9:45 AM - 10:00 AM
[AHW32-10] Biogeochemical cycling of phosphate in the Yasu River Watershed: Insight from oxygen isotope of phosphate
Keywords:Oxygen isotope of phosphate, Freshwater system
Phosphorus (P) is an essential element for all living organisms and can be a limiting factor for primary production in river ecosystems. Therefore, its biogeochemical cycling is very important in proper land management and understanding of natural systems. Recently, oxygen isotope ratio of phosphate (δ18OPO4) has been used as a tool to elucidate the P cycle. Previous studies showed the possibility to evaluate P sources, metabolism by organism in some ecosystems (Paytan & McLaughlin 2011). However, there are few research to show the spatial distribution of δ18OPO4 in the watershed scale, and it is not clear whether δ18OPO4 is useful for evaluating the biogeochemical cycling of P in the watershed scale. The purposes of this study are to show the δ18OPO4 distribution in the watershed scale and to examine the relationship between δ18OPO4 distribution and environmental factors, such as P sources, land use and physical characteristics of a river.
2. Material and Method
The investigation was conducted in the Yasu River Watershed in Shiga prefecture, central Japan. River water samples were collected at 15 sites including tributaries in May 2016. As a P source to the river, rocks (granite, sedimentary rock, accretionary complex), soils from forest and paddy filed, chemical fertilizers mainly used in Shiga prefecture and wastewater treatment plant water were collected. For δ18OPO4 analysis, phosphate in all samples was converted to silver phosphate by McLaughlin et al. (2004) procedure with solid phase extraction method to remove dissolved organic matter. The δ18OPO4 values were measured by a TC/EA-IRMS (thermal conversion elemental analyzer connected to a Delta plus XP isotope ratio mass spectrometer via ConFlo III, Thermo Fisher Scientific). The δ18OPO4 values of biologically cycled phosphate (δ18OPO4 Eq) in the river water samples were calculated by Eq. 1 (Longinelli & Nuti 1973):
T = 111.4 - 4.3 (δ18OPO4 - δ18Ow) (1)
Where T is water temperature (°C); δ18OPO4 and δ18Ow are the δ18O of phosphate and water, respectively.
3. Result and discussion
The δ18OPO4 values in the river water samples ranged from 10.1‰ to 17.8‰. These values were different from the δ18OPO4 Eq values at each site, indicating that the δ18OPO4 values in river water samples can be used as a tracer for P sources in the Yasu River Watershed. Significant correlations were found between the δ18OPO4 values in river water and the proportion area of the agricultural land and each rock. In addition, the direction of the regression line agreed with the δ18OPO4 values in soil from paddy filed and each rock. These data suggest that agricultural land and rocks are main P sources to the river. Our investigation showed that the δ18OPO4 is useful for evaluation of biogeochemical cycling of P in the watershed scale.
Longinelli, A. & Nuti, S., 1973. Revised phosphate-water isotopic temperature scale. Earth and Planetary Science Letters, 19(3), pp.373–376.
McLaughlin, K. et al., 2004. A precise method for the analysis of δ18O of dissolved inorganic phosphate in seawater. Limnology and Oceanography: Methods, 2, pp.202–212.
Paytan, A. & McLaughlin, K., 2011. Tracing the sources and biogeochemical cycling of phosphorus in aquatic systems using isotopes of oxygen in phosphate. In M. Baskaran, ed. Handbook of Environmental Isotope Geochemistry. Berlin: Springer-Verlag, pp. 419–436.