Supply Of NO3 Research Articles

Plastic Allometry in Coarse Root Biomass of Mature Hybrid Poplar Plantations

In this study, we sampled coarse root biomass of three poplar clones in four 13-year-old hybrid poplar (Populus spp.) plantations, with the objective of developing an allometric relationship between diameter at breast height (DBH) and coarse root biomass. A second objective was to test significance of site, clone and ×site clone interaction effects on coarse root biomass, using analysis of covariance (ANCOVA), with DBH as a covariate. Across the four sites, the general allometric relationship between DBH and coarse root biomass was highly significant (R 2 = 0.78, p < 0.001). However, given the high significance of the site effect (p < 0.001) in the ANCOVA and the differences in data distribution between sites, two allometric relationships were developed based on the fertility class of sites (high and moderate). Environment-specific allometric relationships developed for two site fertility classes had a better fit (R 2 = 0.81-0.90, p < 0.001). ANCOVA, with DBH as a covariate and site fertility class as a main effect, also showed that both of these variables significantly affected (p < 0.001) coarse root biomass allocation. Environment-specific equations showed that higher coarse root biomass was allocated in harsher environments for a given DBH, probably to improve access to growth limiting soil nutrients or to build-up larger storage sites for amino acids and non-structural carbohydrates. Consequently, poplar coarse root biomass growth is both driven by ontogeny (size) and environment, reflecting the plasticity of the root system of mature poplars. Implications of using general vs. environment-specific equations in estimating stand-level root biomass and shoot to root ratios are discussed. Shoot to root ratios calculated using environment-specific equations were more strongly correlated to key environmental variables than ratios calculated using the general equation, with soil NO3 supply rate being the strongest predictor of the ratio (R 2 = 0.90, p < 0.001).

Does low stomatal conductance or photosynthetic capacity enhance growth at elevated CO2 in Arabidopsis?

The objective of this study was to determine if low stomatal conductance (g) increases growth, nitrate (NO3 (-)) assimilation, and nitrogen (N) utilization at elevated CO2 concentration. Four Arabidopsis (Arabidopsis thaliana) near isogenic lines (NILs) differing in g were grown at ambient and elevated CO2 concentration under low and high NO3 (-) supply as the sole source of N. Although g varied by 32% among NILs at elevated CO2, leaf intercellular CO2 concentration varied by only 4% and genotype had no effect on shoot NO3 (-) concentration in any treatment. Low-g NILs showed the greatest CO2 growth increase under N limitation but had the lowest CO2 growth enhancement under N-sufficient conditions. NILs with the highest and lowest g had similar rates of shoot NO3 (-) assimilation following N deprivation at elevated CO2 concentration. After 5 d of N deprivation, the lowest g NIL had 27% lower maximum carboxylation rate and 23% lower photosynthetic electron transport compared with the highest g NIL. These results suggest that increased growth of low-g NILs under N limitation most likely resulted from more conservative N investment in photosynthetic biochemistry rather than from low g.

The effects of NO3(-) supply on Mazzaella laminarioides (Rhodophyta, Gigartinales) from southern Chile.

The effects of nitrate supply on growth, pigments, mycosporine-like amino acids (MAAs), C:N ratios and carrageenan yield were investigated in Mazzaella laminarioides cultivated under solar radiation. This species is economically important in southern Chile where an increase of nitrogen in coastal waters is expected as a consequence of salmon aquaculture activity. Apical segments were cultivated in enriched seawater with five different NO3(-) concentrations (0, 0.09, 0.18, 0.38 and 0.75 mm) during 18 days. Although phycoerythrin and phycocyanin content, as well as C:N ratios, were reduced in the control treatment (without NO3(-) supply), when compared to NO3(-) treatments, total MAA concentration, carrageenan yield and growth rates were similar in all tested conditions. Nevertheless, during the experiment, an important synthesis of mycosporine-glycine took place in a nitrate concentration-dependent manner, with accumulation being saturated around 0.18 mm of nitrate. These results indicate that exposure to high NO3(-) concentration of more than 100 times the values observed in the nature did not impair the photoprotection system, as determined by MAAs, nor did it have a deleterious effect on growth or carrageenan yield of M. laminarioides, a late successional species from Chile.

