Nitrogen and phosphorus cycling in relation to stand age of Eucalyptus regnans F. Muell

Lupins, canola, ryegrass and wheat fertilized with Na₂ ³⁵SO₄ and either ¹⁵NH₄Cl or K¹⁵NO₃(N:S=10:1), were grown in the field in unconfined microplots, and the sources of N and S (fertilizer, soil, atmosphere, seed) in plant tops during crop development were estimated. Modelled estimates of the proportion of lupin N derived from the atmosphere, which were obtained independently of reference plants, were used to calculate the proportion of lupin N derived from the soil. Total uptake of N and S and uptake of labelled N and S increased during crop development. Total uptake of S by canola was higher than lupins, but labelled S uptake by lupins exceeded uptake by canola. The form of N applied had no effect on uptake of labelled and unlabelled forms of N or S. Ratios of labelled to unlabelled S and ratios of labelled to unlabelled N derived from soil sources decreased during growth, and were less for S than for N for each crop at each sampling time. Although ratios of labelled to unlabelled soil-derived N were similar between crops at 155, 176 and 190 days after sowing, ratios of labelled to unlabelled S for lupins were higher than for the reference crops and declined during this period. The ratios of labelled to unlabelled S in lupins and the reference plants therefore bore no relationship either to ratios of labelled to unlabelled soil-derived N in the plants, or to total S uptake by the plants. Therefore the hypothesis that equal ratios of labelled N to unlabelled soil-derived N in legumes (Rleg) and reference plants (Rref) would be indicated by equal ratios of labelled to unlabelled S was not supported by the data. The results therefore show that the accuracy of reference plant-derived values of Rleg cannot be evaluated by labelling with ³⁵S.     

The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol,lignin, and nitrogen contents

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and ¹⁵N-recovery methods. Mature tops of alfalfa (Medicago sativa L.), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), leucaena (Leucaena leucocephala Lam., deWit), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde) were incorporated in soil at the rate of 100 mg legume N kg^-1 soil. The medic and vigna were labeled with ¹⁵N. Sorghum-sudan hybrid (Sorghum bicolor, L. Moench) was used as the test crop. A non-amended treatment was used as a control. Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa. With the two legumes that contained less than 20 g kg^-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment. The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 0.05 level and at the 0.01 level at 6 weeks (r²=0.866). Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time. The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r²=0.692). The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized. Estimates of legume-N mineralization by the difference and ¹⁵N recovery methods were significantly different at all sampling dates for both ¹⁵N-labeled legumes. After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the ¹⁵N recovery method. This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from ¹⁵N recovery experiments.     

Biological nitrogen fixation in four Gliricidia sepium genotypes

A field experiment was conducted at the Coconut Research Institute in Sri Lanka to examine the biological nitrogen fixation potential of three Gliricidia sepium provenances (OFI 14/84, 17/84, 12/86) and a local landrace (designated LL), using the ¹⁵N isotope dilution method. There was marked variation in dry matter, total N, nodulation and ¹⁵N enrichment among the Gliricidia genotypes (=0.001), and the dry matter yield of Cassia siamea (syn. Senna siamea), the non-N₂ fixing reference plant was higher than for G. sepium. In all cases, highest biomass and total N were aboveground, with roots on average accounting for < 20 % of total dry matter or the total N in plants. Atom % ¹⁵N excess was highest in C. siamea, and lowest in OFI 14/84. Although atom % ¹⁵N excess was lower in Gliricidia leaves than in the other organs (all of which had similar ¹⁵N enrichments), values of % N derived from atmospheric N₂ fixation (% Ndfa) calculated for any individual organ or for the whole plant were similar. This was because the relative distribution of ¹⁵N in the different parts of the fixing plant followed the same trend as in the reference plant. There were significant differences (p=0.01) in N₂ fixation between the Gliricidia genotypes. The values ranged from 17.8 g N tree^-1 (equivalent to 86 kg N ha^-1 at 5000 trees ha^-1) in OFI 12/86 to 61.7g N tree^-1 (equivalent to 309 kg N ha^-1) in OFI 14/84. Although most of this variability was due to differences in both % Ndfa and total N in plant, amount of N fixed was more correlated with total N in plant (r=0.935) than with % Ndfa (r=0.707). On average, % Ndfa in all three G. sepium provenances and LL was about 55 % or 34.6 g N tree^-1 (equivalent to some 166 kg N ha^-1) in the 9 months within which N₂ fixation was measured. This represents a substantial contribution of N into the soil-plant system.     

