Search

Purpose The ecological importance of arbuscular mycorrhizal fungi (AMF) in plant acquisition of inorganic and organic sources of nitrogen (N) is not clear. To improve understanding of the plant N nutrition ecology, we tested the effect of intraspecific competition and AMF in plant N source use in growth and N acquisition. Methods Solidago virgaurea was grown in microcosms in a fully factorial experiment under greenhouse conditions. The factors tested were intraspecific competition between seedlings and adult plants (yes, no), N source (NH4, glycine) and AMF (inoculated with Glomus hoi, not inoculated). Results When grown separately, non-mycorrhizal seedling growth was highest when grown with ammonium, but non-mycorrhizal adults grew best with glycine as the sole N source. Mycorrhizal symbiosis with Glomus hoi evened out this initial niche partitioning in terms of differences in N source use and all mycorrhizal plants grew best with ammonium. Competition shaped plant benefit from mycorrhizal symbiosis depending on the N source. Competition reduced mycorrhizal growth benefit in glycine-grown seedlings, but not in adults. Plant performance did not show uniform relationship with δ15N, but δ15N was affected by life stage, competition and mycorrhiza. Conclusions Plant competition and AMF shape plant N source use. Plant and AMF benefit of the symbiosis depend on the N source.

Ectomycorrhizal plants trade plant-assimilated carbon for soil nutrients with their fungal partners. The underlying mechanisms, however, are not fully understood. Here we investigate the exchange of carbon for nitrogen in the ectomycorrhizal symbiosis of Fagus sylvatica across different spatial scales from the root system to the cellular level. We provided 15N-labelled nitrogen to mycorrhizal hyphae associated with one half of the root system of young beech trees, while exposing plants to a 13CO2 atmosphere. We analysed the short-term distribution of 13C and 15N in the root system with isotope-ratio mass spectrometry, and at the cellular scale within a mycorrhizal root tip with nanoscale secondary ion mass spectrometry (NanoSIMS). At the root system scale, plants did not allocate more 13C to root parts that received more 15N. Nanoscale secondary ion mass spectrometry imaging, however, revealed a highly heterogenous, and spatially significantly correlated distribution of 13C and 15N at the cellular scale. Our results indicate that, on a coarse scale, plants do not allocate a larger proportion of photoassimilated C to root parts associated with N-delivering ectomycorrhizal fungi. Within the ectomycorrhizal tissue, however, recently plant-assimilated C and fungus-delivered N were spatially strongly coupled. Here, NanoSIMS visualisation provides an initial insight into the regulation of ectomycorrhizal C and N exchange at the microscale.

Over the last 30 years, Denmark has implemented a series of environmental action plans involving regulation of agricultural nitrogen (N) use and management in order to minimize the N pollution of the environment. The local effects of these action plan initiatives depend on various factors such as the efficiency of the implemented measures, and in particular, the hydrogeological structures and geochemical conditions of the subsurface that control the transport and fate of N. In this study, the effects of the Danish agricultural N regulations on shallow oxic groundwater are compared among five small agricultural catchments underlain by two types of lithology (sandy vs. loamy). Long-term spatially dense groundwater monitoring data is compared with monitoring data of nitrate in the root zone leaching and in stream. The results show clear improvements in the environmental state of shallow oxic groundwater in the first two decades since 1989 where the number of monitoring points with significant decreasing nitrate trends gradually increased for both soil types. Such improvements can be attributed to the effects of N mitigation measures implemented as a general regulation all over the country. However, deteriorations have been recorded in the last decade until 2016 where 26–35% of the monitoring points exhibited significant upward nitrate trends in both types of catchments. It is noteworthy that for sandy soils, the major part (93%) of the monitoring points showing significant upward trends in the period 2009–2016, also had concentrations above the groundwater standard of 50 mg/l nitrate in 2016. Altogether, the oxic groundwater in the sandy catchments was more responsive to N regulations than that in the loamy catchments. This might be due to efficient N regulation through statutory norms for the utilization of N in manure, increasing the N use efficiency in areas with a relatively high livestock density. Another reason is the nitrate vulnerability of the aquifers in sandy areas, with widespread oxic conditions from the top soil to the saturated zone. In the loamy catchments, nitrate may be reduced in near-surface localized reduced zones, and the reaction is often fast and the travel time from the root zone to the stream often relatively short. Therefore, stream nitrate concentrations were higher in the loamy catchments than in the sandy catchments. This is attributed to different hydrogeological pathways. Thus, in sandy catchments, deep groundwater is an important pathway, while in loamy catchments tile drains deliver nitrate directly to the streams. Our results indicate that N mitigation measures will help improve groundwater quality in sandy soils, while in loamy soils they will help to reduce surface water N loads. This implies that to achieve optimal environmental N reduction, agricultural N regulations should be strategically implemented according to farming characteristics and site-specific hydrogeological and geochemical conditions of the subsurface.

