Improving Sugarcane Growth and Nutrient Uptake by Inoculating Gluconacetobacter diazotrophicus

Seven Gluconacetobacter diazotrophicus strains from sugarcane roots were screened for their efficiency to promote growth and nutrient uptake in sugarcane at three levels of urea N (0, 75, and 150 kg N ha⁻¹). Inoculation by these strains improved germination, tiller number and plant height. N-uptake and apparent N-recovery increased due to inoculation and the effect was more at N₇₅ level. Gluconacetobacter diazotrophicus isolate IS100 was found to be the most efficient in promoting plant growth and nutrient uptake in sugarcane.     

Influence of nitrogen fertilisation on the population of diazotrophic bacteria Herbaspirillum spp. and Acetobacter diazotrophicus in sugar cane (Saccharum spp.)

We report studies on the possible effects of fertilisation with high level of N (300 kg of N ha^-1) on the occurrence and numbers of the diazotrophic bacteria Herbaspirillum spp. and Acetobacter diazotrophicusin sugar cane plants. In the sugar cane genotype SP79-2312, the N fertilised plants generally showed higher concentrations of this element. These same plants also had lower numbers of A. diazotrophicus, while the population of Herbaspirillum spp. was not affected by N application. These differences in the concentration of N and the numbers of A. diazotrophicus due to N application were not shown in the variety SP70-1143. The numbers of A. diazotrophicus were also shown to be influenced by the harvest time, becoming reduced in the harvests that coincided with dry periods of the year.     

Biological nitrogen fixation associated with sugar cane and rice: Contributions and prospects for improvement

¹⁵N isotope and N balance studies performed over the last few years have shown that several Brazilian varieties of sugarcane are capable of obtaining over 60% of their nitrogen (<150 kg N ha^-1 year^-1) from biological nitrogen fixation (BNF). This may be due to the fact that this crop in Brazil has been systematically bred for high yields with low fertilizer N inputs. In the case of wetland rice, N balance experiments performed both in the field and in pots suggest that 30 to 60 N ha^-1 crop^-1 may be obtained from plant-associated BNF and that different varieties have different capacities to obtain N from this source. ¹⁵N₂ incorporation studies have proved that wetland rice can obtain at least some N from BNF and acetylene reduction (AR) assays also indicate differences in N₂-fixing ability between different rice varieties. However in situ AR field estimates suggest plant-associated BNF inputs to be less than 8 kg N ha^-1 crop^-1. The problems associated with the use of the ¹⁵N dilution technique for BNF quantification are discussed and illustrated with data from a recent study performed at EMBRAPA-CNPAB. Although many species of diazotrophs have been isolated from the rhizosphere of both sugarcane and wetland rice, the recent discovery of endophytic N₂-fixing bacteria within roots, shoots and leaves of both crops suggests, at least in the case of sugarcane, that these bacteria may be the most important contributors to the observed BNF contributions. In sugarcane both Acetobacter diazotrophicus and Herbaspirillum spp. have been found within roots and aerial tissues and these microorganisms, unlike Azospirillum spp. and other rhizospheric diazotrophs, have been shown to survive poorly in soil. Herbaspirillum spp. are found in many graminaceous crops, including rice (in roots and aerial tissue), and are able to survive and pass from crop to crop in the seeds. The physiology, ecology and infection of plants by these endophytes are fully discussed in this paper. The sugarcane/endophytic diazotroph association is the first efficient N₂-fixing system to be discovered associated with any member of the gramineae. As yet the individual roles of the different diazotrophs in this system have not been elucidated and far more work on the physiology and anatomy of this system is required. However, the understanding gained in these studies should serve as a foundation for the improvement/development of similar N₂-fixing systems in wetland rice and other cereal crops.     

