Flooding-induced soil and plant ethylene accumulation and water status response of field-grown tobacco

Summary Ethylene (C₂H₄) accumulation in flooded soil was related to oxygen (O₂), redox potential (Eh), and flooding rate. The water status response of tobacco (Nicotiana, tabacum L.) to these conditions was evaluated from stem diameter, relative water content, leaf water potential, and C₂H₄ content of leaf tissue. Treatments were: flooded with either 0,5, or 15 cm of water per day for 6 days. By the third day, O₂ in the soil decreased to less than 9% in treatments flooded with 5 or 15 cm of water. When O₂ in the soil air was less than 9% and redox potential (Eh) was less than +150 mv, most of the soil air samples contained some C₂H₄ and 16% contained more than 6 ppm. Very little C₂H₄ was present in soil air when O₂ exceeded 9%. Tobacco leaf C₂H₄ peaked 3 days after flooding and then declined to the preflooding level a day later, one day ahead of the rapid increase in soil C₂H₄. Wilting developed progressively beginning with the rise of C₂H₄ in the soil; leaf water potential, stem diameter, and relative leaf water content all were decreased. Soil-and plant-produced C₂H₄ are suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco.Contribution of the USDA-SEA/AR, in cooperation with the South Carolina Experiment Station.     

Nodulation and growth response ofAllocasuarina andCasuarina species to phosphorus fertilization

To examine how soil phosphorus status affects nitrogen fixation by the Casuarinaceae —Frankia symbiosis,Casuarina equisetifolia and two species ofAllocasuarina (A. torulosa andA. littoralis) inoculated or fertilized with KNO₃ were grown in pots in an acid soil at 4 soil phosphate levels. InoculatedC. equisetifolia nodulated well by 12 weeks after planting and the numbers and weight of nodules increased markedly with phosphorus addition. Growth ofC. equisetifolia dependent on symbiotically fixed nitrogen was more sensitive to low levels of phosphorus (30 mg kg^–1 soil) than was growth of seedings supplied with combined nitrogen; at higher levels of phosphorus, the growth response curves were similar for both nitrogen fertilized and inoculated plants. The interaction between phosphorus and nitrogen treatments (inoculated and nitrogen fertilized) demonstrated that there was a greater requirement of phosphorus for symbiotic nitrogen fixation than for plant growth when soil phosphorus was low.WithAllocasuarina species, large plant to plant variation in nodulation occurred both within pots and between replicates. This result suggests genetic variation in nodulation withinAllocasuarina species. Nodulation ofAllocasuarina species did not start until 16 weeks after planting and no growth response due toFrankia inoculation was obtained at the time of harvest. Addition of nitrogen starter is suggested to boost plant growth before the establishment of the symbiosis. Growth ofAllocasuarina species fertilized with nitrogen responded to increasing levels of phosphorus up to 90 mg P/kg soil after which it declined by 69% forA. littoralis. The decrease in shoot weight ofA. littoralis, A. torulosa, C. equisetifolia andC. cunninghamiana at high phosphorus was confirmed in a sand culture experiment, and may be atributable to phosphorus toxicity.     

Biological nitrogen fixation: Investments,expectations and actual contributions to agriculture

Inputs of biologically fixed N into agricultural systems may be derived from symbiotic relationships involving legumes and Rhizobium spp., partnerships between plants and Frankia spp. or cyanobacteria, or from non-symbiotic associations between free-living diazotrophs and plant roots. It is assumed that these N₂-fixing systems will satisfy a large portion of their own N requirements from atmospheric N₂, and that additional fixed N will be contributed to soil reserves for the benefit of other crops or forage species. This paper reviews the actual levels of N₂ fixation attained by legume and non-legume associations and assesses their role as a source of N in tropical and sub-tropical agriculture. We discuss factors influencing N₂ fixation and identify possible strategies for improving the amount of N₂ fixed.     

