Alkalinization and acidification of stream water with changes in atmospheric deposition in a tropical dry evergreen forest of northeastern Thailand (2005-2015)

As the EANET Network Center research activities, joint research projects with research/academic institutions in the participating countries have been conducted.
In particular, in Thailand, Malaysia, and Japan, small catchment plots were established in the respective forest areas. Effects of air-pollutant deposition/input on forest ecosystems and disturbance of biogeochemical cycle processes have been studied in these catchment plots.

The climate in northeastern Thailand is classified as a tropical savanna climate with distinct dry and wet seasons. In accordance with the climate, seasonality of water/material inputs from the atmosphere may largely affect biogeochemical processes in terrestrial ecosystems. In this study, field surveys on atmospheric deposition and stream water chemistry were conducted over 6 years in an evergreen forest of Sakaerat site in northeastern Thailand.

Atmospheric deposition of ion constituents by throughfall and stemflow was shown to increase in the beginning and end of the wet season, reflecting the precipitation pattern. The pH and electrical conductivity of stream water increased with alkalinity and base cation concentrations (alkalinization) due to mineralization of organic matter by the first rain and retention of anions in the soil during the start of the wet season.

After initial alkalinization, the pH and alkalinity declined rapidly with the highest SO₄^2- concentration displayed in the middle towards the end of the wet season (acidification). The magnitude of peaks in SO₄^2- concentration reflects deposition during the first 2 months of the wet season (March and April) in respective years. Release of SO₄^2- with H⁺, which is retained in the soil during the early wet season, may cause acidification later in the season.

The deposition and concentration of SO₄^2- declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO₄^2- and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO₄^2- concentration/deposition and increased precipitation in the middle–late wet season.

The retention‐release cycle of SO₄^2- largely contributed to both seasonal and inter annual variations in stream water chemistry in the tropical savanna climate studied

The study was supported financially by a Grant‐in‐Aid for Scientific Research on Innovative Areas (20120012) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan; the Environment Research and Technology Development Fund (C‐052, C‐082 and B‐0801) from the Ministry of the Environment of Japan; and a grant from the Asia‐Pacific Network for Global Change Research (APN, ARCP2012‐18NMY‐Sase; ARCP2013‐13CMY‐Sase, Sase et al., 2015).

The paper on this study is as follows:

Sase H et al. 2017. Alkalinization and acidification of stream water with changes in atmospheric deposition in a tropical dry evergreen forest of northeastern Thailand.
Hydrological Processes, 31, 836–846.
https://onlinelibrary.wiley.com/doi/abs/10.1002/hyp.11062