Seasonal and interannual variations in carbon fluxes over a rain-fed spring maize cropland in the Loess Plateau, China

Abstract

Under the background of climate change and water resource shortage, it is of great significance to study the dynamics of carbon exchange and its environmental responses for the agroecosystems in coping with increasingly severe drought in water-limited areas. As the largest arid and semi-arid area in China, the Loess Plateau is widely distributed as rain-fed farmlands, which are highly affected by hydrological condition. In this study, the seasonal and interannual variations in carbon fluxes for a rain-fed spring maize field located in the Loess Plateau was analyze based on eddy covariance method during 2019-2020. Precipitation varied substantially in both annual amounts and seasonal distribution during the study period, with 2019 as a drought year and 2020 as a hydrologically normal year. Carbon fluxes showed unimodal seasonal trends, with peaks observed during the growing season. The annual net ecosystem exchange (NEE) were -309.49 and -423.19  g C m-2 in 2019 and 2020, respectively. It indicates that the corn cropland studied here is a carbon sink, and stronger net carbon uptake occurred for the hydrologically normal year compared with the drought year. The gross primary productivity (GPP) was a stronger driver of NEE than ecosystem respiration (Re), and both ecosystem maximum photosynthetic capacity (Amax) and ecosystem respiration rate at 10 °C (Q10) were closely related with vapour pressure deficit (VPD). The light response of NEE was strongly affected by environmental factors, especially soil water content (SWC) and VPD. Based on the light response analysis of NEE, the structural equation models (SEMs) were constructed to investigate the dominant controlling factors for the seasonal variations in NEE, GPP and Re among contrasting hydrological years. Results revealed that the variations in NEE, GPP and Re during growing season were primarily controlled by direct effects of LAI in normal year, while the effects of LAI on carbon fluxes were much smaller or nonsignificant in the normal year. The SWC variation could only produce significant direct effects on Re in normal year, but could strongly indirectly affect GPP and Re variations in the drought year. Overall, NEE variations were mainly affected by LAI, VPD and photosynthetically active radiation (PAR) in the normal year, and were susceptible to variations in air temperature and PAR in the drought year. This study will provide insights into the effects of drought on carbon exchange and its environmental responses for the agroecosystems in the water-limited region.