This could be either because of shifts in CUE within microbial organisms or shifts in microbial community composition to groups that acquire C more efficiently(Riggs and Hobbie, 2016). For example, a decreased fungi:bacteria ratio could decrease CUE, since bacteria tend to have low CUE than fungi(Manzoni et al., 2012). Moreover, high N addition might stimulate enzymes that degrade labile C and inhibit those needed for the decomposition of recalcitrant C, thereby reducing overall microbial activity and CUE (Riggs and Hobbie, 2015;Zang et al., 2017).