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1.
The effects of inoculating soil with a water suspension of Bradyrhizobium japonicum (i) at seeding, (ii) 7, or (iii) 14 days after planting (DAP), (iv) seed slurry inoculation and (v) seed slurry supplemented
with postemergence inoculation of a water suspension of Bradyrhizobium at 7 or (vi) 14 DAP, on nodulation, N 2 fixation and yield of soybean ( Glycine max. [L.] Merrill) were compared in the greenhouse. The 15N isotope dilution technique was used to quantify N 2 fixed at flowering, early pod filling and physiological maturity stages (36, 52 and 70 DAP, respectively). On average, the
water suspension inoculation formed the greatest number of nodules, and seed plus postemergence inoculation formed slightly
more nodules than the seed-only inoculated plants (27, 19 and 12 nodules/plant respectively at physiological maturity). Seed
slurry inoculation followed by postemergence inoculation at 14 DAP gave the highest nodule weight, with the plants fixing
significantly more ( P<0.05) N 2 (125 mg N plant −1 or 56% N) than any other treatment (mean, 75 mg plant −1 or 35% N). However, the higher N 2 fixation was not translated into higher N or dry matter yields. Estimates of N 2 fixed by the ostemergence Bradyrhizobium inoculations as well as plant yield were not significantly different from those
of the seed slurry inoculation. Thus, delaying inoculation ( e.g., by two weeks as in this study) did not reduce the symbiotic ability of soybean plants. 相似文献
2.
The adzuki bean ( Vigna angularis (Wild.) Ohwi and Ohashi) and common bean ( Phaseolus vulgaris L.) have a high physiological demand for N. A 2-year field study was conducted to investigate the seasonal change of available soil N and symbiotic N 2 fixation usage. The beans were seeded at two densities, 22.2 plants m –2 with a row spacing of 0.3 m and 11.1 plants m –2 with a row spacing of 0.6 m. The amount of fixed N 2 in the shoot was calculated using the 15N natural abundance method. The common bean demonstrated low N 2 fixation and the ability to accumulate high levels of soil N. Soil nitrate under the common bean was continually absorbed. The adzuki bean, on the other hand, had a remarkable peak of N accumulation in the early reproductive stage. This was mainly due to N 2 fixation, though the soil nitrate level was high. Narrowing the plant row spacing increased the dry matter yield of both species, but the origin of the increased N differed between the species. For the first 77 DAP in 1999 (73 DAP in 2000) the N increase for both beans was due to both soil and atmospheric N 2. At harvest, though, the increase of N in common bean was mainly due to soil N, while that in adzuki bean was mainly due to atmospheric N 2. It can be concluded that the low symbiotic N 2 fixation ability of common bean was due to its high soil N uptake ability and constant N accumulation, which enabled an efficient soil N absorption. Adzuki bean absorbed N mainly for a short period and depended more on symbiotically fixed N 2 and, in contrast to common bean, left a high level of NO 3-N remaining in the soil after cropping. 相似文献
3.
The effect of inoculation of N 2 fixation by soybean plants, grown in sandy soil was studied in pot experiments. Bradyrhizobium japonicum (Rh) and/or mycorrhizæ, in the presence of basic application of a P fertilizer (super or rock P), and two levels of 15N-labelled ammonium sulfate (20 and 100 mg N per kg soil), were used. Highest N 2 fixation was observed after a dual inoculation (Rh+VAM), followed by single inoculation (Rh) and by mycorrhizal infection. Higher doses of N fertilizer depressed the capacity of the plant nodules and the inoculants for N 2 fixation. 相似文献
4.
