Ken'ichi Osaka

Atmospheric nitrate leached from small forested watersheds during rainfall events: Processes and quantitative evaluation

To determine the availability of atmospheric NO3− deposition on forested ecosystems and to understand the interaction between the nitrogen cycle in a forest ecosystem and atmospheric nitrogen input/output, we quantitatively evaluated the atmospheric NO3− passing through forested watersheds by measuring δ18ONO3 leaching during rainfall events in two forest ecosystems (Su-A and Ab-S). Atmospheric NO3− leaching in rainfall events was clearly higher in Ab-S than in Su-A, even for a similar amount of rainfall, which demonstrated that atmospheric NO3− leaching differs among forested watersheds. Our observations suggest that a large part of the atmospheric NO3− leached from the watersheds was derived from surface soil, which was deposited before rainfall events occurred; however, direct atmospheric NO3− leaching via throughfall discharge also contributed, especially at the beginning of rainfall events. In Ab-S, 2.9–37.8% (average = 15.5%) of atmospheric NO3− deposition passed through the watershed, accounting for 3.1–49.8% (average, 26.4%) of the total NO3− leached during rainfall events. The NO3− input was not large, and the NO3− pool and net nitrification rate were small; therefore, nitrogen was not saturated in the soil at Ab-S. Nevertheless, some of the atmospheric NO3− deposition was not assimilated and was leached immediately. Moreover, our observations suggest that the hydrological characteristics of the watersheds, which control the ease of rainwater discharge, strongly influenced the rate of atmospheric NO3− leaching. This suggests that the hydrological characteristics of watersheds influence the availability of atmospheric NO3− deposition in forested ecosystems and the progression of nitrogen saturation.

Nitrogen availability in the taiga forest ecosystem of northeastern Siberia

Abstract The inorganic nitrogen (N) cycle and its dynamics in the soil were observed in the ecosystem at the Spasskaya Pad experimental forest near Yakutsk in northeastern Siberia in order to estimate the N availability for the larch (Larix cajanderi Mayr.) The soil pool of bulk N in the forest accounted for up to 866 g N m−2 (0–50 cm), whereas up to 1.7% (14.6 g N m−2) was potassium chloride (KCl)-extractable inorganic N. Ammonium was a dominant form of inorganic N. The size of the soil inorganic N pool fluctuated seasonally. It was small in the beginning of the summer and increased rapidly once the cumulative degree day of the soil temperature at 20 cm reached more than 300 (°C days), indicating that active N mineralization began at some point from the middle of July to the beginning of August. This soil pool of inorganic N that had accumulated at the end of the previous summer was consumed by the beginning of the next summer through microbial immobilization, which may have begun in September. Recycling of N in the soil was important because the input of inorganic N by deposition was very small (48 mg N m−2 year−1). A tracer 15N experiment showed that larch did not uptake organic N in the form of amino acid (alanine); rather, it used ammonium and nitrate as N sources. The amount of available N for plants was assessed as water-extractable inorganic N in the soil solution that was transported to the rooting area by mass flow driven by plant transpiration, and it accounted for 1.9 × 103 (1.2 × 103 for trees) mg N m−2 during the growing season−1 (0–50 cm of the mineral soil layer in June, July, and August). Microorganisms in the soil are also expected to be competing for this available N. Our results show that despite a large amount of inorganic N production, N availability for plants is low as the soil pool of inorganic N is built up at the same rate as larch senescence.