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1.
In this study, 30 cowpea genotypes were assessed for symbiotic N2 fixation in 2005, and 15 of them were re-evaluated in 2006 using the 15N natural abundance technique. Shoot dry matter yield of cowpea genotypes increased significantly in cvs. Vuli-1, Glenda, IT93K-2045-29, IT90K-59, Omondaw, Apagbaala, and IT84S-2246 in 2005 producing about 3.0 to 3.6-fold more biomass relative to cv. Vallenga. In 2006, seven out of the 15 cowpea genotypes tested (namely, IT97K-499-39, TVu11424, Botswana White, IT84S-2246, Sanzie, Brown Eye, and Glenda) also produced more dry matter than cv. CH14. Shoot δ15N values ranged from −0.58‰ to 1.49‰ in 2005, and −1.51‰ to 1.40‰ in 2006, and these resulted in %Ndfa values of 63.5–86.7% and 56.2–96.3%, respectively. The amount of N-fixed was 49–178 kg N ha−1 in 2005 and 62–198 kg N ha−1 in 2006. Furthermore, there was a direct relationship between the level of symbiotic N nutrition and plant growth, and between grain yield and amount of N-fixed in 2005 and 2006. As a result, genotypes that fixed the most N also produced the largest biomass and the greatest amount of grain yield. The observed relationship between N2 fixation and biomass confirmed our view that cowpea (and other grain legumes) can be concurrently selected for higher N2 fixation, superior plant growth, and greater grain yield. The high levels of N-fixed by many of the cowpea genotypes in this study suggest that they can contribute large amounts of N to cropping systems in African agriculture.  相似文献   

2.
Biological nitrogen fixation (BNF) of 17 soybean cultivars was comparatively estimated by the δ15N natural abundance technique using two non-nodulation soybeans (Clay and Chippewa) as reference plants. A field study was established on the experimental farm of the University of Abomey-Calavi, Benin on a typical “terre de barre” soil classified by Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organisation as Rhodic Ferralsol. A nitrogen-free pot trial was also carried out using soil substrate sampled from the Atlantic Ocean beach. In the N-free medium, N content of the whole soybean cultivars ranged from 2.6 to 8.1 mg N per plant compared with an average of 1.8 mg N per plant observed with the non-fixing soybeans. Plant δ15N of the nodulating soybeans ranged from −2.7756‰ (Jupiter) to 0.1951‰ (Conquista), while the non-nodulating cultivars Chippewa and Clay had 2.67‰ and 9.30‰, respectively. Percentage and amount of N derived from air (Ndfa) were significantly different (P < 0.01) among soybean cultivars, and values depended highly on the selected reference plants. When Clay was used as the reference plant, the average percentage Ndfa was 1.4 times higher than when Chippewa was the reference plant. Both reference plants consistently ranked promiscuous soybean cvs. TG× 1894 3F and TG× 1908 8F as the best cultivars and cv. TG× 1888 29F as the least in percentage Ndfa, suggesting that any of the reference plants could be used in δ15N method for assessing N2-fixation. The two identified promiscuous soybean cultivars with greatest capacity to fix N could be included in a soybean extension program for West African farming systems.  相似文献   

3.
Nitrogen (N) deposition to the ocean is thought to be increasing worldwide, but the amount of coastal and open ocean measurements is very limited. In this paper, we assess N deposition in the coastal zone of Cayo Coco, in central Cuba, during a multi-annual period (2005–2007). Wet and dry N depositions were estimated based on the NH4+ and NOx– concentrations in the rain. Cold fronts and troughs, coming from the west, contributed most to rain (41%) and to N deposition, followed by tropical waves and storms coming from the east, which caused 31% of the rain. Average concentrations of NH4+ and NOx– in the rain were 8.8 and 8.3 μM. NOx– presented a clearly decreasing trend (0.26 μM per month), decreasing by half during 2005–2007. Total N deposition averaged 3.23 kg N ha−1 year−1, similar to that found in Virgin Islands and Puerto Rico, but lower than previously measured in Cuba and in nearby areas of the USA and than model predictions for the oceanic region around Cuba. These low values and the decreasing trend found are attributed to drastic reduction of fossil fuel and fertilizer use in Cuba since 1990. Because land input has decreased even more drastically, deposition seems to be nowadays the most important N source to the coastal zone of Cayo Coco. The δ15N range of seagrass (Thalassia testudinum) and macroalgae (Penicillus dumetosus) in the area (−1.83‰ to 3.02‰ and +1.02‰ to +4.17‰, respectively) sustain that atmospheric sources (deposition and N2 fixation) comprise 70–90% of the N budget.  相似文献   

