Nitrogen fertilization effects on dinitrogen fixation as influenced by legume species and proportion in legume-grass mixtures in Uruguay

Grasses grown in mixture with nodulated legumes often are N-limited, but N fertilization may result in reductions of N₂ fixation and legume stands. We studied N-fertilizer effects on N₂ fixation for three binary legume-grass mixtures in Uruguay. Replicated swards of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or birdsfoot trefoil (Lotus corniculatus L.) (BT), each in combination with tall fescue (Festuca arundinacea Schreb) (TF) at two legume proportions were sown in 1983 (Exp. 1) and 1984 (Exp. 2). In the fall of 1984, N treatments at 100 kg ha⁻¹ and controls were randomly assigned to subplots in Exp. 1 (established swards) and in Exp. 2 (at seeding). The soil for both experiments was a fine, montmorillonitic, mesic, Typic Argiudolls. Herbage fixed-N was estimated by ¹⁵N isotope-dilution with pure stands of TF as reference. In both experiments, N fertilization reduced the proportion of legume N derived from air (% Ndfa) and increased herbage yield only during the first 18 to 20 weeks after application. Fertilizer-N reduced annual fixed-N yield from 178 to 148 kg ha⁻¹ in Exp. 1 and from 65 to 29 kg ha⁻¹ in Exp. 2 Fixed-N yield for BT was markedly reduced by N in both experiments (33 to 53%), whereas for the clovers reduction was lesser in Exp. 1 (9 to 13%) than in Exp. 2 (46 to 64%). Negative effects of N on % Ndfa were more evident for the high legume proportion. We conclude that fertilization with 100 kg N ha⁻¹ reduced % Ndfa only for the immediate 18 to 20 weeks after application. Fertilizer-induced reduction of fixed-N yields lasted longer because of a more prolonged depression of legume proportion, especially for BT and for newly seeded swards. Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012). Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012).     

Defining phosphorus efficiency in plants

The many different definitions for "nutrient efficiency" make the use of the term ambiguous. We evaluated nutrient efficiency using data from a study of response to phosphorus (P) supply in white clover (Trifolium repens L.) and lucerne (Medicago sativa L.). Application of various criteria identified in the literature as measures of nutrient efficiency did not clarify differences between purportedly P efficient and inefficient germplasm. Germplasm differed in maximum shoot and total dry mass and in solution P concentration ([P]_s) required to achieve 80% maximum yield, but not in P concentration of tissue ([P]_t), internal P utilization, or P uptake per unit of fine root dry mass. Differences in yield may have resulted from factors other than efficient use of P. To reduce the confounding effects that other factors have on nutrient efficiency, it is essential that equivalent yields of germplasm be demonstrated where nutrients are not limiting. Mechanisms that enable enhanced nutrient efficiency can be identified less ambiguously using this approach.Joint contribution of the Minn. Agric. Exp. Stn. and the USDA-ARS     

Interactions between nitrate uptake and N2 fixation in white clover

Summary White clover (Trifolium repens L.) plants grown in pots and supplied with the same concentration x days of¹⁵N labelled nitrate, but in contrasting patterns and doses had similar N concentrations but differed in the proportions devived from N₂ fixation and nitrate. N₂-fixation and nodule dry weight responded rapidly (2–3 days) to changes in nitrate availability. Plants exposed frequently to small doses of nitrate took up more nitrate (and hence relied less on N₂-fixation) and had greater dry weights and shoot: root ratios than those exposed to larger doses less often. In mixed ryegrass (Lolium perenne L.)/clover communities clover's ability to either successfully compete for nitrate or fix N₂ gave it consistently higher N concentrations than grass whether they were given high or low nitrate nutrient. This higher N concentration was accompanied by greater dry weights than grass in the low nitrate swards but not where high levels of nitrate were applied.     

A root observation and sampling chamber (rhizotron) for pot studies

Summary A plant root observation chamber (rhizotron) was designed to examine soil-grown roots under a stereomicroscope and to sample roots and soil during the growth period of a pot study. The mini-rhizotrons are inexpensive to construct and are suitable for replicated, multitreatment experiments. Illustrative data on root hair and lateral development are presented for seedlings of four crop species.Vermont Agric. Exp. Stn. Journal Article No 572.     

