Digital Soil Mapping of Soil Organic Carbon Stock in Bhutan

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Tshering Dorji, Tsheten Dorji, Sangita Pradhan, Dawa Tashi, Karma Dema Dorji

ABSTRACT

Soil organic carbon (SOC) plays an integral part in improving soil security, water security, food security, energy security, climate change abatement, biodiversity protection, and ecosystem services. It is important to understand its stock and spatial distribution for better management. However, not many countries have managed to map their national SOC stock and Bhutan is no exception. There is paucity of SOC information to clearly formulate plans and programs to increase Carbon (C) sequestration and enhance SOC storage in the country. A preliminary mapping of SOC stock of Bhutan for the top 30 cm depth was carried out to establish a baseline and contribute to global SOC mapping. A total of 993 data points was used for mapping SOC stock using regression kriging (RK). Regression tree model and ordinary kriging were used to perform the RK with elevation, land use land cover (LULC), slope, aspect, profile and plan curvatures, normalized difference vegetation index, SAGA wetness index, mean precipitation, mean temperature, geology, and terrain ruggedness index as environmental covariates. The model validation was done by repeated data splitting method. Preliminary results show that for the top 30 cm depth, Bhutan stores about 0.4 giga tonne carbon (GtC) with SOC density ranging from 0.5 to 315.3 ton ha-1. Among the environmental covariates, LULC, topography, and climatic factors had significant influence on SOC stock and its spatial distribution. SOC stock was relatively low in the southern and eastern regions as opposed to the western and northern parts of the country. Under different LULC types, the SOC stock was lowest under agriculture land and highest under forest. These results are based on a small set of soil data and must be used with caution. However, for better SOC stock estimation and mapping, more and well distributed soil data will be necessary.

KEYWORD:

Soil Organic Carbon, Mapping, Land use land cover

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