Nutrient Perturbations Research Articles

Plant responses to elevated CO2 under competing hypotheses of nitrogen and phosphorus limitations.

The future ecosystem carbon cycle has important implications for biosphere-climate feedback. The magnitude of future plant growth and carbon accumulation depends on plant strategies for nutrient uptake under the stresses of nitrogen (N) versus phosphorus (P) limitations. Two archetypal theories have been widely acknowledged in the literature to represent N and P limitations on ecosystem processes: Liebig's Law of the Minimum (LLM) and the Multiple Element Limitation (MEL) approach. LLM states that the more limiting nutrient controls plant growth, and commonly leads to predictions of dramatically dampened ecosystem carbon accumulation over the 21st century. Conversely, the MEL approach recognizes that plants possess multiple pathways to coordinate N and P availability and invest resources to alleviate N or P limitation. We implemented these two contrasting approaches in the E3SM model, and compiled 98 in situ forest N or P fertilization experiments to evaluate how terrestrial ecosystems will respond to N and P limitations. We find that MEL better captured the observed plant responses to nutrient perturbations globally, compared with LLM. Furthermore, LLM and MEL diverged dramatically in responses to elevated CO2 concentrations, leading to a two-fold difference in CO2 fertilization effects on Net Primary Productivity by the end of the 21st century. The larger CO2 fertilization effects indicated by MEL mainly resulted from plant mediation on N and P resource supplies through N2 fixation and phosphatase activities. This analysis provides quantitative evidence of how different N and P limitation strategies can diversely affect future carbon and nutrient dynamics.

Locus-specific proteome decoding reveals Fpt1 as a chromatin-associated negative regulator of RNA polymerase III assembly

Transcription of tRNA genes by RNA polymerase III (RNAPIII) is tuned by signaling cascades. The emerging notion of differential tRNA gene regulation implies the existence of additional regulatory mechanisms. However, tRNA gene-specific regulators have not been described. Decoding the local chromatin proteome of a native tRNA gene in yeast revealed reprogramming of the RNAPIII transcription machinery upon nutrient perturbation. Among the dynamic proteins, we identified Fpt1, a protein of unknown function that uniquely occupied RNAPIII-regulated genes. Fpt1 binding at tRNA genes correlated with the efficiency of RNAPIII eviction upon nutrient perturbation and required the transcription factors TFIIIB and TFIIIC but not RNAPIII. In the absence of Fpt1, eviction of RNAPIII was reduced, and the shutdown of ribosome biogenesis genes was impaired upon nutrient perturbation. Our findings provide support for a chromatin-associated mechanism required for RNAPIII eviction from tRNA genes and tuning the physiological response to changing metabolic demands.

Nutrient sensitive protein O-GlcNAcylation modulates the transcriptome through epigenetic mechanisms during embryonic neurogenesis.

Protein O-GlcNAcylation is a dynamic, nutrient-sensitive mono-glycosylation deposited on numerous nucleo-cytoplasmic and mitochondrial proteins, including transcription factors, epigenetic regulators, and histones. However, the role of protein O-GlcNAcylation on epigenome regulation in response to nutrient perturbations during development is not well understood. Herein we recapitulated early human embryonic neurogenesis in cell culture and found that pharmacological up-regulation of O-GlcNAc levels during human embryonic stem cells' neuronal differentiation leads to up-regulation of key neurogenic transcription factor genes. This transcriptional de-repression is associated with reduced H3K27me3 and increased H3K4me3 levels on the promoters of these genes, perturbing promoter bivalency possibly through increased EZH2-Thr311 phosphorylation. Elevated O-GlcNAc levels also lead to increased Pol II-Ser5 phosphorylation and affect H2BS112O-GlcNAc and H2BK120Ub1 on promoters. Using an in vivo rat model of maternal hyperglycemia, we show similarly elevated O-GlcNAc levels and epigenetic dysregulations in the developing embryo brains because of hyperglycemia, whereas pharmacological inhibition of O-GlcNAc transferase (OGT) restored these molecular changes. Together, our results demonstrate O-GlcNAc mediated sensitivity of chromatin to nutrient status, and indicate how metabolic perturbations could affect gene expression during neurodevelopment.

