Quantifying nitrogen uptake and translocation for mature trees: an in situ whole-tree paired 15N labeling method

Arbuscular mycorrhizal fungi reduce soil N2O emissions by altering root traits and soil denitrifier community composition

Arbuscular mycorrhizal fungi (AMF) increase the ability of plants to obtain nitrogen (N) from the soil, and thus can affect emissions of nitrous oxide (N2O), a long-lived potent greenhouse gas. However, the mechanisms underlying the effects of AMF on N2O emissions are still poorly understood, particularly in agroecosystems with different forms of N fertilizer inputs. Utilizing a mesocosm experiment in field, we examined the effects of AMF on N2O emissions via their influence on maize root traits and denitrifying microorganisms under ammonia and nitrate fertilizer input using 15N isotope tracer. Here we show that the presence of AMF alone or both maize roots and AMF increased maize biomass and their 15N uptake, root length, root surface area, and root volume, but led to a reduction in N2O emissions under both N input forms. Random forest model showed that root length and surface area were the most important predictors of N2O emissions. Additionally, the presence of AMF reduced the (nirK + nirS)/nosZ ratio by increasing the relative abundance of nirS-Bradyrhizobium and Rubrivivax with ammonia input, but reducing nosZ-Azospirillum, Cupriavidus and Rhodopseudomonas under both fertilizer input. Further, N2O emissions were significantly and positively correlated with the nosZ-type Azospirillum, Cupriavidus and Rhodopseudomonas, but negatively correlated with the nirS-type Bradyrhizobium and Rubrivivax. These results indicate that AMF reduce N2O emissions by increasing root length to explore N nutrients and altering the community composition of denitrifiers, suggesting that effective management of N fertilizer forms interacting with the rhizosphere microbiome may help mitigate N2O emissions under future N input scenarios.

An automated workflow that generates atom mappings for large-scale metabolic models and its application to Arabidopsis thaliana

Quantification of reaction fluxes of metabolic networks can help us understand how the integration of different metabolic pathways determines cellular functions. Yet, intracellular fluxes cannot be measured directly but are estimated with metabolic flux analysis (MFA), which relies on the patterns of isotope labeling of metabolites in the network. The application of MFA also requires a stoichiometric model with atom mappings that are currently not available for the majority of large-scale metabolic network models, particularly of plants. While automated approaches such as the Reaction Decoder Toolkit (RDT) can produce atom mappings for individual reactions, tracing the flow of individual atoms of the entire reactions across a metabolic model remains challenging. Here we establish an automated workflow to obtain reliable atom mappings for large-scale metabolic models by refining the outcome of RDT, and apply the workflow to metabolic models of Arabidopsis thaliana. We demonstrate the accuracy of RDT through a comparative analysis with atom mappings from a large database of biochemical reactions, MetaCyc. We further show the utility of our automated workflow by simulating ¹⁵ N isotope enrichment and identifying nitrogen (N)-containing metabolites which show enrichment patterns that are informative for flux estimation in future ¹⁵ N-MFA studies of A. thaliana. The automated workflow established in this study can be readily expanded to other species for which metabolic models have been established and the resulting atom mappings will facilitate MFA and graph-theoretic structural analyses with large-scale metabolic networks.

Interactions between species change the uptake of ammonium and nitrate in Abies faxoniana and Picea asperata

Plant nitrogen (N) uptake is affected by plant-plant interactions, but the mechanisms remain unknown. A 15N-labeled technique was used in a pot experiment to analyze the uptake rate of ammonium (NH4+) and nitrate (NO3-) by Abies faxoniana Rehd. et Wils and Picea asperata Mast. in single-plant mode, intraspecific and interspecific interactions. The results indicated that the effects of plant-plant interactions on N uptake rate depended on plant species and N forms. Picea asperata had a higher N uptake rate of both N forms than A. faxoniana, and both species preferred NO3-. Compared with single-plant mode, intraspecific interaction increased NH4+ uptake for A. faxoniana but reduced that for P. asperata, while it did not change NO3- uptake for the two species. The interspecific interaction enhanced N uptake of both N forms for A. faxoniana but did not affect the P. asperata compared with single-plant mode. NH4+ and NO3- uptake rates for the two species were regulated by root N concentration, root nitrate reductase activity, root vigor, soil pH and soil N availability under plant-plant interactions. Decreased NH4+ uptake rate for P. asperata under intraspecific interaction was induced by lower root N concentration and nitrate reductase activity. The positive effects of interspecific interaction on N uptake for A. faxoniana could be determined mainly by positive rhizosphere effects, such as high soil pH. From the perspective of root-soil interactions, our study provides insight into how plant-plant interactions affect N uptake, which can help to understand species coexistence and biodiversity maintenance in forest ecosystems.

Characterization of Young Shoot Population, Yield, and Nitrogen Demands of Tea (Camellia sinensis L.) Harvested under Different Standards

The quality of green tea is greatly influenced by the harvest standards for young shoots. The present field experiment was conducted to characterize the young shoot populations, yields, and nitrogen (N) demands of tea plants subjected to four different harvest standards, i.e., buds with one, two, or three young expanding leaves (referred to as B1L, B2L, and B3L, respectively) and a combination of B1L and B3L (B1L/B3L) throughout the year. Weight per shoot was closely related to the number of expanding leaves and was greater in B3L than B1L and B2L, and also greater in summer and autumn than in spring, whereas B1L revealed the greatest young shoot density and highest N concentration. Annual shoot yield and shoot N content were largest in B3L and decreased in the following order: B3L > B2L ≈ B1L/B3L > B1L. However, in the early spring the shoot density, yield, and shoot N content of B1L were much higher than those of B3L. The harvest of B3L significantly reduced the biomass of brown roots and its ratio against the above-ground biomass compared to other harvest standards, suggesting a decreased allocation of carbon to the root system due to seasonal removal. The N dilution curve (Nys = a × Yysb, where Nys is the shoot N content and Yys is the shoot yield) of spring tea differed markedly from those of summer and autumn teas, suggesting different coordination properties for shoot growth and N supply among the seasons. The annual harvest index (NHI) measured by 15N traces ranged between 0.18 and 0.23, indicating relatively low N allocation to young shoots, whereby large proportions (58.2–66.9% of the total 15N absorption) remained in the plant at the end of the experiment. In conclusion, the seasonal distribution of the shoot density, weight per shoot, yield, and N demands vary with harvest standards and highlight the importance of N precision management in tea production to be finely tuned to meet the changes in harvest season and requirements.