Growth and nitrogen acquisition strategies of Acacia senegal seedlings under exponential phosphorus additions

There remains conflicting evidence on the relationship between P supply and biological N₂-fixation rates, particularly N₂-fixing plant adaptive strategies under P limitation. This is important, as edaphic conditions inherent to many economically and ecologically important semi-arid leguminous tree species, such as Acacia senegal, are P deficient. Our research objective was to verify N acquisition strategies under phosphorus limitations using isotopic techniques. Acacia senegal var. senegal was cultivated in sand culture with three levels of exponentially supplied phosphorus [low (200 μmol of P seedling⁻¹ over 12 weeks), mid (400 μmol) and high (600 μmol)] to achieve steady-state nutrition over the growth period. Uniform additions of N were also supplied. Plant growth and nutrition were evaluated. Seedlings exhibited significantly greater total biomass under high P supply compared to low P supply. Both P and N content significantly increased with increasing P supply. Similarly, N derived from solution increased with elevated P availability. However, both the number of nodules and the N derived from atmosphere, determined by the ¹⁵N natural abundance method, did not increase along the P gradient. Phosphorus stimulated growth and increased mineral N uptake from solution without affecting the amount of N derived from the atmosphere. We conclude that, under non-limiting N conditions, A. senegal N acquisition strategies change with P supply, with less reliance on N₂-fixation when the rhizosphere achieves a sufficient N uptake zone.     

The contribution of associative and symbiotic nitrogen fixation to the nitrogen nutrition of non-legumes

During the past 10 years estimates of N₂ fixation associated with sugar cane, forage grasses, cereals and actinorhizal plants grown in soil with and without addition of inoculum have been obtained using the ¹⁵N isotope dilution technique. These experiments are reviewed in this paper with the aim of determining the proportional and absolute contribution of N₂ fixation to the N nutrition of non-legumes, and its role as a source of N in agriculture. The review also identifies deficiencies in both the totality of data which are currently available and the experimental approaches used to quantify N₂ fixation associated with non-legumes.Field data indicate that associative N₂ fixation can potentially contribute agronomically-significant amounts of N (>30–40 kg N ha^-1 y^-1) to the N nutrition of plants of importance in tropical agriculture, including sugar cane (Saccharum sp.) and forage grasses (Panicum maximum, Brachiaria sp. and Leptochloa fusca) when grown in uninoculated, N-deficient soils. Marked variations in proportions of plant N derived from the atmosphere have been measured between species or cultivars within species.Limited pot-culture data indicate that rice can benefit naturally from associative N₂ fixation, and that inoculation responses due to N₂ fixation can occur. Wheat can also respond to inoculation but responses do not appear to be due to associative N₂ fixation. ¹⁵N dilution studies confirm that substantial amounts of N₂ can be fixed by actinorhizal plants.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

Influence of soil water potential on respiration and nitrogen fixation ofAzotobacter vinelandii

Summary Respiration and N₂-fixation (acetylene reduction) ofAzotobacter vinelandii have been studied at a variety of soil water potentials. Both processes were strictly linked and strongly reduced at water potentials between –0.6 and –1.3 MPa. Complete inhibition occurred below –2.1MPa. Osmotic potentials in soil compared to matric potentials of the same value were less inhibitory to respiration and acetylene reduction by Azotobacter. The N₂-fixing efficiency (mg N/g glucose) was not influenced by water potentials ranging from –0.1 to –2.1 MPa.     

An intact plant assay for estimating nitrogenase activity (C2H2 reduction) of sorghum and millet plants grown in pots

Summary A non destructive intact-plant assay for estimating nitrogenase activity (C₂H₂ reduction) of pot-grown sorghum and millet plants is described. Plants with intact shoots sustained more activity than plants whose tops were removed prior to the assay. With this technique individual plants can be assayed several times during their life cycle. The C₂H₂ reduction was linear up to 16h incubation in this assay procedure. More rapid diffusion of C₂H₂ was achieved by injection through a Suba seal in the bottom of the pot. The equlibration of injected C₂H₂ in the gas phase of the pots filled with sand and sand:FYM media was completed within 1 h. Significantly higher nitrogenase activity and better growth of sorghum and millet plants occurred when plants were grown in a mixture of sand and farmyard manure (FYM) than when plants were grown in vermiculite, soil, or sand + soil medium. Nitrogenase activity and plant growth were greater in a mixture of sand with 2 and 3% FYM than with 0.5 and 1% FYM. Activity was higher when the plants were incubated at 33°C and 40°C than at 27°C. Activity also increased with increasing soil moisture. There were significant differences amongst 15 sorghum cultivars screened for associated nitrogenase activity. This new technique has good prospects for screening cultivars of millet, sorghum and other grain crops for their nitrogen-fixing ability.Submitted as Journal article No. 358 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).     

