Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short‐term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral‐SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH‐dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short‐term, whereas long‐term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short‐term metabolic adaptation of microbial groups and long‐term shift of microbial communities.

Carbon use efficiency (CUE) is being intensively applied to quantify carbon (C) cycling processes from microbial cell to global scales. Energy use efficiency (EUE) is at least as important as the CUE because (i) microorganisms use organic C mainly as an energy source and not as elemental C per se, and (ii) microbial growth and maintenance are limited by energy, but not by C as a structural element. We conceptualize and review the importance of EUE by soil microorganisms and focus on (i) the energy content in organic compounds depending on the nominal oxidation state of carbon (NOSC), (ii) approaches to assess EUE, (iii) similarities and differences between CUE and EUE, and (iv) discuss mechanisms responsible for lower EUE compared to CUE. The energy content per C atom (enthalpy of combustion, the total energy stored in a compound) in organic compounds is very closely (R² = 0.98) positively related to NOSC and increases by 108 kJ mol⁻¹ C per one NOSC unit. For the first time we assessed the NOSC of microbial biomass in soil (−0.52) and calculated the corresponding energy content of −510 kJ mol⁻¹ C. We linked CUE and EUE considering the NOSC of microbial biomass and element compositions of substrates utilized by microorganisms. The mean microbial EUE (0.32–0.35) is 18% lower than CUE (0.41) using glucose as a substrate. This definitely indicates that microbial growth is limited by energy relative to C. Based on the comparison of a broad range of processes of C and energy utilization for cell growth and maintenance, as well as database of experimental CUE from various compounds, we clearly explained five mechanisms and main factors why EUE is lower than CUE. The two main mechanisms behind lower EUE versus CUE are: (i) microbial recycling: C can be microbially recycled, whereas energy is always utilized only once, and (ii) chemical reduction of organic and inorganic compounds: Energy is used for reduction, which is ongoing without C utilization.