We examined whether variations in photosynthetic capacity are linked to variations in the environment and/or associated leaf traits for tropical moist forests (TMFs) in the Andes/western Amazon regions of Peru. We compared photosynthetic capacity (maximal rate of carboxylation of Rubisco (Vcmax), and the maximum rate of electron transport (Jmax)), leaf mass, nitrogen (N) and phosphorus (P) per unit leaf area (Ma, Na and Pa, respectively), and chlorophyll from 210 species at 18 field sites along a 3300-m elevation gradient. Western blots were used to quantify the abundance of the CO2-fixing enzyme Rubisco. Area- and N-based rates of photosynthetic capacity at 25°C were higher in upland than lowland TMFs, underpinned by greater investment of N in photosynthesis in high-elevation trees. Soil [P] and leaf Pa were key explanatory factors for models of area-based Vcmax and Jmax but did not account for variations in photosynthetic N-use efficiency. At any given Na and Pa, the fraction of N allocated to photosynthesis was higher in upland than lowland species. For a small subset of lowland TMF trees examined, a substantial fraction of Rubisco was inactive. These results highlight the importance of soil- and leaf-P in defining the photosynthetic capacity of TMFs, with variations in N allocation and Rubisco activation state further influencing photosynthetic rates and N-use efficiency of these critically important forests. © 2016 The Authors. New Phytologist © 2016 New Phytologist Trust

Imbalance-p paper contact with Marcos Fernández: m.fernandez@creaf.uab.cat The emission of isoprenoids (e.g. isoprene and monoterpenes) by plants plays an important defensive role against biotic and abiotic stresses. Little is known, however, about the functional traits linked to species-specific variability in the types and rates of isoprenoids emitted and about possible co-evolution of functional traits with isoprenoid emission type (isoprene emitter, monoterpene emitter or both). - We combined data for isoprene and monoterpene emission rates per unit dry mass with key functional traits (foliar nitrogen (N) and phosphorus (P) concentrations, and leaf mass per area) and climate for 113 plant species, covering the boreal, wet temperate, Mediterranean and tropical biomes. - Foliar N was positively correlated with isoprene emission, and foliar P was negatively correlated with both isoprene and monoterpene emission rate. Nonemitting plants generally had the highest nutrient concentrations, and those storing monoterpenes had the lowest concentrations. Our phylogenetic analyses found that the type of isoprenoid emission followed an adaptive, rather than a random model of evolution. - Evolution of isoprenoids may be linked to nutrient availability. Foliar N and P are good predictors of the type of isoprenoid emission and the rate at which monoterpenes, and to a lesser extent isoprene, are emitted.

International audience; Aims We tested here the effect of low nitrogen (N) availability and of UV-C radiations (a physical agent) on the susceptibility of Lactuca sativa L. towards Botrytis cinerea (BC 87) and Sclerotinia minor (SM). Methods We assessed first that a dose of UV-C radiation of 0.85 kJ. m−2 is effective in triggering a positive effect without entailing any negative effect on plants. We then tested the effect of this dose on 62-day-old lettuce plants grown at three N concentrations: 2 (low), 10 (normal) and 20 mM (high). Plants were inoculated 24 h after exposure to UV-C radiation and monitored for 4 days. Results Analysis of lesion areas showed that leaf susceptibility towards Botrytis cinerea (BC 87) and Sclerotinia minor (SM) substantially increased with the increasing N concentrations. Results also showed that the UV-C treatment can reduce MDA and H2O2 concentrations and lesion areas. Reduction of lesion areas reached 67 % on day 4 in the Sclerotinia minor (SM)-10 mM N trial and 52 % in the Botrytis cinerea (BC 87)-20 mM N trial. The UV-C treatment also resulted in an increase in the concentration of total phenolics in leaves which could be observed on days 1, 2 and 4 in Sclerotinia minor SM trial and days 1 and 2 in Botrytis cinerea BC 87 trial. Conclusions Taken together, our observations suggest that UV-C irradiation stimulate the capacity of lettuce plants to resist against Sclerotinia minor (SM) and Botrytis cinerea (BC 87) and that phenolic compounds may be involved in this resistance.

Highlight Maize genotypes producing fewer lateral roots extend them further into the soil, improving overall plant N acquisition., Suboptimal nitrogen (N) availability is a primary constraint for crop production in developing countries, while in developed countries, intensive N fertilization is a primary economic, energy, and environmental cost for crop production. We tested the hypothesis that under low-N conditions, maize (Zea mays) lines with few but long (FL) lateral roots would have greater axial root elongation, deeper rooting, and greater N acquisition than lines with many but short (MS) lateral roots. Maize recombinant inbred lines contrasting in lateral root number and length were grown with adequate and suboptimal N in greenhouse mesocosms and in the field in the USA and South Africa (SA). In low-N mesocosms, the FL phenotype had substantially reduced root respiration and greater rooting depth than the MS phenotype. In low-N fields in the USA and SA, the FL phenotype had greater rooting depth, shoot N content, leaf photosynthesis, and shoot biomass than the MS phenotype. The FL phenotype yielded 31.5% more than the MS phenotype under low N in the USA. Our results are consistent with the hypothesis that sparse but long lateral roots improve N capture from low-N soils. These results with maize probably pertain to other species. The FL lateral root phenotype merits consideration as a selection target for greater crop N efficiency.

