Net Exchanges of carbon dioxide, methane, and nitrous oxide between Terrestrial Ecosystems and the Atmosphere in Tropical Asia during 1901–2010

Abstract

Terrestrial ecosystems act as important sources of the greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), which cause climate warming. Therefore, understanding and quantification of the GHGs emissions has become an urgent task for accurately predicting climate change. Across the globe, several GHGs emission sources are well understood in North America, Europe, and China. However the magnitude and driving factors of GHGs emissions are uncertain in tropical Asia due to several constraints such as lack of reliable land cover and land use (LCLUC) datasets (Banger et al., 2013). Unlike temperate, tropical ecosystems are limited by phosphorus (P); however the C-N-P coupling mechanism is often lacking in the ecosystem models (Yang et al., 2014; Goll et al., 2012), producing uncertainties in GHGs emissions estimates. In this study, I generated the new LCLUC datasets by combining high resolution remote sensing from Indian satellite (Resourcesat-1; 56-m resolution) with multiple inventory archives existing in India (covers 40% area of tropical Asia) during 1880–2010. Our newly developed LCLUC datasets in India were integrated with existing land use datasets at 0.25 degree resolution for other regions in the tropical Asia. In the second step, I have coupled the P cycle with carbon and nitrogen cycles in the DLEM modeling framework and applied it in conjunction with a series of spatial dataset including climate variability, atmospheric carbon dioxide (CO2) concentration, atmospheric nitrogen deposition (NDEP), and land cover and land use change (LCLUC), to quantify the GHGs emissions over the tropical Asia during 1901–2010. The DLEM simulations results have shown that tropical Asia was a net carbon source (13±12 Tg C year-1), with South Asia was a net source (61±7Tg C year-1) while South-East Asia being a net sink (47±9 Tg C year-1) during 1901–2010. Net carbon uptake showed significant increasing trend due to stimulation of plant growth by elevated CO2 concentration, atmospheric nitrogen deposition (NDEP), and cropland management practices, and tropical Asia became a net carbon sink after 1950s. Among the factors, P limitation was the reduced carbon sink by 430±130 Tg C year-1, while the effects of P limitation in reducing carbon uptake was 3-5 folds greater in the South-East Asia than South Asia. Over the study period, CH4 and N2O emissions, with significant increasing trends (p<0.001), and ranged 20.4± 3.6 Tg C year-1 and 0.70 ±0.09 Tg N year-1, respectively. LCLUC was the dominant factor in stimulating CH4 (8.7 TgC year−1) and N2O (0.29 TgC year−1) emissions due to cropland expansion and increase in the nitrogen fertilizer use in tropical Asia. Interestingly, elevated CO2 concentration has stimulated CH4 (2.9 TgC year−1) by increasing plant growth, which however has decreased N2O (0.05TgN year−1) emissions due to progressive nitrogen limitation. By accounting the effects of three GHGs (CO2, CH4, and N2O) together, terrestrial ecosystems of tropical Asia have provided significant warming feedbacks (1063 ± 43Tg CO2 equivalents year-1) to global climate. Though net carbon uptake has increased, global warming potential (GWP) remained similar (p<0.82) due to stimulation of CH4 and N2O emissions, thereby suggesting that benefits from increase in the carbon uptake were offset by stimulation of CH4 and N2O emissions during the study period. This study has generated new LCLUC datasets for India, improved an ecosystem model for P limitation, and provides useful and valuable information to both scientific community and policy makers such as magnitude, spatiotemporal and underlying mechanisms of GHGs emissions in the tropical Asia.