Net ecosystem productivity and carbon dynamics of the traditionally managed Imperata grasslands of North East India

Abstract

There have been few comprehensive descriptions of how fire management and harvesting affect the carbon dynamics of grasslands. Grasslands dominated by the invasive weed Imperata cylindrica are considered as environmental threats causing low land productivity throughout the moist tropical regions in Asia. Imperata grasslands in North East India are unique in that they are traditionally managed and culturally important in the rural landscapes. Given the importance of fire in the management of Imperata grassland, we aimed to assess (i) the seasonal pattern of biomass production, (ii) the eventual pathways for the produced biomass, partitioned between in situ decomposition, harvesting and combustion, and (iii) the effect of customary fire management on the ecosystem carbon cycle. Comparatively high biomass production was recorded during pre-monsoon (154 g m−2 month−1) and monsoon (214 g m−2 month−1) compared to the post-monsoon (91 g m−2 month−1) season, and this is attributed to nutrient return into the soil immediately after fire in February. Post fire effects might have killed roots and rhizomes leading to high belowground litter production 30–35 g m−2 during March to August. High autotrophic respiration was recorded during March–July, which was related to high belowground biomass production (35–70 g m−2) during that time. Burning removed all the surface litter in March and this appeared to hinder surface decomposition and result in low heterotrophic respiration. Annual total biomass carbon production was estimated at 886 g C m−2. Annual harvest of biomass (estimated at 577 g C m−2) was the major pathway for carbon fluxes from the system. Net ecosystem production (NEP) of Imperata grassland was estimated at 91 g C m−2 yr−1 indicating that these grasslands are a net sink of CO2, although this is greatly influenced by weather and fire management.