Effects of extreme events on nitrogen export from forested headwater catchments

Monday, 23 January 2017: 11:00
Ballroom III-IV (San Juan Marriott)
Nobuhito Ohte1, Ken'ichi Osaka2, Masanori Katsuyama1, Tomoki Oda3 and Keitaro Fukushima4, (1)Kyoto University, Kyoto, Japan, (2)University of Shiga Prefec, Hikone, Japan, (3)University of Tokyo, Bunkyo-ku, Japan, (4)Tokyo Metropolitan University, Tokyo, Japan
Abstract:
The effects of the extreme event on the nitrogen (N) export headwater catchments vary with depending on the degree of the disturbance on ecosystem structures and functions:

1) Events without geophysical and biogeochemical disturbances

2) Events without geophysical disturbances, but with biogeochemical disturbances such as the changes of N pool size in soils

3) Events causing geophysical disturbances in addition to biogeochemical disturbances.

Ecosystem changes by disturbances in the types 1 and 2 are recoverable in relatively short time period, while damages in the type 3 are often irreversible or take very long time for recovery. These variations also depend upon the vulnerability of the catchment environments in aspects of biological and geomorphological structures.

Previsouly, field researches have scarcely been conducted on the type 3 in the N export context, while many studies for the types 1 and 2 have been previously performed in temperate regions. The East Asia including Japan has unique climatic characteristics compared to other continental Europe and North America, which is characterized by rainy summer. Also typhoons attack the East Asian regions frequently in summer every year, and those often cause extreme hydrological events.

Previous studies on the types 1 and 2 in Japan indicated that exportability of particulate organic N (PON) was significantly lower than that of dissolved organic and inorganic N, although PON could be transported during larger storm events. The dominant form of exported N was still NO3-, differently from the case of carbon exports. This was explainable by the fact that the NO3- pool was enlarged in summer.

Namely, the major N form in the storm events are determined if the movable pool is dissolved or particulate forms, and spatial distributions of those relative to the pathways of direct runoff. However, the evidencing studies on disturbance of the extreme storm events on the N dynamics (transformations and pool size changes) itself are also limited.

Although, studies on the type 3 level extreme events are still limited, many researches on the other disturbances, such as vegetation dieback, forest cuttings and forest fires. We will have to review carefully those to understand the responding mechanisms to disturbances, and develop the research strategies against the type 3 cases.

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