Enhanced deposition of atmospheric nitrogen (N) has profound impacts on ecosystem processes such as above-ground productivity and community structure in grasslands across the globe. But how N deposition affects below-ground processes of grasslands is less well known. Here, we evaluated the effects of chronic N amendment at a relatively low rate (20 kg ha-1 year-1) on root traits (root productivity, root biomass, root/shoot ratio) in Inner Mongolia steppes by rhizotron and ingrowth core and soil monolith techniques at levels of individual species, functional groups and ecosystem. For 8 years, N amendment suppressed above-ground net primary production (ANPP), photosynthetic rates and root biomass of forbs, but enhanced ANPP and root biomass of grasses. This led to an overall reduction in below-ground productivity of the grassland by 24-33%, while ANPP remained unchanged. Nitrogen amendment acidified soil and subsequently increased extractable soil manganese (Mn) concentration. Nitrogen amendment increased foliar Mn concentrations in forb, but not grass species, leading to a significant inhibition of photosynthetic rates in forb species. Synthesis. These findings highlight the importance of the differentiating responses of plant functional groups to long-term N deposition and the important consequences of these responses for below-ground productivity and long-term soil C sequestration.

Air pollution represented by particulate matter 2.5 (PM2.5) is closely related to diseases of the respiratory system. Although the understanding of its mechanism is limited, pulmonary inflammation is closely correlated with PM2.5-mediated lung injury. Soluble epoxide hydrolase (sEH) and epoxy fatty acids play a vital role in the inflammation. Herein, we attempted to use the metabolomics of oxidized lipids for analyzing the relationship of oxylipins with lung injury in a PM2.5-mediated mouse model, and found that the cytochrome P450 oxidases/sEH mediated metabolic pathway was involved in lung injury. Furthermore, the sEH overexpression was revealed in lung injury mice. Interestingly, sEH genetic deletion or the selective sEH inhibitor TPPU increased levels of epoxyeicosatrienoic acids (EETs) in lung injury mice, and inactivated pulmonary macrophages based on the MAPK/NF-κB pathway, resulting in protection against PM2.5-mediated lung injury. Additionally, a natural sEH inhibitor luteolin from Inula japonica displayed a pulmonary protective effect towards lung injury mediated by PM2.5 as well. Our results are consistent with the sEH message and protein being both a marker and mechanism for PM2.5-induced inflammation, which suggest its potential as a pharmaceutical target for treating diseases of the respiratory system.

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