RESUMO
The dynamics of dissolved organic carbon (DOC) export from headwater catchments are of critical importance for the global carbon balance and are driven by complex runoff processes. Most previous studies have used statistical relationships between runoff and DOC concentration to estimate DOC export dynamics. Thus, the coupling mechanisms between runoff generation and DOC export dynamics at the process level were obscured in the fitting parameters and have rarely been addressed. In this study, high-frequency (hourly) discharge and DOC export from a typical flash flood experimental headwater catchment with an area of 1.8 km2 were simulated using a process-based model (INCA-C). The results showed that the INCA-C model successfully captured the hourly dynamics of both discharge and DOC concentrations with a Nash-Sutcliffe efficiency (NSE) of 0.47-0.81 and 0.28-0.70 among moderate events and 0.81-0.85 and 0.19-0.90 among extreme events, respectively. The DOC was exported with distinct concentration dynamics, fluxes, and contributions from the four flow pathways under different storm intensities. At higher intensities, the DOC fluxes were exported by subsurface flows, particularly from shallow organic soil, with greater peaks and shorter time-to-peaks. Exported DOC is primarily sourced from subsurface runoff from the mineral layer (73 %-77 %) during moderate events, whereas it is primarily sourced from subsurface runoff from the organic layer (61 %-79 %) during extreme events. The two contrasting contributions suggest that hydrological pathway controls and DOC dynamic patterns can shift owing to runoff generation influenced by storm intensity. The distinct and variable controls of different flow pathways on DOC export highlight the need to explain the role of hydrology in regulating DOC storm exports through process-based modelling.
RESUMO
BACKGROUND: Under the escalating threat to sustainable development from the global increase in carbon dioxide concentrations, the variations in carbon flux in the farmland ecosystem and their influencing factors have attracted global attention. Over the past few decades, with the development of eddy covariance technology, the carbon fluxes of farmlands have been determined in many countries. However, studies are very limited for drip irrigation maize the arid regions in northwestern China, which covers a large area where a mixed mode of agriculture and grazing is practiced. RESULTS: To study the effects of drip irrigation on the net ecosystem productivity (NEE), ecosystem respiration (ER), gross primary production (GPP) and net biome productivity (NBP) in the arid regions of northwestern China, we measured the carbon flux annually from 2014 to 2018 using an eddy covariance system. Our results showed that the maize field carbon flux exhibited single-peak seasonal patterns during the growing seasons. During 2014-2018, the NEE, ER and GPP of the drip-irrigated maize field ranged between - 407 ~ - 729 g C m-2, 485.46 ~ 975.46 g C m-2, and 1068.23 ~ 1705.30 g C m-2. In four of the 5 study years, the ER released back to the atmosphere was just over half of the carbon fixed by photosynthesis. The mean daily NEE, ER and GPP were significantly correlated with the net radiation (Rn), air temperature (Ta), leaf area index (LAI) and soil moisture (SWC). The results of path analysis showed that leaf area index is the main driving force of seasonal variation of carbon flux. When harvested removals were considered, the annual NBP was - 234 g C m-2, and the drip-irrigated maize field was a carbon source. CONCLUSIONS: This study shows the variation and influencing factors of NEE, ER and GPP in the growth period of spring maize under film drip irrigation in arid areas of northwest China. The ecosystem was a carbon sink before maize harvest, but it was converted into a carbon source considering the carbon emissions after harvest. The variation of carbon flux was influenced by both environmental and vegetation factors, and its leaf area index was the main driver that affects the seasonal variation of carbon flux.