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1.
Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N2 fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the 15N dilution method. The two Sesbania species as N2-fixing reference plants and four aquatic weed species as non-N2-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with 15N at 0.26 atom % 15N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % 15N excess. The results showed that 15N enrichment of soil NH 4 + -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in 15N enrichment, in both N2-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in 15N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N2-fixing and non-fixing species was estimated by the technique of varying the 15N level. In the second crop, this ratio in non-fixing species was higher than that in N2-fixing species. Comparable estimates of % Ndfa were obtained by using 15N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using 15N enrichment of non-fixing species and those by using soil NH 4 + -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N2 due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the 15N dilution method are discussed with reference to these results.  相似文献   

2.
Summary Biological N2 fixation was estimated in a field experiment following the addition of NH4Cl or KNO3 to unconfined microplots (1.5 m2) at 2.5 g N m-2 (10 atom% 15N). A model of total N and 15N accumulation in lupins and decreasing 15N enrichment in the KCl-extractable soil-N pool (0–0.15 m depth) was used to estimate the proportion of N in lupins derived from biological N2 fixation. Estimates of N2 fixation derived from the model were compared with 15N isotope-dilution estimates obtained using canola, annual ryegrass, and wheat as nonfixing reference plants. Biomass, total N accumulation, or 15N enrichment in the lupin and reference crops did not differ whether NH inf4 sup+ or NO inf3 sup- was added as the labelled inorganic-N source. The decrease in soil 15N enrichment was described by first-order kinetics, whereas total N and 15N accumulation in the lupins were described by logistical equations. Using these equations, the uptake of soil N by lupins was estimated and was then used to calculate fixed N2. Estimates of N2 fixation derived from the model increased from 0 at 50 days after sowing to a maximum of 0.79 at 190 days after sowing. Those based on the 15N enrichment of the NO inf3 sup- pool were 10% higher than those based on the mineral-N pool. 15N isotope-dilution estimates of N2 fixation ranged from 0.37 to 0.55 at 68 days after sowing and from 0.71 to 0.77 at 190 days after sowing. Reference plant-derived values of N2 fixation were all higher than modelled estimates during the early states of growth, but were similar to modelled estimates at physiological maturity. The use of the model to estimate N2 derived from the atmosphere has the intrinsic advantage that the need for a non-fixing reference plant is avoided.  相似文献   

3.
Summary Transport of N by hyphae of a vesicular-arbuscular mycorrhizal fungus was studied under controlled experimental conditions. The N source was applied to the soil as 15NH inf4 sup+ or 15NO inf3 sup- . Cucumis sativus was grown for 25 days, either alone or in symbiosis with Glomus intraradices, in containers with a hyphal compartment separated from the root compartment by a fine nylon mesh. Mineral N was then applied to the hyphal compartment as 15NH inf4 sup+ or 15NO inf3 sup- at 5 cm distance from the root compartment. Soil samples were taken from the hyphal compartment at 1, 3 and 5 cm distance from the root compartment at 7 and 12 days after labelling, and the concentration of mineral N in the samples was measured from 2 M KCl extracts. Mycorrhizal colonization did not affect plant dry weight. The recovery of 15N in mycorrhizal plants was 38 or 40%, respectively, when 15NH inf4 sup+ or 15NO inf3 sup- was applied. The corresponding values for non-mycorrhizal plants were 7 and 16%. The higher 15N recovery observed in mycorrhizal plants than in non-mycorrhizal plants suggests that hyphal transport of N from the applied 15N sources towards the host plant had occurred. The concentration of mineral N in the soil of hyphal compartments was considerably less in mycorrhizal treatments than in controls, indicating that the hyphae were able to deplete the soil for mineral N.  相似文献   