Arbuscular and ectomycorrhizal colonization of two <Emphasis Type="Italic">Eucalyptus</Emphasis> species in semiarid Brazil

Our goal was to evaluate the mycorrhizal colonization, as well as the density of arbuscular mycorrhizal (AM) fungal spores, in Eucalyptus camaldulensis and E. grandis monocultures at 2 years in a semiarid part of Brazil. Soil and root samples were collected in 2 consecutive years. Eucalyptus camaldulensis showed varied AM colonization level according to season of sampling, and Glomus was dominant in spore numbers. Eucalyptus grandis showed dominant ectomycorrhizal (ECM) colonization and lower AM fungal spore density. Overall results suggest that E. camaldulensis has both AM and ECM dependencies, whereas E. grandis is solely ECM dependent in the monocultures.     

Assessment of genetic variability for N2 fixation between and within provenances of Leucaena leucocephala and Acacia albida estimated by 15N labelling techniques

Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by ¹⁵N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N₂ fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N₂ (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida; (equivalent to 20–50 mg N plant^–1 and 4–37 mg N plant^–1 for the two species over three months, respectively) and was correlated with the nodule mass (r=0.91). The time course of N₂ fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N₂ fixed (r=0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants.     

氮素添加和功能群去除对糙隐子草和大针茅根系特征的影响

由于草地不合理利用,中国北方草原严重退化,导致生态系统结构性破坏甚至功能性紊乱。随着草原退化,中能值功能群植物糙隐子草会取代高能值功能群植物大针茅成为群落中优势度最大的植物。为了解大针茅和糙隐子草在氮素添加以及去除功能群的情况下根系特征的变化趋势,选取锡林郭勒盟典型草原中的退化草原群落,分别开展两项实验,实验1:氮素添加梯度实验(0、10.5、17.5、28 gN/m~2),实验2:同时进行功能群的去除和氮素的添加,功能群去除实验(将植物分为高能值植物功能群、中能值植物功能群、低能值植物功能群3个功能群;每个处理分别剪除另外两个功能群,留取单一功能群),并同时进行氮素添加实验(0、17.5 gN/m~2)。在进行两年的实验处理后,通过Delta-T SCAN根系分析仪测量大针茅和糙隐子草根系的长度、直径、面积、体积指标。分析植物根系对氮素添加的响应,以及功能群去除是否改变这两种植物对氮素添加的响应格局。实验1研究结果表明:在受到其他功能群的竞争压力下,大针茅根系长度、面积、体积均在高氮素添加(28 gN/m~2)情况下显著增加,糙隐子草根系直径和体积在中氮素添加(17.5 gN/m~2)的情况下显著高于其他3个处理,退化样地中土壤氮素的增加,促使大针茅根系主要通过增加根系长度扩大在土壤中的空间分布,而促使糙隐子草主要通过增粗生长来扩大土壤分布空间。实验2研究结果表明:功能群的去除,中氮素添加对根系的影响,只有糙隐子草的根系直径显著增加。综合来看,功能群去除实验对大针茅的根系长度和面积有影响,对糙隐子草的根系长度、直径和面积有影响。功能群去除处理与氮素添加的交互作用对大针茅根系没有影响,对糙隐子草的根系直径和体积有影响。     

