Insights into root growth, function, and mycorrhizal abundance from chemical and isotopic data across root orders

Background and aims

Detailed analyses of root chemistry by branching order may provide insights into root function, root lifespan and the abundance of root-associated mycorrhizal fungi in forest ecosystems.

Methods

We examined the nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C) and concentration (%N and %C) in the fine roots of an arbuscular mycorrhizal tree, Fraxinus mandshurica, and an ectomycorrhizal tree, Larix gmelinii, over depth, time, and across five root branching orders.

Results and conclusions

Larix δ¹⁵N increased by 2.3?‰ from 4th order to 1st order roots, reflecting the increased presence of ¹⁵N-enriched ECM fungi on the lower root orders. In contrast, arbuscular mycorrhizal Fraxinus only increased by 0.7?‰ from 4th order to 1st order roots, reflecting the smaller ¹⁵N enrichment and lower fungal mass on arbuscular mycorrhizal fine roots. Isotopic and anatomical mass balance calculations indicate that first, second, and third order roots in ectomycorrhizal Larix averaged 36 %, 23 %, and 8 % fungal tissue by mass, respectively. Using literature values of root production by root branching order, we estimate that about 25 % of fine root production in ECM species like Larix is actually of fungal sheaths. In contrast to %N, %C, and δ¹⁵N, δ¹³C changed minimally across depth, time, and branching order. The homogeneity of δ¹³C suggests root tissues are constructed from a large well-mixed reservoir of carbon, although compound specific δ¹³C data is needed to fully interpret these patterns. The measurements developed here are an important step towards explicitly including mycorrhizal production in forest ecosystem carbon budgets.     

Does the age of fine root carbon indicate the age of fine roots in boreal forests?

To test the reliability of the radiocarbon method for determining root age, we analyzed fine roots (originating from the years 1985?C1993) from ingrowth cores with known maximum root age (1?C6?years old). For this purpose, three Scots pine (Pinus sylvestris L.) stands were selected from boreal forests in Finland. We analyzed root ¹⁴C age by the radiocarbon method and compared it with the above-mentioned known maximum fine root age. In general, ages determined by the two methods (root ¹⁴C age and ingrowth core root maximum age) were in agreement with each other for roots of small diameter (<0.5?mm). By contrast, in most of the samples of fine roots of larger diameter (1.5?C2?mm), the ¹⁴C age of root samples of 1987?C1989 exceeded the ingrowth core root maximum age by 1?C10?years. This shows that these roots had received a large amount of older stored carbon from unknown sources in addition to atmospheric CO₂ directly from photosynthesis. We conclude that the ¹⁴C signature of fine roots, especially those of larger diameter, may not always be indicative of root age, and that further studies are needed concerning the extent of possible root uptake of older carbon and its residence time in roots.     

Ferric Iron Reduction by Bacteria Associated with the Roots of Freshwater and Marine Macrophytes

In vitro assays of washed, excised roots revealed maximum potential ferric iron reduction rates of >100 μmol g (dry weight)⁻¹ day⁻¹ for three freshwater macrophytes and rates between 15 and 83 μmol (dry weight)⁻¹ day⁻¹ for two marine species. The rates varied with root morphology but not consistently (fine root activity exceeded smooth root activity in some but not all cases). Sodium molybdate added at final concentrations of 0.2 to 20 mM did not inhibit iron reduction by roots of marine macrophytes (Spartina alterniflora and Zostera marina). Roots of a freshwater macrophyte, Sparganium eurycarpum, that were incubated with an analog of humic acid precursors, anthroquinone disulfate (AQDS), reduced freshly precipitated iron oxyhydroxide contained in dialysis bags that excluded solutes with molecular weights of >1,000; no reduction occurred in the absence of AQDS. Bacterial enrichment cultures and isolates from freshwater and marine roots used a variety of carbon and energy sources (e.g., acetate, ethanol, succinate, toluene, and yeast extract) and ferric oxyhydroxide, ferric citrate, uranate, and AQDS as terminal electron acceptors. The temperature optima for a freshwater isolate and a marine isolate were equivalent (approximately 32°C). However, iron reduction by the freshwater isolate decreased with increasing salinity, while reduction by the marine isolate displayed a relatively broad optimum salinity between 20 and 35 ppt. Our results suggest that by participating in an active iron cycle and perhaps by reducing humic acids, iron reducers in the rhizoplane of aquatic macrophytes limit organic availability to other heterotrophs (including methanogens) in the rhizosphere and bulk sediments.     

