Root ammonium transport efficiency as a determinant in forest colonization patterns: an hypothesis

Ratios of ammonium (NH₄⁺) to nitrate (NO₃^–) in soils are known to increase during forest succession. Using evidence from several previous studies, we hypothesize that a malfunction in NH₄⁺ transport at the membrane level might limit the persistence of early successional tree species in later seral stages. In those studies, ¹³N radiotracing was used to determine unidirectional fluxes and pool sizes of NH₄⁺ and NO₃^– in seedlings of the late-successional species white spruce ( Picea glauca ) and in the early successional species Douglas-fir ( Pseudotsuga menziesii var. glauca ) and trembling aspen ( Populus tremuloides ). At high external NH₄⁺, the two early successional species accumulated excessive NH₄⁺ in the root cytosol, and exhibited high-velocity, low-efficiency (15% to 22%), membrane fluxes of NH₄⁺. In sharp contrast, white spruce had low cytosolic NH₄⁺ accumulation, and lower-velocity but much higher-efficiency (65%), NH₄⁺ fluxes. Because these divergent responses parallel known differences in tolerance and toxicity to NH₄⁺ amongst these species, we propose that they constitute a significant driving force in forest succession, complementing the discrimination against NO₃^– documented in white spruce (Kronzucker et al. 1997).     

A comparison of NH4+ and NO3– net fluxes along roots of rice and maize

Net fluxes of NH₄⁺ and NO₃^– along adventitious roots of rice ( Oryza sativa L.) and the primary seminal root of maize ( Zea mays L.) were investigated under nonperturbing conditions using ion-selective microelectrodes. The roots of rice contained a layer of sclerenchymatous fibres on the external side of the cortex, whereas this structure was absent in maize. Net uptake of NH₄⁺ was faster than that of NO₃^– at 1 mm behind the apex of both rice and maize roots when these ions were supplied together, each at 0·1 mol m^–3. In rice, NH₄⁺ net uptake declined in the more basal regions, whereas NO₃^– net uptake increased to a maximum at 21 mm behind the apex and then it also declined. Similar patterns of net uptake were observed when NH₄⁺ or NO₃^– was the sole nitrogen source, although the rates of NO₃^– net uptake were faster in the absence of NH₄⁺. In contrast to rice, rates of NH₄⁺ and NO₃^– net uptake in the more basal regions of maize roots were similar to those near the root apex. Hence, the layer of sclerenchymatous fibres may have limited ion absorption in the older regions of rice roots.     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

An alternative explanation for the post-disturbance NO3– flush in some forest ecosystems

The appearance of soil NO₃^– after forest disturbance is commonly ascribed to a higher availability of NH₄⁺ to autotrophic nitrifiers, or to a reduction in available-C resulting in lower microbial assimilation of NO₃^–. Alternatively, it has been proposed that increasing NH₄⁺ pools following disturbance could increase net nitrification by reducing microbial assimilation of NO₃^–. Forest floor material was collected from shelterwood harvest plots which displayed both low available-C and low NH₄⁺ pools, and where previous experiments had suggested the prevalence of heterotrophic nitrification. Subsamples were amended with incremental rates of glucose-C or NH₄⁺, and gross NO₃^– transformation rates were measured by isotope dilution. Glucose-C additions had little effect on the net difference between gross NO₃^– production and consumption rates. On the other hand, NH₄⁺ additions caused gross NO₃^– consumption processes to decrease sharply, while gross NO₃^– production processes remained constant. The results suggest that NH₄⁺ can have an immediate positive effect on net nitrification rates by suppressing NO₃^– assimilation and uptake systems.     

