Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO₃ and NH₄, floodwater NO₃, and N loss from added ¹⁵N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha^–1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH₄ in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha^–1) soil NO₃ accumulations. Soil NO₃, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered ¹⁵N in ¹⁵N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH₃ volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH₄ and NO₃.     

Field investigations of ammonia exchange between barley plants and the atmosphere. I. Concentration profiles and flux densities of ammonia

The exchange of ammonia between the atmosphere and the canopy of spring barley crops growing at three levels of nitrogen application (medium N, high N and excessive N) was studied over two consecutive growing seasons by use of micrometeorological techniques. In most cases, ammonia was emitted from the canopy to the atmosphere. The emission started around 2 weeks before anthesis, and peaked about or shortly after anthesis. The volatilization of ammonia only took place in the daytime. During the night-time, atmospheric ammonia was frequently aborbed by the canopy. Occasionally, plants in the medium and high N treatments also absorbed ammonia from the atmosphere during the daytime. Daytime absorption of ammonia never occurred in the excessive N canopy. The loss of ammonia from the canopy amounted in both years to 0.5–1.5 kg NH₃-N ha^?1 and increased with the N status of the canopy. In agreement with the small losses of ammonia, the content of ¹⁵N-labelled nitrogen in the plants did not decline during the grain-filling period. The experimental years were characterized by very favourable conditions for grain dry matter formation, and for re-utilization of nitrogen mobilized from leaves and stems. Consequently, a very high part of the nitrogen in the mature plants was located in grain dry matter (80–84% in 1989; 74–80% in 1990). The efficient re-utilization of nitrogen may have reduced the volatilization of ammonia.     

Nitrogen relations of the sorghum-Sfriga hermonthica hostparasite association: growth and photosynthesis

The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH₄NO₃ in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m^?3 N-grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m^?3 N-grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m^?3 N-grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m^?3 N-grown plants, and was unaffected in 4 mol m^?3 N-grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m ^?3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g^?1 dwt. in 0.5 mol m^?3 N-grown plants, to 28 mg g^?1 dwt. in 4 mol m^?3 N-grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g^?1 dwt. in 0.5 and 3 mol m^?3 N-grown plants, respectively. The relationship between photosynthesis (CO₂ flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m^?3 N-grown plants, with rates of 16 and 11 μmol m^?2 s^?1 in uninfected and infected plants, respectively (at 1500–1800 μmol m^?2 s^?1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m^?2 s^?1 at 4 mol m^?3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m^?3 N-grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m^?3 N (8 μmol m^?2 s^?1) was double that in 0.5 mol m^?3 N-grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m^?3 and 3 mol m^?3 N-grown plants, respectively.     

Response of small birch plants (Betula pendula Roth.) to elevated CO2 and nitrogen supply

Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol^-1) or elevated (700 μmol mol^-1) concentrations of CO₂. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO₂ concentration at sub-optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO₂. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N-concentration. At all N-supply rates, elevated CO₂ resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO₂. Specific leaf area (and leaf area ratio) was less at higher CO₂ and at lower rates of N-supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO₂. At sub-optimal N-supply, the higher net assimilation rate at elevated CO₂ was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N-supply. In the treatment with lowest N-supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N-supplies was greater at elevated CO₂. These responses of the root system to lower N-supply and elevated CO₂ may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO₂. The advantage of maintaining steady-state nutrition in small plants while investigating the effects of elevated CO₂ on growth is emphasized.     

Characteristics of inorganic nitrogen assimilation of plants in fire-prone Mediterranean-type vegetation

A range of approaches was used to investigate how species within a fire-prone Banksia woodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline-dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow-rooted herbaceous, non-mycorrhizal or VAM-positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen-fixing species were active in reducing and transporting nitrate, others were virtually inactive in these respects.     

Long-term effects of elevated CO2 and nutrients on photosynthesis and rubisco in loblolly pine seedlings

The effects of long-term CO₂ enhancement and varying nutrient availability on photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) were studied on loblolly pine (Pinus taeda L.) seedlings grown in two atmospheric CO₂ partial pressures (35 and 65 Pa) and three nutrient treatments (low N, low P, and high N and P). Measurements taken in late autumn (November) after 2 years of CO₂ enrichment and nutrient addition showed that photosynthetic rates were higher for plants grown at elevated CO₂ only when they received supplemental N. Total rubisco activity and rubisco content decreased at elevated CO₂, but there was an increase in activation state. At elevated CO₂, proportionately less N was found in rubisco and more N was found in the light reaction components. These results demonstrate acclimation of photosynthetic processes to elevated CO₂ through reallocation of N. Loblolly pine grown in nutrient conditions similar to native soils (low N availability) had lower needle N and chlorophyll content, lower total rubisco activity and content, and lower photosynthetic rates than plants grown at high N and P. This suggests that the magnitude of the photosynthetic response to a future, high-CO₂ environment will be dependent on soil fertility in the system.     

