Abstract
Abstract. Soil respiration (SR) is a major component of ecosystems' carbon cycles and represents the second largest CO2 flux in the terrestrial biosphere. Soil temperature is considered to be the primary abiotic control on SR, whereas soil moisture is the secondary control factor. However, soil moisture can become the dominant control on SR in very wet or dry conditions. Determining the trigger that makes soil moisture as the primary control factor of SR will provide a deeper understanding on how SR changes under the projected future increase in droughts. Specific objectives of this study were (1) to investigate the seasonal variations and the relationship between SR and both soil temperature and moisture in a Mediterranean riparian forest along a groundwater level gradient; (2) to determine soil moisture thresholds at which SR is controlled by soil moisture rather than by temperature; (3) to compare SR responses under different tree species present in a Mediterranean riparian forest (Alnus glutinosa, Populus nigra and Fraxinus excelsior). Results showed that the heterotrophic soil respiration rate, groundwater level and 30 cm integral soil moisture (SM30) decreased significantly from the riverside moving uphill and showed a pronounced seasonality. SR rates showed significant differences between tree species, with higher SR for P. nigra and lower SR for A. glutinosa. The lower threshold of soil moisture was 20 and 17% for heterotrophic and total SR, respectively. Daily mean SR rate was positively correlated with soil temperature when soil moisture exceeded the threshold, with Q10 values ranging from 1.19 to 2.14; nevertheless, SR became decoupled from soil temperature when soil moisture dropped below these thresholds.
Highlights
Soil is the largest pool of terrestrial organic carbon in the biosphere, storing around 2344 Pg C in the top 3 m (Jobbágy and Jackson, 2000)
Our results showed that the seasonal variations of soil heterotrophic respiration (SRH) and soil respiration rates (SRtot) were mainly controlled by soil temperature, with a secondary influence of soil moisture (SM5)
This research demonstrates how soil moisture constrains the relationship between Soil respiration (SR) and soil temperature
Summary
Soil is the largest pool of terrestrial organic carbon in the biosphere, storing around 2344 Pg C in the top 3 m (Jobbágy and Jackson, 2000). SR plays an important role in the global carbon balance (Schimel et al, 2001; Raich et al, 2002), and even small changes of SR may induce positive feedbacks to climate change (Schlesinger and Andrews, 2000). Information of how SR interacts with environmental conditions, such as the response of specific components of soil respiration to temperature and moisture changes, will be a key part of the improvement of process-based models. On large scales, such as in ecosystems and biomes, net primary production (NPP) may be the most important factor controlling SR (Wardle, 2002).
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