5种相思树和尾巨桉人工林土壤养分和酶活性特征

为揭示固氮树种土壤养分转化的酶学机制,对固氮树种[厚荚相思(Acacia crassicarpa)、黑木相思(A. melanoxylon)、卷荚相思(A.cincinnata)、大叶相思(A.auriculiformis)和马占相思(A.mangium)]及非固氮树种尾巨桉(Eucalyptusurophylla×E.grandis)人工林的土壤养分含量、酶活性及其相关性进行研究。结果表明,相思林40～60cm土层的pH均高于尾巨桉林;5种相思林土壤各土层的TP、TK含量均低于尾巨桉林,而20～40 cm土层的TC、TN含量均高于尾巨桉林,黑木相思林和马占相思林各土层的有效养分均显著高于尾巨桉林(P0.05)。0～10 cm土层中,相思林的土壤酸性磷酸酶和纤维素酶活性均高于尾巨桉林,大叶相思林的土壤脲酶、蔗糖酶、纤维素酶和芳基硫酸酯酶活性显著高于尾巨桉林(P0.05),卷荚相思林的土壤脲酶、纤维素酶、几丁质酶和淀粉酶活性显著高于尾巨桉林(P0.05)。相关分析结果表明,土壤脲酶、蔗糖酶和几丁质酶活性与AP显著负相关(P0.05),蔗糖酶和纤维素酶活性与NH4+-N显著负相关(P 0.05),脲酶、纤维素酶、芳基硫酸酯酶与土壤TK显著负相关(P0.05),几丁质酶活性与TN含量呈显著正相关(P0.05),土壤淀粉酶活性与NH4+-N呈显著正相关(P 0.05),过氧化氢酶活性与土壤TK含量呈显著正相关。可见,与尾巨桉人工林相比,在我国南方退化山地引种相思树可提高土壤关键酶的活性,对土壤有效养分具有明显改良作用,有利于退化地土壤的生态修复及人工林长期生产力的维持。     

Sporothrix eucalypti (sp. nov.), a shoot and leaf pathogen ofEucalyptus in South Africa

A species ofSporothrix was consistently isolated from leaf spots and serious shoot infections on a clone ofEucalyptus grandis in Northern Natal, South Africa. The fungus was morphologically distinct from other species in the genus and is consequently described as a new taxon,S. eucalypti. Sporothrix eucalypti was shown to be highly virulent in pathogenicity tests on a number ofE. grandis clones. Significant differences amongst susceptibility of clones were also detected in these tests.Sporothrix eucalypti represents a new pathogen ofEucalyptus that has the potential to cause substantial damage to this host in South Africa and probably elsewhere in the world.     

Assessment of potential sources of error in nitrogen fixation measurements by the nitrogen-15 isotope dilution technique

Although the use of ¹⁵N fertilizers to measure nitrogen (N₂) fixed in crops has increased substantially in recent years, some methodological uncertainties still remain unresolved. The results obtained from a greenhouse study of soybean [Glycine max. (L.) Merrill] inoculated by six different methods have been examined for potential errors arising from incorporating ¹⁵N labelled fertilizer into soil to estimate N₂ fixed in pods or shoots or the whole plant at three growth stages (50% flowering, pod-initiation and physiological maturity) using as reference crops, an uninoculated soybean cultivar and a non-nodulating soybean isoline. At the first harvest when N₂ fixed was very low, the estimates of N₂ fixed by the two reference crops did not match. At this stage the uninoculated soybean estimated about four times as much N₂ fixed in the symbiotic soybean as that measured using the non-nodulating soybean. For the second and third harvests, there were substantial increases in N₂ fixed, and both the non-nodulating and uninoculated soybean were equally suitable as reference crops for assessing N₂ fixed in the symbiotic soybean. These results indicate how critical and difficult the choice of the reference crop could be at early harvests, or when N₂ fixed is low. Even though there were significant differences in ¹⁵N enrichments in different organs (generally nodules < pods < roots < shoots), the estimates of N₂ fixed in soybean plants obtained by excluding roots and nodules did not differ much from those based on the whole plant. Of the above-ground organs, % N₂ fixed in pods (containing seeds) was closest to that of the whole plant (similar at P<0.05 at physiological maturity). However, the total N₂ fixed in pods or shoots was substantially lower than that fixed by the whole plant (P<0.05), although that for the pods and enclosed seeds once again was closer to N₂ fixed in the whole plant than that in the shoots.     