4.
A pot experiment was conducted to compare the uptake and dry matter production potential of NH inf4 sup+ and NO inf3 sup- and to study the effect of Baythroid, a contact poison for several insect pests of agricultural crops, on growth and N uptake of maize (Zea mays L.). Nitrogen was applied as (15NH4)2SO4, K15NO3, or 15NH4NO3 and in one treatment Baythroid was combined with 15NH4NO3. Source of N had, in general, a nonsignificant effect on dry matter and N yield, but uptake of NO inf3 sup- was significantly higher than that of NH inf4 sup+ when both N sources were applied together. Substantial loss of N occurred from both the sources, with NH inf4 sup+ showing greater losses. Baythroid was found to have a significant positive effect on dry matter yield of both root and shoot; N yield also increased significantly. Uptake of N from both the applied and native sources increased significantly in the presence of Baythroid and a substantial added nitrogen interaction (ANI) was determined. The positive effect of Baythroid was attributed to: (1) a prolonged availability of NH inf4 sup+ due to inhibition of nitrification, (2) an increased availability of native soil N through enhanced mineralization, and (3) an enhanced root proliferation.  相似文献   

5.
Appropriate 15N-labeling methods are crucial for estimating N2-fixation in trees used in agroforestry systems. A 4-year field experiment was conducted on an Alfisol in Southwestern Nigeria to compare the estimates of N2 fixed in Leucaena leucocephala, using two non-N2-fixing leguminous trees, Senna siamea and S. spectabilis, as reference plants and three different methods of introducing 15N into soil. The atom % 15N uptake pattern (as reflected in the leaves) was identical in both N2- and non-N2-fixing tree species irrespective of the 15N-application method. There was a significant decline in atom % 15N excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sampling 12 months after planting and the second sampling 18 months after sampling. From the second harvest in 1991 until the end of the experiment (fifth) harvest in 1993, however, the atom 15N % excess decline in leaves of the three species was less pronounced and depended on the method of 15N application. In those plants to which the tracer was applied once at planting, the 15N decline was steady between the second and the last prunings. In the split-application treatment, the atom 15N % excess increased slightly at the third pruning and decreased during the subsequent two prunings. The reference tree and the method of 15N application influenced the estimated proportion of N derived from atmospheric N2 by L. leucocephala, calculated as 73 and 64%, corresponding to 119 and 98 kg N ha-1 of N2 fixed per 6 months, when S. spectabilis and S. siamea were used as reference trees, respectively. The approach by which 15N-labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atmosphere but this was of little agronomic significance because total N2 fixed was similar for all methods.  相似文献   

6.
Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NH inf4 sup+ , total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NO inf3 sup- levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH inf4 sup+ but not with available soil water, pH, electrical conductivity, NO inf3 sup- , or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.Contribution No. R754 of the Saskatchewan Center of Soil Research  相似文献   

7.
A laboratory incubation experiment was conducted to compare the effects of NH inf4 sup+ and NO inf3 sup- on mineralization of N from 15N-labelled vetch (Vicia villosa Rotn) in an Illinois Mollisol, and to determine the effect of a nitrification inhibitor (nitrapyrin) on mineralization of vetch N when used with NH inf4 sup+ . The addition of either NH inf4 sup+ or NO inf3 sup- (100 and 200 mg N kg-1 soil) significantly increased mineralization of vetch N during incubation for 40 days. The effect was greater with NH inf4 sup+ than with NO inf3 sup- , and a further increase occurred in the presence of nitrapyrin (10 mg kg-1 soil). The addition of NO inf3 sup- retarded the nitrification of NH inf4 sup+ -N derived from vetch.  相似文献   

8.
In 1991, field experiments on loess (with winter wheat) and sandy soils (with summer barley) were conducted to study N dynamics in the microbial biomass and non-exchangeable NH inf4 sup+ . The measurements showed a mass change in microbial N, with a maximum increase of 100 kg N ha-1 30 cm-1 from March to July in the loess soil, and a change for only 1 month (May) in the sandy soil. Plots treated with conventional levels of N fertilizer (213 kg N ha-1 on a loess soil to winter wheat and 130 kg ha-1 on the sandy soil to summer barley), reduced levels of N (83% and 62% of the conventional N application), or no N showed no consistent fertilizer N effect on microbial biomass N. From March to July, non-exchangeable NH inf4 sup+ in loess soils under winter wheat decreased by 110 kg N ha-1 30 cm-1 in conventionally fertilized plots and by 200 kg N ha-1 30 cm-1 in a plot with no N fertilizer. After harvest, the pool of non-exchangeable NH inf4 sup+ increased due to increasing mineral N concentrations in the soil.  相似文献   

