Enhanced deposition of atmospheric nitrogen (N) leads to loss of plant diversity in grassland ecosystems. Numerous theories have provided potential explanations for the negative effects of N enrichment on plant diversity. However, the relative importance of each mechanism and the time‐scales of responses for the different functional groups remain unclear. We investigated the temporal responses of plant community in a temperate steppe to N enrichment by linking above‐ground to below‐ground processes using a series of field N‐addition and greenhouse experiments. The N enrichment‐induced declines in plant diversity of grasslands were phase‐based, functional group‐dependent and driven by three below‐ground processes. The rapid accumulation of NH4+-N by N addition inhibited photosynthetic rates of broad‐leaf non‐rhizomatous forbs, contributing to loss of these N‐sensitive species during early phase of N enrichment (≤3 years). The N‐induced changes in this phase were independent of soil pH as evidenced by results from application of lime to mitigate N‐evoked soil acidification. With progression of N addition, manganese (Mn) toxicity to narrow‐leaf non‐rhizomatous forbs due to soil acidification‐induced Mn2+ mobilization in soil accounted for their loss in the second phase of N enrichment (~4–9 years). When N addition proceeded longer than ~10 years, N enrichment stimulated below‐ground meristem differentiation and rhizome growth of the rhizomatous species, leading to the dominance by rhizomatous sedges/grasses in the community at the later phase of N enrichment. Synthesis. The hierarchical mechanisms not only provide a comprehensive explanation for the N enrichment‐induced diversity decline in grasslands, but can also facilitate us to understand the differential sensitivities of ecosystems to chronic N enrichment, and predict future ecosystem dynamics.

Background Medicago ruthenica, a wild and perennial legume forage widely distributed in semi-arid grasslands, is distinguished by its outstanding tolerance to environmental stress. It is a close relative of commonly cultivated forage of alfalfa (Medicago sativa). The high tolerance of M. ruthenica to environmental stress makes this species a valuable genetic resource for understanding and improving traits associated with tolerance to harsh environments. Results We sequenced and assembled genome of M. ruthenica using an integrated approach, including PacBio, Illumina, 10×Genomics, and Hi-C. The assembled genome was 904.13 Mb with scaffold N50 of 99.39 Mb, and 50,162 protein-coding genes were annotated. Comparative genomics and transcriptomic analyses were used to elucidate mechanisms underlying its tolerance to environmental stress. The expanded FHY3/FAR1 family was identified to be involved in tolerance of M. ruthenica to drought stress. Many genes involved in tolerance to abiotic stress were retained in M. ruthenica compared to other cultivated Medicago species. Hundreds of candidate genes associated with drought tolerance were identified by analyzing variations in single nucleotide polymorphism using accessions of M. ruthenica with varying tolerance to drought. Transcriptomic data demonstrated the involvements of genes related to transcriptional regulation, stress response, and metabolic regulation in tolerance of M. ruthenica. Conclusions We present a high-quality genome assembly and identification of drought-related genes in the wild species of M. ruthenica, providing a valuable resource for genomic studies on perennial legume forages.

Arbuscular mycorrhizal (AM) fungi are important components of grassland ecosystems and are sensitive to enhanced atmospheric nitrogen (N) deposition. Enhanced N deposition has been widely reported to reduce species richness and alter species composition across different types of grasslands world‐wide. Despite extensive studies on effects of N deposition on AM fungal communities of grasslands, few studies have specifically focused on effects of N deposition on AM fungi associated with dominant species in grasslands. We investigated long‐term (12‐year) effects of N addition (80 kg ha−1 year−1) on AM fungal community richness in roots and rhizosphere biomass of two dominant plant species (forb Artemisia frigida and grass Stipa krylovii) in temperate steppes of northern China. We found that AM fungi associated with the two dominant plant species differed in their responses to N addition. Nitrogen addition led to a significant reduction in AM fungal richness colonized in roots, and biomass in the rhizosphere of A. frigida, while N addition had little impacts on AM fungi colonized in roots and rhizosphere of S. krylovii. Nitrogen addition significantly reduced AM fungal colonization in roots and AM fungal spore density in the rhizosphere soils of A. frigida. In contrast, N addition had no effect on AM fungal colonization in roots and spore density in the rhizosphere of S. krylovii. Nitrogen addition markedly suppressed photosynthetic rates in A. frigida due to excessive foliar accumulation of manganese. The N‐induced reduction in photosynthetic rates reduced allocation of C into roots in A. frigida, leading to a lower root/shoot ratio, and suppression of AM fungal community associated with A. frigida. The inhibition of AM fungi would render A. frigida less competitive in terms of acquisition of mineral nutrients in soils, thus contributing to its loss in the steppe community under conditions of elevated N deposition. These findings provide a mechanistic explanation for N‐evoked differential responses of AM fungal communities associated with A. frigida and S. krylovii by linking soil properties and host plants to AM fungal communities. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13081/suppinfo is available for this article.