Impact of anthropogenic disturbances on carbon cycle changes in terrestrial-aquatic-estuarine continuum by using an advanced process-based model

Recent research has shown that inland water-including rivers, lakes, and groundwater-may play some role in carbon cycling, although the extent of its contribution has remained uncertain due to the limited amount of reliable data available. To evaluate global changes in the carbon cycle due to anthropogenic factors, such as application of fertilizer and manure, in major rivers including 130 tidal estuaries over an 18-year period (1998-2015), the present study used an advanced process-based model derived by coupling a process-based national integrated catchment-based eco-hydrology (NICE) model with various biogeochemical cycle models (NICE-BGC). Generally, total nitrogen and phosphorus transports in overland flow were found to increase. In contrast, total suspended sediment in overland flow decreased in some regions because the vegetation was able to expand to cover the ground, resulting in less erosion. NICE-BGC simulated the difference in carbon budget in major rivers with and without nutrient application. Generally, CO2 degassing across the water-air interface decreased and particulate organic carbon (POC) increased in most rivers through variations in carbon budget, where excess nutrients might stimulate gross primary production of carbon-rich algal biomass. The simulated result also showed that the estuarine carbon cycle was sensitive to intense anthropogenic disturbances reflected by nutrient load, seawater temperature, increases in sea level, and ocean acidification. Extension of previous studies only by categorizing MARgins and CATchments Segmentation (MARCATS) segment numbers showed that the estimated total CO2 flux from the world's estuaries was 0.14 Pg C/year. Generally, the simulation showed that incorporation of the nutrient cycle into the terrestrial-aquatic-estuarine continuum improved estimates of net land flux and carbon budget in inland waters, thus emphasizing that the effect of estuarine inland water should be explicitly included in the global carbon model to minimize the range of uncertainty.