Soil carbon and nitrogen stocks along the altitudinal gradient of the Darjeeling Himalayas, India.

Soil is the largest terrestrial carbon pool and has been increasingly recognized to play crucial role to mitigate global warming resulting from climate change and land use and land cover change. The carbon cycle is closely linked with nitrogen cycles and needs to be studied together for their important implications for mitigating land degradations and associated declining productivity. Within the global biodiversity hotspot of Himalayas, which constitutes more than one third of India's carbon pool, the Eastern Himalayas in spite of having highest forest cover, protected area network coverage, biodiversity, and endemicity have been understudied for soil carbon and nitrogen dynamics. The present study was designed to assess the patterns and determinants of soil carbon stock, SOC stocks, nitrogen stocks, and carbon/nitrogen (C:N) ratio along the altitudinal gradient, forest type, and depth in Darjeeling Himalayas, India. We followed standard protocol for soil sampling and analysis. The soil carbon stocks (257.02 to 527.79 MgC ha), SOC stocks (152.55 to 398.88 MgC ha), and soil nitrogen stocks (15.10 to 32.38 MgN ha) increased (but C:N ratio 15.13 to 19.12 declined) along the altitudinal gradient (154 to 3170 m), forest types (tropical moist deciduous forest: MWLS < East Himalayan temperate forest: NVNP < East Himalayan sub-alpine forest: SNP) and annually (year 1 < year 2); however, opposite pattern was observed with increase in depths. The soil carbon stocks, SOC stocks, soil nitrogen stocks, and C:N ratio showed strong effects of forest type, depth, elevation, NDVI, bulk density, MI, and AET. Additionally, there was strong relationship of MAP with soil carbon stock and SOC stock, MAT with C:N ratio, and year of sampling with SOC stocks and C:N ratio. The soil carbon stocks, SOC stocks, and soil nitrogen stocks showed negative correlation with different environmental factors (MAT, MAP, NDVI, MI, AET), but positive correlation with elevation, however, C:N ratio had weak positive correlation. We conclude that the different forests types of Darjeeling Himalayas encompassing wide elevation gradient have high levels of soil carbon stocks, SOC stocks, soil nitrogen stocks, and C:N ratio, and hence must be properly managed to maximize their soil carbon sequestration potential.