Nitrogen fixation by white clover when competing with grasses at moderately low temperatures

It was the aim of this study to determine the way in which low temperature modifies the effect of a competing grass on nitrogen fixation of a forage legume. White clover (Trifolium repens L.) was grown in monoculture or in different planting ratios with timothy (Phleum pratense L.) or perennial ryegress (Lolium perenne L.) in growth chambers at either 7.5/5°C (LoT) or 15/10°C (HiT) average day/night temperatures, and with 2.5 or 7.5 mM ¹⁵N-labelled nitrate in the nutrient solution.Competition with grass led to a marked increase in the proportion of clover nitrogen derived from symbiosis (% N_sym). This increase was slower at LoT where % N_sym was reduced considerably; it was closely related to the reduction in the amount of available nitrate as a result of its being utilized by the grass.Nitrogen concentration in white clover herbage and dry matter yield per clover plant were reduced, for the most part, when a competing grass was present. The amount of nitrogen fixed per plant of white clover decreased markedly with temperature. Low temperature consequently accentuated competition for nitrate. The capacity of white clover to compete successfully was limited by its slower growth and nitrogen accumulation.     

Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae

Summary Drought resistance of wheat (Triticum aestivum L.) as influenced by two vesiculararbuscular mycorrhizal (VAM) fungi,Glomus fasciculatum ¹⁰ andGlomus deserticola ¹⁹, was evaluated. Soil columns 0.15 m diam. by 1.20 m length were used to reduce the influence of limited rooting space. With initial soil water at 0.5 MPa (0.145 kg kg^–1), plants were subjected to low-level water stress throughout the experiment and severe water stress for 24 h at one (55 days after transplanting, Feekes scale 10.1) two (55 and 63 days, Feekes 10.1 and 10.2), or three (55, 63, and 70 days, Feekes 10.1, 10.1, and 10.2) periods. After each stress period, one set of plants was watered and grown to maturity without subsequent water stress. A second set of plants was harvested 1 week after stress.G. fasciculatum-inoculated plants harvested 7 days after stress at 55 days had greater leaf area and leaf, total plant, and root weight than non-VAM plants.G. deserticola-inoculated plants had greater leaf area and leaf weight than non-VAM plants. After stress at 55 and 63 days, leaf area, and leaf and total dry weight were again greater for VAM than for non-VAM plants. However, after stress at 55, 63, and 70 days, differences in aboveground biomass between VAM and non-VAM plants were not significant at P=0.05. Aboveground biomass was not affected by VAM species in plants stressed at 55 or 55 and 63 days, butG. fasciculatum-inoculated plants produced more tillers atter stress at 55 days. When grown to maturity, VAM plants which had undergone three stress periods had twice the biomass and grain yield as non-VAM plants subjected to the same stress. The three stress periods reduced number of heads and kernel numbers of weight of non-VAM plants compared to VAM plants.G. fasciculatum-inoculated plants consistently had increased root weight and rooting depth.Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebr. Agric. Exp. Stn., Univ. Nebr.-Lincoln, Lincoln, Nebr. Published as Paper No. 7571 Journal Series, Nebr. Agric. Exp. Stn.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Response of wheatgrasses and wheat × wheatgrass hybrids to barley yellow dwarf virus

Summary Resistance to barley yellow dwarf virus (BYDV), manifested by low enzyme-linked immunosorbent assay (ELISA) values in plants exposed to viruliferous aphids, was identified in several wheatgrasses (Agropyron spp.). ELISA results were similar for root and leaf extracts of infested plants. No difference in reaction to BYDV was found between plants grown in the field and those in the growth chamber. Interspecific hybrids were generated using pollen from single resistant plants of Agropyron spp. to pollinate soft red winter wheat spikes. Resistance in hybrids appeared to be at the level of virus replication rather than at the level of vector inoculation. The hybrids varied in their reaction to BYDV. Expression of BYDV resistance in hybrids was influenced not only by wheat genotype and Agropyron species but, in some cases, reaction varied even among hybrids between the same wheat genotype and Agropyron plant. Implications of these results are discussed.Contribution from the Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907, and USDA-ARS. The research was supported in part by Public Varieties of Indiana. Purdue Univ. Agric. Exp. Stn. Journal No. 11656     