Nitric oxide accumulation and glycinebetaine metabolism in two osmotically stressed maize cultivars supplied with different nitrogen forms

Hydroponic experiments were conducted to investigate the effects of nitrogen forms on nitric oxide accumulation and glycinebetaine (GB) metabolism under osmotic stress (OS) simulated by the addition of polyethylene glycol (PEG-6000) to two maize (Zea mays L.) cultivars Zhengdan 958 (ZD958; drought-tolerant) and Jundan 20 (JD20; drought-sensitive). OS led to the accumulation of NO, GB, and choline, as well as to an increase of betaine aldehyde dehydrogenase (BADH) activity in both the cultivars. The responses of both the maize cultivars to OS were significantly higher under nitrate nutrition than under the mixed supply of ammonium and NO3 − or sole NH4 +. Cultivar ZD958 was superior to JD20 under the osmotic stress. Furthermore, compared with sole NH4 +, sole NO3 − and the mixed supply of NH4 + and NO3 − increased all the parameters in cv. ZD958 but only the GB and choline content in JD20. Therefore, it is concluded that ZD958 showed a higher accumulation of NO and enhanced GB metabolism under OS than JD20 regardless of N form.

Hot moments of N2O transformation and emission in tropical soils from the Pantanal and the Amazon (Brazil)

Tropical wetland soils emit large amounts of nitrous oxide (N2O), especially following wetting of drained soil. We investigated seasonally drained wetland soils from the Pantanal and the Amazon, both with a natural high nitrate content and low pH. Here we report the effect of wetting on the production, emission and consumption of N2O on these soils. Intact soil cores were wetted to simulate natural water logging events, and microsensor measurements were used to i) characterize the vertical microscale distribution of O2 and N2O, ii) monitor the accumulation of N2O in the anoxic soil volume, and iii) quantify the N2O efflux out of the soil. Flux chamber measurements of N2O emission confirmed the effluxes calculated from microsensor measurements.The N2O concentration dynamics in the soil cores were characterized by three distinct phases: 1) an initial slow N2O production, 2) a higher N2O production ending abruptly when the supply of NO3− and NO2− (NOx−) was exhausted, and 3) a final phase where the accumulated N2O was reduced to N2. This evolution of the N2O pool in an intact soil core could be accurately simulated by a simple diffusion-reaction model with the presence of O2 and NOx− as determining factors.Approximately one third of the initial NO3− present in the soil was lost as N2O or N2. As the soil was depleted for NO3− by the end of the experiment we suggest that dissimilatory nitrate reduction to ammonia (DNRA) was responsible for reducing the remaining NO3−.

Summer nitrogenous nutrient transport and its fate in the Taiwan Strait: A coupled physical‐biological modeling approach

In order to understand the fate of nutrients in the Taiwan Strait during summer, we built a coupled physical‐biological numerical ocean model, which can capture the basic hydrographic and biological features within the strait. The nutrient that we chose to model is dissolved inorganic nitrogen (DIN). The model includes individual reservoirs for nitrate (NO3) and ammonium (NH4). Both the observational evidence and model results show that NO3 in the strait originates primarily from the upwelling subsurface water in the northern South China Sea (SCS) that enters the strait via the eastern and western routes separated by the Taiwan Bank. The coupled physical and biological effects on the NO3 transport at these two routes are highlighted in the study. For the western route, the shallow topography and the coastal upwelling intensify the biological uptake of NO3 in the whole water column. Consequently, the nitrogenous contribution by this route is mainly in form of the particulate organic nitrogen (PON). In contrast, NO3 is transported conservatively below the nitricline at the deep eastern route, contributing the whole NO3 supply in the TWS. The model estimates the fluxes of DIN and PON into the TWS, from the northern SCS, are 1.8 and 4 kmol s−1, respectively. Over half (∼1 kmol s−1) of the DIN is synthesized into PON by the phytoplankton in the strait. Overall, this study estimates the physical and biological effects on the nutrient transport in the TWS during summer.