Non-additive effects of foundation species determine the response of aquatic ecosystems to nutrient perturbation.

Eutrophication is a persistent threat to aquatic ecosystems worldwide. Foundation species, namely those that play a central role in the structuring of communities and functioning of ecosystems, are likely important for the resilience of aquatic ecosystems in the face of disturbance. However, little is known about how interactions among such species influence ecosystem responses to nutrient perturbation. Here, using an array (N=20) of outdoor experimental pond ecosystems (15,000L), we manipulated the presence of two foundation species, the macrophyte Myriophyllum spicatum and the mussel Dreissena polymorpha, and quantified ecosystem responses to multiple nutrient disturbances, spread over two years. In the first year, we added five nutrient pulses, ramping up from 10 to 50μg P/L over a 10-week period from mid-July to mid-October, and in the second year, we added a single large pulse of 50μg P/L in mid-October. We used automated sondes to measure multiple ecosystems properties at high frequency (15-minute intervals), including phytoplankton and dissolved organic matter fluorescence, and to model whole-ecosystem metabolism. Overall, both foundation species strongly affected the ecosystem responses to nutrient perturbation, and, as expected, initially suppressed the increase in phytoplankton abundance following nutrient additions. However, when both species were present, phytoplankton biomass increased substantially relative to other treatment combinations: non-additivity was evident for multiple ecosystem metrics following the nutrient perturbations in both years but was diminished in the intervening months between our perturbations. Overall, these results demonstrate how interactions between foundation species can cause surprisingly strong deviations from the expected responses of aquatic ecosystems to perturbations such as nutrient additions.

Bacterial cell shape control by nutrient-dependent synthesis of cell division inhibitors

By analyzing cell size and shapes of the bacterium Bacillus subtilis under nutrient perturbations, protein depletion, and antibiotic treatments, we find that cell geometry is extremely robust, reflected in a well-conserved scaling relation between surface area (S) and volume (V), S∼Vγ, with γ=0.85. We develop a molecular model supported by single-cell simulations to predict that the surface-to-volume scaling exponent γ is regulated by nutrient-dependent production of metabolic enzymes that act as cell division inhibitors in bacteria. Using theory that is supported by experimental data, we predict the modes of cell shape transformations in different bacterial species and propose a mechanism of cell shape adaptation to different nutrient perturbations. For organisms with high surface-to-volume scaling exponent γ, such as B. subtilis, cell width is not sensitive to growth-rate changes, whereas organisms with low γ, such as Acinetobacter baumannii, cell shape adapts readily to growth-rate changes.

Effects of nutrient enrichments on oligotrophic phytoplankton communities: a mesocosm experiment near Hawai‘i, USA

A large-volume mesocosm-based nutrient perturbation experiment was conducted off the island of Hawai‘i, USA, to investigate the response of surface ocean phytoplankton communities to the addition of macronutrients, trace metals, and vitamins and to assess the feasibility of using mesocosms in the open ocean. Three free-drifting mesocosms (~60 m3) were deployed: one mesocosm served as a control (no nutrient amendments); a second (termed +P) was amended with nitrate (N), silicate (Si), phosphate (P), and a trace metal + vitamin mixture; and a third (termed -P) was amended with N, Si, and a trace metal + vitamin mixture but no P. These mesocosms were unreplicated due to logistical constraints and hence differences between treatments are qualitative. After 6 d, the largest response of the phytoplankton community was observed in the +P mesocosm, where chlorophyll a and 14C-based primary production were 2-3× greater than in the -P mesocosm and 4-6× greater than in the control. Comparison between mesocosm and ‘microcosm’ incubations (20 l) revealed differences in the magnitude and timing of production and marked differences in community structure with a reduced response of diatoms in microcosm treatments. Notably, we also observed pronounced declines in Prochlorococcus populations in all treatments, although these were greater in microcosms (up to 99%). Overall, this study confirmed the feasibility of deploying free-drifting mesocosms in the open ocean as a potentially powerful tool to investigate ecological impacts of nutrient perturbations and constitutes a valuable first step towards scaling plankton manipulation experiments.

