How Carbon Footprint Responds to Water Circulation Rates and Availability at Different Timescales in a Subtropical Forest Ecosystem

In the subtropics, the influence of water on the net ecosystem carbon exchange (NEE) is critical, subsequently determining net primary productivity (NPP). Water also contributes the majority of interannual variability in atmospheric carbon dioxide (CO2) concentrations in the subtropics and helps to mitigate climate change. Using the technology of vorticity correlation, this study continuously monitored NEE, ecosystem respiration (RE), and the gross ecosystem exchange (GEE) of a small subtropical watershed in the middle and lower reaches of the Yangtze River from 2014 to 2017. Water parameters were concurrently monitored, such as rainfall (RF), runoff (RO), evapotranspiration (ET), vapor-pressure deficit (VPD), and soil water content (SWC). Results showed that an increase in the water circulation rate and water availability will promote an increase in GEE, RE, and NEE on both hourly and daily timescales, but will inhibit these carbon (C) exchanges on an annual timescale. For years that experience low annual rainfall but increased continuous rainfall, a decrease in ET may inhibit plant respiration, thus enabling subtropical forest ecosystems to fix more C. Additionally, more intense and irregular rainfall patterns caused by climate change may weaken the overall C sequestration capacity of subtropical forest ecosystems. Results from this study are intended to help us better understand coupled C and water cycling mechanisms within terrestrial ecosystems, while also providing a reference to assess the impact of climate change on ecosystem C cycling processes.

Automated summary

This study continuously monitored NEE, RE, and GEE in a small subtropical watershed in the middle and lower reaches of the Yangtze River. The results showed that an increase in water availability promoted an increase in carbon exchange on hourly and daily scales, but inhibited it on an annual scale. Climate change-induced irregular rainfall patterns may weaken the overall carbon sequestration capacity of subtropical forest ecosystems.