |
DOP | 0.804** | 0.371** | 0.284** | 0.321** | <0.001 | |
DON | 0.136** | 0.151** | 0.135** | 0.190** | 0.081 |
**Correlation is significant at the
Since many environmental factors (including climate, topography, and soil) interact on a regional scale [
Means for nutrient concentrations (
Site | Number of samples | DTP | DTN | DIP | DIN | DOP | DON | N : P ratio |
---|---|---|---|---|---|---|---|---|
EC-A3 | 19 | 0.07 (0.06) | 31.73 (4.71) | 0.02 (0.01) | 21.38 (7.56) | 0.06 (0.05) | 10.35 (10.11) | 638 (296.37) |
DB-A4 | 53 | 0.06 (0.04) | 11.95 (3.92) | 0.02 (0.01) | 6.16 (3.57) | 0.04 (0.03) | 5.79 (1.44) | 241 (129.75) |
DB-K2 | 24 | 0.48 (0.29) | 14.86 (3.22) | 0.14 (0.16) | 12.75 (2.77) | 0.34 (0.14) | 2.11 (0.94) | 40 (23.19) |
DB-K3 | 64 | 0.30 (0.16) | 20.90 (14.27) | 0.09 (0.12) | 17.24 (12.48) | 0.22 (0.12) | 3.66 (2.88) | 79 (47.65) |
EC-K4 | 4 | 0.06 (0.03) | 1.74 (0.27) | 0.20 (0.00) | 0.26 (0.16) | 0.04 (0.23) | 1.47 (0.30) | 40 (23.36) |
EC-K5 | 5 | 0.41 (0.16) | 37.67 (2.86) | 0.04 (0.05) | 34.58 (2.53) | 0.37 (0.18) | 3.09 (0.79) | 107 (48.05) |
EC-K6 | 21 | 0.42 (0.11) | 18.94 (1.65) | 0.05 (0.05) | 16.56 (1.51) | 0.37 (0.12) | 2.38 (0.69) | 49 (15.97) |
EC-K7 | 19 | 0.35 (0.11) | 54.53 (15.14) | 0.16 (0.99) | 47.90 (11.98) | 0.19 (0.15) | 6.62 (5.04) | 169 (70.19) |
EC-M1 | 29 | 0.06 (0.08) | 15.17 (11.63) | 0.02 (0.00) | 12.01 (8.47) | 0.05 (0.08) | 53.97 (50.00) | 352 (221.19) |
EC-M2 | 40 | 0.13 (0.10) | 13.87 (11.89) | 0.04 (0.02) | 10.01 (9.86) | 0.10 (0.10) | 53.77 (39.13) | 117 (76.00) |
EC-M3 | 32 | 0.08 (0.09) | 16.92 (8.67) | 0.03 (0.04) | 13.18 (7.43) | 0.06 (0.08) | 52.36 (31.13) | 370 (248.20) |
EC-M4 | 4 | 0.10 (0.08) | 20.56 (10.33) | 0.02 (0.00) | 9.31 (0.74) | 0.08 (0.07) | 157.4 (150.69) | 433 (471.02) |
EC-M5 | 10 | 0.12 (0.13) | 37.93 (12.19) | 0.02 (0.00) | 34.46 (11.77) | 0.11 (0.13) | 48.70 (26.98) | 670 (493.82) |
DB-M8 | 39 | 0.09 (0.09) | 9.46 (4.75) | 0.03 (0.18) | 5.44 (2.69) | 0.07 (0.08) | 4.01 (3.61) | 182 (107.83) |
EC-N2 | 24 | 0.56 (0.50) | 68.36 (6.18) | 0.14 (0.46) | 61.91 (8.01) | 0.42 (0.31) | 6.45 (4.79) | 190 (112.23) |
EC-N4 | 9 | 1.01 (0.22) | 53.34 (14.74) | 0.24 (0.07) | 5.05 (2.86) | 0.78 (0.17) | 48.29 (12.11) | 52.67 (7.04) |
EC-N5 | 20 | 0.61 (0.24) | 56.75 (13.60) | 0.34 (0.06) | 50.20 (13.68) | 0.27 (0.25) | 6.55 (2.13) | 100 (32.62) |
DB-N6 | 15 | 1.20 (0.29) | 11.50 (4.32) | 0.87 (0.20) | 0.77 (0.83) | 0.33 (0.10) | 10.73 (3.66) | 9 (2.67) |
EC-T5 | 57 | 0.51 (0.33) | 133.92 (28.21) | 0.30 (0.32) | 126.93 (30.28) | 0.22 (0.16) | 6.98 (4.83) | 483 (464.92) |
DB-T6 | 60 | 0.70 (0.46) | 161.54 (31.80) | 0.29 (0.27) | 156.38 (32.97) | 0.41 (0.25) | 5.16 (5.38) | 390 (297.56) |
The concentrations of all the forms of P in the storm runoff in Nagano were the highest amongst the five regions with averages of 0.77, 0.37, and 0.40
As for the N : P ratio in the five regions, a distinct regional pattern was shown, with significantly lower N : P ratios in Kochi than those in the four other sites (
We found no significant differences in the DIP concentration between the DB and EC in the four regions including Aichi, Kochi, and Mie while significantly higher DIP was shown in the DB than that of the EC in Nagano and Tokyo (
Forested stream nutrient concentrations for the different vegetation types in the five regions during storm events. The symbol on the top of columns shows a significant difference at either ***
