Despite increased attention to the need for sustainable agriculture, fertilizer application rates above crop requirements remain common agricultural practices in South Korea, causing eutrophication of freshwater and coastal ecosystems. The aim of this study is to quantify phosphorus (P) inputs, outputs, and retention in a forested-agricultural watershed. The P budget showed that the combined use of chemical fertilizer and organic compost was the largest source of P (97.6% of the total) followed by atmospheric wet deposition (2.1% of the total P), whereas forest export (0.2% of the total) and sewage treatment plants (STPs) (0.1% of the total) were negligible. The P outputs were crop harvesting and hydrologic export to surface water. The P balance showed that P inputs are higher than the P outputs; approximately 87% of the total P input was retained in the soils within the watershed. However, P concentrations in drainage water were still high enough to cause eutrophication of downstream reservoirs. The results provide important details on the proportion of P export and retention in the watershed. This will help efforts to improve water quality and design better management strategies for agricultural nonpoint source pollution.

The decomposition rates of organic nitrogen and organic phosphorus in lakes and rivers can be important parameters in water quality model calibrations. Commonly, decomposition rate coefficients are calibrated by observing the changes in each component in the field survey. However, in natural field conditions, production and decomposition occur at the same time, and the calibrations may result in uncertainties. In this study, water samples collected from six major rivers in Korea were incubated for 20 days in a dark incubator and the decomposition rates of POP, DOP, LDOP, PON, DON, LPON, LDON, ammonia, and nitrite were determined. By excluding the interference of photosynthesis, animal grazing, and excretion, the decomposition rate coefficients were determined with less uncertainty than in natural conditions. The decomposition data were fit to first-order models of several decomposition pathways, including POP ! DIP,The decomposition rates of dissolved forms were five-seven times higher than particulate forms. And the rates of organic P (0.149 day �1 for LPOP and 0.718 day �1 for LDOP) were 1.6 times higher than those of organic N (0.093 day �1 for LPON and 0.472 day �1 for LDON). Traditionally, the decomposition rates of organic phosphorus and organic nitrogen are not discriminated; the same rate is applied to organic carbon, nitrogen, and phosphorus. The result of this study shows that organic phosphorus and nitrogen should be given different decomposition rates for more accurate models. The rate coefficients measured in this study will provide a guide to the selection of model parameters for water quality modeling.

Background: Stoichiometry plays an important role in understanding nutrient composition and cycling processes in aquatic ecosystems. Previous studies have considered C:N:P ratios constant for both DOM (dissolved organic matter) and POM (particulate organic matter). In this study, water samples were collected in the six major rivers in Korea and were incubated for 20 days. C:N:P ratios were determined during the time course of the incubations. This allowed us to examine the changes in N and P contents of organic matter during decomposition. Results: POM and DOM showed significant differences in N and P content and the elemental ratios changed during the course of decomposition; DOM showed higher C:N and C:P ratios than POM, and the C:N and C:P ratios increased during decomposition, indicating the preferential mineralization of P over N and N over C. Conclusions: The N and P contents of organic matter in aquatic ecosystem are far from constant and vary significantly during decomposition. More detailed information on the changes in C:N:P ratios will provide improved understanding of decomposition processes and improved modeling of aquatic ecosystems.

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Dissolved organic carbon (DOC) concentrations and zooplankton and particulate organic matter (POM) δ 13C values were measured in five reservoirs in Korea. Zooplankton δ 13 C and POM δ 13C showed large range from -33‰ to -22‰ and a significant difference among the reservoirs. One eutrophic reservoir, Lake Masan, showed unique characteristics with the highest zooplankton density, the highest δ 13 C, and the highest DOC. Zooplankton δ 13 C was similar to POM δ 13 C, implying that zooplankton occupies substantial portion of POM or that zooplankton isotopic composition is related to selective grazing and assimilation of food sources from bulk POM. Except Lake Masan zooplankton δ 13 C values were negatively correlated to DOC concentration in four reservoirs with mostly forest land use. This pattern can be probably attributed to intensive agricultural land use in the watershed of Lake Masan compared to the mostly forest land use in the other watersheds. Understanding the relationship between zooplankton δ 13 C values and the origin of organic matter associated with watershed characteristics will be valuable to better understand trophic relationships in reservoirs in the summer monsoon region.