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Atmospheric nitrogen (N) deposition has been substantially reduced due to declines in the reactive N emission in major regions of the world. Nevertheless, the impact of reduced N deposition on soil microbial communities and the mechanisms by which they are regulated remain largely unknown. Here, we examined the effects of N addition and cessation of N addition on plant and soil microbial communities through a 17‐year field experiment in a temperate grassland. We found that extreme N input did not irreversibly disrupt the ecosystem, but ceasing high levels of N addition led to greater resilience in bacterial and fungal communities. Fungi exhibited diminished resilience compared to bacteria due to their heightened reliance on changes in plant communities. Neither bacterial nor fungal diversity fully recovered to their original states. Their sensitivity and resilience were mainly steered by toxic metal ions and soil pH differentially regulating on functional taxa. Specifically, beneficial symbiotic microbes such as N‐fixing bacteria and arbuscular mycorrhizal fungi experienced detrimental effects from toxic metal ions and lower pH, hindering their recovery. The bacterial functional groups involved in carbon decomposition, and ericoid mycorrhizal and saprotrophic fungi were positively influenced by soil metals, and demonstrated gradual recovery. These findings could advance our mechanistic understanding of microbial community dynamics under ongoing global changes, thereby informing management strategies to mitigate the adverse effects of N enrichment on soil function. [ABSTRACT FROM AUTHOR]

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.

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.

Abstract Background MicroRNAs (miRNAs) are small, endogenous RNAs that play important regulatory roles in development and stress response in plants by negatively affecting gene expression post-transcriptionally. Identification of miRNAs at the global genome-level by high-throughout sequencing is essential to functionally characterize miRNAs in plants. Drought is one of the common environmental stresses limiting plant growth and development. To understand the role of miRNAs in response of plants to drought stress, drought-responsive miRNAs were identified by high-throughput sequencing in a legume model plant, Medicago truncatula. Results Two hundreds eighty three and 293 known miRNAs were identified from the control and drought stress libraries, respectively. In addition, 238 potential candidate miRNAs were identified, and among them 14 new miRNAs and 15 new members of known miRNA families whose complementary miRNA*s were also detected. Both high-throughput sequencing and RT-qPCR confirmed that 22 members of 4 miRNA families were up-regulated and 10 members of 6 miRNA families were down-regulated in response to drought stress. Among the 29 new miRNAs/new members of known miRNA families, 8 miRNAs were responsive to drought stress with both 4 miRNAs being up- and down-regulated, respectively. The known and predicted targets of the drought-responsive miRNAs were found to be involved in diverse cellular processes in plants, including development, transcription, protein degradation, detoxification, nutrient status and cross adaptation. Conclusions We identified 32 known members of 10 miRNA families and 8 new miRNAs/new members of known miRNA families that were responsive to drought stress by high-throughput sequencing of small RNAs from M. truncatula. These findings are of importance for our understanding of the roles played by miRNAs in response of plants to abiotic stress in general and drought stress in particular.

Nutrient limitation and metal toxicity have been implicated in changes of grassland communities by nitrogen (N) deposition. Below‐ground processes, especially those at the soil–root interface, play important roles in determining variation in nutrient concentrations in plants. However, few studies have specifically focused on the roles of these processes in mineral‐element acquisition in grassland plants in response to N enrichment.Here we investigated the contributions of below‐ground processes at the soil–root interface to the differential acquisition of phosphorus (P), calcium (Ca) and manganese (Mn) by forbs and grasses of a temperate steppe in response to N addition by combining field and glasshouse experiments.Nitrogen addition increased the concentrations of both leaf P ([P]) and Mn ([Mn]) and decreased leaf [Ca] of forbs while it had little effects on leaf concentrations of these elements in grasses. Nitrogen addition led to a higher activity of acid phosphatase in the rhizosphere of forbs, and greater release of protons and carboxylates from forb roots than grass roots, contributing to the differential [P], [Ca] and [Mn] in the leaves of forbs and grasses. Applying oxalate to soil to simulate the release of carboxylates by N enrichment enhanced [P] and [Mn], and decreased [Ca] in the soil solution. However, addition of hydrogen‐ion increased [P], [Mn] and [Ca] in the soil solution. Lime addition mitigated the N‐addition‐induced soil acidification while it did not abolish the stimulatory effect of short‐term N addition on leaf [P] and [Mn] of forbs. Therefore, we conclude that differences in the eco‐physiological processes at the soil–root interface account for changes in leaf [P], [Ca] and [Mn] under short‐term N addition, and that soil acidification aggravates the responses of these elements, especially [Ca] and [Mn], to long‐term N enrichment.Synthesis. Our results highlight the contribution of below‐ground processes, especially those at the soil–root interface, to variation in plant element concentrations between dominant forbs and grasses in the temperate steppe. These findings greatly enhance our mechanistic understanding of the effects of N deposition on grassland communities. [ABSTRACT FROM AUTHOR]

Aluminum (Al) toxicity is a major factor limiting plant growth in acid soils. Elevated atmospheric CO2 [CO2] enhances plant growth. However, there is no report on the effect of elevated [CO2] on growth of plant genotypes differing in Al tolerance grown in acid soils. We investigated the effect of short-term elevated [CO2] on growth of Al-tolerant (ET8) and Al-sensitive (ES8) wheat plants and malate exudation from root apices by growing them in acid soils under ambient [CO2] and elevated [CO2] using open-top chambers. Exposure of ET8 plants to elevated [CO2] enhanced root biomass only. In contrast, shoot biomass of ES8 was enhanced by elevated [CO2]. Given that exudation of malate to detoxify apoplastic Al is a mechanism for Al tolerance in wheat plants, ET8 plants exuded greater amounts of malate from root apices than ES8 plants under both ambient and elevated [CO2]. These results indicate that elevated [CO2] has no effect on malate exudation in both ET8 and ES8 plants. These novel findings have important implications for our understanding how plants respond to elevated [CO2] grown in unfavorable edaphic conditions in general and in acid soils in particular.