Modifications of the carbon and nitrogen allocations in the plant (Triticum aestivum L.) soil system in response to increased atmospheric CO2 concentration

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO₂ concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ¹⁴CO₂ concentration of 350 or 700 L L^–1 at two levels of soil nitrogen fertilization. Doubling CO₂ availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO₂ enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO₂ increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO₂ availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO₂ enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-¹⁴C came from respiration, leaving only 10 to 15% in the soil as organic ¹⁴C. However, when total root-derived ¹⁴C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO₂, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO₂ concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO₂ increase.Abbreviations N soil fertilization without nitrogen - N soil fertilization with nitrogen     

Root respiration associated with nitrate assimilation by cowpea

Nitrate uptake by roots of cowpea (Vigna unguiculata) was measured using ¹⁵NO₃⁻, and the energy cost to the root was estimated by respirometry. Roots of 8-day-old cowpea seedlings respired 0.6 to 0.8 milligram CO₂ per plant per hour for growth and maintenance. Adding 10 millimolar NO₃⁻ to the root medium increased respiration by 20 to 30% during the following 6 hours. This increase was not observed if the shoots were in the dark. Removal of NO₃⁻ from the root medium slowed the increase of root respiration. The ratios of additional respiration to the total nitrogen uptake and reduced nitrogen content in roots were 0.4 gram C per gram N and 2.3 grams C per gram N, respectively. The latter value is close to theoretical estimates of nitrate assimilation, and is similar to estimates of 1 to 4 grams C per gram N for the respiratory cost of symbiotic N₂ fixation.     

Fine root dynamic characteristics and effect on plantation’s carbon sequestration of three Salix shrub plantations in Tibetan Plateau alpine sandy land

Desertification land in Gonghe Basin of Tibetan Plateau, China accounts for 91.9% of the total land area. Vegetation restoration and reconstruction with desert shrubs in degraded ecosystem are effective ways to prevent and control desertification. However, the evaluation studies of fine root dynamic characteristics of desert shrubs and their contribution to carbon sequestration of plantation are limited. To gain a better understanding of vegetation restoration, the vertical distribution of fine root biomass, fine root decomposition, fine root turnover was investigated, as well as their coupling effect on carbon sequestration of plantation in three desert vegetation. The results estimated that the total decomposition time of fine roots of Salix cheilophila (S. cheilophila), Salix psammophila (S. psammophila), and Salix microstachya (S. microstachya) are 39.00, 27.99 and 35.95 years. Biomass carbon density for three Salix plantations ranged from 1.42 to 2.39 t/hm², which showed that three Salix plantations in alpine sandy land are an important carbon pool. In addition, fine root biomass carbon density for the three shrub plantations varied significantly. Fine root biomass carbon density for S. psammophila reached the largest among the three plantations, which was 1.48 t/hm², accounting for the ratio of 62% of the plantation total biomass carbon density. The results indicated that the root system of S. psammophila, especially the fine roots, was very developed, which was conducive to soil water transportation and carbon sequestration. Therefore, S. psammophila might be a better species for carbon sequestration of plantation in alpine sandy areas. The carbon input from the fine roots of the three shrub plantations through decomposition and turnover into the plantations accounts for 11.5% to 15.5% of total carbon sequestration of plantations. Therefore, the fine roots dynamics must be considered for long‐term carbon pool estimations in three Salix plantations, otherwise the total carbon sequestration of plantations would be underestimated.     

Response of long-, medium- and short-term processes of the carbon budget to overgrazing-induced crusts in the Tibetan Plateau