4.
A survey of N2 fixation in farmers’ fields of Northern (>1,000 mm rainfall), Central (800–1,000 mm rainfall), and Southern (<800 mm rainfall) Zambia revealed some significant differences in plant growth and symbiotic performance of different food grain legumes. Of the three grain legumes (i.e., Bambara groundnut (Vigna subterranea L. Verdc.), cowpea (Vigna unguiculata L. Walp.), and groundnut (Arachis hypogaea L.)) grown in Southern Zambia, cowpea showed greater shoot biomass and significantly lower shoot δ15N values than groundnut and Bambara groundnut. The lower shoot δ15N resulted in greater %Ndfa (59%) in shoots and higher amounts of N-fixed, whether per square meters (6,394.0 mg N), per plant (650.8 mg N), or per hectare (63.9 kg N) relative to groundnut and Bambara groundnut, even though the number of cowpea plants per square meter was significantly lower than that of groundnut or Bambara groundnut. Although the shoot δ15N values of cowpea, Bambara groundnut and common bean (Phaseolus vulgaris L.) were significantly lower than those of groundnut in Central Zambia and their %Ndfa values, therefore, greater, the higher number of groundnut plants per square meter resulted in significantly greater shoot N content, as well as N-fixed per square meter and per hectare relative to the other species. Despite having similar plant density as cowpea in Central Zambia, common bean could fix only 6.0 kg N ha−1 compared with 35.4 kg N ha−1 by cowpea. In Northern Zambia, Bambara groundnut showed the lowest mean shoot δ15N value (0.54 ± 0.3‰), followed by groundnut (1.59 ± 1.0‰), and then common bean (the three grain legumes grown in that region). As a result, %Ndfa and N-fixed were significantly greater in groundnut (69.7% and 566.0 mg N per plant) and Bambara groundnut (62.9% and 440.1 mg N per plant) than in common bean (2.6% and 2.4 mg N per plant). In Northern Zambia, groundnut, Bambara groundnut and common bean fixed 78.7, 67.6, and 0.9 kg N ha−1, respectively, even though the plant density per square meter of common bean (which fixed the lowest amount of N per hectare) was twice that of groundnut and Bambara groundnut. A species × site analysis showed that cowpea fixed relatively greater amounts of N per plant, per square meter, and per hectare in Southern than Central Zambia. Bambara groundnut and common bean also had significantly lower δ15N values and higher %Ndfa in Central than Northern Zambia.  相似文献   

5.
Different theories have been brought forward to explain the commonly observed δ15N enrichment with depth in soil profiles, including the discrimination against 15N during N decomposition and the buildup of 15N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, 15N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the 15N-enrichment pattern and to link 15N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate (<150 μm) than for the macroaggregate (>150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light (<1.37 g cm−3) and heavy (>1.37 g cm−3) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential 15N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced 15N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large 15N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of 15N-enriched microbial (especially fungal) compounds was an additional driver for 15N enrichment of this soil profile.  相似文献   

6.
The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ13C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ13C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ13C was narrowly constrained in the forest (from −27.96 to −26.09‰) but varied from −15.09 to −28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ13C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ13C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.  相似文献   