The uptake of zinc by erythrocytes under near-physiological conditions

The in vitro uptake of zinc by erythrocytes was measured under near-physiological conditions, using⁶⁵Zn as a radioactive tracer. Because of the presence of serum albumin—a strong zinc ligand—a low concentration of medium free zinc was maintained. Under these conditions a high-affinity carrier for zinc transport over the cell membrane was identified. With human erythrocytes, a Michaelis constant (K _m ) of 0.2 nM with respect to free medium zinc was measured and aV _max of 4.5 nmoles Zn transported per h/g dry wt. TheK _m for medium Zn increases when the size of the internal erythrocytic Zn pool is augmented, whereasV _max remains virtually unchanged. A model to explain this phenomenon is proposed. It is suggested that this phenomenon could underlie observations, confirmed here, that the in vitro uptake of Zn by animal erythrocytes depends on the Zn status of the animal.     

Fertile plants regenerated from suspension culture-derived protoplasts of an indica type rice (Oryza sativa L.)

Calluses were induced from immature embryos of an indica type rice and finely dispersed cell suspension cultures were initiated from the callus using modified AA medium (S₁ medium). The suspension cultures were maintained alternatively (1–2 passages in each medium) in S₁ medium and S₂ medium, the latter containing KNO₃, NH₄NO₃, proline and glutamine as nitrogen source. Protoplasts of high quality were isolated form suspension cells cultured in S₂ medium supplemented with ABA. Embedding the protoplasts in agarose blocks containing NH₄NO₃-free modified KM8P(PM₁) medium and immersing the blocks in NH₄NO₃-containing modified KM8P(PM₃) medium were most effective for obtaining protoplast division and callus formation. The protoplast-derived calluses were precultured in potato extract-aand/or ABA-containing N₆(D₁, D₂ or D₃) media and many embryo-like structures were formed. These structures developed into plantlets after being transferred to N₆ differentiation (D₄) medium. The regenerated plantlets grew into mature plants and beard seeds normally.Abbreviations AA medium amino acids based medium - ABA abscisic acid - BA benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - DF division frequency - IAA indoleacetic acid - KIN kinetin - NAA naphthaleneacetic acid - PE planting efficiency     

The fate of nutrients during fires in a tropical savanna

Abstract The nutrient loads contained in the grassy fuel before fires, and of ash subsequently, were compared to determine the fluxes of macronutrients, copper and zinc during fires at Kapalga in Kakadu National Park. The fluxes were estimated in three vegetation types: forest, woodland and open woodland. The magnitudes of the fluxes were greatest in the forest community where grassy fuel loads were highest at about 6.3 t ha^?1. In these sites, 54–94% of all measured nutrients in the fuel were transferred to the atmosphere during the fires. For each nutrient, the proportion transferred to the atmosphere as entrained ash was calculated by assuming that calcium was not volatilized during the fires. If the transfer of entrained ash represents local redistribution only, then rainfall accession and the deposition of these particu-lates should replace most of the losses of all nutrients except nitrogen (N). Estimated rates of biological fixation of N appear to be insufficient to replace the annual losses of N. It is therefore concluded that a regime of annual fires that completely burn the available grassy fuel would deplete N reserves in these savannas, unless there are other sources of biologically fixed N, which are unknown at present.     

Transpiration by an emergent macrophyte: source of water and implications for nutrient supply

The contribution of sediment interstitial water and the water column to the transpiration stream of Myriophyllum aquaticum (Vellozo) Verdcourt was determined to estimate the significance of mass flow in supply of sediment nutrients for plant growth. Sediment interstitial water accounted for about 2% of the water transpired over a 37 day period. Because of the small volume of water that originated in the sediment we concluded that mass flow did not significantly enhance nutrient supply to the roots of M. aquaticum. Relative growth rate (RGR) of adventitious, water roots was greater than whole plant RGR, and RGR of sediment roots was not significantly different from zero, indicating a shift in the biomass allocation after emergence of the apical meristem into the air. Water use, measured by the transpiration coefficient, averaged 260 ml H₂O mg DW^-1, which is similar to C-4 terrestrial plants. M. aquaticum has leaf characteristics commonly associated with xerophytic habitats. These characteristics may be necessary if a high transpiration rate and a mechanical requirement for high cell turgor pressure, required by a reliance upon hydrostatic pressure for support of the aerial stems, are mutually exclusive because of morphological constraints on hydraulic conductivity.