The Kobresia pastures of the Tibetan Plateau represent the world’s largest alpine grassland ecosystem. These pastures remained stable during the last millennia of nomadic animal husbandry. However, strongly increased herds’ density has promoted overgrazing, with unclear consequences for vegetation and soils, particularly for cycles of carbon (C), nutrients and water. Vegetation-free patches of dead root-mat covered by blue-green algae and crustose lichens (crusts) are common in overgrazed Kobresia pastures, but their effect on C turnover processes is completely unknown. We tested the hypothesis that the crusts strongly affect the C cycle by examining: (i) the long-term C stock measured as soil organic matter content; (ii) medium-term C stock as dead roots; (iii) recent C fluxes analyzed as living roots and CO₂ efflux; and (iv) fast decomposition of root exudates. Up to 7.5 times less aboveground and 1.9 times less belowground living biomass were found in crust patches, reflecting a much smaller C input to soil as compared with the non-crust Kobresia patches. A lower C input initially changed the long-term C stock under crusts in the upper root-mat horizon. Linear regression between living roots and CO₂ efflux showed that roots contributed 23% to total CO₂ under non-crust areas (mean July–August 5.4 g C m^?2 day^?1) and 18% under crusts (5.1 g C m^?2 day^?1). To identify differences in the fast turnover processes in soil, we added ¹³C labeled glucose, glycine and acetic acid, representing the three main groups of root exudates. The decomposition rates of glucose (0.7 day^?1), glycine (1.5 day^?1) and acetic acid (1.2 day^?1) did not differ under crusts and non-crusts. More ¹³C, however, remained in soil under crusts, reflecting less complete decomposition of exudates and less root uptake. This shows that the crust patches decrease the rates of medium-term C turnover in response to the much lower C input. Very high ¹³C amounts recovered in plants from non-crust areas as well as the two times lower uptake by roots under crusts indicate that very dense roots are efficient competitors with microorganisms for soluble organics. In conclusion, the altered C cycle in the overgrazing-induced crustose lichens and blue-green algae crusts is connected with strongly decreased C input and reduced medium-term C turnover.     

Different Bacterial Populations Associated with the Roots and Rhizosphere of Rice Incorporate Plant-Derived Carbon

Microorganisms associated with the roots of plants have an important function in plant growth and in soil carbon sequestration. Rice cultivation is the second largest anthropogenic source of atmospheric CH₄, which is a significant greenhouse gas. Up to 60% of fixed carbon formed by photosynthesis in plants is transported below ground, much of it as root exudates that are consumed by microorganisms. A stable isotope probing (SIP) approach was used to identify microorganisms using plant carbon in association with the roots and rhizosphere of rice plants. Rice plants grown in Italian paddy soil were labeled with ¹³CO₂ for 10 days. RNA was extracted from root material and rhizosphere soil and subjected to cesium gradient centrifugation followed by 16S rRNA amplicon pyrosequencing to identify microorganisms enriched with ¹³C. Thirty operational taxonomic units (OTUs) were labeled and mostly corresponded to Proteobacteria (13 OTUs) and Verrucomicrobia (8 OTUs). These OTUs were affiliated with the Alphaproteobacteria, Betaproteobacteria, and Deltaproteobacteria classes of Proteobacteria and the “Spartobacteria” and Opitutae classes of Verrucomicrobia. In general, different bacterial groups were labeled in the root and rhizosphere, reflecting different physicochemical characteristics of these locations. The labeled OTUs in the root compartment corresponded to a greater proportion of the 16S rRNA sequences (∼20%) than did those in the rhizosphere (∼4%), indicating that a proportion of the active microbial community on the roots greater than that in the rhizosphere incorporated plant-derived carbon within the time frame of the experiment.     

Influence of Helminthosporium maydis, Race T, Toxin on Potassium Uptake in Maize Roots

The effect of a toxin extract of Helminthosporium maydis, race T on K⁺ (⁸⁶Rb) uptake by excised root segments of normal (N) and Texas cytoplasmic male-sterile (T) versions of corn inbred W64A was investigated. The uptake of K⁺ was inhibited in both N and T roots by the toxin. This was true for both basal (freshly excised) and augmented (pretreated with aeration) K⁺ uptake. Augmented uptake was more toxin-sensitive than basal uptake (irrespective of cytoplasm type), and the augmented uptake in T roots was seven to eight times more toxin-sensitive than in N roots.     