7.
 N2 fixation by leguminous crops is a relatively low-cost alternative to N fertilizer for small-holder farmers in developing countries. N2 fixation in faba bean (Vicia faba L.) as affected by P fertilization (0 and 20 kg P ha–1) and inoculation (uninoculated and inoculated) with Rhizobium leguminosarium biovar viciae (strain S-18) was studied using the 15N isotope dilution method in the southeastern Ethiopian highlands at three sites differing in soil conditions and length of growing period. Nodulation at the late flowering stage was significantly influenced by P and inoculation only at the location exhibiting the lowest soil P and pH levels. The percentage of N derived from the atmosphere ranged from 66 to 74%, 58 to 74% and 62 to 73% with a corresponding total amount of N2 fixed ranging from 169 to 210 kg N ha–1, 139 to 184 kg N ha–1 and 147 to 174 kg N ha–1 at Bekoji, Kulumsa and Asasa, respectively. The total N2 fixed was not significantly affected by P fertilizer or inoculation across all locations, and there was no interaction between the factors. However, at all three locations, N2 fixation was highly positively correlated with the dry matter production and total N yield of faba bean. Soil N balances after faba bean were positive (12–58 kg N ha–1) relative to the highly negative N balances (–9–44 kg N ha–1) following wheat (Triticum aestivum L.), highlighting the importance of rotation with faba bean in the cereal-based cropping systems of Ethiopia. Received: 13 January 2000  相似文献   

8.
Studies on Alnus rubra Bong. (red alder) were carried out to assess its potential for use as a component of a silvopastoral system. Comparison was between two treatments: red alder at 400 stems ha−1 (silvopasture) and red alder at 2,500 stems ha−1 (forestry control). δ15N values close to zero were recorded in all red alder plant parts except for root nodules, indicating that a large proportion of N in red alder was fixed from the atmosphere. Overall, it was estimated that there was 63.45 kg N ha−1 fixed N accumulated in red alder trees, and the rate of N fixation was estimated at 30.95 kg ha−1 year−1 in the silvopasture treatment. The total amount of N that could potentially be added to the soil in the silvopasture treatment as a result of decomposition of senescent leaves, roots, and dead nodules was estimated at 40.56 kg ha−1 year−1. Of the total N added to the soil, 27.1 kg ha−1 year−1 was due to N fixation from the atmosphere. These results show that red alder has a potential to improve and maintain soil fertility in a silvopastoral system.  相似文献   

9.
Summary Three15N isotopic dilution methods (15N natural abundance, labelled mineral fertilizer, and organic matter) were used to determine the proportion of N derived from different available sources in seedlines ofAlnus glutinosa andPopulus nigra planted together or in monoculture under natural climatic conditions. The proportion of N derived from N2 fixation in associated alders was appreciably higher than that determined in monoculture. The reduction of soil N uptake by associated alders contributed to an increase in total plant N and biomass production in associated poplars. When slightly N-labelled organic matter (alder leaf litter) was incorporated into the soil, 10–15% of its initial N content was recovered in poplar tissues, showing that this N source makes an important contribution to the N yield of associated non-fixing plants. There were no significant differences between the results obtained by15N natural abundance and those obtained by labelled fertilizer methods, suggesting that the 15N method could be used to evaluate annual N budgets in natural ecosystems.  相似文献   

10.
Atmospheric biological nitrogen fixation (BNF) by cowpea (Vigna unguiculata) and groundnut (Arachis hypogea) was evaluated using a 2-year (2000–2001) experiment with different fertilizer treatments. The 15N isotopic dilution method with a nonfixing cowpea as test reference crop was used. The effects of the two legumes on soil N availability and succeeding sorghum (Sorghum bicolor) yields were measured. Groundnut was found to fix 8 to 23 kg N ha-1 and the percentage of N derived from the atmosphere varied from 27 to 34%. Cowpea fixed 50 to 115 kg N ha−1 and the percentage of N derived from the atmosphere varied from 52 to 56%. Compared to mineral NPK fertilizer alone, legumes fixed more N from the atmosphere when dolomite or manure was associated with mineral fertilizers. Compared to soluble phosphate, phosphate rock increased BNF by cowpea. Significant correlation (p<0.05, R 2=0.94) was observed between total N yields of legumes and total N derived from the atmosphere. Compared to monocropping of sorghum, the soils of cowpea–sorghum and groundnut–sorghum rotations increased soil mineral N from 15 and 22 kg N ha−1, respectively. Cowpea–sorghum and groundnut–sorghum rotations doubled N uptake and increased succeeding sorghum yields by 290 and 310%, respectively. Results suggested that, despite their ability to fix atmospheric nitrogen, N containing fertilizers (NPK) are recommended for the two legumes. The applications of NPK associated with dolomite or cattle manure or NK fertilizer associated with phosphate rock were the better recommendations that improved BNF, legumes, and succeeding sorghum yields.  相似文献   

