Response of carbon fluxes under long-term warming, and its relationship with plant biomass

Background/Question/Methods

The magnitude of future warming climate will vary according to feedbacks between climate warming and ecosystem carbon cycle; however, these feedbacks are not very clear yet. To examine the climate-carbon feedbacks, our research group conducted the warming and clipping experiment in a tall grass prairie in central Oklahoma, US since November 1999. In this experiment, we are investigating whether and how warming and clipping affect different carbon fluxes, i.e., total, auto and heterotrophic soil respiration (Resp_Auto, Resp_Auto and Resp_Hetero)_, ecosystem respiration (ER), gross primary production (GPP), and net ecosystem CO₂ exchange (NEE), and fixed carbon amount, i.e., aboveground net primary production (ANPP). The treatment included four combinations – Unwarming and Unclipped, Unwarming and Clipped, Warming and Unclipped, and Warming and Clipped. The warming plot’s air and soil temperature are 1.1 °C and 2.3 °C above ambient, respectively. The carbon fluxes measured monthly, and the plant biomass harvested and weighted every fall by sorting C₃ and C₄ plants. The data was collected from 2009 to 2015.

Results/Conclusions

Our overall hypothesis is that the ecosystem subjected to different treatment combinations could result in changes in carbon flux in different ways. Our results show that the experimental warming significantly enhanced ecosystem respiration by 0.34 µmol m^-2s^-1 , total soil respiration by 0.47 µmol m^-2s^-1, and autotrophic respiration by 0.23 µmol m^-2s^-1 (p < 0.05). But warming did not cause any statistically significant effects on GPP, NEE, and Resp_Hetero. The clipping significantly stimulated ER, GPP and NEE by 0.75, 1.65, and -0.90 µmol m^-2s^-1, respectively; however, there were no statistically significant difference in Resp_Total, Resp_Auto and Resp_Hetero between unclipped and clipped plots. The biomass was significantly different between unwarmed and warmed treatment, ANPP (97.0 g m^-2), ANPP_C3 (40.9 g m^-2) and ANPP_C4 (56.3 g m^-2), respectively. On the other hand, the ANPP included C₃ and C₄ plant biomass were unchanged under clipping. Our results show that soil respiration has high sensitivity to warming. The increasing trend of respiration is similar with ANPP under warming. Our correlation analysis reveals how the carbon fluxes are related to changes plant biomass in different treatment combinations.