Effect of cyclic wetting and drying of a soil on root hair growth of maize roots

The objective of this study was to investigate the effect of cyclic soil wetting and drying on maize (Zea mays L.) root hair growth. Three soils, Chalmers silty clay loam (Typic Haplaquolls), Raub silt loam (Aquic Argiudolls) and Aubbeenaubbee sandy loam (Aric Ochraqualfs) and two soil moisture contents, −175 (M₀) and −7.5 kPa (M₁), were used to study root hair growth in a controlled-climate chamber. Increasing soil moisture after 7d from M₀ and M₁ resulted in a cessation of root hair growth behind the root cap while drying the soil after 7d from M₁ and M₀ promoted root hair growth on new but not old or existing roots. By maintaining liquid continuity under cyclic wetting and drying of a soil, root hairs may be of far greater significance to the nutrition of the plant than originally thought. Journal Paper No. 11023, Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron.     

Association of two measures of vegetative growth rate with other traits in inter and intraspecific matings of oats

Summary F₂-derived oat lines from inter (Avena sativa L. x A. sterilis L.) and intraspecific (among A. sativa cultivars) matings were evaluated in the F₃, F₄, and F₅ generations for heading date, grain and straw yields, biomass, vegetative dry weight at anthesis, vegetative growth rates until anthesis (GRA) and until maturity (GRM), and harvest index. The associations of GRA and GRM with harvest index ranged from zero to slightly negative. The positive correlations of GRA and GRM with grain yield were stronger in inter than in intraspecific matings. Grain yield was positively associated with harvest index in both inter and intraspecific matings. The results suggest the use of A. sterilis x A. sativa matings to improve vegetative growth rate, grain yield, and, to a certain extent, harvest index simultaneously without affecting the growth duration of the crop.Journal Paper No. J 12130 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011, USA. Project No. 2447. Part of a study conducted by the senior author while he was a visiting scientist at Iowa State University     

Soybean yield response to residual NO3−N and applied N

Summary During 1976 through 1978, 10N treatments (combinations of N application times and rates) were used in a corn study. Those treatments created different levels of soil NO ₃ ^– –N content that were well-suited to a study of the influence of residual NO ₃ ^– –N and applied N on soybean yield. In April 1979 we applied ammonium nitrate at rates of 0, 75, or 150 kg N/ha to three subplots formed from each of the whole plots (previous N treatment plots). With N fertilization in 1979, seed yield increased where the residual NO ₃ ^– –N amount was less than 190 kg/ha but decreased where the residual amount was greater than 190 kg/ha. As the NO ₃ ^– –N content in the soil increased by 1 kg/ha, the soybean yield increase due to N fertilization in 1979 decreased by approximately 4 kg/ha.Contribution no. 82-368-J, Dep. of Agronomy, Kansas Agric. Exp. Stn., Manhattan, KS 66506, USA     

The effect of earthworms and snails in a simple plant community

Snails and earthworms affected the dynamics of a simple, three-species plant community, in the Ecotron controlled environment facility. Earthworms enhanced the establishment, growth and cover of the legume Trifolium dubium, both via the soil and interactions with other plant species. Worms increased soil phosphates, increased root nodulation in T. dubium, and enabled T. dubium seedlings to establish in the presence of grass (Poa annua) litter, by increasing soil heterogeneity. Worms also buried the seeds of Poa annua and Senecio vulgaris, reducing the germination of new seedlings. Snails reduced nitrogen-fixing Trifolium dubium and increased cover of plant litter, thereby reducing ammonia-nitrogen concentrations in the soil. These effects and their interactions demonstrate that the detritivore food chain, and earthworms in particular, cannot be ignored if we are to understand the spatial and temporal dynamics of plant communities.     

The effect ofAcremonium coenophialum on the growth and nematode infestation of tall fescue

The presence of the endophytic fungusAcremonium coenophialum Morgan-Jones et Gams in tall fescue (Festuca arundinacea Schreb.) induces toxicity when this grass is grazed by cattle; however, there is evidence that removing the endophyte reduces the stand vigor and longevity of fescue. A field trial was conducted to determine the effects of water supply and the presence of the endophytic fungus on plant growth, drought tolerance, and soil nematode populations in Kentucky 31 tall fescue. The design included two factors, level of endophyte infection (0 and 75%) and irrigation regime (none, low, and high). Where water deficits occurred, herbage yield and leaf area were lower, and percentage dead tissue and canopy minus air temperature were greater in endophyte-free compared with endophyte-infected fescue. Soil populations ofPratylenchus scribneri andTylenchorhynchus acutus were substantially higher in the noninfected than in the endophyte-infected plots. The endophyte apparently confers drought tolerance to Kentucky 31 tall fescue, and this effect may be at least partially mediated through enhanced resistance to soil-borne nematodes.Published with the approval of the Director of the Ark. Agric. Exp. Stn.     