Application of 15N and xylem ureide methods for assessing N2 fixation of three shrub legumes periodically pruned for forage

The apparently diminished capacity for N₂ fixation by the shrub legume Calliandra calothyrsus (Calliandra) relative to other woody perennial legumes was investigated in a field experiment in northern Queensland, Australia. In this trial, (i) the proportion of plant nitrogen (N) derived from symbiotic N₂ fixation (%Pfix) and the amounts of N₂ fixed were compared in Calliandra, Gliricidia sepium (Gliricidia) and Codariocalyx gyroides (Codariocalyx), (ii) variations in N₂ fixation due to season or tree age were determined, (iii) estimates of Pfix derived with the ¹⁵N natural abundance technique were compared with values obtained from ¹⁵N enrichment or xylem sap ureide procedures to determine whether the previous conclusions about Calliandra's ability to fix N had resulted from specific problems with the natural abundance methodology used in the earlier studies.Inoculated seedlings of each of the three shrub legume species were planted in dense stands (1.5 m rows, 0.5 m between trees) in two randomised blocks. The northern block was used solely for natural abundance measurements, while ¹⁵N-enriched KNO₃ (10 atom % ¹⁵N excess) was applied four times over a 52 week period to plots in the southern block. The non-nodulating tree legume Senna spectabilis (formally Cassia spectabilis) was used as a non-N₂-fixing reference for the ¹⁵N-based procedures, with Guinea grass (Panicum maximum) included as an additional non-fixing check. Growth by the trees above 75 cm was first cut and removed after 22 weeks and regrowth was subsequently pruned periodically for another 95 weeks. Sampling for dry matter production, N yield and estimates of Pfix were restricted to the central four of the 32 plants which constituted each replicate plot. Information generated during the 117 week study indicated that estimates of Pfix by ¹⁵N natural abundance were closely similar to values derived with ¹⁵N-enrichment or sap ureides. The data indicated that Calliandra had a reduced reliance upon N₂ fixation relative to Gliricidia and Codariocalyx for the first 65 weeks after establishment. This appeared to be due to more prolifc root growth by Calliandra than either of the other N₂-fixing species and an ability to extract a greater proportion of its N requirements from soil mineral N. However, after week 65 and for the remainder of the experiment, estimates of Pfix for Calliandra were similar to the other shrub legumes. Over 117 weeks, prunings from Calliandra and Gliricidia had removed 52–58 t dry matter ha^-1, and between 1471 and 1678 kg N ha^-1, of which 1026–1063 kg N ha^-1 was estimated to have been derived from N₂ fixation. At the time of final harvest, 65–73% of the fixed N was present in shoot regrowth of the N₂ fixing shrubs, 9–18% in the roots, 15% in the trunk, and 2–6% in fallen leaves.     

Chloroplast DNA variation and population structure in the widespread forest tree, Eucalyptus grandis

Recognition of genetic structure of populations and the ability to identify vulnerable populations is useful for the formation of conservation management strategies for plants. Eucalyptus grandis is a tall forest tree that has a major area of occurrence in subtropical eastern Australia, with smaller populations located in the east coast tropics. Many widespread forest species exhibit population differentiation that corresponds to geographic regions. However, Eucalyptus grandis appears to be an exception based on isozyme and morphological data. This is intriguing given a large discontinuity between northern populations and those in the southern part of the species range. In this study, the distribution of a maternally inherited chloroplast locus was examined because it was more likely to reveal genetic structure due to the slower evolution of the chloroplast genome and limited dispersal of seed in eucalypts. As expected, the G _ST for chloroplast DNA was higher than that for nuclear DNA but indicated low population differentiation for a forest tree species. Phylogeographic analysis indicated that the 15 populations grouped into three broad geographical regions; however, overall population structure was weak suggesting that the large geographical disjunction in the distribution of E. grandis may be relatively recent. A paradigm for conservation management of E. grandis based on chloroplast DNA haplotype distribution would take into account the low differentiation among populations.     

