Confounding Effects of Soil Moisture on the Relationship Between Ecosystem Respiration and Soil Temperature in Switchgrass

Understanding the response of ecosystem respiration (ER) to major environmental drivers is critical for estimating carbon sequestration and large-scale modeling research. Temperature effect on ER is modified by other environmental factors, mainly soil moisture, and such information is lacking for switchgrass (Panicum virgatum L.) ecosystems. The objective of this study was to examine seasonal variation in ER and its relationship with soil temperature (Ts) and moisture in a switchgrass field. ER from the nighttime net ecosystem CO2 exchange measurements by eddy covariance system during the 2011 and 2012 growing seasons was analyzed. Nighttime ER ranged from about 2 (early growing season) to as high as 13 μmol m−2 s−1 (peak growing period) and showed a clear seasonality, with low rates during warm (>30 °C) and dry periods (<0.20 m3 m−3 of soil water content). No single temperature or moisture function described variability in ER on the seasonal scale. However, an exponential temperature–respiration function explained over 50 % of seasonal variation in ER at adequate soil moisture (>0.20 m3 m−3), indicating that soil moisture <0.20 m3 m−3 started to limit ER. Due to the limitation of soil–atmosphere gas exchange, ER rates declined markedly in wet soil conditions (>0.35 m3 m−3) as well. Consequently, both dry and wet conditions lowered temperature sensitivity of respiration (Q10). Stronger ER–Ts relationships were observed at higher soil moisture levels. These results demonstrate that soil moisture greatly influences the dynamics of ER and its relationship with Ts in drought prone switchgrass ecosystems.