Flash drought effects on carbon dynamics in an agroecosystem – insights from the eastern German lowlands

Flash droughts pose a serious threat to agriculture due to their abrupt onset and rapid intensification. The increasing frequency of such events presents considerable challenges and uncertainties for agricultural carbon sequestration, particularly during the growing season. To effectively assess carbon dynamics in agroecosystems under ongoing climate change, it is crucial to understand the complex interactions among energy, water, and carbon fluxes, as well as their connection to vegetation.

An open-path eddy-covariance (EC) measurement system (Licor Smartflux Measurement Suite) was installed over an apple tree stock in eastern Germany (Brandenburg) to monitor microclimatic conditions and the water and carbon budgets at the ecosystem level. Brandenburg, one of the driest regions in Germany, has approximately 48.79% of its land used for agriculture. These agricultural landscapes are highly vulnerable to climate-related stresses, such as water scarcity and flash droughts. The stress conditions can increase irrigation demands, disrupt phenological cycles, and influence atmospheric water and carbon cycles, making them critical areas of investigation in the context of land use and climate change. This study investigates the interactions between microclimatic conditions and soil water levels, and their effect on gross primary production (GPP) in an agricultural region of eastern Germany during a flash drought event that occurred during the growing season. Key variables analyzed include surface energy fluxes (sensible heat H, latent heat LE), the net-ecosystem exchange (NEE) with its partitioned components of GPP and ecosystem respiration (Reco), as well as the water-related variables such as precipitation, irrigation, and soil water content (SWC).

The results indicated that during the flash drought event, the concurrence of low SWC and high vapor pressure deficit (VPD) led to elevated net radiation (Rn), which caused high H and depleted soil water availability. This exacerbated plant water stress and suppressed GPP. Furthermore, irrigation efforts during the event were insufficient to mitigate these adverse impacts, underscoring the vulnerability of agroecosystems to extreme climatic events. The findings highlight the critical role of the coupling between SWC and VPD in regulating carbon sequestration during flash droughts.