Plant-available soil nutrients have a limited influence on cone production patterns of individual white spruce trees.

The resource budget model for mast seeding hypothesizes that soil nutrients proximately influence reproduction. Plants in high soil nutrient (particularly N) areas are predicted to have lower reproductive variability over time and higher mean reproduction. While often examined theoretically, there are relatively few empirical tests of this hypothesis. We quantified cone production of 110 individual white spruce (Picea glauca) trees over seven years and quantified plant-available soil macronutrients (N, Ca, K, Mg, P, S) in natural forest conditions across three years with different cone crop conditions. Each of these plant-available soil nutrients were correlated across years (rs = 0.55-0.89; all > 0.81 for total-N); spatially, total-N availability varied 366-fold across trees. Plant-available soil nutrients did not influence variability or mean annual reproduction, contrary to nutrient perturbation experiments. We examined within-year nutrient and cone-production relationships, and observed significant positive relationships between reproduction and plant-available soil nutrients only in a low-reproduction year preceding a mast event. Both during a mast event and the following year, when overall cone production was very high or very low, there were no relationships. Both external drivers (e.g., weather) and internal resource budgets likely influence soil nutrient-reproduction relationships. These results suggest that plant-available soil nutrients may not be a large factor influencing mast-seeding patterns among individuals in this species.

Nutrient-Dependent Trade-Offs between Ribosomes and Division Protein Synthesis Control Bacterial Cell Size and Growth

Cell size control emerges from a regulated balance between the rates of cell growth and division. In bacteria, simple quantitative laws connect cellular growth rate to ribosome abundance. However, it remains poorly understood how translation regulates bacterial cell size and shape under growth perturbations. Here, we develop a whole-cell model for growth dynamics of rod-shaped bacteria that links ribosomal abundance with cell geometry, division control, and the extracellular environment. Our study reveals that cell size maintenance under nutrient perturbations requires a balanced trade-off between ribosomes and division protein synthesis. Deviations from this trade-off relationship are predicted under translation inhibition, leading to distinct modes of cell morphological changes, in agreement with single-cell experimental data on Escherichia coli. Furthermore, by calibrating our model with experimental data, we predict how combinations of nutrient-, translational-, and shape perturbations can be chosen to optimize bacterial growth fitness and antibiotic resistance.

Integrated network modeling approach defines key metabolic responses of soil microbiomes to perturbations

The soil environment is constantly changing due to shifts in soil moisture, nutrient availability and other conditions. To contend with these changes, soil microorganisms have evolved a variety of ways to adapt to environmental perturbations, including regulation of gene expression. However, it is challenging to untangle the complex phenotypic response of the soil to environmental change, partly due to the absence of predictive modeling frameworks that can mechanistically link molecular-level changes in soil microorganisms to a community’s functional phenotypes (or metaphenome). Towards filling this gap, we performed a combined analysis of metabolic and gene co-expression networks to explore how the soil microbiome responded to changes in soil moisture and nutrient conditions and to determine which genes were expressed under a given condition. Our integrated modeling approach revealed previously unknown, but critically important aspects of the soil microbiomes’ response to environmental perturbations. Incorporation of metabolomic and transcriptomic data into metabolic reaction networks identified condition-specific signature genes that are uniquely associated with dry, wet, and glycine-amended conditions. A subsequent gene co-expression network analysis revealed that drought-associated genes occupied more central positions in a network model of the soil community, compared to the genes associated with wet, and glycine-amended conditions. These results indicate the occurrence of system-wide metabolic coordination when soil microbiomes cope with moisture or nutrient perturbations. Importantly, the approach that we demonstrate here to analyze large-scale multi-omics data from a natural soil environment is applicable to other microbiome systems for which multi-omics data are available.