Soybean ( Glycine max [L.] Merr.) N 2 fixation is a primary plant mechanism responsible for meeting plant-N demand during seed development. Nitrogen fixation is recognized as a drought-sensitive mechanism; however, N 2 fixation response to water deficit and N 2 fixation recovery at different reproductive stages are not well documented. We tested the hypothesis that water deficit during late reproductive stages would inhibit N 2 fixation and lead to the breakdown of essential leaf proteins and an inability to recover N 2 fixation. Acetylene reduction activity (ARA) and N redistribution response to a 5-d drought period at flowering (R2), early seed fill (R5), and late seed fill (R6) were evaluated in one genotype (Hendricks, maturity group 0). Control plants maintained high rates of nodule activity until late seed fill. Plants drought stressed at R2 and R5 recovered ARA after rewatering and in some cases had higher nitrogenase activity than control plants during mid-seed fill. Recovery of ARA on plants stressed at R2 and R5 was associated with higher shoot N concentration than control plants at maturity. Drought stress at R6 reduced ARA, and the inability to recover ARA after stress alleviation at R6 resulted in decreased individual seed mass, which was likely caused by an acceleration of leaf N redistribution and a shorter seed-fill period. Results emphasized the importance of soybean N 2 fixation during late seed development on seed yield and that the ability to recover N 2 fixation following drought is dependent upon crop developmental stage. 相似文献
5.
Background and Aims Understanding the impact of soil rhizobial populations and inoculant rhizobia in supplying sufficient nodulation is crucial to optimising N 2 fixation by legume crops. This study explored the impact of different rates of inoculant rhizobia and contrasting soil rhizobia on nodulation and N 2 fixation in faba bean ( Vicia faba L.). Methods Faba beans were inoculated with one of seven rates of rhizobial inoculation, from no inoculant to 100 times the normal rate of inoculation, sown at two field sites, with or without soil rhizobia present, and their nodulation and N 2 fixation assessed. Results At the site without soil rhizobia, inoculation increased nodule number and increased N 2 fixation from 21 to 129 kg shoot N ha ?1, while N 2 fixation increased from 132 to 218 kg shoot N ha ?1 at the site with high background soil rhizobia. At the site without soil rhizobia, inoculation increased concentrations of shoot N from 14 to 24 mg g ?1, grain N from 32 to 45 mg g ?1, and grain yields by 1.0 Mg (metric tonne) ha ?1. Differences in nodulation influenced the contributions of fixed N to the system, which varied from the net removal of 20 kg N ha ?1 from the system in the absence of rhizobia, to a net maximum input of 199 kg N ha ?1 from legume shoot and root residues, after accounting for removal of N in grain harvest. Conclusions The impact of inoculation and soil rhizobia strongly influenced grain yield, grain N concentration and the potential contributions of legume cropping to soil N fertility. In soil with resident rhizobia, N 2 fixation was improved only with the highest inoculation rate. 相似文献
6.
This study tested the hypothesis that inoculation of soybean ( Glycine max Merr.) with a Bradyrhizobium japonicum strain (USDA110) with greater N 2 fixation rates would enhance soybean response to elevated [CO 2]. In field experiments at the Soybean Free Air CO 2 Enrichment facility, inoculation of soybean with USDA110 increased nodule occupancy from 5% in native soil to 54% in elevated [CO 2] and 34% at ambient [CO 2]. Despite this success, inoculation with USDA110 did not result in greater photosynthesis, growth or seed yield at ambient or elevated [CO 2] in the field, presumably due to competition from native rhizobia. In a growth chamber experiment designed to study the effects of inoculation in the absence of competition, inoculation with USDA110 in sterilized soil resulted in nodule occupation of >90%, significantly greater 15N 2 fixation, photosynthetic capacity, leaf N and total plant biomass compared with plants grown with native soil bacteria. However, there was no interaction of rhizobium fertilization with elevated [CO 2]; inoculation with USDA110 was equally beneficial at ambient and elevated [CO 2]. These results suggest that selected rhizobia could potentially stimulate soybean yield in soils with little or no history of prior soybean production, but that better quality rhizobia do not enhance soybean responses to elevated [CO 2]. 相似文献
7.