Compartmentalization and ultrastructural alterations induced by chromium in aquatic macrophytes

The aim of the present study was to identify the sites of accumulation of Cr in the species of macrophytes that are abundant in the Cachoeira river, namely, Alternanthera philoxeroides, Borreria scabiosoides, Polygonum ferrugineum and Eichhornia crassipes. Plants were grown in nutritive solution supplemented with 0.25 and 50 mg l^?1 of CrCl₃·6H₂O. Samples of plant tissues were digested with HNO₃/HCl in a closed-vessel microwave system and the concentrations of Cr determined using inductively-coupled plasma mass spectrometry (ICP-MS). The ultrastructure of root, stem and leaf tissue was examined using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) in order to determine the sites of accumulation of Cr and to detect possible alterations in cell organelles induced by the presence of the metal. Chromium accumulated principally in the roots of the four macrophytes (8.6?C30 mg kg^?1 dw), with much lower concentrations present in the stems and leaves (3.8?C8.6 and 0.01?C9.0 mg kg^?1 dw, respectively). Within root tissue, Cr was present mainly in the vacuoles of parenchyma cells and cell walls of xylem and parenchyma. Alterations in the shape of the chloroplasts and nuclei were detected in A. philoxeroides and B. scabiosoides, suggesting a possible application of these aquatic plants as biomarkers from Cr contamination.     

Mineralisation of carbon and plant uptake of phosphorus from microbially-derived organic matter in response to 19 years simulated nitrogen deposition

Here we test the hypotheses that 19 years of simulated pollutant N deposition increases both losses of carbon (C) and the ability of plants to access P from organic material in upland heathland. The grass, Dactylis glomerata, and the dwarf shrub, Calluna vulgaris, were grown in soil containing microbial-derived organic matter labelled with ¹⁴C and ³³P. We found that both soil and root-surface phosphatase activity increased significantly in response to N deposition. We also found a significant positive relationship between root-surface phosphatase activity and ³³P uptake for Calluna, but a negative relationship for Dactylis. Efflux of ¹⁴C from the microbial-derived organic matter was strongly dependent on an interaction among plant presence, plant species and N deposition. Our results show that mineralisation of C and P, and subsequent plant uptake of P from organic sources is decoupled. In our experimental conditions, stimulation of P turnover coupled with subsequent plant uptake through up-regulation of root phosphatases is little affected by N addition. However, our data indicate that root-surface phosphatases are likely to be more important for uptake of P derived from organic sources for Calluna than for Dactylis.     

Desiccation of sediments affects assimilate transport within aquatic plants and carbon transfer to microorganisms

• With the projected increase in drought duration and intensity in future, small water bodies, and especially the terrestrial–aquatic interfaces, will be subjected to longer dry periods with desiccation of the sediment. Drought effects on the plant–sediment microorganism carbon continuum may disrupt the tight linkage between plants and microbes which governs sediment carbon and nutrient cycling, thus having a potential negative impact on carbon sequestration of small freshwater ecosystems. However, research on drought effects on the plant–sediment carbon transfer in aquatic ecosystems is scarce. We therefore exposed two emergent aquatic macrophytes, Phragmites australis and Typha latifolia, to a month‐long summer drought in a mesocosm experiment.
• We followed the fate of carbon from leaves to sediment microbial communities with ¹³CO₂ pulse labelling and microbial phospholipid‐derived fatty acid (PLFA) analysis.
• We found that drought reduced the total amount of carbon allocated to stem tissues but did not delay the transport. We also observed an increase in accumulation of ¹³C‐labelled sugars in roots and found a reduced incorporation of ¹³C into the PLFAs of sediment microorganisms.
• Drought induced a switch in plant carbon allocation priorities, where stems received less new assimilates leading to reduced starch reserves whilst roots were prioritised with new assimilates, suggesting their use for osmoregulation. There were indications that the reduced carbon transfer from roots to microorganisms was due to the reduction of microbial activity via direct drought effects rather than to a decrease in root exudation or exudate availability.

     

Experimental study of solute transport and extraction by a single root in soil

The fate of ¹⁴C-2,4,6-trinitrotoluene ([U-¹⁴C]TNT) in soil/plant systems was studied using onion (Allium cepa L.) plants with only a single root. It was found that the single roots grew exponentially and that the rate of water uptake of the onion plants increased exponentially, as well. The concentration of [U-¹⁴C] in the roots at first increased and then appeared to reach a steady state, while the [U-¹⁴C] concentration in the leaves was found to increase linearly with time. The [U-¹⁴C] concentration in the rhizosphere increased gradually, while in the bulk soil it decreased slowly. The accumulation of [U-¹⁴C] in the rhizosphere is likely to difference between movement into the rhizosphere (through advective mass flow of soil water by root uptake) and its uptake into the roots. The distribution of ¹⁴C in the soil/plant system was found to be 60–85% in the soil solid phase, 7–11% in the soil liquid phase, <1% in the soil air phase, <1% in the root compartment, and <0.01% in the leaf compartment. The maximum RCF (root concentration factor) value for TNT and its derivates was found to be about 20, and the maximum TSCF (transpiration stream concentration factor) was 0.18. These values can be changed by a variety of factors in soil-plant systems     