Nitrogen fixation by soybeans (Glycine max L.) in Zambia

Soybeans (Glycine max L.) are being introduced as a cash crop to small scale farmers in Zambia for rotation in their farming systems. The objectives of this study were to compare and select the most approriate non-fixing reference crop for estimating N₂ fixation by soybeans and assess yields and N₂ fixation of soybeans in Zambia. Nitrogen isotope dilution techniques using¹⁵N-labelled organic or inorganic materials were utilized. Two nonnodulating soybean cultivars, Clark RJ1 and N77 or in their absence Pearl millet (Panicum glaucum L.) were judged to be appropriate reference crops. A local soybean fixing cultivar (Glycine max L. cv. Magoye) rated highest among three cultivars tested for its ability to support symbiotic N₂ fixation byB. japonicum under the experimental conditions. Values of percent N derived from atomosphere for this cultivar were in the order of 65 to 70%.deceased.Contribution no R531 of the Saskatchewan Institute of Pedology. Present address (REK): Esso Chemical Canada, P.O. Box 3010, Lethbridge, Alberta Canada T1J 4A9.     

Manipulation of quality and mineralization of tropical legume tree prunings by varying nitrogen supply

The effect of N supply on the quality of Calliandra calothyrsus and Gliricidia sepium prunings was studied in a glasshouse over a 7-month growing period. Increasing the concentration of N supplied from 0.625 to 10.0 mM NO₃-N resulted in increased N concentration but decreased polyphenol concentration, protein-binding capacity and C:N ratio of prunings from both species. Lignin concentration was not consistently altered by the N treatment. Mineralization of N from the prunings was measured over a 14-week period under controlled leaching and non-leaching conditions. The results indicated a strong interaction between legume species and concentration of N supply in their influence on N mineralization of the prunings applied to the soil. Differences in the %N mineralized were dictated by the quality of the prunings. The (lignin + polyphenol):N ratio was the pruning quality factor which could be used most consistently and accurately to predict N mineralization of the legume prunings incubated under leaching conditions, and the relationship was best described by a linear regression. Under non-leaching conditions, however, the protein-binding capacity appeared to be the most important parameter in determining the patterns of N release from the prunings studied. The relationship between the N mineralization rate constant and the protein-binding capacity was best described by a negative exponential function, y=0.078 exp(–0.0083x). The present study also indicated that the release of N from legume prunings containing a relatively high amount of polyphenol could be enhanced by governing the N availability conditions under which the plant is grown, for example whether or not it is actively fixing nitrogen. Estimates of pruning N mineralization after 14 weeks with the difference method averaged 6% (leaching conditions) and 22% (nonleaching conditions) more than with the ¹⁵N method for all legume prunings studied. The recovery of pruning by maize (4–38%) was well correlated with the % pruning N mineralized suggesting that incubation data closely reflect the pruning N value for a given catch crop under non-leaching conditions.     

Effect of nitrogen fertilization and cropping system of the reference crop on estimation of N2 fixation by vetch using15N methodology

This study was conducted to examine the effects of varying N rates and cropping systems (mixedversus pure stand) on the suitability of oats (Avena sativa L.) for estimating N₂ fixed in sequentially harvested vetch (Vicia sativa L.) over two growing seasons (1984–85 and 1985–86). The N rates were, 20 and 100 kg N ha^–1 in 1984–85 and 15 and 60 kg N ha^–1 in 1985–86. In the 1984–85 season, vetch at maturity derived 76 and 63% N from fixation at the high and low N rates respectively. The corresponding values for the second season were 66 and 42%. Except in the 1985–86 season when some significantly higher values of % N₂ fixed were estimated by using the reference crop grown at the higher (A-value approach) than at the lower N rate (isotope-dilution approach), both approaches resulted in similar measurements of N₂ fixed. In the 1984–85 season, similar values of N₂ fixed were obtained using either the pure or mixed stand oats reference crops. Although in the 1985–86 season, the mixed reference crop occasionally estimated lower % N₂ fixed than pure oats, total N₂ fixed estimates were always similar (P<0.05). Thus, in general, N fertilization and cropping system of the reference crop did not significantly influence estimates of N₂ fixation.     