Nutrient-Dependent Trade-Offs between Ribosomes and Division Protein Synthesis Control Bacterial Growth and Cell Shape

Cell size control emerges from a regulated balance between the rates of cell growth and division. In bacteria, simple quantitative laws connect cellular growth rate to ribosome abundance. However, it remains poorly understood how translation regulates bacterial cell size and shapes under growth perturbations. Here we develop a whole-cell model for growth dynamics in rod-shaped bacteria that links ribosomal abundance with cell geometry, division control, and the extracellular environment. Our study reveals that cell shape maintenance under nutrient perturbations requires a balanced trade-off between ribosomes and division protein synthesis. Deviations from this trade-off relationship is predicted under translational perturbations, leading to distinct modes of cell morphological changes, in agreement with single-cell experimental data on Escherichia coli. Furthermore, by calibrating our model with experimental data, we predict how combinations of nutrient-, translational- and shape perturbations can be chosen to optimize bacterial growth fitness and drug resistance.

Surface-to-volume scaling and aspect ratio preservation in rod-shaped bacteria.

Rod-shaped bacterial cells can readily adapt their lengths and widths in response to environmental changes. While many recent studies have focused on the mechanisms underlying bacterial cell size control, it remains largely unknown how the coupling between cell length and width results in robust control of rod-like bacterial shapes. In this study we uncover a conserved surface-to-volume scaling relation in Escherichia coli and other rod-shaped bacteria, resulting from the preservation of cell aspect ratio. To explain the mechanistic origin of aspect-ratio control, we propose a quantitative model for the coupling between bacterial cell elongation and the accumulation of an essential division protein, FtsZ. This model reveals a mechanism for why bacterial aspect ratio is independent of cell size and growth conditions, and predicts cell morphological changes in response to nutrient perturbations, antibiotics, MreB or FtsZ depletion, in quantitative agreement with experimental data.

Functional properties of bacterial communities in water and sediment of the eutrophic river-lake system of Poyang Lake, China.

In river-lake systems, sediment and water column are two distinct habitats harboring different bacterial communities which play a crucial role in biogeochemical processes. In this study, we employed Phylogenetic Investigation of Communities by Reconstruction of Unobserved States to assess the potential functions and functional redundancy of the bacterial communities in sediment and water in a eutrophic river-lake ecosystem, Poyang Lake in China. Bacterial communities in sediment and water had distinct potential functions of carbon, nitrogen, and sulfur metabolisms as well as phosphorus cycle, while the differences between rivers and the lake were inconspicuous. Bacterial communities in sediment had a higher relative abundance of genes associated with carbohydrate metabolism, carbon fixation pathways in prokaryotes, methane metabolism, anammox, nitrogen fixation, and dissimilatory sulfate reduction than that of water column. Bacterial communities in water column were higher in lipid metabolism, assimilatory nitrate reduction, dissimilatory nitrate reduction, phosphonate degradation, and assimilatory sulfate reduction than that of sediment bacterial communities. Furthermore, the variations in functional composition were closely associated to the variations in taxonomic composition in both habitats. In general, the bacterial communities in water column had a lower functional redundancy than in sediment. Moreover, comparing to the overall functions, bacterial communities had a lower functional redundancy of nitrogen metabolism and phosphorus cycle in water column and lower functional redundancy of nitrogen metabolism in sediment. Distance-based redundancy analysis and mantel test revealed close correlations between nutrient factors and functional compositions. The results suggested that bacterial communities in this eutrophic river-lake system of Poyang Lake were vulnerable to nutrient perturbations, especially the bacterial communities in water column. The results enriched our understanding of the bacterial communities and major biogeochemical processes in the eutrophic river-lake ecosystems.