Seasonal accumulation and partitioning of nitrogen by lentil

Although wheat (Triticum aestivum L.) is the dominant crop of the semi-arid plains of Canada and the western United States, lentil (Lens culinaris Medik.) has become an important alternative crop. Sources and seasonal accumulation of N must be understood in order to identify parameters that can lead to increased N₂-fixing activity and yield. Inoculated lentil was grown in a sandy-loam soil at an irrigated site in Saskatchewan, Canada. Wheat was used as the reference crop to estimate N₂ fixation by the A-value approach. Lentil and wheat received 10 and 100 kg N ha⁻¹ of ammonium nitrate, respectively. Crops were harvested six times during the growing season and plant components analyzed. During the first 71 days after planting the wheat had a higher daily dry matter and N accumulation compared to lentil. However, during the latter part of the growing season, daily dry matter and N accumulation were greater for lentil. The maximum total N accumulation for lentil at maturity was 149 kg ha⁻¹. In contrast, wheat had a maximum N accumulation of 98 kg ha⁻¹ in the Feekes 11.1 stage, or 86 days after planting. The maximum daily rates of N accumulation were 3.82 kg N ha⁻¹ day⁻¹ for lentil and 2.21 kg N ha⁻¹ day⁻¹ for wheat. The percentage of N derived from N₂ fixation (% Ndfa) ranged from 0 at the first harvest to 92 % at final harvest. Generative plant components had higher values for % Ndfa than the vegetative components which indicates that N in the reproductive plant parts was derived largely from current N₂ fixation and lentil continued to fix N until the end of the pod fill stage. At final harvest, lentil had derived 129 kg N ha⁻¹ from N₂ fixation with maximum N₂-fixing activity (4.4 kg N ha⁻¹ day⁻¹) occurring during the early stages of pod fill. Higher maximum rates of N₂-fixing activity than net N accumulation (3.82 kg N ha⁻¹ day⁻¹) may have been caused by N losses like volatilization. In addition, lentil provided a net N contribution to the soil of 59 kg ha⁻¹ following the removal of the grain.     

Symbiotically fixed nitrogen from field- grown white and red clover mixed with ryegrasses at low levels of15N-fertilization

A field study was carried out near Zürich (Switzerland) to determine the yield of symbiotically fixed nitrogen (¹⁵N dilution) from white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L) and from red clover (Trifolium pratense L.) grown with Italian ryegrass (Lolium multiflorum Lam.). A zero N fertilizer treatment was compared to a 30 kg N/ha per cut regime (90 to 150 kg ha⁻¹ annually). The annual yield of clover N derived from symbiosis averaged 131 kg ha⁻¹ (49 to 227 kg) without N fertilization and 83 kg ha⁻¹ (21 to 173 kg) with 30 kg of fertilizer N ha⁻¹ per cut in the seeding year. Values for the first production year were 308 kg ha⁻¹ (268 to 373 kg) without N fertilization and 232 kg ha⁻¹ (165 to 305 kg) with 30 kg fertilizer N ha⁻¹ per cut. The variation between years was associated mainly with the proportion of clover in the mixtures. Apparent clover-to-grass transfer of fixed N contributed up to 52 kg N ha⁻¹ per year (17 kg N ha⁻¹ on average) to the N yield of the mixtures. Percentage N derived from symbiosis averaged 75% for white and 86% for red clover. These percentages were affected only slightly by supplemental nitrogen, but declined markedly during late summer for white clover. It is concluded that the annual yield of symbiotically fixed N from clover/grass mixtures can be very high, provided that the proportion of clover in the mixtures exceeds 50% of total dry mass yield.     

Dinitrogen-fixation by three neotropical agroforestry tree species under semi-controlled field conditions

Cultivating dinitrogen-fixing legume trees with crops in agroforestry is a relatively common N management practice in the Neotropics. The objective of this study was to assess the N₂ fixation potential of three important Neotropical agroforestry tree species, Erythrina poeppigiana, Erythrina fusca, and Inga edulis, under semi-controlled field conditions. The study was conducted in the humid tropical climate of the Caribbean coastal plain of Costa Rica. In 2002, seedlings of I. edulis and Vochysia guatemalensis were planted in one-meter-deep open-ended plastic cylinders buried in soil within hedgerows of the same species. Overall tree spacing was 1 × 4 m to simulate a typical alley-cropping design. The ¹⁵N was applied as (NH₄)₂SO₄ at 10% ¹⁵N atom excess 15 days after planting at the rate of 20 kg [N] ha⁻¹. In 2003, seedlings of E. poeppigiana, E. fusca, and V. guatemalensis were planted in the same field using the existing cylinders. The ¹⁵N application was repeated at the rate of 20 kg [N] ha⁻¹ 15 days after planting and 10 kg [N] ha⁻¹ was added three months after planting. Trees were harvested 9 months after planting in both years. The ¹⁵N content of leaves, branches, stems, and roots was determined by mass spectrometry. The percentage of atmospheric N fixed out of total N (%N_f) was calculated based on ¹⁵N atom excess in leaves or total biomass. The difference between the two calculation methods was insignificant for all species. Sixty percent of I. edulis trees fixed N₂; %N_f was 57% for the N₂-fixing trees. Biomass production and N yield were similar in N₂-fixing and non-N₂-fixing I. edulis. No obvious cause was found for why not all I. edulis trees fixed N₂. All E. poeppigiana and E. fusca trees fixed N₂; %N_f was ca. 59% and 64%, respectively. These data were extrapolated to typical agroforestry systems using published data on N recycling by the studied species. Inga edulis may recycle ca. 100 kg ha⁻¹ a⁻¹ of N fixed from atmosphere to soil if only 60% of trees fix N₂, E. poeppigiana 60–160 kg ha⁻¹ a⁻¹, and E. fusca ca. 80 kg ha⁻¹ a⁻¹.     

Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N₂+N₂O)−¹⁵N from 98 atom %¹⁵N-labeled urea broadcast at 29 kg N ha⁻¹ into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N₂+N₂O)−¹⁵N during the 19 days following urea application never exceeded 28 g N ha⁻¹ day⁻¹. The total recovery of (N₂+N₂O)−¹⁵N evolved from the field was only 0.51% of the applied N, whereas total gaseous¹⁵N loss estimated from unrecovered¹⁵N in the¹⁵N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m⁻³ or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha⁻¹, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N₂+N₂O)−¹⁵N and total¹⁵N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N₂+N₂O)−¹⁵N and total¹⁵N loss from 27 kg applied nitrate-N ha⁻¹ were 20% and 53% during the same 19-day period. The failure of directly-recovered (N₂+N₂O)−¹⁵N to match total¹⁵N loss from added nitrate-¹⁵N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N₂+N₂O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g⁻¹ in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.     

Acetobacter diazotrophicus,an indoleacetic acid producing bacterium isolated from sugarcane cultivars of México

Thirteen cane cultivars grown on fields in México were sampled to assess the occurrence of Acetobacter diazotrophicus, a recently identified N₂-fixing bacterium. Results showed that the isolation frequencies extended over a broad range (1.1 to 67%), likely to be related to the nitrogen fertilization level. The lowest isolation frequencies (1.1 to 2.5%) were obtained from plants growing at high nitrogen doses (275–300 kg ha^-1) and the highest values (10–67%) from plants cultivated with 120 kg N ha^-1. All eighteen strains of A. diazotrophicus produced indoleacetic acid (IAA) in defined culture medium. Estimates obtained from HPLC analyses revealed that A. diazotrophicus strains produced from 0.14 to 2.42 g IAA mL^-1 in culture medium. Considering that A. diazotrophicus is found within the plant tissue, the biosynthesis of IAA suggests that the bacteria could promote rooting and improve sugarcane growth by direct effects on metabolic processes, in addition to their role in N₂ fixation.     

Effect of varying periods of inoculation ofAzolla pinnata R. Brown on its growth,nitrogen fixation and response to rice crop

Summary Inoculation of water fernAzolla pinnata R. Brown (Bangkok isolate) at the rate of 500kg fresh weight ha⁻¹ in rice fields at weekly intervals after planting in addition to 30 kg N ha⁻¹ as urea showed a decrease in its growth and N₂-fixation with delay in application. Use of Azolla up to 3 weeks after planting (WAP) during wet and 4 WAP during dry season produced significantly more grain yield than 30 kg N ha⁻¹, whereas its application upto one WAP produced more grain yield than 60 kg N ha⁻¹. Grain yield with Azolla applied at the time of planting was similar to that of 60 kg N treatment during the wet season. Higher grain yields in zero and one WAP Azolla treatments resulted due to increase in both number of panicles m⁻² and number of grains/panicle while the subsequent Azolla inoculations increased grain yield mainly by producing more number of grains/panicle. Dry matter and total N yields at maturity of rice crop were more with Azolla application upto 3 WAP during wet and 2 WAP during dry season while the reduction in sterility (%) was observed upto one WAP over 30 kg N ha⁻¹ during both seasons. Number of tillers m⁻² and dry matter production at maximum tillering and flowering were more than 30 kg N ha⁻¹ with the use of Azolla upto one WAP. Increased grain N yield was observed with the use of Azolla upto 4 WAP during two seasons whereas straw N yield increased upto one WAP during wet and 2 WAP during dry season.     