Pigeon peas [ Cajanus cajan (L.) Millsp.] were grown in soil columns containing 15N-enriched organic matter. Seasonal N 2 fixation activity was determined by periodically assaying plants for reduction of C 2H 2. N 2 fixation rose sharply from the first assay period at 51 days after planting to a peak of activity between floral initiation and fruit set. N 2 fixation (acetylene reduction) activity dropped concomitantly with pod maturation but recovered after pod harvests. Analysis of 15N content of plant shoots revealed that approximately 91 to 94% of plant N was derived from N 2 fixation. The effect of inoculation with hydrogenase-positive and hydrogenase-negative rhizobia was examined. Pigeon peas inoculated with strain P132 (hydrogenase-positive) yielded significantly more total shoot N than other inoculated or uninoculated treatments. However, two other hydrogenase-positive strains did not yield significantly more total shoot N than a hydrogenase-negative strain. The extent of nodulation by inoculum strains compared to indigenous rhizobia was determined by typing nodules according to intrinsic antibiotic resistance of the inoculum strains. The inoculum strains were detected in almost all typed nodules of inoculated plants. Gas samples were taken from soil columns several times during the growth cycle of the plants. H2 was never detected, even in columns containing pigeon peas inoculated with hydrogenase-negative rhizobia. This was attributed to H2 consumption by soil bacteria. Estimation of N2 fixation by acetylene reduction activity was closest to the direct 15N method when ethylene concentrations in the gas headspace (between the column lid and soil surface) were extrapolated to include the soil pore space as opposed solely to measurement in the headspace. There was an 8-fold difference between the two acetylene reduction assay methods of estimation. Based on a planting density of 15,000 plants per hectare, the direct 15N fixation rates ranged from 67 (noninoculated) to 134 kilograms per hectare, while grain yields ranged from 540 to 825 kilograms per hectare. Grain yields were not increased with N fertilizer. 相似文献
8.
Field and greenhouse experiments were conducted to assess the nitrogen fixation rates of four cultivars of common bean ( Phaseolus vulgaris L.) at different growth stages. The 15N isotope dilution technique was used to quantify biological nitrogen fixation. In the greenhouse, cultivars M4403 and Kallmet accumulated 301 and 189 mg N plant –1, respectively, up to 63 days after planting (DAP) of which 57 and 43% was derived from atmosphere. Under field conditions, cultivars Bayocel and Flor de Mayo RMC accumulated in 77 DAP, 147 and 135 kg N ha –1, respectively, of which approximately one-half was derived from the atmosphere. The rates of N 2 fixation determined at different growth stages increased as the plants developed, and reached a maximum during the reproductive stage both under field and greenhouse conditions. Differences in translocation of N were observed between the cultivars tested, particularly under field conditions. Thus, the fixed N harvest index was 93 and 60 for cultivars Flor de Mayo and Bayocel, respectively. In early stages of growth, the total content of ureides in the plants correlated with the N fixation rates. The findings reported in the present paper can be used to build a strategy for enhancing biological N 2 fixation in common bean. 相似文献
9.
The relationship between ureide N and N 2 fixation was evaluated in greenhouse-grown soybean ( Glycine max L. Merr.) and lima bean ( Phaseolus lunatus L.) and in field studies with soybean. In the greenhouse, plant N accumulation from N 2 fixation in soybean and lima bean correlated with ureide N. In soybean, N 2 fixation, ureide N, acetylene reduction, and nodule mass were correlated when N 2 fixation was inhibited by applying KNO 3 solutions to the plants. The ureide-N concentrations of different plant tissues and of total plant ureide N varied according to the effectiveness of the strain of Bradyrhizobium japonicum used to inoculate plants. The ureide-N concentrations in the different plant tissues correlated with N 2 fixation. Ureide N determinations in field studies with soybean correlated with N 2 fixation, aboveground N accumulation, nodule weight, and acetylene reduction. N 2 fixation was estimated by 15N isotope dilution with nine and ten soybean genotypes in 1979 and 1980, respectively, at the V9, R2, and R5 growth stages. In 1981, we investigated the relationship between ureide N, aboveground N accumulation, acetylene reduction, and nodule mass using four soybean genotypes harvested at the V4, V6, R2, R4, R5, and R6 growth stages. Ureide N concentrations of young stem tissues or plants or aboveground ureide N content of the four soybean genotypes varied throughout growth correlating with acetylene reduction, nodule mass, and aboveground N accumulation. The ureide-N concentrations of young stem tissues or plants or aboveground ureide-N content in three soybean genotypes varied across inoculation treatments of 14 and 13 strains of Bradyrhizobium japonicum in 1981 and 1982, respectively, and correlated with nodule mass and acetylene reduction. In the greenhouse, results correlating nodule mass with N 2 fixation and ureide N across strains were variable. Acetylene reduction in soybean across host-strain combinations did not correlate with N 2 fixation and ureide N. N 2 fixation, ureide N, acetylene reduction, and nodule mass correlated across inoculation treatments with strains of Bradyrhizobium spp. varying in effectiveness on lima beans. Our data indicate that ureide-N determinations may be used as an additional method to acetylene reduction in studies of the physiology of N 2 fixation in soybean. Ureide-N measurements also may be useful to rank strains of B. japonicum for effectiveness of N 2 fixation. 相似文献
10.