9.
Summary Acetylene reduction activity by Azospirillum brasilense, either free-living in soils or associated with wheat roots, was determined in a sterilised root environment at controlled levels of O2 tension and with different concentrations of mineral N. In an unplanted, inoculated soil nitrogenase activity remained low, at approximately 40 nmol C2H4 h-1 per 2kg fresh soil, increasing to 300 nmol C2H4 h-1 when malic acid was added as a C source via a dialyse tubing system. The N2 fixation by A. brasilense in the rhizosphere of an actively growing plant was much less sensitive to the repressing influence of free O2 than the free-living bacteria were. An optimum nitrogenase activity was observed at 10 kPa O2, with a relatively high level of activity remaining even at an O2 concentration of 20 kPa. Both NO inf3 sup- and NH inf4 sup+ repressed nitrogenase activity, which was less pronounced in the presence than in the absence of plants. The highest survival rates of inoculated A. brasilense and the highest rates of acetylene reduction were found in plants treated with azospirilli immediately after seedling emergence. Plants inoculated at a later stage of growth showed a lower bacterial density in the rhizosphere and, as a consequence, a lower N2-fixing potential. Subsequent inoculations with A. brasilense during plant development did not increase root colonisation and did not stimulate the associated acetylene reduction. By using the 15N dilution method, the affect of inoculation with A. brasilense in terms of plant N was calculated as 0.067 mg N2 fixed per plant, i.e., 3.3% of the N in the root and 1.6% in the plant shoot were of atmospheric origin. This 15N dilution was comparable to that seen in plants inoculated with non-N2-fixing Psudomonas fluorescens.  相似文献   

10.
Summary We studied the effect of three successive cuttings on N uptake and fixation and N distribution in Leucaena leucocephala. Two isolines, uninoculated or inoculated with three different Rhizobium strains, were grown for 36 weeks and cut every 12 weeks. The soil was labelled with 50 ppm KNO3 enriched with 10 atom % 15N excess soon after the first cutting. Except for the atom % 15N excess in branches of K28 at the second cutting, both the L. leucocephala isolines showed similar patterns of total N, fixed N2, and N from fertilizer distribution in different parts of the plant at each cutting. The Rhizobium strain did not influence the partitioning of 15N among the different plant parts. Significant differences in 15N enrichment occurred in different parts. Live nodules of both isolines showed the lowest atom % 15N excess values (0.087), followed by leaves (0.492), branches (0.552), stems (0.591), and roots (0.857). The roots contained about 60% of the total plant N and about 70% of the total N derived from fertilizer over the successive cuttings. The total N2 fixed in the roots was about 60% of that fixed in the whole plant, while the shoots contained only 20% of the fixed N2. We conclude that N reserves in roots and nodules constitute another N source that must be taken into account when estimating fixed N2 or the N balance after pruning or cutting plants. 15N enrichment declined up to about fivefold in the reference and the N2-fixing plants over 24 weeks following the 15N application. The proportion and the amounts of N derived from fertilizer decreased, while the amount derived from N2 fixation increased with time although its proportion remained constant.  相似文献   

11.
Summary Three Illinois Mollisols were incubated for 2 weeks at 25°C after treatment with different amounts of glucose and/or 15N-labelled (NH4)2SO4 or 15N-labelled KNO3. The objectives were: (1) to compare the immobilization and interaction of NH inf4 sup+ –N and NO inf3 sup- –N with the native soil N, and (2) to study the relationship between immobilization of applied N and the added N interaction. As determined, immobilized N refers to forms not extractable with 2 MKCl (immobilized 15N+clay-fixed 15NH inf4 sup+ ). In all cases, both NH inf4 sup+ –N and NO inf3 sup- –N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH inf4 sup+ than NO inf3 sup- was recovered in organic forms. Although the three soils differed considerably in the amounts of applied N immobilized, similar trends in N immobilization were observed. A positive added N interaction occurred with all soils, the magnitude increasing with the rate of N addition. In the absence of glucose, higher added N interactions were obtained for NH inf4 sup+ than NO inf3 sup- , whereas there was very little difference between NH inf4 sup+ and NO inf3 sup- in the presence of glucose. The results indicate that under conditions of rapid immobilization (e.g., in the presence of glucose), NH inf4 sup+ and NO inf3 sup- will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH inf4 sup+ than with NO inf3 sup- . Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.  相似文献   

