The behavior of dissolved and particulate components of stream nitrogen (N) and phosphorus (P) were measured for 2 years in a small mountainous watershed covered primarily with a plantation forest of Japanese cypress (Chamaecyparis obtusa). The load of dissolved N and P bore a consistent relationship to discharge while the load of particulate N and P varied by up to two orders of magnitude at a given discharge level. Most N was exported in a dissolved form (DN) while most P was exported in a particulate form (PP), which bears similarity to loads from agricultural watersheds. Owing to the different behaviors of DN and PP, changes in the total nitrogen (TN) load were primarily attributed to variations in discharge unlike changes in the total phosphorus (TP) load. High flow conditions, resulting from heavy rainfall, displayed PP release significantly larger than expected. The TP load in high flow conditions was severely underestimated using a regression equation expressed as a function of discharge, which was based on the weekly sampling data biased toward low flow conditions. In addition, the TN load during peak discharges in heavy rain events was underestimated by the regression equation because of unpredictable increases in the particulate component. Our study shows that the particulate component ratio determines whether discharge can explain changes in load regardless of chemical species. The results suggest that plantation forests in rainy regions can be a diffuse source of particulate nutrients depending on soil surface conditions.

The effects of elevated atmospheric nitrogen (N) deposition on N cycling in Japanese forest ecosystems are reviewed here to assess the sensitivity of nitrate (NO₃⁻) leaching from forests to streams in response to this deposition. Long-term monitoring of atmospheric N deposition throughout Japan suggests that long-range transport of air pollutants from East Asia accounts for recent increases in atmospheric N deposition in Japan. A new conceptual model of N saturation proposes that kinetic N saturation can be related to unavoidable hydrological nitrate (NO₃⁻) loss from Japanese forests having an Asian monsoon climate with warm and wet summers. Soil microbes, including fungi and ammonia-oxidizing archaea, are important players in these forest ecosystems, affecting nutrient cycling in response to increased atmospheric N deposition. Similarly, a new framework based on a mycorrhizal-associated nutrient economy provides an understanding of NO₃⁻ leaching in soils from temperate forests, suggesting that arbuscular mycorrhizal (AM)-dominated forests leach more NO₃⁻ in response to N deposition than ectomycorrhizal fungi-dominated forests. Because Japanese cedar and cypress are AM-associated trees, they are likely susceptible to NO₃⁻ leaching. Maturation and poor management of such plantations may accelerate NO₃⁻ leaching. Grazing by increasing deer populations also enhances NO₃⁻ leaching from the soil system. We argue that several factors, including long-range transport of air pollutants, an Asian monsoon climate setting, as well as maturation and poor management of AM-associated plantations, will make Japanese temperate forests more sensitive to increased atmospheric N deposition in East Asia over the next decades.

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20kgNha(-1)yr(-1)) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8kgNha(-1)yr(-1), an additional 8, 24, and 56kgNha(-1)yr(-1) of either ammonium (NH4(+)) or nitrate (NO3(-)) has been applied for 11years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32kgNha(-1)yr(-1) even after 11years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56kgNha(-1)yr(-1) of NH4(+) addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH4(+) dominates wet deposition for high inputs (56kgNha(-1)yr(-1)). The N filter function in response to NO3(-) uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH4(+) uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO3(-) and NH4(+), may lead to species change.