Environmental factors regulating stream nitrate concentrations at baseflow condition in a large region encompassing a climatic gradient

Abstract

Though high rates of nitrate (NO 3 À ) leaching from forests are undesirable, the factors significantly regulating stream NO 3 À concentration is not clarified yet. In Japan, not only near metropolitan areas but also the Japan Sea-side area with heavy snowfall is well known for receiving more than 10 kg-N ha À1 year À1 of nitrogen (N) deposition.However, NO 3 À concentration in stream water is relatively low in the Japan Sea-side area compared with its concentration in other areas. We examined important environmental factors regulating stream NO 3 À concentrations at baseflow condition in a large region of Japan, the Kinki region (KIN) including a part of Japan Sea-side (JSK)using Random Forest regression. The amounts of N deposition and precipitation were common regulating factors for stream NO 3 À concentration at baseflow condition. Random forest showed the significant correlation between the factors related to ecosystem N retention and stream NO 3 À concentration at baseflow condition, and it suggests that large N deposited during the growing season was incorporated into the ecosystem in the entire KIN. Heavy rain and snow flush N and wash out N accumulated in the surface soil, causing small N accumulation in forests. Also, large precipitation dilute NO 3 À concentration in baseflows. These things lowered stream NO 3 À concentration at baseflow condition. Especially in JSK, most of N deposed with the heavy snow flushed out during the snowmelt period. We provided the first statistical confirmation using Random Forest regression that N accumulation and cycling in forest ecosystems were related to NO 3 À leaching from forests into streams.

Causal Relationships between Stream NO₃⁻ Concentrations at Baseflow Conditions and Key Regulating Factors in the Kinki Region of Japan, Including the Japan-sea Side Area and Encompassing a Climatic Gradient

“…The averaged N deposition in the Japan-sea side in the KIN (hereafter, the JSK) was larger than the one in the entire KIN though both were relatively large as described below in the "Study area and database development" section. However, the stream NO 3 − concentrations in JSK were lower than the entire KIN [13,14]. Though all possible factors which can affect NO 3 − leaching such as topography [15,16], climate [8,[17][18][19][20][21], vegetation [9,22,23], soil chemistry [9,22,23], soil type [24,25], geology [26][27][28], and historical land-use and disturbance [8,9,[29][30][31] were examined, Makino et al [14] demonstrated that both the amount of N deposition and several climatic factors such as precipitation and temperature were important for regulating stream NO 3 − concentrations at baseflow conditions.…”

“…In a previous study, the key factors regulating NO 3 − leaching from watersheds without human-created features were identified across the Kinki region (KIN), a large region (28,914 km 2 ) of Japan, using random forest (RF) regression, a machine learning technique [14]. The KIN includes a portion of the Japan-sea side and experiences considerable variation in N deposition and climatic conditions.…”

“…Based on the previous results, Makino et al [14] suggested several possible mechanisms of NO 3 − leaching. For example, they proposed that high amounts of N deposited during the growing season over the entire KIN as well as N deposited during the dormant season in the JSK were incorporated into the ecosystem and contributed to increased stream NO 3 − concentrations [14].…”

“…Based on the previous results, Makino et al [14] suggested several possible mechanisms of NO 3 − leaching. For example, they proposed that high amounts of N deposited during the growing season over the entire KIN as well as N deposited during the dormant season in the JSK were incorporated into the ecosystem and contributed to increased stream NO 3 − concentrations [14]. In contrast, much lower levels of N deposition during the dormant season over the entire KIN compared to levels in the JSK did not strongly affect N ac-cumulation in the ecosystems.…”

“…In contrast, much lower levels of N deposition during the dormant season over the entire KIN compared to levels in the JSK did not strongly affect N ac-cumulation in the ecosystems. Instead, N flush by rain and/ or snow contributed greatly to NO 3 − export and reduced N accumulation in the ecosystems, resulting in negative correlations between both precipitation and N deposition and stream NO 3 − concentrations at baseflow conditions [14].…”