11.
Dinitrogen fixation by biological soil crusts in an Inner Mongolian steppe   总被引：1，自引：0，他引：1  
Eurasian steppe ecosystems are nitrogen-limited and suffer additionally from high grazing intensities in many areas. Soil surface-bound cyanobacteria are able to fix nitrogen and can be the major source of plant available nitrogen in such ecosystems. In this study, the abundance and dinitrogen fixation capacity of the most common soil surface-bound microbial and lichen species were determined at an ungrazed, a winter-grazed, and a heavily grazed steppe site in the Xilin River catchment, Inner Mongolia, People’s Republic of China. The microorganisms were identified as Nostoc spec. and the lichen species as Xanthoparmelia camtschadalis (Ach.) Hale by a combination of classical light microscopy, confocal laser scanning microscopy and molecular analysis of the internal transcribed spacer (ITS1) region of ribosomal RNA. Both species were found exclusively at grazed steppe sites, with a clear difference in abundance depending on the grazing intensity. At the winter-grazed site, Nostoc was more abundant than Xanthoparmelia; for the heavily grazed site, the opposite was found. N2 fixation was quantified with both the acetylene reduction method and 15N2 incubation. Cyanobacterial colonies of Nostoc fixed N2 vigorously, whereas X. camtschadalis did not at all. The fraction of nitrogen derived from the fixation of molecular nitrogen in Nostoc was 73%, calculated from 15N natural abundance measurements of Nostoc with X. camtschadalis as reference. The conservatively calculated N2 uptake by Nostoc was 0.030–0.033 kg N ha−1 for the heavily grazed site and 0.080–0.087 kg N ha−1 for the winter-grazed site for the growing seasons of 2004 and 2005, respectively. Together with previous findings, this study demonstrates that N2 fixation by Nostoc can potentially replace significant amounts, if not all, of the nitrogen lost in the form of N2O and NO soil emissions in this steppe ecosystem.  相似文献   

12.
Many studies of the population dynamics of the planarian Arthurdendyus triangulatus have been confounded because it has not been possible to define if small flatworms are adults that have been starved or young ones being recruited into the population. Body shrinkage is a process exhibited by planarians under nutritional stress. In birds and the few invertebrate species studied, starvation leads to a 15N-enrichment of body tissues (i.e. an increased δ 15N). Under laboratory conditions, the posterior parts of flatworms were removed at day 0 and frozen and compared isotopically with the anterior parts of flatworms, which were starved for 67, 108, 171 and 243 days. With one exception, δ 13C 108 days after starvation, the impact of starvation on A. triangulatus %N, %C and δ 13C was not significant statistically. In contrast, δ 15N increased significantly (to a maximum enrichment of 2.8‰ by day 243) on all sampling dates. This is supported by significant positive correlations between the changes in δ 15N between the anterior and posterior body parts and number of days after starvation (p≤0.05) and decrease in body weight (p≤0.01), respectively. Existing δ 15N data from field populations of animals show intra-population variability of around 5.0‰, greater than that noted as a result of starvation. Thus, as a single measure, it is unlikely that δ 15N can be used to accurately characterise the proportion of starved A. triangulatus within a field population.  相似文献   