Effects of elevated CO2 and nitrogen on nutrient uptake in ponderosa pine seedlings

This paper reports on the results of a controlled-environment study on the effects of CO₂ (370, 525, and 700 mol mol^-1) and N [0, 200, and 400 g N g soil^-1 as (NH₄)SO₄] on ponderosa pine (Pinus ponderosa) seedlings. Based upon a review of the literature, we hypothesized that N limitations would not prevent a growth response to elevated CO₂. The hypothesis was not supported under conditions of extreme N deficiency (no fertilizer added to a very poor soil), but was supported when N limitations were less severe but still suboptimal (lower rate of fertilization). The growth increases in N-fertilized seedlings occurred mainly between 36 and 58 weeks without any additional N uptake. Thus, it appeared that elevated CO₂ allowed more efficient use of internal N reserves in the previously-fertilized seedlings, whereas internal N reserves in the unfertilized seedlings were insufficient to allow this response. Uptake rates of other nutrients were generally proportional to growth. Nitrogen treatment caused reductions in soil exchangeable K⁺, Ca²⁺, and Mg²⁺ (presumably because of nitrification and NO₃ ^- leaching) but increases in extractable P (presumably due to stimulation of phosphatase activity).The results of this and other seedling studies show that elevated CO₂ causes a reduction in tissue N concentration, even under N-rich conditions. The unique response of N is consistent with the hypothesis that the efficiency of Rubisco increases with elevated CO₂. These results collectively have significant implications for the response of mature, N-deficient forests to evevated CO₂.     

Differentiation among populations of Sedum wrightii (Crassulaceae) in response to limited water availability: water relations,CO2 assimilation,growth and survivorship

Summary Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO₂ assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO₂ exchange patterns, in biomass ¹³C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO₂ uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.     

Growth and nitrogen fixation ofAeschynomene under water stressed conditions

Summary Studies on the tolerance ofAeschynomene americana L. to periods of flooding or soil moisture deficit were conducted in an attempt to elucidate nitrogen fixation as affected by soil moisture. Nitrogenase activity was not reduced significantly in pot-grown Aeschynomene plants subjected to flooding in greenhouse conditions. After 20 days of withholding water from the soil, nitrogenase activities of the drought-stressed plants were much lower than those of either the well-watered or flooded plants. Leaf water potentials were similar in flooded and control plants; however, the droughted plants had leaf water potentials that were 4 bars lower than those of the control plants. Aeschynomene plants were tolerant to long-term periods of flooding, but exhibited a reduction in nitrogenase activity and leaf water status when subjected to soil moisture deficits.     

Effect of light and atmospheric carbon dioxide concentration on nitrogen fixation by herbage legumes

Summary Lucerne, red clover and white clover were grown at two atmospheric concentrations of CO₂ (300 and 1000 μl l⁻¹) and the effects on N₂ fixation, nodule mass/number and root/shoot dry matter production determined. Pea plants were similarly evaluated as a comparison with grain legumes. CO₂ enrichment increased N₂ fixation activity in all cases but activity/unit nodule mass was significantly increased only in the pea. The enhancement of N₂ fixation in herbage legumes by CO₂ enrichment reflected an increase in nodule mass which in turn was attributed to increased nodule number, and results show that under the experimental conditions obtaining here photosynthate supply did not limit nodule N₂ fixation in these plants though it was limiting in the case of peas. White clover growing in a 6 and 14 hour photoperiod was studied for response of the N₂ fixing system to light. Long photoperiod (14 hour) plants assayed at constant temperature (20°C) did not show a significant response to light at the end of the dark period either in terms of fixation per plant or per unit nodule mass, in contrast with short photoperiod (6 hour) plants which showed significant responses. Short photoperiod plants compensated for reduced photosynthates by maintaining only half the root nodule mass and fixation activity of 14 hour photoperiod plants though plants in both systems supported similar rates of N₂ fixation per unit mass of nodule during the photoperiod. Comparison of N₂ fixation activities in whole and decapitated plant systems indicates the importance of shoot reserves for sustaining nitrogenase activity in white clover during short-term interruption of photosynthesis. These results support the conclusion of the CO₂ enrichment studies, that herbage legumes have the potential for supplying their nodule photosynthate requirements for sustaining optimum rates of N₂ fixation and excess carbon supply is used solely to promote further nodulation. Nodules of short photoperiod white clover plants were less efficient in N₂ fixation in that they evolved more H₂ relative to N₂ (C₂H₂) reduced than did long photoperiod plants.     