In Situ Diazotroph Population Dynamics Under Different Resource Ratios in the North Pacific Subtropical Gyre.

Major advances in understanding the diversity, distribution, and activity of marine N2-fixing microorganisms (diazotrophs) have been made in the past decades, however, large gaps in knowledge remain about the environmental controls on growth and mortality rates. In order to measure diazotroph net growth rates and microzooplankton grazing rates on diazotrophs, nutrient perturbation experiments and dilution grazing experiments were conducted using free-floating in situ incubation arrays in the vicinity of Station ALOHA in March 2016. Net growth rates for targeted diazotroph taxa as well as Prochlorococcus, Synechococcus and photosynthetic picoeukaryotes were determined under high (H) and low (L) nitrate:phosphate (NP) ratio conditions at four depths in the photic zone (25, 45, 75, and 100 m) using quantitative PCR and flow cytometry. Changes in the prokaryote community composition in response to HNP and LNP treatments were characterized using 16S rRNA variable region tag sequencing. Microzooplankton grazing rates on diazotrophs were measured using a modified dilution technique at two depths in the photic zone (15 and 125 m). Net growth rates for most of the targeted diazotrophs after 48 h were not stimulated as expected by LNP conditions, rather enhanced growth rates were often measured in HNP treatments. Interestingly, net growth rates of the uncultivated prymnesiophyte symbiont UCYN-A1 were stimulated in HNP treatments at 75 and 100 m, suggesting that N used for growth was acquired through continuing to fix N2 in the presence of nitrate. Net growth rates for UCYN-A1, UCYN-C, Crocosphaera sp. (UCYN-B) and the diatom symbiont Richelia (associated with Rhizosolenia) were uniformly high at 45 m (up to 1.6 ± 0.5 d-1), implying that all were growing optimally at the onset of the experiment at that depth. Differences in microzooplankton grazing rates on UCYN-A1 and UCYN-C in 15 m waters indicate that the grazer assemblage preyed preferentially on UCYN-A1. Deeper in the water column (125 m), both diazotrophs were grazed at substantial rates, suggesting grazing pressure may increase with depth in the photic zone. Constraining in situ diazotroph growth and mortality rates are important steps for improving parameterization for diazotrophs in global ecosystem models.

Sedimentary Environment Influences Ecosystem Response to Nutrient Enrichment

As coastal catchment land use intensifies, estuaries receive increased nutrient and sediment loads, resulting in habitats that are dominated by muddy organic-rich sediments. Increased mud (i.e. silt-clay (particles < 63 μm)) content has been associated with negative effects on soft sediment biodiversity and ecosystem functioning, but the simultaneous impact of nutrient enrichment on ecosystem response is unclear. Nutrient recycling and denitrification in estuarine soft sediments represent important ecosystem functions regenerating nutrients for primary producers and regulating the ability to remove excess terrestrially derived nitrogen. To test the effect of sedimentary environment on ecosystem resilience to nutrient perturbation, we experimentally enriched sediments with slow release fertiliser across an intertidal sedimentary gradient (0–24% mud content). The enrichment successfully elevated pore water ammonium concentrations (median 36 × control) to levels representative of enriched estuaries. Findings show that the sedimentary environment can influence ecosystem function response to nutrient stress. In particular, denitrification enzyme activity was suppressed by nutrient enrichment, but the effect was greater as sediment mud content increased. Furthermore, compared with sandy sediments, sediments with high mud content may restrict nutrient processing (release, uptake or transformation of organic nutrients by the benthos) facilitating ecosystem shifts toward eutrophication. These results show the value of investigating the impacts of stressors in different environmental settings and demonstrate that land use practices that increase the proportion of muddy habitats in estuaries may reduce denitrification which in turn may reduce ecosystem resilience to eutrophication.

Determinants and Regulation of Protein Turnover in Yeast.