Management of biological N2 fixation in alley cropping systems: Estimation and contribution to N balance

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N₂ symbiotically with Rhizobium. Measurements of biological N₂ fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha^-1 yr^–1 from atmospheric N₂, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha^-1 yr^-1. Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N₂. The long-term evaluation of the potential or actual amounts of N₂ fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N₂ fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of ¹⁵N procedures and the selection criteria for appropriate reference plants have been discussed.Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the ¹⁵N labelling techniques have indicated that up to 50% or more of the tree's N may be below ground after pruning. In this case, quantification of N₂ fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N₂ fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (>300 kg N ha^-1 yr^-1) but N contribution to crops is commonly in the range of 40–70 kg N ha^-1 season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5–10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25–102 kg N ha^-1 season^-1. Timing and severity of pruning may allow for some management of underground transfer of fixed N₂ to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on ¹⁵N labelling techniques or ¹⁵N natural abundance measurements are outlined. These would lead to estimates of N₂ fixation and N saving resulting from the management of N₂ fixation in alley cropping systems.     

Persistence and recovery of introduced Rhizobium ten years after inoculation on Leucaena leucocephala grown on an Alfisol in southwestern Nigeria

Establishment of Leucaena leucocephala was poor at Ibadan (Transition forest-savanna zone) and Fashola (savanna zone, 70 km north of Ibadan) in southwestern Nigeria as a result of low soil fertility and the presence of only a few native rhizobia capable of nodulating it. Inoculation with L. leucocephala at these two locations in 1982 resulted in striking responses with Rhizobium strains IRc 1045 and IRc 1050 isolated from L. leucocephala grown in Nigeria. The persistence of inoculated effective Rhizobium strains after inoculation is desirable since it removes the need for reinoculation. Because of the perennial nature of L. leucocephala and its use in long-term alley farming experiments, we examined the persistence of inoculated rhizobial strains after inoculation, and their ability to sustain N₂-fixation and biomass production at Ibadan. In 1992, ten years after Rhizobium introduction, uninoculated, L. leucocephala fixed about 150 kg N ha^-1 yr^-1 or about 41% of total plant N compared to 180 kg N ha^-1 yr^-1 or 43% measured in 1982. Serological typing of the nodules using the Enzyme-Linked-Immunosorbent Assay (ELISA) and intrinsic resistance to the streptomycin test revealed that most of the nodules (96%) formed on L. leucocephala in 1992 were by Rhizobium strains IRc 1045 and IRc 1050, which were inoculated in 1982. Nodules were absent on uninoculated L. leucocephala grown on the adjacent field with no history of L. leucocephala cultivation. We conclude that the N₂ fixed by Rhizobium strains IRc 1045 and IRc 1050 persisted for many years in the absence of L. leucocephala and sustained effectively fixed N₂ which growth and yield of L. leucocephala after several years, thus encouraging a possible low-input alley farming system by smallholder farmers in Nigeria.     

Intra-specific variation in growth and P accumulation of Leucaena leucocephala and Gliricidia sepium as influenced by soil phosphate status

Twenty-three provenances of Gliricidia sepium and eleven isolines of Leucaena leucocephala were examined at a low and at high phosphate levels (20 and 80 mg P kg^-1 soil) for growth, phosphate (P) uptake and use efficiency. Large differences in growth at the low P level, and in growth response to the higher P rate occurred among L. leucocephala isolines and G. sepium provenances. Shoot dry weight at low P varied from 1.30 to 3.01 g plant^-1 for L. leucocephala and from 1.44 to 3.07 g plant^-1 for G. sepium.Leucaena isolines had only half the root weight of G. sepium provenances yet produced approximately 90% of the shoot weight of the corresponding G. sepium treatments, i.e. more than a 2-fold difference in root/shoot ratios. Total P in shoots of G. sepium was some 85% greater than of the respective L. leucocephala isolines in corresponding treatments. Physiological phosphate use efficiency (g shoot/mg P in shoots) (PPUE) was not a simple reciprocal relation, being markedly lower at higher shoot % P and content. However, for the same shoot P both species produced the same shoot weight. Nevertheless, there were apparent genotypic differences within species in the root development, shoot P and PPUE.In another study, the numbers of rhizobia in the rhizosphere of L. leucocephala, nodulation, N₂ fixation at five different levels of P were determined. The numbers of rhizobia in the rhizosphere of inoculated L. leucocephala during the first two weeks were lower when P was added but later became similar to those without added P. Nodules formed earlier than inoculated plants fertilized with P and in greater numbers (4- to 5-fold) and dry weights than in those without P. However, the percentage of N₂ derived from fixation did not change with increasing levels of P application. These results suggest that the observed P effect did not operate via stimulated growth of rhizobia in the rhizosphere, nor through increased N₂ fixation rate. The major effect appeared to be due to effects via plant growth.     