Evaluating the effects of gross nitrogen mineralization,immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg⁻¹ diesel, and amended with nitrogen (15.98 atom% ¹⁵N) at 0, 250, 500, and 1000 mg kg⁻¹ to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were −0.20, −0.47, −0.85, and −1.50 MPa in the 0, 250, 500, and 1000 mg kg⁻¹ nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O₂ kg⁻¹ soil day⁻¹ were observed in the 250, 500, control and 1000 mg kg⁻¹ nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg⁻¹ soil day⁻¹ in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg⁻¹ soil day⁻¹, and mineralization rates of 0.5, 0.3, and 0.3 mg N kg⁻¹ soil day⁻¹ in the 1000, 500, and 250 mg kg⁻¹ nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.     

Contribution of N2 fixing cyanobacteria to rice production: availability of nitrogen from 15N-labelled cyanobacteria and ammonium sulphate to rice

This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N₂-fixing cyanobacteria abundance and N₂ fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha^-1. No further increase was observed with 140 kg N ha^-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from ¹⁵N fertilizer. Recovery of applied ¹⁵N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of ¹⁵N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application. This revised version was published online in June 2006 with corrections to the Cover Date.     

Glyoxylate decreases the oxygen sensitivity of nitrogenase activity and photosynthesis in the cyanobacterium Anabaena cylindrica

Raising the pO₂ reduced nitrogenase activity (C₂H₂ reduction) of Anabaena cylindrica for both glyoxylate-treated (5 mM) and untreated cells. The stimulation caused by glyoxylate, however, increased with increases of pO₂ from 2 to 99 kPa. As the pO₂ increased the net CO₂ fixation was lowered (Warburg effect) while the CO₂ compensation point increased. Glyoxylate partly relieved this sensitivity of net photosynthesis to oxygen and reduced the compensation point considerably. The cells used were preincubated in the dark to exhaust photosynthetic pools. A more pronounced reduction in sensitivity of nitrogenase to oxygen for glyoxylate-treated cells was evident when a preincubation in air with reduced pCO₂ (13 l l^-1) was used. This was, however, not evident until after a 10-h incubation in air. Before this point 2 kPa O₂ sustained the highest nitrogenase activity. Addition of 0.5 and 5 mM of HCO ₃ ^- to Anabaena cultures preincubated at low CO₂ levels (29 l l^-1) abolished the stimulatory effect of glyoxylate on the nitrogenase. Thus, the results sustain the suggestion that glyoxylate may act as an inhibitor of photorespiratory activities in cyanobacteria and can be used as a means of increasing their nitrogen and CO₂ fixation capacities.Abbreviation RuBP ribulose 1,5-bisphosphate     

Variation with soil depth,topographic position and host species in the capacity of soils from an Australian locale to nodulateCasuarina andAllocasuarina seedlings