Copper Uptake and Its Effect on Metal Distribution in Root Growth Zones of Commelina communis Revealed by SRXRF

To explore the copper uptake mechanisms by the Cu-tolerant plant Commelina communis, the contents of Cu and other metals (including Fe, Zn, and Mn) in roots were detected using atomic absorption spectrometer under transporter inhibitors, partial element deficiency, or Cu excess treatments, while distribution characters of Cu and other metals in root growth zones were investigated by synchrotron radiation X-ray fluorescence spectroscopy (SRXRF). Cu uptake was inhibited by the uncoupler DNP and P-type ATPase inhibitor Na₃VO₄, not by the Ca²⁺ ion channel inhibitor LaCl₃, suggesting that Cu could probably be assimilated actively by root and be related with P-type ATPase, but not through Ca²⁺ ion channel. Fe or Zn deficiency could enhance Cu uptake, while 100 μM Cu inhibited Fe, Zn, and Mn accumulation in roots significantly. Metal distribution under 100 μM Cu treatment was investigated by SRXRF. High level of Cu was found in the root meristem, and higher Cu concentrations were observed in the vascular cylinder than those in the endodermis, further demonstrating the initiative Cu transport in the root of C. communis. Under excess Cu stress, most Fe was located in the epidermis, and Fe concentrations in the endodermis were higher than those in the vascular cylinder, suggesting Cu and Fe competition not only in the epidermal cells but also for the intercellular and intracellular transport in roots. Zn was present in the meristem and the vascular cylinder similar to Cu. Cu and Zn showed a similar pattern. Mn behaves as Zn does, but not like Fe.     

Characterization of phosphate absorption in maize root cortex segments

Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH₂PO₄ in 0.2 mM CaSCO₄ at the average rate of 34.3 ±3.2 μg P_i g^?1 (fr. m.) h^?1,i.e. 0.35± 0.02 μmol P_i g^?1 (fr. m.) h^?1. Phosphate uptake considerably increases after a certain period of “augmentation”,i.e. washing in aerated 0.2 mM CaSO₄. This increase is completely blocked by the presence of 10 μg ml^?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K_m and V_max values were found: K_m[mM] : 0.37 - 3.82 - 27.67 V_max[μg P_i g^?1 (fr. m.) h^?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH₂PO₄, while uptake from 5 and 10 mM KH₂PO₄ was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10^?3 M NEM by 81.6%, 10⁴ M NEM by 42% and 10^?4 M PCMB by 42%.     

Assimilation dynamics of soil carbon and nitrogen by wheat roots and Collembola

It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled (¹³C and ¹⁵N) green manure by analysing for excess ¹³C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess ¹³C and ¹⁵N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna. The experimental setup consisted of soil microcosm with wheat and dual-labelled green manure additions. Plant growth, plant N and recoveries of ¹³C and ¹⁵N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake of green manure derived ¹³C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided the main C source for collembolans (54−79%). We conclude that there is no solid support for claiming that roots assimilated green manure derived C due to very small or no recoveries of excess ¹³C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition. However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was released throughout the incubation. Responsible Editor: A. C. Borstlap.     

Uptake and Assimilation of NO(3) and NH(4) by Nitrogen-Deficient Perennial Ryegrass Turf

Assimilation of NO₃⁻ and NH₄⁺ by perennial ryegrass (Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH₄⁺, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO₃⁻ had been reduced, whereas 97% of the NH₄⁺ had been assimilated. During the early stages (0 to 8 hours) of NO₃⁻ uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO₃⁻, NH₄⁺ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO₃⁻ uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.     

