Large amounts of nitrogen fertilizers were applied into the soil in a tea plantation. The pH values of spring and well water originated from the tea plantation were below 7. Although acidic spring water flowed into a local pond, the pH value of the pond water remained above 7. The increase in pH values within the pond was associated with an increase in DO (dissolved oxygen) and δ 15N values and the partial disappearance of NO3-. As DO values within the pond remained high and nitrogen assimilation was associated with the production of oxygen by the photosynthesis of algae, nitrogen assimilation was concluded to give rise to increases in pH values within the pond.