13.
 The 15N natural abundance (δ15N) of white clover (Trifolium repens L.) grown in pasture under different management practices was determined. Plants were split into leaflets, petioles and stolons and the 15N signature of each tissue was measured. The δ15N of leaflet tissue from plants of two non-N2-fixing species (Lolium perenne L. and Ranunculus repens L.), growing in close proximity to the sampled T. repens, was also measured. By using T. repens plants grown in the absence of mineral N to provide reference material, the proportion of N derived from N2 fixation (%Ndfa) in pasture plants was calculated. Within a plot, variation was present in the δ15N between the tissues of T. repens. Variation was also present between the same tissues under different management practices. The %Ndfa in the leaf material of T. repens varied from 34% to 100% between the plots. The use of different reference species did not affect the estimate of %Ndfa. Received: 14 December 1998  相似文献   

14.
A baseline hydrogeochemical investigation was carried out as a contribution to the inventory of wetlands in Büyük Menderes River (BMR) coastal wetland which is the most important wetland in Turkey that is protected as “Natural Park” by government. In addition to the wetland values, agricultural activities are also important at this delta. From inland to coastal zone, the groundwater type is changed from Ca–HCO3 to Na–Cl. Perfect correlation of Cl with Na and SO4 indicated the mixing process between saline and fresh water. The seawater intrusion and “winter-leaching” is the main causes of salinization. Minor ions as B, Br, and Li versus Cl relationship also showed the marine influence at the BMR coastal wetland. The δ18O and δD composition of the waters are range between −8.11‰ and −1.65‰ and −49.2‰ and 8.1‰, respectively. Isotope values show the seawater mixing line and also dissolution of salts in the BMR coastal wetland. Weathering of aluminosilicates in the Paleozoic schist and clays resulted in the release of silica to the water. Due to the organic activity, iron concentrations reached to the tolerance limits. On the other hand, the use of phosphate fertilizers increases the phosphorus content in the water. The high value of As (from 0.6 to 134 μg/l) can be resulted as an evaporation process in the study area.  相似文献   

15.
Abstract

A study was carried out to compare the difference or N-yield method with the 15N natural abundance method for the estimation of the fractional contribution of biological N2 fixation in the different plant parts of nodulating and non-nodulating isolines of soybeans. The results indicated that the δ15N values of most plant parts of soybeans were significantly lower (p<0.05) in the nodulating than in the non-nodulating isoline. However, in the case of the root+nodule component, the δ15N value was higher in the nodulating than in the non-nodulating isoline possibly due to isotopic discrimination of 15N over 14N which may have occurred in the nodules. Inoculation of soybeans with the Bradyrhizobium japonicum strain CB 1809 increased significantly (p<0.05) the δ15N value of the root+nodule component implying that the effectiveness of the soybean-rhizobium symbiosis had increased by inoculation.

Percentage of plant N derived from atmospheric N2 fixation (%Ndfa) estimated by the 15N natural abundance method was highly correlated (r=0.762, p<0.01) with that by the difference or N-yield method and the differences between the two methods were not statistically significant. The agreement between the two methods was closer at maturity than at the early reproductive stage.

The %Ndfa obtained by the difference method ranged from 48.4 to 92.6% whereas the %Ndfa obtained by the 15N natural abundance method ranged from 43.2 to 92.4% in the different plant parts. Based on the 15N natural abundance method, approximately 15% of the N in pod, shoot, grain, and shell was derived from the soil but in the case of stover, this fraction was about 55%.  相似文献   

16.
The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha−1) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N2 fixation (P atm). A yield-independent, time-integrated 15N-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca++ and the Ca++:Mg++ ratio in the 0–10-cm soil layer were measured 30 months after application of 10 t ha−1 gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P atm = 0.74 at zero gypsum and 0.81–0.82 at 2.5–10 t ha−1 gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha−1 of gypsum, faba bean fixed more than 200 kg N ha−1 in above-ground biomass.  相似文献   

