Acid rain reduced soil carbon emissions and increased the temperature sensitivity of soil respiration: A comprehensive meta-analysis

Soil respiration the second-largest carbon flux in terrestrial ecosystems, has been extensively studied across a wide range of biomes. Surprisingly, no consensus exist on how acid rain (AR) impacts the spatiotemporal pattern of soil respiration. Therefore, we conducted a meta-analysis using 318 soil respiration and 263 soil respiration temperature sensitivity (Q10) data points obtained from 48 studies to assess the impact of AR on soil respiration components and their Q10. The results showed that AR reduced soil total respiration (Rt) and soil autotrophic respiration (Ra) by 7.41 % and 20.75 %, respectively. As the H+ input increased, the response rates of Ra to AR (RR-Ra) and soil heterotrophic respiration (Rh) to AR (RR-Rh) decreased and increased, respectively. With increased AR duration, the RR-Ra increased, whereas the RR-Rh did not change. AR increased the Q10 of Rt (Rt-Q10) and Rh (Rh-Q10) by 1.92 % and 9.47 %, respectively, and decreased the Q10 of Ra (Ra-Q10) by 2.77 %. Increased mean annual temperature, mean annual precipitation, and initial soil organic carbon increased the response rate of Ra-Q10 to AR (RR-Ra-Q10) and decreased the response rate of Rh-Q10 to AR (RR-Rh-Q10). However, as the AR frequency and initial soil pH increased, both RR-Ra-Q10 and RR-Rh-Q10 also increased. In summary, AR decreased Rt but increased Q10, likely due to soil acidification (soil pH decreased by 7.84 %), reducing plant root biomass (decreased by 5.67 %) and soil microbial biomass (decreased by 5.67 %), changing microbial communities (increased fungi to bacteria ratio of 15.91 %), and regulated by climate, vegetation, soil and AR regimes. To the best of our knowledge, this is the first study to reveal the large-scale, varied response patterns of soil respiration components and their Q10 to AR. It highlights the importance of applying the reductionism theory in soil respiration research to enhance our understanding of soil carbon cycling processes with in the context of global climate change.

Estimating influences of environmental drivers on soil heterotrophic respiration in the Athabasca River Basin, Canada

Soil heterotrophic respiration (R_H) is a crucial component of the atmospheric carbon dioxide (CO₂) budget, as R_H accounts for ∼10 times more CO₂ than burning fossil fuels. However, modelling of R_H is primarily based upon empirical/semi-empirical approaches. Here, we developed a mechanistic model based on microbial kinetics and thermodynamics processes (MKT) to model soil chemical environment and soil R_H in the Athabasca River Basin, Canada. MKT was coupled with the Soil and Water Assessment Tool (SWAT) for a regional-scale hydro-biogeochemical simulation. Dissolved oxygen, redox potential and meteorological variables were simulated for the first time at a regional scale. Annual mean simulated R_H varied from 20 to 320 kg CO₂-C/ha/yr across Athabasca River basin (ARB) in 2000-2013. Our results show that dissolved oxygen, air temperature, and soil temperature have more influence on R_H than redox potential, precipitation, and water-filled pore space (WFPS). A significant (p < 0.01) causal relationship exists between the dissolved oxygen, air temperature, soil temperature, redox potential and precipitation with R_H. Our results show that the role of environmental drivers are essential and should be considered in future estimations of R_H.