Summary Soybeans ( Glycine max (L.) Merr.) have a high N requirement which is fulfilled by soil N uptake and N 2-fixation. This study was concerned with the effects of past yield selection on N 2-fixation in soybeans.The soybean cultivars, Lincoln, Shelby, and Williams, which represent successive improvements in the Lincoln germplasm, and a non-nodulating control were planted in a soil containing 15N labelled organic matter. Two replications occurred on soil previously cropped to alfalfa and two on soil previously cropped to soybeans. Plants were harvested at five growth stages and leaf area, plant weight, total N, and atom percent 15N were determined. Mature grain was harvested and yield components were also determined, as well as the total N and 15N content.Cultivar differences in total dry matter were only evident at physiological maturity, when Williams contained the greatest dry matter. Williams exhibited the longest period of seed formation and seed fill and also had the highest grain yield which resulted from a larger weight per seed.The N content of the cultivars did not vary until physiological maturity when Williams contained the highest percent N. The quantity of N fixed at physiological maturity was highest for Williams and lowest for Lincoln. Fixed N contained in the harvested grain was greater for Williams than for the other two cultivars. The fraction of the total plant N derived from fixation was not greatly affected by cultivar and all cultivars acquired an average of 50% of their total N through N 2-fixation.Previous cropping history greatly affected the quantity of N fixed and the fraction of the total plant N derived from fixation. Soybeans following soybeans were more dependent upon N 2-fixation than soybeans following alfalfa with the former deriving 65% of the total plant N from fixation and the latter only 32%. These soybean cultivars apparently utilized soil N first and then used N 2-fixation to satisfy their N requirement.The past selection for higher yield has resulted in soybean cultivars with improved capacities to fix atmospheric N 2 and an improved ability to take up available soil N. 相似文献
11.
During the past 10 years estimates of N 2 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 N 2 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 N 2 fixation associated with non-legumes.Field data indicate that associative N 2 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 N 2 fixation, and that inoculation responses due to N 2 fixation can occur. Wheat can also respond to inoculation but responses do not appear to be due to associative N 2 fixation. 15N dilution studies confirm that substantial amounts of N 2 can be fixed by actinorhizal plants. 相似文献
12.
Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N 2 fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine ( Lupinus nootkatensis Donn) and gray alder ( Alnus incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH +
4 and NO –
3 as collected on ion exchange membranes. We also estimated N 2 fixation activity of the N 2-fixing plants by the natural abundance of 15N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N 2-fixing plants was almost totally derived from N 2 fixation. Plots containing N 2-fixing species generally had significantly higher soil N and N availability than a control plot without N 2-fixing plants. Taken together, all measurements indicated that N 2-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden. 相似文献
13.