12.
Summary Gross rates of N mineralization, assimilation, nitrification, and NO in3 sup- reduction were determined in soil from a wet riparian fen by 1-day incubations of soil cores and slurries with 15N-labelled substrates. N mineralization transformed 0.1% of the total organic N pool daily in the soil cores, of which 25% was oxidized through autotrophic nitrification and 53%–70% was incorporated into microorganisms. N mineralization and nitrification were markedly inhibited below 5 cm in soil depth. At least 80% of the NO in3 sup- reduction in aerated cores occurred through dissimilatory processes. Dissimilatory reduction to NH in4 sup+ (DNRA) occurred only below 5 cm in depth. The results show that NH in4 sup+ oxidation was limited by available substrate and was itself a strong regulator of NO in3 sup- -reducing activity. NO in3 sup- reduction was significantly increased when the soil was suspended under anaerobiosis; adding glucose to the soil slurries increased NO in3 sup- reduction by 2.4–3.7 times. Between 3% and 9% (net) of the added NO in3 sup- was reduced through DNRA in the soil slurries. The highest percentage was observed in soil samples from deeper layers that were pre-incubated anaerobically.  相似文献   

13.
Summary The application of liquid anhydrous NH3 to soil leads to chemical fixation of NH3 by organic matter and of NH inf4 sup+ by clay minerals. A laboratory study was conducted to ascertain the biological transformations of newly fixed liquid anhydrous 15NH3 in a Drummer silty clay loam by incubation of the 15N-labelled soil with glucose for 0, 7, 30, and 90 days and by sequential extraction of organic-matter-fixed 15NH3 with 0.15 M Na4P2O7, 0.15 M KOH, 0.1 M NaOH, and acidified dimethyl sulfoxide. About 16% of the 15NH3 injected was fixed, of which 52% was accounted for by clay fixation. The various humic fractions (fulvic acid, humic acid, and humin) were obtained, and the distribution patterns of the fixed 15NH3-N in these fractions were compared. The potential availability of the fixed 15NH3-N was also estimated. The percentage of the 15NH3 recovered as organic-matter-fixed 15NH3 decreased as the length of incubation increased (to 28% after 90 days); the decrease was attributed in part to an increase in the amount recovered as clay-fixed NH inf4 sup+ (from 52 to 64%). Changes in the distribution of the organic-matter-fixed 15NH3-N in the humic fractions included: (1) an increase in the relative amount of the fixed 15NH3 as humic acid in both the Na4P2O7 and KOH extracts, (2) an increase in the percentage of organic-matter-fixed 15NH3-N in the fulvic acid fractions as high-molecular-weight components (determined by dialysis) or as generic fulvic acid (determined by sorption-desorption from XAD-8 resin), and (3) an increase in the percentage of the organic-matter-fixed 15NH3 as humin. The potential availability of the organic-matter-fixed 15NH3-N decreased as the length of the incubation increased, from 22 to 4% over the 90-day incubation period, and was correlated significantly (0.05 level) with Na4P2O7-extractable N. These results suggest that organic-matter-fixed liquid anhydrous NH3 is initially more labile than the native soil N but becomes less labile with time.  相似文献   

14.
Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using 15N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N2-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with 15N by adding cellulose and (15NH4)2SO4. The results of these studies showed fixation values of 6%–32% when estimated by 15N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of 15N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N2.  相似文献   