Dual inoculation withRhizobium sp. andGlomus fasciculatum enhances nodulation,yield and nitrogen fixation in chickpea (Cicer arietinum Linn.)

Summary Seed inoculation with Rhizobium and soil inoculation withGlomus fasciculatum increased nodulation, nitrogen and phosphorus concentration in plants and yield of chickpea (Cicer arietinum) var. BG 212 in pots containing unsterilized soil especially with 50kgP₂O₅ ha⁻¹ in the form of superphosphate. Inoculation with Rhizobium orG. fasciculatum separately or in combination significantly increased the N₂ fixed in straw and grain than uninoculated controls as determined by¹⁵N atom percent excess of plants grown in soil amended with labelled ammonium sulphate (¹⁵NH₄)₂SO₄) at the rate of 20kg N ha⁻¹. These increases were most pronounced when P was applied at 50kgP₂O₅ ha⁻¹.     

Evaluation of N2-fixation and nitrogen economy of a maize/cowpea intercrop system using15N dilution methods

Yields of above ground biomass and total N were determined in summer-grown maize and cowpea as sole crops or intercrops, with or without supplementary N fertilizer (25 kg N ha⁻¹, urea) at an irrigated site in Waroona, Western Australia over the period 1982–1985. Good agreement was obtained between estimates of N₂ fixation of sole or intercrop cowpea (1984/85 season) based on the¹⁵N natural abundance and¹⁵N fertilizer dilution techniques, both in the field and in a glasshouse pot study. Field-grown cowpea was estimated to have received 53–69% of its N supply from N₂-fixation, with N₂-fixation onlyslightly affected by intercropping or N fertilizer application. Proportional reliance on N₂-fixation of cowpea in glasshouse culture was lower (36–66%) than in the field study and more affected by applied N. Budgets for N were drawn up for the field intercrops, based on above-ground seed yields, return of crop residues, inputs of fixed N and fertilizer N. No account was taken of possible losses of N through volatilization, denitrification and leaching or gains of N in the soil from root biomass. N₂-fixation was estimated tobe 59 kg N ha⁻¹ in the plots receiving no fertilizer N, and 73 kg N ha⁻¹ in plots receiving 25 kg N ha⁻¹ as urea. Comparable fixation by sole cowpea was higher (87 and 82 kg N ha⁻¹ respectively) but this advantage was outweighed by greater land use efficiency by the intercrop than sole crops.     

Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of¹⁵N-labeled, urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha⁻¹ in 1986 and 1987, respectively. The incorporated residue contained 54 and 49 kg N ha⁻¹ in 1986 and 1987, respectively. The unrecovered¹⁵N in the¹⁵N balances for 58 kg N ha⁻¹ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammoniapNH₃, and floodwater (nitrate + nitrite)-N following application of 29 kg N ha⁻¹ as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha⁻¹, to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha⁻¹, respectively. Coating urea with 0.2g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4 Mg ha⁻¹) and total plant N (11 kg ha⁻¹) in 1987 but not in 1986.     

Endophytic nitrogen fixation in sugarcane: present knowledge and future applications

In Brazil the long-term continuous cultivation of sugarcane with low N fertiliser inputs, without apparent depletion of soil-N reserves, led to the suggestion that N₂-fixing bacteria associated with the plants may be the source of agronomically significant N inputs to this crop. From the 1950s to 1970s, considerable numbers of N₂-fixing bacteria were found to be associated with the crop, but it was not until the late 1980s that evidence from N balance and ¹⁵N dilution experiments showed that some Brazilian varieties of sugarcane were able to obtain significant contributions from this source. The results of these studies renewed the efforts to search for N₂-fixing bacteria, but this time the emphasis was on those diazotrophs that infected the interior of the plants. Within a few years several species of such `endophytic diazotrophs' were discovered including Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, H. rubrisubalbicansand Burkholderia sp. Work has continued on these endophytes within sugarcane plants, but to date little success has been attained in elucidating which endophyte is responsible for the observed BNF and in what site, or sites, within the cane plants the N₂ fixation mainly occurs. Until such important questions are answered further developments or extension of this novel N₂-fixing system to other economically important non-legumes (e.g. cereals) will be seriously hindered. As far as application of present knowledge to maximise BNF with sugarcane is concerned, molybdenum is an essential micronutrient. An abundant water supply favours high BNF inputs, and the best medium term strategy to increase BNF would appear to be based on cultivar selection on irrigated N deficient soils fertilised with Mo.     