Controlled environment and field studies were conducted to determine relationships between various measurements of N 2 fixation using soybeans and to use these measures to evaluate a number of Bradyrhizobium japonicum strains for effectiveness in N 2 fixation in Thai soils. 15N dilution measurements of N 2 fixation showed levels of fixation ranging from 32 to 161 kg N ha −1 depending on bacterial strain, host cultivar and location. Midseason measures of N 2 fixation were correlated with each other, but not related measures taken at maturity. Ranking of B. japonicum strains based on performance under controlled conditions in N-free media were highly correlated with rankings based on soybean
seed yields and N 2 fixation under field conditions. This study showed that inoculation of soybeans with effective B. japonicum strains can result in significant increases in yield and uptake of N through fixation. The most effective strains tested
for use in Thai conditions were those isolated from Thai soils; however, effective strains from other locations were also
of benefit. 相似文献
14.
Short-term studies of tree growth at elevated CO 2 suggest that forest productivity may increase as atmospheric CO 2 concentrations rise, although low soil N availability may limit the magnitude of this response. There have been few studies of growth and N 2 fixation by symbiotic N 2-fixing woody species under elevated CO 2 and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO 2 on growth, tissue N accretion, and N 2 fixation of nodulated Alnus glutinosa (L.) Gaertn. grown under low soil N conditions for 160 d. Root, nodule, stem, and leaf dry weight (DW) and N accretion increased significantly in response to elevated CO 2. Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta- 15N analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO 2 grown plants fixed approximately 40% more N than did ambient CO 2 grown plants. Leaves from both CO 2 treatments showed similar relative declines in leaf N content prior to autumnal leaf abscission, but total N in leaf litter increased 24% in elevated compared to ambient CO 2 grown plants. These results suggest that with rising atmospheric CO 2 N 2-fixing woody species will accumulate greater amounts of biomass N through N 2 fixation and may enhance soil N levels by increased litter N inputs. 相似文献
15.
Summary A two-year field study was undertaken using 15N isotope techniques to differentiate between stimulation of N uptake and N 2 fixation in Western Canadian cultivars of spring wheat ( Triticum aestivum L. emend Thell) and durum ( T. turgidum L. emend Bowden) in response to inoculation with N 2-fixing bacteria.
Bacterial inoculation either had no effect or lowered the % N derived from the fertilizer and the fertilizer use efficiency.
Despite the depression of fertilizer uptake, inoculants did not alter the relative uptake from soil and fertilizer-N pools
indicating that bacterial inoculation did not alter rooting patterns. Nitrogen-15 isotope dilution indicated that N 2 fixation did occur.
In 1984, % plant N derived from the atmosphere (% Ndfa) due to inoculation with Bacillus C-11-25 averaged 23.9% while that
with Azospirillum brasilense ATCC 29729 (Cd) averaged 15.5%. In 1985, higher soil N levels reduced these values by approximately one-half. Cultivar x
inoculant interactions, while significant, were not consistent across years. However, these interactions did not affect cultivars
‘Cadet’ and ‘Rescue’. In agreement with previous results, ‘Cadet’ performed well with all inoculants in both years while ‘Rescue’
performed poorly.
Among 1984 treatments, the N increament in inoculated plants was positively correlated with % Ndfa but no such correlation
existed in 1985. N 2 fixation averaged over all cultivars and strains was 17.9 and 6.7 kg N fixed ha −1 in 1984 and 1985, respectively. Highest rates of N 2 fixation were estimated at 52.4 kg N ha −1 for ‘Cadet’ in 1984 and 31.3 kg N ha −1 for ‘Owens’ in 1985, both inoculated with Bacillus C-11-25, an isolate from southern Alberta soils. Inoculation with either
of Azospirillum brasilense strain Cd (ATCC29729) or 245 did not result in as consistent or as high N 2 fixation, suggesting that these wheats had not evolved genetic compatability with this exogenous microorganism. These agronomically
significant amounts of N 2 fixation occurred under optimally controlled experimental conditions in the field. It is yet to be determined if N 2 fixation would occur in response to bacterial inoculation under dryland conditions commonly occurring in Western Canada.
Contribution from Agriculture Canada Research Station, Lethbridge, Alberta, Canada. 相似文献
16.