Effect of nitrogen and phosphorus status on the translocation of three herbicides in field bindweed (Convolvulus arvensis L.)

Twenty-eight day old field bindweed plants grown in culture solutions deficient in nitrogen (N) or phosphorus (P) for the last seven days of growth translocated significantly less foliarly applied dicamba (3,6-dichloro-o-anisic acid) and 2,4-D [(2,4-dichlorophenoxy) acetic acid] to their roots than did plants grown in complete nutrient solutions. In contrast, N deficiency stimulated basipetal translocation of glyphosate [N-(phosphonomethyl) glycine] and inhibited its acropetal translocation in field bindweed. Deficiencies of both N and P decreased translocation of dicamba from the treated area, but had no influence on translocation of glyphosate or 2,4-D from the treated area.Journal Article No. 4406 of the Agric. Exp. Stn., Oklahoma State University.     

Nitrogen release from roots of alfalfa and soybean grown in sand culture

An enclosed root chamber containing sterile sand medium was used to study net nitrogen (N) release from actively growing root systems of ‘Saranac’ alfalfa (Medicago sativa L.) and ‘Fiskeby V’ soybean (Glycine max L. Merr.). Plants were inoculated with a rhizobial strain appropriate to each host, irrigated with N-free nutrient solution, and grown either to 85 or to 173 d after germination (alfalfa) or to physiological maturity (soybean). Alfalfa released 4.5% of symbiotically-fixed plant N into the root zone over its growth period; soybean released 10.4% of plant N. Root zone leachates were analyzed for total N and for amino acid and ammonium content. Significant ammonium-N release occurred from the alfalfa but not the soybean root system; little amino-N was released by root systems of either species. Shoot harvest and water deficit caused increased release of N from alfalfa roots. The results provide evidence that alfalfa and soybean released significant proportions of their N into the root zone, and indicate that while substantial ammonium-N was released from alfalfa roots, passive leakage of amino-N was not a primary mechanism for N release from root systems of either species. Cooperative investigation of USDA-ARS and the Minnesota Agric. Exp. Stn. (Scientific J. Series No. 16048). This research was supported in part by a Graduate School Fellowship to LSB from the Univ. of Minnesota.     

Genetic variation in tissue cultures of red clover

Summary Design II matings were made among randomly selected clones of Arlington red clover (Trifolium pratense L.). Progeny were evaluated in vitro on two regeneration media for callus growth and differentiation. Additive genetic variance was a significant source of variability for nearly all traits evaluated, including somatic embryogenesis. In vitro traits, such as rapid callus growth, colony vascularization, root initiation, chlorophyll production and embryogenesis were highly heritable and should respond to breeding and selection. Dominance genetic variance was significant for only a few in vitro characters. Maternal and cytoplasmic factors were significant primarily in the early subcultures. Highly significant additive genetic correlation of performance on two regeneration media was found. A population selected on one of the regeneration media for such characteristics as improved plantlet regeneration, rapid callus growth, long term colony viability or the frequency of root initiation should show correlated improvement on the other medium. No significant differences for embryogenesis were attributable to differences in the regeneration media used. Furthermore, no interaction of additive genetic effects with regeneration media were observed. These data indicate that improvement in the frequency of plantlet regeneration from callus of red clover could effectively be achieved by breeding and selection for embryogenic types.The research reported in this paper (No. 80-3-152) is in connection with a project of the Kentucky Agric. Exp. Stn. and the paper is published with the approval of the director. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree     