Inoculation of forage grasses with N2-fixing Enterobacteriaceae

Summary Previous investigations indicated some forage grass roots in Texas are heavily colonized with N₂-fixing bacteria. The most numerous N₂-fixing bacteria were in the genera Klebsiella and Enterobacter. In the present investigation inoculation experiments were conducted using 18 isolates of these bacteria to determine if a N₂-fixing association could be established between the bacteria and the grassesCynodon dactylon andPanicum coloratum. Plants were grown in soil for approximately 5 months in a greenhouse and were measured periodically for dry matter, nitrogen accumulation, and acetylene reduction activity. Results of the investigation indicated that 25% of the plant-soil systems were active in acetylene reduction and the activity was high enough to indicate agronomically significant quantities of N₂ were being fixed (>8kg N ha⁻¹). However, plant systems extrapolated to fix>8 kg N ha⁻¹ contained less nitrogen and accumulated less dry matter than plants less active in acetylene reduction. Inocula could not be re-isolated from healthy grass roots indicating that the N₂-fixing activity may have not have been closely assiciated with plant roots. Future research is needed to determine factors limiting colonization of grass roots.     

Seasonal patterns of growth and nitrogen fixation in field-grown pea

The seasonal patterns of growth and symbiotic N₂ fixation under field conditions were studied by growth analysis and use of¹⁵N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage. The patterns of fertilizer N-uptake were almost identical in pea and barley (the non-fixing reference crop), but more fertilizer-N was recovered in barley than in pea. The estimated rate of N₂ fixation in pea gradually increased during the pre-flowering and flowering growth stages and reached a maximum of 10 kg N fixed per ha per day nine to ten weeks after seedling emergence. This was the time of early pod-development (flat pod growth stage) and also the time for maximum crop growth rate and maximum green leaf area index. A steep drop in N₂ fixation rate occurred during the following week. This drop was simultaneous with lodging of the crop, pod-filling (round pod growth stage) and the initiation of mobilization of nitrogen from vegetative organs. The application of fertilizer-N inhibited the rate of N₂ fixation only during that period of growth, when the main part of fertilizer-N was taken up and shortly after. Total accumulation of fixed nitrogen was estimated to be 244, 238 and 213 kg N ha⁻¹ in pea supplied with nil, 25 or 50 kg NO ₃ ⁻ −N ha⁻¹, respectively. About one-fourth of total N₂ fixation was carried out during preflowering, one fourth during the two weeks of flowering and the remainder during post-flowering. About 55% of the amount of N present in pods at maturity was estimated to be derived from mobilization of N from vegetative organs. “Starter” N (25 or 50 kg NO ₃ ⁻ −N ha⁻¹) did not significantly influence either dry matter and nitrogen accumulation or the development of leaf area. Neither root length and root biomass determined 8 weeks after seedling emergence nor the yield of seed dry matter and nitrogen at maturity were influenced by fertilizer application.     

Dinitrogen fixation in white clover grown in pure stand and mixture with ryegrass estimated by the immobilized 15N isotope dilution method

Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using ¹⁵N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing ¹⁵N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which ¹⁵N fertilizer is added after sward establishment. Immobilization of ¹⁵N in the soil organic matter has not previously been used in studies of N₂ fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the ¹⁵N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the applied ¹⁵N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N₂ fixation. In pure stand clover, 75–94% of the N was derived from N₂ fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N₂ fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha⁻¹ day⁻¹ in autumn to more than 2.6 kg N ha⁻¹ day⁻¹ in June. For clover in pure stand the average N₂ fixation rate was greater and varied between 0.5 and 3.3 kg N ha⁻¹ day⁻¹, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha⁻¹ in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha⁻¹ in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha⁻¹ to the grass in the first and second production year, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