巨桉连栽对土壤微生物生物量和数量的影响

为探讨巨桉连栽对土壤微生物生物量和数量的影响,采用时空互换法,研究了马尾松(Pinus massoniana)林和不同连栽代次的巨桉(Eucalyptus grandis)人工林的微生物生物量、数量与土壤理化性质的关系。结果表明,巨桉一代林的土壤微生物生物量碳、生物量氮和土壤真菌、细菌数量与马尾松林的差异不显著,放线菌数量则显著增加。随巨桉连栽代次增加,土壤微生物生物量碳、生物量氮和土壤细菌、放线菌、真菌数量均递减。回归分析表明,土壤全磷能独立解释微生物生物量氮、细菌和真菌数量71.7%、86.1%和63.0%的变异,与总孔隙度共同解释微生物生物量碳87.9%的变异,与全氮共同解释放线菌数量89.6%的变异。可见,土壤全磷较大程度解释了微生物的变化。     

Influence of pruning management on P and N distribution and use efficiency by N2 fixing and non-N2 fixing trees used in alley cropping systems

Pruning of hedgerow trees is an important management practice for the successful establishment of an alley cropping system. Although pruning affects biomass production, only meager evidence of this management on distribution of nutrients among the different plant organs after tree regrowth is available. This study examined the effect of pruning on the distribution and use efficiency of N and P in a N₂ fixing leguminous tree species, Gliricidia sepium, and two non-N₂ fixing leguminous tree species, Senna siamea and S. spectabilis, grown in a field on an Alfisol (low in P) at Fashola (Guinea Savanna Zone), Southwestern Nigeria. Four P rates, 0, 20, 40 and 80 kg P ha^–1 as single superphosphate were used and management treatments included pruned versus unpruned plants. The ¹⁵N isotope dilution technique was used to measure N₂ fixation in G. sepium. Partitioning of total P among different plant organs was influenced by plant species and pruning management, but was not affected by P application rates. The distribution of total P in the various plant organs followed that of dry matter yield while N partitioning had a different pattern. Pruned plants distributed about 118% more total P to branches and had a higher physiological P use efficiency (PPUE) than unpruned plants. Leaves were the biggest sink for total N and N allocation in the other plant organs was influenced by plant species and pruning management, G. sepium had relatively more of its total N and P partitioned into roots (about double that of the non-N₂ fixing trees) but had a lower PPUE. Unpruned and pruned G. sepium derived 35 and 54% respectively of their total N from atmospheric N2, with about 54% of the fixed N2 being allocated to leaves and roots. Results showed that N and P pools turned over in the branches during plant regrowth after pruning but the causative factors associated with this phenomenon were not clear.     

Accumulation of nutrients by eucalyptus seedlings from acidic and calcareous soils of the cerrado region of Central Brazil

Summary The accumulation of nutrients by seedlings ofEucalyptus grandis andE. saligna from a latosol and two other fertile soils was compared. Both species performed well in a calcareous soil and an acidic but fertile gallery forest soil but not in the dystrophic latosol.E. grandis accumulated more nutrients from the calcareous soil thanE. saligna. In the other two soils these two species showed similar growth and nutrient accumulation. In spite of the high acidity, manganese toxicity did not seem to be a limiting factor in the latosol or the gallerty forest soil. Correlations among the shoot concentrations were not always the same as those among the total contents of the nutrients in the aerial biomass.