Different effects of supplied ammonium on glutamine synthetase activity in mustard (Sinapis alba) and pine (Pinus sylvestris) seedlings

The activity of glutamine synthetase (GS) in mustard ( Sinapis alba L.) and Scots pine ( Pinus sylvestris L.) seedlings was used as an index to evaluate the capacity to cope with excessive ammonium supply. In these 2 species GS activity was differently affected by the application of nitrogen compounds (NH₄⁺ or NO₃⁻). Mustard seedlings older than 5 days showed a considerable increase in GS activity after NH₄⁺ or NO₃⁻ application. This response was independent of the energy flux, but GS activity in general was positively affected by light. Endogenous NH₄⁺ did not accumulate greatly after nitrogen supply. In contrast, seedlings of Scots pine accumulated NH₄⁺ in cotyledons and roots and showed no stimulation of GS activity after the application of ammonium. In addition, root growth was drastically reduced. Thus, the pine seedlings seem to have insufficient capacity to assimilate exogenously supplied ammonium. NO₃⁻, however, did not lead to any harmful effects.     

Fixation of molecular nitrogen by Methanosarcina barkeri

Abstract Methanosarcina barkeri cells were observed in ammonia-free anaerobic acetate enrichments for sulfate-reducing bacteria. The capacity of Methanosarcina to grow diazotrophically was proved with a pure culture in mineral media with methanol. The cell yields with N₂ or NH₄⁺ ions as nitrogen source were 2.2 g and 6.1 g dry weight, respectively, per mol of methanol. Growth experiments with ¹⁵N₂ revealed that 84% of the cell nitrogen was derived from N₂. Acetylene was highly toxic to Methanosarcina and only reduced at concentrations lower than 100 μmol dissolved per 1 of medium. Assimilation of N₂ and reduction of acetylene were inhibited by NH₄⁺ ions. The experiments show that N₂ fixation occurs not only in eubacteria but also in archaebacteria. The ecological significance of diazotrophic growth of Methanosarcina is discussed.     

Methane-dependent nitrate production by a microbial consortium enriched from a cultivated humisol

Abstract A consortium was enriched from a humisol incubated with 3.6 kPa CH₄ and NH₄⁺. This consortium oxidized NH₄⁺ to NO₂⁻ and NO₃⁻ (NO₃⁻/NO₂⁻ ratio about 20) with smaller amounts of N₂O. This oxidation stopped in the stationary phase after depletion of CH₄. CH₃OH or CO₂ did not support oxidation. Growth and resting cell experiments suggested that nitrification was associated with methanotrophic activity and that chemoautotrophic nitrifiers were absent.     

Photosynthate costs associated with the utilization of different nitrogen–forms: influence on the carbon balance of plants and shoot–root biomass partitioning

The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of nitrate (NO₃⁻), ammonium (NH₄⁺) or glutamine (Gln) for plant growth were estimated. Based on these estimates, the effects of these forms of nitrogen (N) on the carbon balance of plants and on shoot–root biomass allocation were evaluated. The results indicated that NO₃⁻ as an N source for plant growth is not substantially more expensive to utilize than either NH₄⁺ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is also suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and hence on shoot–root biomass allocation.     

Patterns of ammonium absorption and acetylene reduction during soybean developmental growth

Nodulated and unnodulated soybean plants ( Glycine max (L.) Merr. cv. Amsoy 71) were grown in nutrient solution either lacking or containing N. Nodulated plants, dependent on N₂ fixation, exhibited a generalized N-stress and were less vigorous than unnodulated plants dependent on inorganic N assimilation.
Starting at preflowering throughout mid pod-filling, NH₄⁺ absorption, expressed on the basis of root dry weight, was determined for intact nodulated and unnodulated plants in short-term kinetic experiments. Depletion of NH₄⁺ was measured from the liquid phase of a mist chamber. Maximum NH₄⁺ absorption occurred for both nodulated and unnodulated plants during vegetative growth. A pattern of progressive decrease in NH₄⁺ absorption was similar in nodulated and unnodulated plants, however. NH₄⁺ absorption was consistently greater in unnodulated plants. Simultaneous measurements of C₂H₂ reduction from the gas phase of the mist chamber revealed and 41-day-old plants, corresponding to late flowering and early pod-filling.