The concept of critical nitrogen concentration(%N _c) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%N _c declines as a function of aerial biomassaccumulation (W) and the %N _c –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as: If W < 1 t ha^-1%N _c = 3.40 If 1 t ha^-1≤ W ≤ 22 t ha^-1%N _c = 3.40(W)^−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %N _c wasallowed. A critical N uptake model (Nu_c, kg Nha^-1) is proposed as Nu_c = 34 (W)^0.63 A comparison between Nu_c and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha^-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen accumulation in sole and mixed stands of sweet-blue lupin (Lupinus angustifolius L.), ryegrass and oats

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the ¹⁵N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average of 92% or 195 kg N ha⁻¹ of its N from N₂ fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N₂ (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N₂ fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar estimates of N₂ fixed in lupin. It was deduced from the ¹⁵N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the ¹⁵N fertilizer rate from 30 to 60 kg N ha⁻¹ decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N₂ fixation at low temperatures, and can maintain higher rates of N₂ fixation in soils of high N than many other forage and pasture legumes.     

Symbiotic nitrogen fixation in eight Acacia senegal provenances in dryland clays of the Blue Nile Sudan estimated by the 15N natural abundance method

The symbiotic biological N₂fixation by Acacia senegal was estimated using the ¹⁵N natural abundance (δ ¹⁵N) procedure on eight provenances collected from different environments and soil types grown in a clay soil in the Blue Nile region, Sudan. Balanites aegyptiaca (a non-legume) was used as a non-N₂-fixing reference plant to allow ¹⁵N-based estimates of the proportion of the Acacia N derived from atmospheric N₂ (N_dfa) to be calculated. Results show variation in leaf δ ¹⁵N between A. senegal and the reference plant and among years. The relative δ ¹⁵N values (‰) were higher in B. aegyptiaca than in the N₂-fixing acacia provenances. Provenances originally collected from clay soils fixed little N in the first year, but the amount fixed increased as the trees aged. All provenances showed a decrease in δ ¹⁵N with age. The N_dfa varied between 24% (Mazmoom provenance) and 61% (Rahad provenance) 4 years after planting. There was no significant difference in δ ¹⁵N between provenance groups based on soil type or rainfall at original growing site. The amount of N_dfa increased significantly with age in all provenances. The above-ground contribution of fixed N to foliage growth in a 4-year-old A. senegal was highest in the Rahad sand–soil provenance (46.7 kg N ha⁻¹) and lowest in the Mazmoom clay-soil provenance (28.7 kg N ha⁻¹). Our study represents the first use of the δ ¹⁵N method for estimating the N input by A. senegal to the clay plain soils of the gum belt in the Sudan.     

Turfgrass (Cynodon dactylon L.) sod production on sandy soils: I. Effects of irrigation and fertiliser regimes on growth and quality

The effects on growth, quality and N uptake by turfgrass (Cynodon dactylon L.) during sod production of four fertiliser types applied at three application rates (100, 200 or 300 kg N ha⁻¹ per ‘crop’) under two irrigation treatments (70% and 140% daily replacement of pan evaporation) were investigated. The fertiliser types were: water-soluble (predominately NH₄NO₃), control-release, pelletised poultry manure, and pelletised biosolids; and the experiment was conducted on a sandy soil in a Mediterranean-type climate. Plots were established from rhizomes, with the turfgrass harvested as sod every 16–28 weeks depending upon the time of the year. Four crops were produced during the study. Applying water-soluble and control-release fertilisers doubled shoot growth and improved turfgrass greenness by up to 10% in comparison with plots receiving pelletised poultry manure and pelletised biosolids. Nitrogen uptake into the shoots after four crops (averaged across irrigation treatments and N rates) was 497 kg N ha⁻¹ for the water-soluble fertiliser, 402 kg N ha⁻¹ for the control-release, 188 kg N ha⁻¹ for the pelletised poultry manure and 237 kg N ha⁻¹ for the pelletised biosolids. Consequently, the agronomic nitrogen-use efficiency (NAE, kg DM kg⁻¹ N applied) of the inorganic fertilisers was approximately twice that of the organic fertilisers. Increasing irrigation from 70% to 140% replacement of pan evaporation was detrimental to turfgrass growth and N uptake for the first crop when supplied with the water-soluble fertiliser. Under the low irrigation treatment, inorganic N fertilisers applied at 200–300 kg N ha⁻¹ were adequate for production of turfgrass sod. Section Editor: P. J. Randall