Soil carbon stocks under different forest types in Bhutan, Eastern Himalayas

Abstract

This dissertation focusses on quantifying C stocks from forest ecosystems in the Eastern Himalayan. Total soil C and N stocks significantly increased with altitude and decreased with soil depth. Carbon and N stocks were significantly correlated with altitude which accounted for 73% and 47% of the variation in C and N stocks, respectively. To elucidate the driving processes of C and N stocks, inputs and stability, C and N isotopes in soil and biomass were measured. Overstorey vegetation contributes significantly to the soil C, as 13C of overstorey and soil showed similar trends. The slope of soil δ13C versus the C concentration, indicative of organic matter decomposition, was smallest at the highest altitude forest. This suggests slow turnover of C and N in the high altitude forest soils. Sequential density fractionation, DRIFT spectroscopy and IRMS were used to determine the different proportion and forms of C in forest soils. Lighter soil density fractions had a greater proportion of aliphatic C, while the heavier soil density fractions had a greater proportion of aromatic C. The larger proportion of aromatic C in the higher soil density fractions suggests that SOC in this fraction has been more processed, corroborated by the accompanied decreased C:N ratio and enrichment of δ13C with increasing soil density fractions. Aboveground biomass (AGB) allometric equations were developed to estimate forest AGB C stocks for the study area. Estimated AGB C stocks increased with altitude from 57 to 207 Mg C ha-1. The use of measured C concentration rather than an assumed 50% C for biomass reduced estimated AGB C stocks between 6.8 and 8.6%. The estimation of C stocks in the forest soils and biomass allometric equations for the different forest types in Bhutan will enable the region to better monitor its C stocks and emission to benefit from the United Nations REDD programs.