17.
The aim of this work was to obtain pure extracellular DNA molecules so as to estimate their longevity in soil by an isotope-based approach. Extracellular DNA molecules were extracted from all horizons of a forest soil and purified by the procedure of Davis (Purification and precipitation of genomic DNA with phenol–chloroform and ethanol. In: Davis LG, Dibner MD, Battey JF (eds) Basic methods in molecular biology. Appleton & Lange, Norwalk, 16–22, 1986) without (DNA1) or with (DNA2) a successive treatment with binding resins followed by elution. The two differently purified DNA samples were compared for their A260/A280 ratio, polymerase chain reaction (PCR) amplification and natural abundance of stable (13C and 15N) and radioactive (14C) isotopes. The purity index and the PCR amplification did not differentiate the efficiency of the two purification procedures. The isotopic signature of DNA was more sensitive and was strongly affected by the purification procedures. The isotopic measurements showed that the major contaminant of extracellular DNA1 was the soil organic matter (SOM), even if it is not possible to exclude that the similar δ 13C, δ 15N and Δ14C values of DNA and SOM could be due to the use of SOM-deriving C and N atoms for the microbial synthesis of DNA. For extracellular DNA2, extremely low values of Δ14C were obtained, and this was ascribed to the presence of fossil fuel-derived substances used during the purification, although in amounts not revealed by gas chromatography-mass spectrometry analysis. The fact that it is not possible to obtain contaminant-free DNA molecules and the potential use of soil native organic compounds during the microbial synthesis of DNA make it not achievable to estimate the age of soil extracellular DNA by radiocarbon dating.  相似文献   

18.
The interactive impacts of arbuscular mycorrhizal fungi (AMF, Glomus intraradices) and earthworms (Aporrectodea trapezoides) on maize (Zea mays L.) growth and nutrient uptake were studied under near natural conditions with pots buried in the soil of a maize field. Treatments included maize plants inoculated vs. not inoculated with AMF, treated or not treated with earthworms, at low (25 mg kg−1) or high (175 mg kg−1) P fertilization rate. Wheat straw was added as feed for earthworms. Root colonization, mycorrhiza structure, plant biomass and N and P contents of shoots and roots, soil available P and NO3−–N concentrations, and soil microbial biomass C and N were measured at harvest. Results indicated that mycorrhizal colonization increased markedly in maize inoculated with AMF especially at low P rate, which was further enhanced by the addition of earthworms. AMF and earthworms interactively increased maize shoot and root biomass as well as N and P uptake but decreased soil NO3−–N and available P concentrations at harvest. Earthworm and AMF interaction also increased soil microbial biomass C, which probably improved root N and P contents and indirectly increased the shoot N and P uptake. At low P rate, soil N mobilization by earthworms might have reduced potential N competition by arbuscular mycorrhizal hyphae, resulting in greater plant shoot and root biomass. Earthworms and AMF interactively enhanced soil N and P availability, leading to greater nutrient uptake and plant growth.  相似文献   

19.
 A 15N isotope dilution technique was applied to quantify the extent of N2 fixation in lentil (Lens culinaris Medik.) cultivars as influenced by Rhizobium leguminosarum bv. viciae strains in a field experiment in Pakistan. The experiment was conducted on a soil with a very small indigenous rhizobial population and where N was a limiting factor for crop production. Significant variations in number of nodules, dry weight of nodules, biomass yield, grain yield, total N yield, proportion of plant N derived from N2 fixation (Pfix) and amount of N derived from the atmosphere (Ndfa) were observed among combined treatments of four rhizobial strains and six lentil varieties. In a field previously labelled with 15N, to which a basal dose of 75 kg P2O5 ha–1 was applied as single super phosphate, Ndfa ranged from 15 to 24 kg N ha–1 when calculated according to rhizobial strain and from 4 to 38 kg N ha–1 when calculated according to lentil variety. Lc 26 was the most effective strain and fixed 243% more N than the indigenous population in the uninoculated control. In treatments with the lentil variety PL-406, Ndfa was 38 kg N ha–1, which was 850% higher than with the lentil variety Precoz/F6-20-1×M-85. Generally, the varieties with greater Pfix produced a higher dry matter yield. Received: 26 May 1999  相似文献   

20.
 The 15N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha–1 year–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g–1 day–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g–1 day–1. In the F treatment, the turnover of NH4 +-N and NO3 –-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO3 –-N pools turning over twice as fast as the NH4 +-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added 15N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C2H2) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for 15N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. Received: 25 May 1999  相似文献   

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