During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N 2 and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N 2 fixation and endogenous N remobilization were estimated by 15N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N 2 or from NH 4NO 3 during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N 2 fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH 4NO 3 grown and in N 2 fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine. 相似文献
17.
In 2005 and 2006, 30 and 15 cowpea genotypes were respectively evaluated for plant growth and symbiotic performance at Manga in Northern Ghana, in order to identify N 2-fixing potential of these cowpea genotypes as source of N for cropping systems. The results showed differences in biomass production by the 30 or 15 cowpea genotypes. In 2005, cultivars Fahari, Mchanganyiko, IT97K-499-39, IT93K-2045-29 and IT84S-2246 produced the most shoot biomass, while Apagbaala, Brown Eye, ITH98-46, Vita 7 and Iron Grey produced the least. Of the 15 genotypes tested in 2006, cv. TVu11424 produced the largest amount of biomass, and ITH98-46, the least. Isotopic analysis of 15N in plant parts also revealed significant differences in δ 15N of the cowpea genotypes studied. As a result, the percent N derived from fixation (% Ndfa) also differed among the cowpea genotypes tested in 2005, with only 5 out of the 30 cultivars obtaining over 50% of their N from symbiotic fixation. Whether expressed as mg N.plant ?1 or kg N.ha ?1, the levels of N 2 fixation by the cowpea genotypes varied considerably during 2005 and 2006, with values of N contribution ranging from 14.1 kg N.ha ?1 by cv. TVu1509 to 157.0 kg N.ha ?1 by IT84S-2246 in 2005. The amounts of N-fixed in 2006 ranged from 16.7 kg N.ha ?1 by cv. ITH98-46 to 171.2 kg N.ha ?1 by TVu11424, clearly indicating genotypic differences in symbiotic N yield. Re-evaluating 15 out of the 30 cowpea genotypes for N 2 fixation in 2006, revealed higher % Ndfa values (>50%) in all (15 cowpea genotypes) relative to those tested in 2005, indicating greater dependence on N 2 fixation for their N nutrition even though, the actual amounts of fixed-N were lower in 2006. This was due, in part, to reduced plant biomass as a result of very late sampling in 2006, close to physiological maturity (72 DAP in 2006 vs. 46 DAP in 2005) when considerable leaf matter was lost. The amount of N-fixed in 2006 can therefore be considered as being under-estimated. 相似文献
18.
The response of legumes to inoculation with rhizobia can be affected by many factors. Little work has been undertaken to examine how indigenous populations or rhizobia affect this response. We conducted a series of inoculation trials in four Hawaiian soils with six legume species ( Glycine max, Vigna unguiculata, Phaseolus lunatus, Leucaena leucocephala, Arachis hypogaea, and Phaseolus vulgaris) and characterized the native rhizobial populations for each species in terms of the number and effectiveness of the population for a particular host. Inoculated plants had, on average, 76% of the nodules formed by the inoculum strain, which effectively eliminated competition from native strains as a variable between soils. Rhizobia populations ranged from less than 6 × 10 0/g of soil to 1 × 10 4/g of soil. The concentration of nitrogen in shoots of inoculated plants was not higher than that in uninoculated controls when the most probable number MPN counts of rhizobia were at or above 2 × 10 1/g of soil unless the native population was completely ineffective. Tests of random isolates from nodules of uninoculated plants revealed that within most soil populations there was a wide range of effectiveness for N 2 fixation. All populations had isolates that were ineffective in fixing N 2. The inoculum strains generally did not fix more N 2 than the average isolate from the soil population in single-isolate tests. Even when the inoculum strain proved to be a better symbiont than the soil rhizobia, there was no response to inoculation. Enhanced N 2 fixation after inoculation was related to increased nodule dry weights. Although inoculation generally increased nodule number when there were less than 1 × 10 2 rhizobia per g of soil, there was no corresponding increase in nodule dry weight when native populations were effective. Most species compensated for reduced nodulation in soils with few rhizobia by increasing the size of nodules and therefore maintaining a nodule dry weight similar to that of inoculated plants with more nodules. Even when competition by native soil strains was overcome with a selected inoculum strain, it was not always possible to enhance N 2 fixation when soil populations were above a threshold number and had some effective strains. 相似文献
19.
