Does calculation method affect the nutrient-addition effect on priming?

Abstract Priming effect (PE) plays critical roles in soil organic matter (SOM) decomposition and terrestrial carbon cycling, and is largely affected by nutrient availability. Many studies found that nutrient addition can affect PE, but the calculations for such effects differed among studies. This inconsistence partly lies in which treatment (unamended control vs. sole nutrient addition) should be treated as a reference to calculate the PE under nutrient addition (PEnutrient), as PE is not a directly measured but indirectly estimated variable. Such inconsistent calculations for PEnutrient may affect the results of how nutrient addition affects PE and associated interpretation. By conducting a global synthesis of 52 nutrient-manipulation studies worldwide, we found that the PEnutrient generated by different calculations are very different, particularly when nutrient addition alone has a large effect on SOM decomposition (or microbial basal respiration). Moreover, different calculations affected the direction, magnitude and interpretation of nutrient-addition effects on PE. We recommend that future studies investigating the effects of nutrient addition on PE should design full factorial experiments (e.g., two-factor experiment with treatments of unamended control, carbon substrate addition, nutrient addition, and carbon substrate plus nutrient addition) and treat sole nutrient addition treatment as a reference to calculate the PEnutrient.