Nitrogen utilization by Typha latifolia L. as affected by temperature and rate of nitrogen application

Greenhouse and growth chamber studies were conducted to evaluate growth and N utilization by Typha latifolia L. in flooded organic soil under varying temperatures and rates of N additions. Elevation of temperature from 10 to 25°C increased shoot biomass yields by 275%. Root biomass yields were lowest at 10°C and increased linearly as a function of temperature. Shoot/root ratios were low (0.72–0.82) at lower temperatures (10–15°C) and ratios increased by about three times at higher temperatures (20–30°C). Biomass yields were increased by addition of N fertilizers, while the shoot/root ratios were directly related to plant-available N present in the soil.Fertilizer ¹⁵N uptake (expressed as % of applied N) by the whole plant was 5.3% at 10°C, 37.5% at 20°C and at 30°C decreased to 20.8%. Fertilizer N accumulation in shoots was 2.1–29.8% of applied N, while roots accumulated 3.2–7.7%. Under greenhouse conditions, N uptake by T. latifolia was found to increase with increased rate of N application. Fertilizer N uptake by both shoots and roots was in the range of 61–77%. Plants cultured in growth chambers were affected by low light conditions resulting in poor growth and low fertilizer ¹⁵N uptake, as compared to plants grown under greenhouse conditions. Added fertilizer N was the major source of N during the early part of the growing season, while soil organic N was the major and perhaps the sole source of N during the latter part of the growing season.     

Links between Belowground and Aboveground Resource-Related Traits Reveal Species Growth Strategies that Promote Invasive Advantages

Belowground processes are rarely considered in comparison studies of native verses invasive species. We examined relationships between belowground fine root production and lifespan, leaf phenology, and seasonal nitrogen dynamics of Lonicera japonica (non-native) versus L. sempervirens (native) and Frangula alnus (non-native) versus Rhamnus alnifolia (native), over time. First and second order fine roots were monitored from 2010 to 2012 using minirhizotron technology and rhizotron windows. ¹⁵N uptake of fine roots was measured across spring and fall seasons. Significant differences in fine root production across seasons were seen between Lonicera species, but not between Frangula and Rhamnus, with both groups having notable asynchrony in regards to the timing of leaf production. Root order and the number of root neighbors at the time of root death were the strongest predictors of root lifespan of both species pairs. Seasonal ¹⁵N uptake was higher in spring than in the fall, which did not support the need for higher root activity to correspond with extended leaf phenology. We found higher spring ¹⁵N uptake in non-native L. japonica compared to native L. sempervirens, although there was no difference in ¹⁵N uptake between Frangula and Rhamnus species. Our findings indicate the potential for fast-growing non-native Lonicera japonica and Frangula alnus to outcompete native counterparts through differences in biomass allocation, root turnover, and nitrogen uptake, however evidence that this is a general strategy of invader dominance is limited.     

Occurrence and coexistence of arbuscular mycorrhizal fungi and dark septate fungi in aquatic macrophytes in a tropical river-floodplain system

In this study, carried out in four water bodies in the Upper Paraná River floodplain, we assessed the occurrence of root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate fungi (DSF), as well as the AMF species richness associated with 24 species of aquatic macrophytes belonging to different life forms. AMF were found in nine species of macrophytes and DSF in 16 species among the 24 investigated. AM colonization occurred mainly in eudicotiledons (five of the six species evaluated) and the Paris morphology was the most common type. Co-occurrence of AMF and DSF was observed in seven species of macrophytes (Commelinaceae sp. 1, Limnobium laevigatum (H.B.K. ex Willd) Heine, Hygrophila cf. costata, Myriophyllum brasiliense (Camb), Polygonum acuminatum Kunth, P. ferrugineum Wedd and P. stelligerum Cham). Four species of macrophytes (Pistia stratiotes L., Eichhornia crassipes (Mart.) Solms, Egeria najas Planch and Nymphaea amazonum Mart. & Zucc) were not colonized by any type of fungi. In total, 27 morphotypes of AMF were recorded, and spores occurred both in the rhizosphere of macrophytes whose roots were internally colonized by AMF and in non-colonized macrophytes. Acaulospora delicata, Acaulospora aff. laevis, Acaulospora longula, Glomus lamellosum, Glomus luteum and NID 1 (a non-identified species) were the most frequent species. Samples collected close to the roots of N. amazonum had the highest AMF richness (20 species), but this plant was not colonized by fungi. A species richness curve indicated that more root-associated fungi than reported here are likely present in this floodplain.