A novel soil manganese mechanism drives plant

Loss of plant diversity with increased anthropogenic nitrogen (N) deposition in 40 grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of 41 light or space is invoked as a key mechanism. Here, we provide evidence from a 9-year N 42 addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to 43 increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply 44 shifted plant community composition from grass-forb codominance (primarily Stipa krylovii and 45 Artemisia frigida, respectively) to exclusive dominance by grass, with associated declines in 46 overall species richness. Reduced abundance of forbs was linked to soil acidification that 47 increased mobilization of soil Mn, with a 10-fold greater accumulation of Mn in forbs than that 48 in grasses. The enhanced accumulation of Mn in forbs was correlated with their reduced 49 photosynthetic rates and growth, and is consistent with the loss of forb species. Differential 50 accumulation of Mn between forbs and grasses can be linked to fundamental differences between 51 dicots and monocots in the biochemical pathways regulating metal transport. These findings 52 provide a mechanistic explanation for N-induced species loss in temperate grasslands by linking 53 metal mobilization in soil to differential metal acquisition and impacts on key functional groups 54 in these ecosystems.