The δ 15N natural abundance (‰) of the total soil N pool varies at the landscape level, but knowledge on short-range variability and
consequences for the reliability of isotopic methods are poorly understood. The short-range spatial variability of soil δ 15N natural abundance as revealed by the 15N abundance in spring barley and N 2-fixing pea was measured within the 0.15–4 m scale at flowering and at maturity. The short-range spatial variability of soil
δ 15N natural abundance and symbiotic nitrogen fixation were high at both growth stages. Along a 4-m row, the δ 15N natural abundance in barley reference plants varied up to 3.9‰, and sometimes this variability was observed even between
plants grown only 30 cm apart. The δ 15N natural abundance in pea varied up to 1.4‰ within the 4-m row. The estimated percentage of nitrogen derived from the atmosphere
(%Ndfa) varied from 73–89% at flowering and from 57–95% at maturity. When increasing the sampling area from 0.01 m 2 (single plants) and up to 0.6 m 2 (14 plants) the %Ndfa coefficient of variation (CV) declined from 5 to 2% at flowering and from 12 to 2% at maturity. The
implications of the short-range variability in δ 15N natural-abundance are that estimates of symbiotic N 2-fixation can be obtained from the natural abundance method if at least half a square meter of crop and reference plants is
sampled for the isotopic analysis. In fields with small amounts of representative reference crops (weeds) it might be necessary
to sow in reference crop species to secure satisfying N 2-fixation estimates. 相似文献
20.
Grain legumes such as field pea are known to have high variability of yield and dinitrogen (N 2) fixation between seasons, but less is known about the yearly spatial variability within a field. The objective of this study was to improve the understanding of spatial field scale variability of field pea dry matter (DM) yield and nitrogen (N) acquisition from fixation and soil within a 10 ha farmer’s field. A 42 m systematic random grid providing 56 plant sampling locations across 10 ha supplemented by soil data provided from an existing database were used to determine whether the observed spatial variability could be explained by the variability in selected abiotic soil properties. All measured soil variables showed substantial variability across the field and the pea dry matter production ranged between 4.9 and 13.8 Mg ha ?1 at maturity. The percent of total N derived from the atmosphere (%Ndfa) at flowering, estimated using the 15N natural abundance method, ranged from 65% to 92% with quantitative N 2-fixation estimates from 93 kg to 202 kg N ha ?1. At maturity %Ndfa ranged from 26% to 81% with quantitative N 2-fixation estimates from 48 kg to 167 kg N ha ?1. Significant correlations were found between pea dry matter production and humus content, potassium content (collinear with humus) and total N in the 0–25 cm topsoil. No correlation was found between any individual soil property and %Ndfa or kg N fixed ha ?1. It was not possible to create a satisfactory global multi-regression model for the field dry matter production and N 2-fixation. A number of other models were tested, but the best was only able to explain less than 40% of the variance in %Ndfa using seven soil properties. Together with the use of interpolated soil data, high spatial variation of soil 15N natural abundance, a mean increase in pea 15N natural abundance of 1 δ unit between flowering and maturity and a reference crop decline of 1.3 δ 15N unit over the same period increased noise of derived variables, making modeling of N 2-fixation difficult. Furthermore, complex interactions with other soil variables and biotic stresses not measured in this study may have contributed significantly to the variability of fixation and yield of pea within the field. Pea N 2-fixation obtained from two additional 10 ha farmer fields was in agreement with the other findings highlighting that N 2-fixation takes place under a range of physical and chemical soil properties and is controlled by local site specific conditions. In future studies addressing field scale variability we recommend that soil variables wherever possible should be measured in the same plots as the sampled crop. Sampling designs that optimize the use of a priori information about the field soil and landscape properties for positioning plots and that facilitate estimates of local variances should be considered. 相似文献
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