Protein turnover maintains the recycling needs of theproteome, and its malfunction has been linked to aging and age-related diseases. However, not allproteins turnover equally, and the factors that contribute to accelerate or slow down turnover are mostly unknown. We measured turnover rates for 3,160 proteins in exponentially growing yeast and analyzed their dependence on physical, functional, and genetic properties. We found that functional characteristics, including protein localization, complex membership, and connectivity, have greater effect on turnover than sequence elements. We also found that protein turnover and mRNA turnover are correlated. Analysis under nutrient perturbation and osmotic stress revealed that protein turnover highly depends on cellular state and is faster when proteinsare being actively used. Finally, stress-induced changes in protein and transcript abundance correlated with changes in protein turnover. This study provides a resource of protein turnover rates and principles to understand the recycling needs of the proteome under basal conditions and perturbation.

Proteogenomic analyses indicate bacterial methylotrophy and archaeal heterotrophy are prevalent below the grass root zone.

Annually, half of all plant-derived carbon is added to soil where it is microbially respired to CO2. However, understanding of the microbiology of this process is limited because most culture-independent methods cannot link metabolic processes to the organisms present, and this link to causative agents is necessary to predict the results of perturbations on the system. We collected soil samples at two sub-root depths (10–20 cm and 30–40 cm) before and after a rainfall-driven nutrient perturbation event in a Northern California grassland that experiences a Mediterranean climate. From ten samples, we reconstructed 198 metagenome-assembled genomes that represent all major phylotypes. We also quantified 6,835 proteins and 175 metabolites and showed that after the rain event the concentrations of many sugars and amino acids approach zero at the base of the soil profile. Unexpectedly, the genomes of novel members of the Gemmatimonadetes and Candidate Phylum Rokubacteria phyla encode pathways for methylotrophy. We infer that these abundant organisms contribute substantially to carbon turnover in the soil, given that methylotrophy proteins were among the most abundant proteins in the proteome. Previously undescribed Bathyarchaeota and Thermoplasmatales archaea are abundant in deeper soil horizons and are inferred to contribute appreciably to aromatic amino acid degradation. Many of the other bacteria appear to breakdown other components of plant biomass, as evidenced by the prevalence of various sugar and amino acid transporters and corresponding hydrolyzing machinery in the proteome. Overall, our work provides organism-resolved insight into the spatial distribution of bacteria and archaea whose activities combine to degrade plant-derived organics, limiting the transport of methanol, amino acids and sugars into underlying weathered rock. The new insights into the soil carbon cycle during an intense period of carbon turnover, including biogeochemical roles to previously little known soil microbes, were made possible via the combination of metagenomics, proteomics, and metabolomics.

Autophagy requires poly(adp-ribosyl)ation-dependent AMPK nuclear export.

AMPK is a central energy sensor linking extracellular milieu fluctuations with the autophagic machinery. In the current study we uncover that Poly(ADP-ribosyl)ation (PARylation), a post-translational modification (PTM) of proteins, accounts for the spatial and temporal regulation of autophagy by modulating AMPK subcellular localisation and activation. More particularly, we show that the minority AMPK pool needs to be exported to the cytosol in a PARylation-dependent manner for optimal induction of autophagy, including ULK1 phosphorylation and mTORC1 inactivation. PARP-1 forms a molecular complex with AMPK in the nucleus in non-starved cells. In response to nutrient deprivation, PARP-1 catalysed PARylation, induced the dissociation of the PARP-1/AMPK complex and the export of free PARylated nuclear AMPK to the cytoplasm to activate autophagy. PARP inhibition, its silencing or the expression of PARylation-deficient AMPK mutants prevented not only the AMPK nuclear-cytosolic export but also affected the activation of the cytosolic AMPK pool and autophagosome formation. These results demonstrate that PARylation of AMPK is a key early signal to efficiently convey extracellular nutrient perturbations with downstream events needed for the cell to optimize autophagic commitment before autophagosome formation.

