Divergent effects of hydrological alteration and nutrient addition on greenhouse gas emissions in the water level fluctuation zone of the Three Gorges Reservoir, China.

Changes in global rainfall patterns and construction of artificial dams have led to widespread alteration of hydrological processes in riparian ecosystems. At the same time, many riparian ecosystems, such as those associated with the Yangtze, are being subjected to enhanced inputs of nitrogen (N) and phosphorus (P) due to intensified agricultural activity in surrounding uplands. Together, these environmental changes may alter the magnitude and direction of greenhouse gasses (GHGs) fluxes from riparian soils. We conducted an in situ experiment combined with quantitative PCR approach (qPCR) to elucidate the effects of hydrological alterations (continuous flooding (CF), periodic flooding (PF), and no flooding (NF)) and nutrient addition (N addition (urea, 100 kg N ha y), P addition (PO, 20 kg hay), N + P addition, and control (CK)) on three major GHGs including carbon dioxide (CO), methane (CH) and nitrous oxide (NO) fluxes as well as the underlying mechanisms. Our results showed that hydrological alterations greatly affected GHGs emissions, possibly by altering soil moisture, soil organic C, and C:N ratios. The CF, with higher soil moisture and lower C:N ratio, increased CH emissions 13-fold and reduced CO and NO emissions by 37.3% and 72.2% averaged over the growing seasons compared with no flooding. PF enhanced CH emissions 5.7-fold and decreased NO emissions by 69.0% in comparison with no flooding. Nutrient additions had no significant effect on CO or CH flux, but P addition significantly lowered NO flux. Interactions between hydrological alterations and nutrient additions were not detected for any GHGs. As a result, hydrological alterations and nutrient additions affected the global warming potential (GWP) of growing season GHG budgets on a 100-year time horizon, mainly by changing the CO emissions. CF reduced GWP from 597 to 439 g CO-eq m, and N + P addition enhanced GWP from 489 to 625 g CO-eq m. The qPCR analysis revealed that decreased CH oxidation potential may lead to the enrichment of CH emissions under the hydrological alterations, and reduced nitrification and denitrification potential contributed to the reduction of NO fluxes under all the treatments. Our study indicates that continuous flooding could curb the contribution of riparian GHGs fluxes to global warming but that the combination of N and P additions may increase the greenhouse effect mainly by regulating the CO emissions of growing season in riparian ecosystem.