Root litter is the dominant soil carbon and nutrient input in many ecosystems, yet few studies have considered how root decomposition is regulated at the landscape scale and how this is mediated by land‐use management practices. Large herbivores can potentially influence below‐ground decomposition through changes in soil microclimate (temperature and moisture) and changes in plant species composition (root traits). - To investigate such herbivore‐induced changes, we quantified annual root decomposition of upland grassland species in situ across a landscape‐scale livestock grazing experiment, in a common‐garden experiment and in laboratory microcosms evaluating the influence of key root traits on decomposition. - Livestock grazing increased soil temperatures, but this did not affect root decomposition. Grazing had no effect on soil moisture, but wetter soils retarded root decomposition. Species‐specific decomposition rates were similar across all grazing treatments, and species differences were maintained in the common‐garden experiment, suggesting an overriding importance of litter type. Supporting this, in microcosms, roots with lower specific root area (m2 g−1) or those with higher phosphorus concentrations decomposed faster. - Our results suggest that large herbivores alter below‐ground carbon and nitrogen dynamics more through their effects on plant species composition and associated root traits than through effects on the soil microclimate. © 2014 The Authors. New Phytologist © 2014 New Phytologist Trust This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2016 The Authors. New Phytologist © 2016 New Phytologist Trust Orchids are highly dependent on their mycorrhizal fungal partners for nutrient supply, especially during early developmental stages. In addition to organic carbon, nitrogen (N) is probably a major nutrient transferred to the plant because orchid tissues are highly N-enriched. We know almost nothing about the N form preferentially transferred to the plant or about the key molecular determinants required for N uptake and transfer. We identified, in the genome of the orchid mycorrhizal fungus Tulasnella calospora, two functional ammonium transporters and several amino acid transporters but found no evidence of a nitrate assimilation system, in agreement with the N preference of the free-living mycelium grown on different N sources. Differential expression in symbiosis of a repertoire of fungal and plant genes involved in the transport and metabolism of N compounds suggested that organic N may be the main form transferred to the orchid host and that ammonium is taken up by the intracellular fungus from the apoplatic symbiotic interface. This is the first study addressing the genetic determinants of N uptake and transport in orchid mycorrhizas, and provides a model for nutrient exchanges at the symbiotic interface, which may guide future experiments.

Nitrogen (N) and phosphorus (P) concentrations and N:P ratios critically influence periphyton productivity and nutrient cycling in aquatic ecosystems. In coastal wetlands, variations in hydrology and water source (fresh or marine) influence nutrient availability, but short-term effects of drying and rewetting and long-term effects of nutrient exposure on periphyton nutrient retention are uncertain. An outdoor microcosm experiment simulated short-term exposure to variation in drying-rewetting frequency on periphyton mat nutrient retention. A 13-year dataset from freshwater marshes of the Florida Everglades was examined for the effect of long-term proximity to different N and P sources on mat-forming periphyton nutrient standing stocks and stoichiometry. Field sites were selected from one drainage with shorter hydroperiod and higher connectivity to freshwater anthropogenic nutrient supplies (Taylor Slough/Panhandle, TS/Ph) and another drainage with longer hydroperiod and higher connectivity to marine nutrient supplies (Shark River Slough, SRS). Total P, but not total N, increased in periphyton mats exposed to both low and high drying-rewetting frequency with respect to the control mats in our experimental microcosm. In SRS, N:P ratios slightly decreased downstream due to marine nutrient supplies, while TS/Ph increased. Mats exposed to short-term drying-rewetting had higher nutrient retention, similar to nutrient standing stocks from long-term field data. Periphyton mat microbial communities may undergo community shifts upon drying-rewetting and chronic exposure to nutrient loads. Additional work on microbial species composition may further explain how periphyton communities interact with drying-rewetting dynamics to influence nutrient cycling and retention in wetlands.

While nitrate acquisition has been extensively studied, less information is available on transport systems of urea. Furthermore, the reciprocal influence of the two sources has not been clarified, so far. In this review, we will discuss recent developments on plant response to urea and nitrate nutrition. Experimental evidence suggests that, when urea and nitrate are available in the external solution, the induction of the uptake systems of each nitrogen (N) source is limited, while plant growth and N utilization is promoted. This physiological behavior might reflect cooperation among acquisition processes, where the activation of different N assimilatory pathways (cytosolic and plastidic pathways), allow a better control on the nutrient uptake. Based on physiological and molecular evidence, plants might increase (N) metabolism promoting a more efficient assimilation of taken-up nitrogen. The beneficial effect of urea and nitrate nutrition might contribute to develop new agronomical approaches to increase the (N) use efficiency in crops.

EFM Research Note 04/2007