Nutrient dynamics in the western Canadian Arctic. I. New production in spring inferred from nutrient draw-down in the Cape Bathurst Polynya

Ice-retreat from the Cape Bathurst Polynya occurred in late May and early June 2004, and was quickly followed by a decrease in dissolved nutrient concentration at the sea sur- face. Concentration plots of NO3 � versus PO4 3� , and Si(OH)4 versus NO3 � showed the ratios of nutrient uptake in the surface layer were in proportions expected for diatom growth (N:P 13.1:1; Si:N 1.8:1) and that water-column NO3 � was depleted before PO4 3� and Si(OH)4. The temporal changes in integrated nitrate, phosphate and silicate concentrations were well described by logis- tic models that showed maximum consumption rates of 11.8, 0.82, and 17.8 mmol m �2 d �1 , respec- tively, and a total seasonal draw-down of 210 ± 19 mmol NO3 � m �2 . If we include estimates of NO3 � supply by advection, nitrification and freshwater dilution, then the amount of NO3 � consumed could be 25�33% higher. Uptake of NO3 � above the 1% isolume was balanced by an equivalent (94%) increase in particulate N over a 15 d period beginning at ice break-up. Thus, the amount of particulate spring new production associated with NO3 � disappearance was estimated to be 16.1 ± 1.5 g C m �2 .

High supply of NO3 − mitigates salinity effects through an enhancement in the efficiency of photosystem II and CO2 assimilation in Jatropha curcas plants

This study was performed to determine if a high supply of N-NO3 (-) is capable of mitigating negative salinity effects on photosynthesis and growth through the stimulation of nitrate assimilation, which could act as an sink from photosynthetic electron transport chain and restrict the over reduction in thylakoid membrane in Jatropha curcas leaves. The experiment was arranged in a factorial design with two nitrate concentrations (1 and 10 mM) and two NaCl levels (0 and 100 mM). Salt-stressed plants supplied with high NO3 (-) demonstrated a higher nitrate uptake rate, nitrate reductase activity and soluble-protein content when compared with plants that presented low nitrate uptake. High nitrate assimilation was associated with higher leaf growth, CO2 assimilation and lower membrane damage in salt-stressed plants. The superior performance of salt-stressed plants grown with high NO3 (-) was indicated by a higher effective quantum yield of PSII and electron transport rate and lower energy excess at the PSII level and non-photochemical quenching. Interestingly, a high NO3 (-) level in the absence of NaCl did not alter the leaf growth, photochemical activity and gas exchange parameters when compared with plants supplied with low nitrate. The proline and glycinebetaine contents were similarly increased in both low- and high-NO3 (-) salt-stressed plants. Our data suggest that the favorable effects induced by high nitrate supply were possibly associated with stimulation in the nitrate assimilatory pathway. This process might have acted as a sink of electrons from the thylakoid membranes minimizing photo-damage and stimulating CO2 assimilation under salinity in J. curcas.

Effect of homogeneous and heterogeneous supply of nitrate and ammonium on nitrogen uptake and distribution in tomato seedlings

To evaluate the effects of homogeneous or heterogeneous supply of different sources of nitrogen (N) on N uptake and distribution in tomato seedlings, hydroponic experiments with 15N tracers were conducted. Under homogeneous condition, NO3 − and NH4 + were supplied as the ratio of 100:0 (100-0NA), 75:25 (75-25 NA), and 50:50 (50-50NA) for all roots of tomato seedlings. Heterogeneous supply of N (localized N supply) was conducted by a split-root experiment with three treatments: both half of the roots supplied with NO3 − (N|N, as the control), a half of the roots supplied with NH4 + (N|A), and a half of the roots supplied with NO3 − and NH4 + at a ratio of 75:25 (N|AN). The results showed that (1) homogeneous supply of NO3 − and NH4 + at the ratio of 75:25 led to the highest root and shoot biomass and a higher total N uptake. 15NO3 − absorption was not affected by decreased NO3 − percentage from 100 to 75 %, and increase of NH4 + ratio from 25 to 50 % showed no effect on 15NH4 + absorption. Similar amount of 15NH4 +-N was absorbed by roots under N|A and N|AN treatments in the localized supply. Spatially separated supply of NO3 − and NH4 + led to a more than 3 times higher 15N accumulation in NO3 −-fed root than NH4 +-fed root in N|A. Half-roots supplied by mixed NO3 − and NH4 + at the ratio of 75:25 in N|AN resulted in higher 15N accumulation in the shoot. (2) In plant tissues, the amount of 15N deriving from NO3 − in root, stem, and leaves was all decreased by 50 % NH4 + in the whole-root system. In N|A treatment in the split-root system, however, the amount of leaf 15N deriving from sole NO3 − was increased by 66 % and that coming from sole NH4 + decreased by 73 %, in comparison with N|N. The amount of 15N coming from the mixed N source was found to be more than 30 % higher in stem, leaves, and flower. Taken together, our results suggested that whether homogeneous or heterogeneous supply of NO3 − and NH4 + significantly influenced the N uptake and distribution in the plant and combined application of NO3 − and NH4 + was approved to be superior to plant growth and N uptake.