15.
Erythrina poeppigiana, a woody tropical plant, was inoculated with vesicular-arbuscular mycorrhizal (VAM) fungiGlomus etunicatum Becker and Gerdeman,G. mosseae Nicol. and Gerd. Gerdeman and Trappe, orG. intraradices Schenk and Smith. Growth, N uptake, and nutrition were evaluated in VAM-inoculated plants and controls fertilized with two levels (3 or 6 mM) of either NH inf4 sup+ -N or NO inf3 sup- -N. The response by the mycorrhizal plants to N fertilization, according to N source and/or level differed significantly from that of the control plants. In general, the growth of the mycorrhizal plants was similar to that of the non-mycorrhizal plants when N was provided as NH inf4 sup+ . When the N source was NO inf3 sup- the control plants grew significantly less than the VAM plants. Inoculation with VAM fungi gave yield increases of 255 and 268% forG. etunicatum-colonized plants, 201 and 164% forG. mosseae-colonized plants and 286 and 218% forG. intraradices-colonized plants fertilized with 3 and 6 mM NO inf3 sup- -N, respectively. The increased growth and acquisition of nutrients by plants fertilized with NO inf3 sup- -N and inoculated with VAM shows that VAM mycelium has a capacity for NO inf3 sup- absorption. The results also showed thatE. poeppigiana seedlings preferred NH inf4 sup+ as an N source.G. etunicatum was the most effective endophyte, not only increasing N, P, Ca, Mg, and Zn uptake in the presence of NO inf3 sup- fertilizer but also P and Mg in the presence of NH inf4 sup+ applications. From these results we conclude that VAM symbiosis affects N metabolism inE. poeppigiana plants and that this species can overcome limitations on the use of NO inf3 sup- -N by the mediation of VAM fungi.  相似文献   

16.
Below-ground transfer of nitrogen (N) fixed by legume trees to associated non-N2-fixing crops has received little attention in agroforestry, although the importance of below-ground interactions is shown in other ecosystems. We used 15N natural abundance to estimate N transfer from the legume tree Gliricidia sepium (Jacq.) Kunth ex Walp. to C4 grass Dichanthium aristatum (Poir.) C.E. Hubb. in a silvopastoral system, where N was recycled exclusively by below-ground processes and N2 fixation by G. sepium was the sole N input to the system. Finding a suitable reference plant, a grass without contact with tree roots or litter, was problematic because tree roots invaded adjacent grass monocrop plots and soil isotopic signature in soil below distant grass monocrops differed significantly from the agroforestry plots. Thus, we used grass cultivated under greenhouse conditions in pots filled with agroforestry soil as the reference. A model of soil 15N fractionation during N mineralization was developed for testing the reliability of that estimate. Experimental and theoretical results indicated that 9 months after greenhouse transplanting, the percentage of fixed N in the grass decreased from 35% to <1%, due to N export in cut grass and dilution of fixed N with N taken up from the soil. The effect of soil 15N fractionation on the estimate of the reference value was negligible. This indicates that potted grass is a suitable reference N transfer studies using 15N natural abundance. About one third of N in field-grown grass was of atmospheric origin in agroforestry plots and in adjacent D. aristatum grassland invaded by G. sepium roots. The concentration of fixed N was correlated with fine root density of G. sepium but not with soil isotopic signature. This suggests a direct N transfer from trees to grass, e.g. via root exudates or common mycorrhizal networks.  相似文献   

17.
Management of N fertilization depends not only on the mineral N measured at the beginning of the growing season but also on the status of the low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the 15N applied in labeled cornshoot tissue would be recovered in 0.01 M CaCl2-extractable 15N fractions and wheter a decrease in the CaCl2-extractable 15N fraction quantitatively followed the trend in net mineralization of the 15N applied in corn-shoot tissue during an incubation period. The effects of adding 15N-labeled young corn-shoot tissue to a sandy soil and a clay soil were investigated for 46 days in an aerobic incubation experiment at 25°C. The application of 80 mg N kg-1 soil in the form of labeled corn-shoot tissue (24.62 mg 15N kg-1 soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in comparison with the untreated soils during the incubation. The labeled organic-N fraction was significantly higher in the sandy soil than in the clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in 15NO inf3 sup- during the incubation. Ammonification of applied plant N resulted in a significant increase in the 1 M HCl-extractable non-exchangeable 15NH inf4 sup+ fraction in the clay soik, owing to the vermiculite content. The 15N recovery was analyzed by the 0.01 M CaCl2 extraction method; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incubation, recovery increased to 47.2 and 43.8% in the sandy and clay soils, respectively. Net mineralization of the 15N applied in corn-shoot tissue determined after the 46-day incubation was 6.60 mg 15N kg-1 soil (=34.9% of the applied organic 15N) and 4.37 mg 15N kg-1 soil (=23.1% of the applied organic 15N) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl2 over the whole incubation period was 3.14 and 2.33 mg 15N kg-1 soil in the sandy and clay soil, respectively. These results indicate that net mineralization of 15N was not consistent with the decrease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl2 to extract or desorb all of the applied organic 15N that was mineralized during the incubation period.  相似文献   

