Several studies have shown that dissolved organic carbon (DOC), the most active form of fresh carbon, plays an important role in the carbon balance. However, the natural mechanism underlying the effects of the hydrologically transported DOC flux on CO₂ emissions remains unclear. 

Prof. ZHANG Yiping and his team of Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences investigated the relative contribution of hydrologically transported DOC to soil respiration (SR) in a rainforest in Xishuangbanna compared with the contributions of soil temperature and moisture. They determined the soil respiration, heterotrophic respiration (HR), and DOC fluxes in the rainfall, throughfall, litter leachate, and surface soil water (0-20 cm depth). They then compared the seasonality of δ¹³C_DOC in each hydrological process, and δ¹³C in leaves, litter, and surface soil, and analyzed the throughfall, litter leachate, and surface soil water (0–20 cm) effect on soil respiration. 

The researchers found that the seasonal and annual water fluxes decreased from the rainfall to the surface soil. During the transfer of rainfall to soil water, δ¹³C DOC was highest in the rainfall DOC and lowest in the throughfall DOC in both the rainy and dry seasons. 

They also found that soil respiration was dominated by heterotrophic respiration and there were significant correlations between the standardized weekly SR and HR and the standardized weekly water fluxes and DOC fluxes through the hydrological processes. 

The results showed that the surface soil was a sink for the DOC transported by hydrological processes, and that HR and SR were sensitive to the DOC flux through these processes. Soil water (0-20 cm) was the most sensitive indicator of SR and soil-water DOC flux was the second. These two exceeded the sensitivity of the soil temperature, soil water content, and other water flux, and DOC flux along all the hydrological processes.  

The DOC transported by hydrological processes thus played a more important role in the SR processes. 

 The study entitled “Hydrologically transported dissolved organic carbon influences soil respiration in a tropical rainforest” has been published in Biogeosciences.