Taking the pulse of snowmelt: in situ sensors reveal seasonal, event and diurnal patterns of nitrate and dissolved organic matter variability in an upland forest stream

Highly resolved time series data are useful to accurately identify the timing, rate, and magnitude of solute transport in streams during hydrologically dynamic periods such as snowmelt. We used in situ optical sensors for nitrate (NO3 −) and chromophoric dissolved organic matter fluorescence (FDOM) to measure surface water concentrations at 30 min intervals over the snowmelt period (March 21–May 13, 2009) at a 40.5 hectare forested watershed at Sleepers River, Vermont. We also collected discrete samples for laboratory absorbance and fluorescence as well as δ18O–NO3 − isotopes to help interpret the drivers of variable NO3 − and FDOM concentrations measured in situ. In situ data revealed seasonal, event and diurnal patterns associated with hydrological and biogeochemical processes regulating stream NO3 − and FDOM concentrations. An observed decrease in NO3 − concentrations after peak snowmelt runoff and muted response to spring rainfall was consistent with the flushing of a limited supply of NO3 − (mainly from nitrification) from source areas in surficial soils. Stream FDOM concentrations were coupled with flow throughout the study period, suggesting a strong hydrologic control on DOM concentrations in the stream. However, higher FDOM concentrations per unit streamflow after snowmelt likely reflected a greater hydraulic connectivity of the stream to leachable DOM sources in upland soils. We also observed diurnal NO3 − variability of 1–2 μmol l−1 after snowpack ablation, presumably due to in-stream uptake prior to leafout. A comparison of NO3 − and dissolved organic carbon yields (DOC, measured by FDOM proxy) calculated from weekly discrete samples and in situ data sub-sampled daily resulted in small to moderate differences over the entire study period (−4 to 1% for NO3 − and −3 to −14% for DOC), but resulted in much larger differences for daily yields (−66 to +27% for NO3 − and −88 to +47% for DOC, respectively). Despite challenges inherent in in situ sensor deployments in harsh seasonal conditions, these data provide important insights into processes controlling NO3 − and FDOM in streams, and will be critical for evaluating the effects of climate change on snowmelt delivery to downstream ecosystems.

Effects of NO 3 − -N on the growth and salinity tolerance of Tamarix laxa Willd

The influence of NO 3 − -N on growth and osmotic adjustment was studied in Tamarix laxa Willd., a halophyte with salt glands on its twigs. Seedlings of T. laxa Willd. were exposed to 1 mM (control) or 300 mM NaCl, with 0.05, 1 or 10 mM NO 3 − -N for 24 days. The relative growth rate of seedlings at 300 mM NaCl was lower than that of control plants at all NO 3 − -N levels, but the concentrations of organic N and total N in the twigs did not differ between the two NaCl treatments. Increasing NO 3 − supply under 300 mM NaCl improved the growth of T. laxa, indicating that NO 3 − played positive roles in improving salt resistance of the plant. The twigs of T. laxa Willd. accumulated mainly inorganic ions, especially Na+ and Cl−, to lower osmotic potential (Ψs): the contributions of Na+ and Cl− to Ψs were estimated at 31% and 27% respectively, at the highest levels of supply of both NaCl and NO 3 − -N. The estimated contribution of NO 3 − -N to Ψs was as high as 20% in the twigs in these conditions, indicating that NO 3 − was also involved in osmotic adjustment in the twigs. Furthermore, increases in tissue NO 3 − were accompanied by decreases in tissue Cl− and proline under 300 mM NaCl. The estimated contribution of proline to Ψs declined as with NO 3 − -N supply increased from 1 to 10 mM, while the contributions of nitrate to Ψs were enhanced under 300 mM NaCl. This suggested that higher accumulation of nitrate in the vacuole alleviated the effects of salinity stress on the plant by balancing the osmotic potential. In conclusion, NO 3 − -N played both nutritional and osmotic roles in T. laxa Willd. in saline conditions.

