Authors: Qing Wang，Nianpeng He，Yuan Liu，Meiling Li，Li Xu，Xuhui Zhou
Abstract: Background and aims Increasing the emission of carbon dioxide by heterotrophic respiration (Rh) might lead to global warming. However, issues remain on how Rh responds to changing temperatures, especially with respect to the hysteresis loop in the relationship between Rh and temperature at the daily scale, along with elucidating the underlying mechanisms.
Method We investigated hysteresis loop by measuring Rh in subtropical forest soil at the daily scale (12 h for warm-up (6–30 °C) and cool-down processes (30–6 °C), respectively) using continuous temperature variation and high resolution of measurements over a 56-day incubation period. The ratios of R20 and Q10 between warm-up and cooldown were calculated as the characteristics of diel hysteresis. We measured chemical (pH, conductivity,oxidation-reduction potential), microbial biomass and dissolved substrate (carbon and nitrogen) parameters to explain variation of diel hysteresis.
Results Rh was strongly dependent on temperature, with a clockwise hysteresis loop of Rh between the warm-up and cool-down daily processes. The average value of R20 [at a reference temperature of 20 °C] during the whole incubation period under the warm-up process was significantly higher (46.05 ± 0.96 μgC g−1 d−1) than that under the cool-down process (14.74 ± 0.03 μgC g−1 d−1). In comparison, the average value of Q10 under the cool-down process (5.27 ± 0.2) was significantly higher than that under the warm-up process (1.66 ± 0.02). Redundancy analysis showed that the interaction effects of soil chemical, microbial biomass, and dissolved substrate parameters explain most variation of diel hysteresis:98% variation in R20 and 93.5% variation in Q10.Compared with the weak effect of chemistry parameters on the diel hysteresis, the sole and interactive effects of microbial biomass and substrate were more important,especially their interaction.
Conclusions Interactions of chemical, microbial biomass,and dissolved substrate parameters dominated the variation in diel hysteresis of Rh with temperature,especially the interaction of microbial biomass and dissolved substrate. Of note, Q10 during the warm-up process might be overestimated when using the highly fitted temperature-dependent function of cool-down period.Furthermore, using a constant value of Q10 (Q10=2) in carbon cycle models might be an important source of uncertainty.