18.
The natural abundance of 15N and 13C, conventional soil analyses, and biomass production by maize were used to study the influence of five tropical tree species on soils and their fertility. The experiment was conducted in Morogoro, Tanzania, to compare Cassia (Senna) siamea, Eucalyptus camaldulensis, E. tereticornis (all non-N2-fixing), Leucaena leucocephala, Prosopis chilensis (both N2-fixing), and a grass fallow. Maize biomass production, which was correlated with N uptake (P=0.001), was higher on soils from plots with 5-year-old Leucaena and Prosopis spp. compared to the grass fallow, while other tree species had less favourable effects on maize growth. The per cent N was higher in soil and 15N of soil total N was lower under Prosopis sp. compared to soil under other tree species, which suggests an input from N2 fixation by Prosopis sp. A transfer of fixed N to maize or to understorey grass species was, however, not indicated by the 15N natural abundance. Prosopis sp. contributed more C to the soil than the other four tree species; the difference in 13C between soils from Prosopis sp. plots and from grass fallow plots showed that the tree contributed 11% to the total C of the soil over a period of 8 years. The leaves of the N2-fixing species had a low ratio of lignin+phenols to N, and maize growth was negatively correlated with this parameter. The Eucalyptus spp. had leaves with a high lignin+phenols to N ratio, contributed very little C to the soil, and lowered the soil pH.  相似文献   

19.
Our understanding of leaf litter carbon (C) and nitrogen (N) cycling and its effects on N management of deciduous permanent crops is limited. In a 30-day laboratory incubation, we compared soil respiration and changes in mineral N [ammonium (NH4+-N) + nitrate (NO3–-N)], microbial biomass nitrogen (MBN), total organic carbon (TOC) and total non-extractable organic nitrogen (TON) between a control soil at 15N natural abundance (δ15N = 1.08‰) without leaf litter and a treatment with the same soil, but with almond (Prunus dulcis (Mill.) D.A. Webb) leaf litter that was also enriched in 15N (δ15N = 213‰). Furthermore, a two-end member isotope mixing model was used to identify the source of N in mineral N, MBN and TON pools as either soil or leaf litter. Over 30 d, control and treatment TOC pools decreased while the TON pool increased for the treatment and decreased for the control. Greater soil respiration and significantly lower (p < 0.05) mineral N from 3 to 15 d and significantly greater MBN from 10 to 30 d were observed for the treatment compared to the control. After 30 d, soil-sourced mineral N was significantly greater for the treatment compared to the control. Combined mineral N and MBN pools derived from leaf litter followed a positive linear trend (R2 = 0.75) at a rate of 1.39 μg N g?1 soil day?1. These results suggest early-stage decomposition of leaf litter leads to N immobilization followed by greater N mineralization during later stages of decomposition. Direct observations of leaf litter C and N cycling assists with quantifying soil N retention and availability in orchard N budgets.  相似文献   

20.
Summary The common bean (Phaseolus vulgaris L.) is generally regarded as a poor N2 fixer. This study assessed the sources of N (fertilizer, soil, and fixed N), N partitioning and mobilization, and soil N balance under field conditions in an indeterminate-type climbing bean (P. vulgaris L. cv. Cipro) at the vegetative, early pod-filling, and physiological maturity stages, using the A-value approach. This involved the application of 10 and 100 kg N ha-1 of 15N-labelled ammonium sulphate to the climbing bean and a reference crop, maize (Zea mays L.). At the late pod-filling stage (75 days after planting) the climbing bean had accumulated 119 kg N ha-1, 84% being derived from fixation, 16% from soil, and only 0.2% from the 15N fertilizer. N2 fixation was generally high at all stages of plant growth, but the maximum fixation (74% of the total N2 fixed) occurred during the interval between early (55 days after planting) and late podfilling. The N2 fixed between 55 and 75 days after planting bas a major source (88%) of the N demand of the developing pod, and only about 11% was contributed from the soil. There was essentially no mobilization of N from the shoots or roots for pod development. The cultivation of common bean cultivars that maintain a high N2-fixing capacity especially during pod filling, satisfying almost all the N needs of the developing pod and thus requiring little or no mobilization of N from the shoots for pod development, may lead to a net positive soil N balance.  相似文献   

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