In Japan’s mountains, torrents of water cascade down steep ridges, flow beneath cedar boughs in unmanaged forests, stream through farms maintained by an aging rural population, and, finally, course past the urban areas to which many younger citizens have flocked. By the time this water reaches the rivers that feed local drinking supplies, it has picked up a lot of evidence of the land it has traversed, including nutrient pollution.
Nutrient pollution, or an excess of nutrients such as nitrogen and phosphorus in the water, can lead to a host of health and environmental problems. Many nutrients enter rivers as runoff from farms and residential areas. Scientists have also suggested that forest soils may generate nutrient runoff, especially in coniferous forests where soil surfaces are often bare and prone to erosion.
As global climate warms and strong rainstorms become more frequent throughout the world, researchers are wondering how the increase in heavy precipitation might affect nutrient pollution. In a new study, Ide et al. studied the Hii River basin in western Japan to understand how nitrogen and phosphorus levels there fluctuate with rainfall.
Over an 18-year period, the team collected water samples from subbasins within the larger river basin and from the area where the entire basin drains into a single river. They measured concentrations of nitrate, phosphate, total nitrogen, and total phosphorus and then analyzed their data with a model designed to evaluate many possible factors, including land cover, and relationships that could explain patterns seen in their results.
The researchers found that the relationship between nutrient concentrations and surrounding land types was strongest during times of heavy rain and high river flow. Agricultural land always leached phosphorus and nitrogen but did so even more during heavy rains. In contrast, forests of all types helped dilute excess nutrients, reducing nutrient concentrations even more during periods of heavy rain.
On farms, nutrients from fertilizers accumulate over time and are flushed out in high doses with rainfall—an issue that may be exacerbated in Japan. Proper fertilizer use tends to require a lot of work, and the aging farming population of Japan often opts instead for fewer, heavier fertilizer applications, the researchers noted. The team found that even a small stretch of agricultural or residential land had a disproportionately large effect on nutrient levels in the rivers studied.
The scientists predict that as heavy rainstorms occur more frequently and as young people continue migrating from rural to urban areas, nitrogen and phosphorus levels in Japan’s rivers will go on rising, affecting the drinking water and lakes downstream. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004513, 2019)
—Elizabeth Thompson, Freelance Writer