Ammonium transport and CitAMT1 expression are regulated by N in Citrus plants

Citrus seedlings (Citrus sinensis L. Osbeck x Poncirus trifoliata Blanco) were used to describe the effects of different N treatments on the NH4+ influx mediated by high- and low-affinity transport systems (HATS and LATS, respectively) and CitAMT1 gene expression. Results show that Citrus plants favor NH4+ over NO3- influx mediated by HATS and LATS when both N sources are present in the nutrient solution and Citrus plants display a much higher capacity to take up NH4+ than NO3-. Furthermore, NH4+ exerts a regulatory effect on NH4+ HATS activity and CitAMT1 expression, both are down-regulated by high N status of the plant, but specifically stimulated by NH4+ and the balance between these two opposite effects depends on the prior nutrition regime of the plant. On the other hand, supply of NO3- inhibits CitAMT1 expression but doesn't affect NH4+ HATS activity on the roots. To explain this discrepancy, it is possible that other CitAMT1 transporters, up-regulated by N limitation, but not repressed by NO3- could be involved in the stimulation of NH4+ HATS activity under pure NO3- nutrition or CitAMT1 transporter could be regulated at the post-transcriptional level.

Effects of different nitrogen forms on the growth and cytokinin content in xylem sap of tomato (Lycopersicon esculentum Mill.) seedlings

In order to investigate the effects of homogeneous and localized supply of different nitrogen forms (nitrate, NO3 − vs ammonium, NH4 +) on the growth of tomato seedlings, root morphology and six cytokinin (CTK) fractions in xylem sap were analyzed. Whole roots were supplied with different ratios of NO3 − to NH4 + (100:0, as 100-0NA; 75:25, as 75-25NA; 50:50, as 50-50NA) under homogeneous supply. In split-root experiments, three treatments were compared: a sole NO3 − supply (N|N), a spatially separated supply of NO3 − and NH4 + (N|A), and a spatially separated supply of NO3 − and a mixture of NO3 − and NH4 + nutrition at a ratio of 75:25 (N|AN). All concentrations of total N were set at 5 mM. The results showed that (1) homogeneous 75% NO3 − plus 25% NH4 + supply to the whole root zone led to maximum shoot and root dry matter (DM), root surface area (RS) and root volume (RV). The spatially separated supply of NO3 − and NH4 + (N|A) resulted in a contrasting effect on root morphology: in comparison to N|N, root DM in the NO3 −-containing pot was increased by 50% whereas it was depressed by 50% in the NH4 +-containing pot. The 75% NO3 − plus 25% NH4 + supply in the split-root experiment led to no significant effects either on shoot DM and root DM, or on RS and RV when compared to N|N. (2) The presence of NH4 + in the external medium led to a significantly reduced total xylem-CTK concentration, and a close negative correlation was found between xylem NH4 + and total CTK concentration irrespective of culture mode. A relatively high level of zeatin riboside (ZR) was maintained both in 75-25NA and N|A treatments. It was concluded that, in addition to the percentage of NH4 + to NO3 − in the nutrient solution, whether NH4 + was supplied to the whole root system or to only part of the root system was also an important factor affecting plant growth. The fact that the 75-25NA and N|A treatments resulted in optimal growth of tomato seedlings might be attributed to the higher ZR concentration in xylem.

Nitrous oxide emissions from denitrification and the partitioning of gaseous losses as affected by nitrate and carbon addition and soil aeration

National inventories of N2O emissions from agricultural situations are being developed; however, the factors controlling such emissions may vary with soil and environmental conditions and management practices. This study determined the relative importance of soil aeration, as measured by water-filled pore space (WFPS), NO3− addition, C addition and C source on the amount and partitioning of gaseous N losses from denitrification from an arable soil in Atlantic Canada. Denitrification (N2O + N2) and N2O emissions were measured on repacked soil cores using acetylene inhibition.The N2O:(N2O + N2) ratio was frequently 0.7 or higher, indicating that most emissions occurred as N2O. N2O emissions and denitrification were negligible at a WFPS of 0.45 m3 m-3, and high at WFPS of 0.75 m3 m-3, regardless of NO3−or C addition treatments. At a WFPS of 0.60 m3 m-3, N2O emissions and denitrification were low and were increased by both NO3− and C addition treatments. Carbon source was investigated by amendment with glucose, red clover or barley straw. Based on the quantity of soil respiration per unit of C added in the amendment, C in the red clover and barley straw was estimated to be 48 and 28% as available as glucose C. When corrected for C availability, cumulative N2O emissions averaged 0.010, 0.011 and 0.002 mg N kg-1 soil, and cumulative denitrification averaged 0.014, 0.014 and 0.003 mg N kg-1 soil, for each 1.0 mg C kg-1 soil of available C added as glucose, red clover or barley straw, respectively. NO3− addition had no effect on denitrification, but increased N2O emissions, especially where C availability was high. The amount of denitrification was controlled primarily by soil O2 supply, as controlled by WFPS and C availability. The N2O:(N2O + N2) ratio was generally high in cases where the supply of O2 or NO3− was sufficient to meet the demand for terminal electron acceptors. Key words: Denitrification, nitrous oxide, glucose, red clover, barley straw, carbon availability, terminal electron acceptor, aeration

