Daisuke Akaishi

Abstract Mountain streamwater chemistry is an effective indicator of forest condition. In 2022, we conducted a nationwide investigation of mountain streamwater chemistry in Japan, leveraging citizen‐participatory sampling. This approach involved 629 individuals with regular exposure to mountain and natural environments. Although our primary aim was to sample at locations from a 2003 study, we also welcomed samples from new sites. In total, 1414 streamwater samples were collected one time from each forested watershed at the baseflow condition. Our study focused on stream nitrate (NO₃⁻) concentration as a key indicator of anthropogenic nitrogen (N) loading impacts on forests. We compared NO₃⁻ concentrations in 2022 with those from 2003 at identical sampling points. After excluding 179 points with evident human‐created features upstream, the mean NO₃⁻ concentration in 2022 was 0.328 mg N L⁻¹ (n = 1236). Comparing data from 1088 points sampled in both years, the mean value in 2022 (0.324 mg N L⁻¹) was significantly lower than that in 2003 (0.359 mg N L⁻¹, p < 0.05). Notably, 88.5% of sampling points showed differences within ±0.25 mg N L⁻¹. The spatial distribution pattern of mountain stream NO₃⁻ concentrations in 2022 did not consistently align with large cities, industrial areas, or N deposition sources. This unique approach marked the first nationwide participatory survey for collecting mountain streamwater in Japan. Our success in ensuring sample quality through accessible explanations, manuals, and videos demonstrates the potential of citizen science. However, the quantitative evaluation of scientific accuracy remains a forthcoming challenge.