To compare the relative impacts of the two main variables, that is, region and vegetation (DB, EC), we calculated the weighting of the two factors with respect to the nutrient concentration and the N : P ratio using the whole dataset from the five regions. All
To quantitatively describe the limiting nutrients, we assumed that phototrophic bacteria are likely to be limited by N when the ambient N : P ratio is less than 20, and limited by P when the N : P ratio is greater than 34 [
Relationships between the runoff concentrations of DTN and DTP in the EC and DB forests during storm events in each region.
In this study, the concentrations of dissolved N and P in storm water from forested watersheds in five regions in Japan during storm events were analyzed and compared. First, the concentrations of DIN, DON, DOP, DTN, DTP, and the N : P ratio were higher during storm events relative to baseflow conditions at all the sites, and the same spatial pattern for DIN in storm water was found as for baseflow conditions. Additionally, the spatial patterns across the five regions for all P constituents including DIP, DOP, and DTP followed the same order as being the greatest in Nagano > Tokyo > Kochi > Mie > Aichi, a little different from that of the trends in DIN during both storm and baseflow conditions, in which there was an inverse order for Nagano and Tokyo. The differences in concentrations and spatial patterns of nutrients between stormflow and baseflow conditions across the five regions verified the different mechanisms controlling the nutrient runoff during these two stream states.
Second, significantly higher concentrations of DIN were found in storm water from the EC sites than from the DB with the exception of Tokyo. However, the DTP concentrations from the EC were significantly lower than those from the DB in Nagano and Tokyo, with no significant differences evident between the two types of vegetation in the other three regions. Interestingly, a consistently higher N : P ratio in the storm water from the EC than those of the DB among each region were indicated, which is also in accordance with those measured during baseflow conditions.
Finally, almost all the N : P ratios in the storm water were generally higher than 34 with the exception of those from the DB in Nagano and a very small fraction of the sites in Kochi and Mie, implying that forested stream water in storm events is P-limited, especially in Tokyo, and at the EC sites in Nagano. When comparing the N : P ratios during baseflow conditions, there should be a greater sensitivity to P limitation in the storm water from forested watersheds to increasing trends in atmospheric N deposition and extreme storm events, more of which are possible in the future.
This paper was funded by the Chinese National Science Foundation projects (no. 41101074), the Fundamental Research Funds for the Central Universities, the National Basic Research Program of China (2012CB955404), State Key Laboratory of Earth Surface Processes and Resource Ecology (2010-ZY-10, 2011-KF-06), and International Cooperation Project, the Ministry of Science and Technology of China (40821140354, 2010DFB20880). This paper was also funded by the Japan Science and Technology Agency, Core Research for Evolutional Science and Technology Research for Evolutional Science and Technology.