Effect of split application of fertilizer nitrogen on N2O emissions from potatoes

The timing of fertilizer nitrogen (N) application influences the availability of NO3− as a substrate for denitrification. This study examined the effect of split application of fertilizer N on N2O emissions and denitrification rate in potato (Solanum tuberosumL.) production over 2 yr. Three treatments were used: 0 or 200 kg N ha-1 at planting, and 120 kg N ha-1 at planting plus 80 kg N ha-1 at final hilling. Fertilizer N application increased cumulative N2O emissions. Split fertilizer N application decreased cumulative N2O emissions in 2003, but not in 2002, compared with all fertilizer N applied at planting. A greater proportion of N2O emissions occurred between planting and hilling in 2003 (67%) compared with 2002 (17%). In 2003, the higher emissions during this period resulted from the coincidence of high soil NO3− availability and increased rainfall resulting in reduced aeration. Split N application was effective in reducing N2O emissions by minimizing the supply of NO3− when demand for terminal electron acceptors was high. N2O emissions were higher in the potato hill relative to the furrow; however, denitrification rate was higher in the furrow. Nitrate intensity (NI) expresses the exposure of the soil microbial population to NO3− and was calculated as the summation of daily soil nitrate concentration over the monitoring period. Cumulative N2O emissions were positively related to NI across year, N fertility treatment and row location. Denitrification was not related to NI, reflecting the primary role of NO3− in influencing the N2O:N2 ratio of denitrification rather than the magnitude of the overall process. Split N application was an effective strategy for reducing N2O emissions in years where there was significant rainfall during the period between planting and hilling. Key words: Denitrification, nitrous oxide, aeration, nitrate intensity, split N application, terminal electron acceptor, Solanum tuberosum

Ammonium nutrition increases photosynthesis rate under water stress at early development stage of rice (Oryza sativa L.)

An hydroponic experiment with a simulated water stress induced by PEG (6000) was conducted in a greenhouse to study the effects of nitrate (NO3 −), ammonium (NH4 +) and the mixture of NO3 − and NH4 +, on water stress tolerance of rice seedlings. Rice (Shanyou 63) was grown under non- or simulated water stress condition (10% (w/v) PEG, MW6000) with the 3 different N forms during 4 weeks. Under non-stressed condition no difference was observed among the N treatments. Under simulated water stress, seedlings grown on N-NO3 − were stunted. Addition of PEG did not affect rice seedling growth in the treatment of only NH4 + supply but slightly inhibited the rice seedling growth in the treatment of mixed supply of NO3 − and NH4 +. Simulated water stress, when only N-NH4 + was present, did not affect leaf area and photosynthesis rate, however, both parameters decreased significantly in the NO3 − containing solutions. Under water stress, Rubisco content in newly expanded leaves significantly increased in the sole NH4 + supplied plants as compared to that in plants of the other two N treatments. Under water stress, the ratio of carboxylation efficiency to Rubisco content was, respectively, decreased by 13 and 23% in NH4 + and NO3 − treatments, respectively. It is concluded that, water stress influenced the Rubisco activity than stomatal limitation, and this effects could be regulated by N forms.

Short-term interactions between nitrate and iron nutrition in cucumber.