Sandy alluvial soils in a floodplain supporting a native stand ofCasuarina cunninghamiana Miq. produced about three times as many nodulated seedlings and more than twice as many nodules per nodulated seedling on roots of baitedCasuarina spp. than did clay loam red earth soils from the adjacent valley slope. Moist and well-aerated subsurficial alluvial sands had the greatest nodulation capacity of all the soils sampled. For all topographic positions, soil samples from depths greater than 20 cm promoted 76% more nodulated Casuarina seedlings than samples from the surficial 20 cm.Seedlings of three provenances ofC. cunninghamiana, together with seedlings ofC. glauca Sieb. ex Spreng.,C. cristata F. Muell ex Miq. andC. obesa Miq. developed significantly more nodules per pot and nodules per nodulated seedling in soils from this locale than seedlings of twoCasuarina equisetifolia Forst. provenances. Seedlings of two provenances ofAllocasuarina torulosa (Ait.) L. Johnson had fewer than 1% nodulated seedlings, a significantly lower level by far than that ofCasuarina seedlings.A. torulosa provenances also had significantly fewer nodulated seedlings per pot and nodules per nodulated seedling than all Casuarina hosts excepting one poorly-nodulated provenance ofC. equisetifolia.Nodulated seedlings of allCasuarina species had the capacity to fix atmospheric N₂, as indicated by acetylene-reduction capability. The presence of yellow cladodes and low rates of acetylene reduction per plant forC. cristata Miq. suggest that this association was poorly effective.     

Growth and shoot: root ratio of seedlings in relation to nutrient availability

The influence of mineral nutrient availability, light intensity and CO₂ on growth and shoot:root ratio in young plants is reviewed. Special emphasis in this evaluation is given to data from laboratory experiments with small Betula pendula plants, in which the concept of steady-state nutrition has been applied.Three distinctly different dry matter allocation patterns were observed when growth was limited by the availability of mineral nutrients: 1, Root growth was favoured when N, P or S were the major growth constraints. 2, The opposite pattern obtained when K, Mg and Mn restricted growth. 3, Shortage of Ca, Fe and Zn had almost no effect on the shoot:root ratio. The light regime had no effect on dry matter allocation except at very low photon flux densities (< 6.5 mol m^-2 day^-1), in which a small decrease in the root fraction was observed. Shortage of CO₂, on the other hand, strongly decreased root development, while an increase of the atmospheric CO₂ concentration had no influence on dry matter partitioning. An increased allocation of dry matter to below-ground parts was associated with an increased amount of starch in the tissues. Depletion of the carbohydrate stores occurred under all conditions in which root development was inhibited. It is concluded that the internal balance between labile nitrogen and carbon in the root and the shoot system determines how dry matter is being partitioned in the plant. The consistency of this statement with literature data and existing models for shoot:root regulation is examined.     

Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability

Pools and annual fluxes of carbon (C) were estimated for a mature Eucalyptus pauciflora (snowgum) forest with and without phosphorus (P) fertilizer addition to determine the effect of soil P availability on allocation of C in the stand. Aboveground biomass was estimated from allometric equations relating stem and branch diameters of individual trees to their biomass. Biomass production was calculated from annual increments in tree diameters and measurements of litterfall. Maintenance and construction respiration were calculated for each component using equations given by Ryan (1991a). Total belowground C flux was estimated from measurements of annual soil CO₂ efflux less the C content of annual litterfall (assuming forest floor and soil C were at approximate steady state for the year that soil CO₂ efflux was measured). The total C content of the standing biomass of the unfertilized stand was 138 t ha^-1, with approximately 80% aboveground and 20% belowground. Forest floor C was 8.5 t ha^-1. Soil C content (0–1 m) was 369 t ha^-1 representing 70% of the total C pool in the ecosystem. Total gross annual C flux aboveground (biomass increment plus litterfall plus respiration) was 11.9 t ha^-1 and gross flux belowground (coarse root increment plus fine root production plus root respiration) was 5.1 t ha^-1. Total annual soil efflux was 7.1 t ha^-1, of which 2.5 t ha^-1 (35%) was contributed by litter decomposition.The short-term effect of changing the availability of P compared with C on allocation to aboveground versus belowground processes was estimated by comparing fertilized and unfertilized stands during the year after treatment. In the P-fertilized stand annual wood biomass increment increased by 30%, there was no evidence of change in canopy biomass, and belowground C allocation decreased by 19% relative to the unfertilized stand. Total annual C flux was 16.97 and 16.75 t ha^-1 yr^-1 and the ratio of below- to aboveground C allocation was 0.43 and 0.35 in the unfertilized and P-fertilized stands, respectively. Therefore, the major response of the forest stand to increased soil P availability appeared to be a shift in C allocation; with little change in total productivity. These results emphasise that both growth rate and allocation need to be estimated to predict changes in fluxes and storage of C in forests that may occur in response to disturbance or climate change.