Source waters for the highly productive Patagonian shelf in the southwestern Atlantic

Possible nutrient sources and delivery mechanisms for the highly productive Patagonian shelf in the southwest Atlantic are identified. Using a passive tracer adjoint sensitivity experiment, we identify three source waters: waters local to the Patagonian shelf, coastal waters near the Chilean coast and the subsurface waters in the southeast Pacific. We perform a series of forward simulations of a biogeochemical model to investigate the impact of nutrient perturbations in these source regions to productivity on the Patagonian shelf.Positive nitrate perturbations from local waters have an immediate impact elevating productivity. Iron perturbations local to the shelf, however, do not change productivity because the shelf region is limited by nitrate. Additional nutrient supply from the other source regions leads to increases in productivity. We find that positive nutrient perturbations in subsurface waters in the southeast Pacific result in the largest boost of productivity over the shelf. These source waters are rich in nutrients and upwelled from the depth where light levels are so low that they cannot be consumed. Finally, we identify wintertime intense vertical mixing as the key process which draws nutrients from below 300–500m to the surface before being delivered to the shelf.

Nitrogen deposition but not climate warming promotes Deyeuxia angustifolia encroachment in alpine tundra of the Changbai Mountains, Northeast China

Vegetation in the alpine tundra area of the Changbai Mountains, one of two alpine tundra areas in China, has undergone great changes in recent decades. The aggressive herb species Deyeuxia angustifolia (Komarov) Y. L. Chang, a narrow-leaf small reed, was currently encroaching upon the alpine landscape and threatening tundra biota. The alpine tundra of the Changbai Mountains has been experiencing a warmer climate and receiving a high load of atmospheric nitrogen deposition. In this study, we aimed to assess the respective roles of climate warming and atmospheric nitrogen deposition in promoting the upward encroachment of D. angustifolia. We conducted experiments for three years to examine the response of D. angustifolia and a native alpine shrub, Rhododendron chrysanthum, to the conditions in which temperature and nitrogen were increased. Treatments consisting of temperature increase, nitrogen addition, temperature increase combined with nitrogen addition, and controls were conducted on the D. angustifolia communities with three encroachment levels (low, medium, and high levels). Results showed that 1) D. angustifolia grew in response to added nutrients but did not grow well when temperature increased. R. chrysanthum showed negligible responses to the simulated environmental changes. 2) Compared to R. chrysanthum, D. angustifolia could effectively occupy the above-ground space by increasing tillers and growing rapidly by efficiently using nitrogen. The difference in nitrogen uptake abilities between the two species contributed to expansion of D. angustifolia. 3) D. angustifolia encroachment could deeply change the biodiversity of tundra vegetation and may eventually result in the replacement of native biota, especially with nitrogen addition. Our research indicated that nutrient perturbation may be more important than temperature perturbation in promoting D. angustifolia encroachment upon the nutrient- and species-poor alpine tundra ecosystem in the Changbai Mountains.

A new application of conditional nonlinear optimal perturbation approach to boundary condition uncertainty

AbstractThe conditional nonlinear optimal perturbation (CNOP) approach was mainly used to investigate the effects of the uncertainties in initial condition and model parameters on model results. This study presents a new application of the CNOP approach to the investigation of the effects of boundary condition uncertainty. Specifically, we first give the general formulation of the CNOP approach for uncertainties of initial and boundary conditions and model parameters. The method is then applied to analyze the effects of nutrient perturbations at the bottom boundary of the water column on the modeled deep chlorophyll maximum (DCM) using an ocean ecosystem model. The results show that nutrient perturbations at the bottom boundary have significant impacts on the modeled DCM. Interestingly, the approach also reveals that nonlinear processes play important roles in the evolution of phytoplankton perturbations caused by nutrient perturbations at the bottom boundary. This implies that the CNOP approach is useful for investigating the effects of boundary condition uncertainty.