Cucumber (Cucumis sativus L.) plants were precultured for 7 days in either optimal (10 µm) or low (0.5 µm) Fe conditions and then grown for further 5 days in a N-free nutrient solution with (+Fe) or without (-Fe) 10 µm Fe. Thereafter NO3- (4 mm) was added to the nutrient solution for 24 h and, concomitantly, half of the -Fe plants were treated with 1 µm Fe complexed to water extractable humic substances (WEHS). Supply of NO3- to +Fe-N-deprived plants caused a large induction in the capacity to take up the anion by roots, which was accompanied by a rise in root-shoot NO3- concentration. The -Fe plants showed a lower level of induction of NO3- uptake and hence a lower accumulation of the anion in the tissues, these effects being reversed by supply of Fe-WEHS. Supply of either NO3-- or NH4+-N (+/- Fe-WEHS) to -Fe plants promoted the development of the root FeIII-chelate reductase activity, but the capacity of roots to take up the Fe2+ remained unaffected. Results show that an inadequate Fe supply can limit the acquisition of NO3-, whereas NO3- supply can affect Fe uptake by influencing the development and maintenance of a high FeIII-chelate reducing capacity.

Relationship Between Gross Nitrogen Cycling and Nitrous Oxide Emissionin Grass-clover Pasture

Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N2O emissions. This study examined the relationship between N-cycling rates and N2O production and evolution from the root zone of grass-clover pastures of various ages (production year 1, 2 and 8). The experimental approach included cross-labelling pasture monoliths with 15N-enriched substrates to identify sources of N2O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136μg N2O–N m−2 d−1, independent of pasture age. The 15N labelling indicated that at least 50% of the N2O was derived from the soil NH 4 + pool, approaching 100% in June. In the two year old pasture the NH 4 + pool contributions to N2O emissions varied significantly with sampling time. Emission rates of N2O correlated positively with soil NH 4 + concentrations and the NH 4 + supply as expressed by gross mineralization. The N2O emissions showed a significant inverse relationship with soil NO 3 − , but was not correlated with the supply of NO 3 − as expressed by gross nitrification. The ratio N2O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.

HELD EXPERIMENTS WITH MESOCOSM ON DIEL VERTICAL MIGRATION OF BLOOM OF GOIVYOSTOMUM SEMEN (RAPHIDOPHYCEAE)

By the field observation on water quality in Kawahara Lake, it was found that phytoplankton biomass, of which dominant species was Gonyostomum semen (G. semen), grew and aggregated densely in the middle layer in summer season. To clarify the influence factors on the diel migration of aggregated layer of G. semen, the field experiments with 3 mesocosms was conducted. A part of the lower water body in a mesocosm was transferred into the other mesocosm to change the condition of stratification for the water quality. In daytime G. semen aggregated at the depth of about 53m and in nighttime about 75m. As the results of the mesocosm experiment, it was found that G. semen moved downward for the nutrient salts at nighttime and stopped the downward migration when reached the anaerobic layer in strong reduction. Also, in daytime, the upward migration of G. semen was influenced by pH and the photon flux density.In addition, it was found that the bloom of G. semen was generated after the heavy rainfall from the frequent field researches, so that the trigger of a rapid proliferation of G. semen was the supply of NO3 with inflowing of river discharge.

Responses to shading in Chenopodium album: the effect of the maternal environment and the N source supplied

To study the effect of the maternal environment and N sources in responses to shading in Chenopodium album L., seeds from plants grown at an ambient red/far red (R/FR) ratio (1.07) or a low R/FR ratio (0.39) were germinated in a greenhouse at ambient light. At 34 d after sowing (d.a.s.), they were exposed to either ambient R/FR (0.98) or low R/FR (0.21) progeny light treatments and fertilized with 2.7 mmol/L nitrogen supplied at 1:3 or 3:1 NO3– : NH4+ ratios. Two harvests were performed at 67 and 113 d.a.s. At each harvest, stem elongation traits and biomass of plant organs were recorded. In the first harvest, tissue N% and free NO3–-N concentrations were analyzed. In the first harvest, plants from the low R/FR maternal treatment were heavier and taller than the maternal controls when grown at an ambient R/FR and with a high NO3– supply. At the second harvest this difference persisted in most of the conditions tested. Stem elongation and relative biomass allocation to the stem increased in the low R/FR treated plants, although the effect varied during ontogeny and with the maternal and nutrient treatment considered. The studied factors also affected N acquisition and metabolism. It is concluded that responses to the light spectral quality as well as plant N economy are influenced by the R/FR ratio of the maternal environment and that their expression may be altered by the relative availability of NO3–-N and NH4+-N.Key words: ammonium, Chenopodium album, light spectral quality, maternal effects, nitrate, shade-avoidance responses.