Effect of soil pH and sewage sludge on VA mycorrhizal infection of soybeans

Summary Small plots were amended in 1976 or 1978 with four kinds of sewage sludge. The sludges represented samples considered to be relatively free of heavy metals as well as sludges highly contaminated with heavy metals. Sludges were added to a silt loam soil at rates of 224 or 448 Mgha⁻¹. The soils were maintained at a high or low pH regime. In 1984, soybeans (Glycine max L. Merril. var. ‘Clark’) were planted and grown to the R4 stage. After harvest, roots were removed from the soil, washed, and examined for VA mycorrhizal infection. It was found that the heavy metal content of the sludge alone was generally not related to determining the extent of mycorrhizal infection. A heat treated sludge, high in heavy metals, exhibited the highest degree of mycorrhizal infection when the soil was maintained at a pH of 6.2. With this treatment, 52% of the root segments examined were infected by mycorrhiza. When the same sludge was added to a soil with a slightly lower pH (5.7) none of the roots examined were infected by mycorrhiza. When soybean roots were examined from soils that received no sludge and were maintained at either a low (5.6) or high (6.2) pH, there was no significant difference in mycorrhizal infection between the pH regimes. These results therefore indicate that sewage sludge may inhibit mycorrhizal infection if the sludge contains a high concentration of heavy metals and the sludge is applied to the soil with a low pH. Scientific Article No. A-4093 and Contribution No 7078 of the Maryland Agric. Exp. Stn., Dept. of Agronomy, University of Maryland, College Park, MD 20742.     

Effects of crop rotation and fertilization on catalase activity in a soil of the southeastern United States

Summary Catalase activity of a loamy sand under a 3-year crop rotation in the southeastern U.S.A. was monitored. Corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean [Glycine max (L.) Merr.] were the summer crops in the rotation. Winter wheat (Triticum aestivum L.) was planted after corn, and soybean was followed by a winter fallow period. Cotton was followed by a mixture of common vetch (Vicia sativa L.) and crimson clover (Trifolium incarnatum Gibelli & Belli) which was eventually plow-incorporated as a green manure. Highest mean catalase activities were recorded in soil under the wheat, soybean, and winter legume crops; lowest activities were found in soil bearing corn and cotton, and during the winter fallow period. The fertilization regime influenced soil catalase activity independently of the crop. Soil deficient in any of the major elements showed low enzyme activity. Highest activity was found in soil fertilized with P and K, and with N supplied by a winter legume crop. Addition of supplementary mineral nitrogen to this regime reduced catalase activity. Elimination of the winter legume crop from an otherwise complete fertilization regime resulted in a drastic reduction in enzyme activity. In soil receiving a complete fertilization regime there was a close correlation between soil catalase and xylanase activities. A similar correlation between these two enzymes was not found in soil receiving incomplete fertilization.     

Inoculant made of encapsulated Frankia: assessment of Frankia growth within alginate beads

The growth of Frankia cells within alginate beads was inhibited when the amount encapsulated exceeded 0.5 to 2.5 g protein/ml of beads. Frankia growth was observed not only in the beads incubated in nutrient media (with of without combined N), but also in those incubated in air provided they retained enough nutrients. The results allow some recommendations to be made for the preparation of Frankia inoculants.L. Frioni is with the Facultad de Agronomia, Av. Garzon 780, Montevideo, Uruguay. C. Le Roux, H.G. Diem and Y.R. Dommergues are with ORSTOM/CIRAD-Forêts BSFT Laboratory, 45 bis Av. de la Belle Gabrielle, 94736 Nogent-sur-Marne Cedex, France     

Improving cultivated oats (A vena sativa L.) with alleles for vegetative growth index from A. sterilis L.

Summary Ten Avena sterilis L. lines of Mediterranean origin were crossed with six A. sativa L. cultivars from the North Central USA. Additionally, six intervarietal crosses were made among the A. sativa cultivars. F₂- derived lines from each cross type (interspecific and intraspecific) were evaluated for transgressive segregation for grain yield and several vigor traits. Mean percentages of transgressive segregates one LSD_0.05 above the high parent for vegetative growth index and biomass were 9.0% and 9.8%, respectively, from interspecific crosses, but only 4.5% and 2.9%, respectively, from intraspecific crosses. However, there were two and a half times more high transgressive segregates for grain yield from intra than from interspecific crosses. The maximum vegetative growth index among segregates from interspecific crosses was 0.2 q/day/ha greater than the highest segregate from intraspecific crosses. However, mean harvest index was reduced materially by the introgression of A. sterilis germplasm. Because there was no genetic association between vegetative growth index and harvest index, however, it should be possible to improve both harvest index and vegetative growth index and, thus, the grain yield of cultivated oats.Journal Paper No. J-11228 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011. Project 2447