Digital soil mapping of soil organic carbon stocks under different land use and land cover types in montane ecosystems, Eastern Himalayas

Quantification of soil organic carbon (SOC) stocks is quite useful for accurate monitoring of C sequestration. However, there are still substantial gaps in our knowledge of SOC stocks in many parts of the world, including the Himalayas. We investigated the total SOC stocks and its spatial distribution under different land use and land cover (LULC) types in montane ecosystems of Bhutan. 186 Soil profiles were described and sampled by genetic horizons at sites located using conditioned Latin hypercube sampling. SOC concentrations at the standard depths designated for the GlobalSoilMap.Net were estimated with an equal-area spline profile function. SOC concentrations at these depth intervals were digitally mapped to a fine resolution matrix of 90 m grid using regression kriging. We found significant influence of LULC categories on SOC concentration, SOC density, SOC stocks and their spatial distributions, although this influence decreased with increasing soil depth. The estimated mean SOC density in the top 1 m were highest in fir forest soils (41.4 kg m−2) and lowest in paddy land (12.0 kg m−2). Allowing for LULC relative areas, mixed conifer forest had the highest SOC stocks in the upper meter (12.4 Mt) with orchards the lowest (0.1 Mt). The total SOC stocks for the whole study area for the 0-5, 5-15, 15-30, 30-60 and 60-100 cm depths were 2.6, 5.0, 6.5, 7.5 and 5.4 Mt, respectively. The overall SOC stock of the study area for the upper meter was approximately 27.1 Mt. The combined forests accounted for more than 77.5% of the total SOC stocks of the study area. This and the relative SOC densities indicate that the conversion of even a fraction of forests to other LULC types could lead to substantial loss of SOC stocks. This loss of SOC stock is even greater when the decrease in aboveground biomass is also taken into consideration. However, appropriate management of the agricultural lands could increase their sequestration of atmospheric CO2.