Amino acid content in xylem sap of regrowing alfalfa (Medicago sativa L.): Relations with N uptake,N2 fixation and N remobilization

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N₂ and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N₂ fixation and endogenous N remobilization were estimated by ¹⁵N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N₂ or from NH₄NO₃ during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N₂ fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH₄NO₃ grown and in N₂ fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.     

Source of the soybean N credit in maize production

Nitrogen response trials throughout the United States Corn Belt show that economic optimum rates of N fertilization are usually less for maize (Zea mays L.) following soybean (Glycine max L.) than for maize following maize; however, the cause of this rotation effect is not fully understood. The objective of this study was to investigate the source of the apparent N contribution from soybean to maize (soybean N credit) by comparing soil N mineralization rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields, plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha^–1) and above-ground N accumulation (97 vs. 71 kg ha^–1) were greatly increased when maize followed nodulated soybean compared with maize following maize. A partial benefit to yield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha^–1, respectively. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil inorganic N (especially in soil 5 cm from the row) was greater following nodulated soybean than non-nodulated soybean or maize. Based on these data, the soybean N credit appears to result from a combination of a decrease in net soil mineralization in continuous maize production and an increase in residual soil N from symbiotic fixation.     

Comparison of isotope techniques and non-nodulating isolines to study the effect of ammonium fertilization on dinitrogen fixation in soybean,Glycine max

Summary Two experiments were carried out with two nodulating and non-nodulating soybean isolines, with three different levels of N as (¹⁵NH₄)₂SO₄ at the equivalent of 0, 25 and 50 kg N/ha. In the first experiment three seeds were sown in each pot and the plants harvested at 35, 55 and 75 days. In the second experiment only one seed was sown per pot and harvested at 75 days.Isotope dilution technique and in certain cases natural isotope variation (¹⁵N) was used to determine directly the origin of nitrogen in the plant, whether from soil, fertilizer or biological N₂-fixation. The use of nodulating and non-nodulating isolines enabled comparison with the classical method of estimating N₂-fixation by difference from total plant N. Results at the 75 day harvest were similar for either method, but at the earlier harvests, particularly at 35 days, the total-N method was inadequate. The isotope method appeared more sensitive while the total-N method suffered from greater variability with correspondingly high standard errors and significant differences.It was found that by the 35 and 55 day harvests hardly any N₂-fixation had taken place, plant nitrogen being almost entirely derived from soil or fertilizer N. Plants in competition used up soil fertilizer N more rapidly, thus stimulating symbiotic nitrogen fixation. When only one plant was grown in each pot it had a greater proportion of N derived from soil or fertilizer, and less N derived from fixation. In general the¹⁵N data showed that only about 25% of the applied fertilizer N was absorbed by the plant.The nodulating isoline absorbed more N than the non-nodulating plants. This suggests a possible synergistic effect of N₂-fixation on N derived from other sources, giving an increase in total-N content of nudulated plants. The N derived from N₂-fixation was scarcely detectable in the roots but appeared to be translocated almost entirely to shoots and pods.With 25 kg N/ha the greater proportion of the nitrogen in the pods was derived from N₂-fixation. Even with 50 kg N/ha the nitrogen in the pods derived from fixation remained high, that being derived from fertilizer being less than 15%. About 80% of the nitrogen in the nodules was due to fixation.In the present experiment the application of 25 kg N/ha appeared sufficient to give maximum N absorption by both isolines. At this level symbiotic fixation by Rhizobium remained high in nodulating plants, while the proportion of total N due to fixation was reduced with 50 kg N/ha.UNDP/IAEA Project BRA 78/006.     

Utilization of 15NO3 − by nodulated soybean plants under conditions of root hypoxia

Waterlogging of soils is common in nature. The low availability of oxygen under these conditions leads to hypoxia of the root system impairing the development and productivity of the plant. The presence of nitrate under flooding conditions is regarded as being beneficial towards tolerance to this stress. However, it is not known how nodulated soybean plants, cultivated in the absence of nitrate and therefore not metabolically adapted to this compound, would respond to nitrate under root hypoxia in comparison with non-nodulated plants grown on nitrate. A study was conducted with ¹⁵N labelled nitrate supplied on waterlogging for a period of 48 h using both nodulated and non-nodulated plants of different physiological ages. Enrichment of N was found in roots and leaves with incorporation of the isotope in amino acids, although to a much smaller degree under hypoxia than normoxia. This demonstrates that nitrate is taken up under hypoxic conditions and assimilated into amino acids, although to a much lesser extent than for normoxia. The similar response obtained with nodulated and non-nodulated plants indicates the rapid metabolic adaptation of nodulated plants to the presence of nitrate under hypoxia. Enrichment of N in nodules was very much weaker with a distinct enrichment pattern of amino acids (especially asparagine) suggesting that labelling arose from a tissue source external to the nodule rather than through assimilation in the nodule itself.     

Effect of nodulation on assimilate remobilization in soybean

The objectives of this work were to determine the effect of nodulation on dry matter, reduced-N, and phosphorus accumulation and partitioning in above-ground vegetative parts and pods of field-grown soybean (Glycine max [L.] Merr. cv Harosoy).

From comparison of nodulated and nonnodulated isolines, it was estimated that nodulated plants attained 81 and 71% of total-plant (above ground) N from uptake of soil N in 1981 and 1982, respectively. These data, along with visibly greener leaves of nodulated plants, led us to assume that nonnodulated plants were under a moderate N stress relative to nodulated plants. Nonnodulated plants accumulated less total-plant N and partitioned less dry matter and N to the pods, compared with nodulated plants. This occurred even though net photosynthesis, as estimated by rate and amount of dry matter accumulation, was the same for both nonnodulated and nodulated plants. Rate of dry matter and reduced-N accumulation in pods was less for nonnodulated than for nodulated plants while duration of podfill was similar for both isolines. From these data we concluded that moderate N stress affected partitioning of photosynthate rather than net photosynthesis, and that N played a role in translocation of photosynthate to the pods. Total plants (above-ground portion) and pods of both nodulated and nonnodulated plants accumulated similar amounts of phosphorus, which indicated that phosphorus and N accumulation were independent.

Remobilization of nitrogen and phosphorus from vegetation to pods preceded dry matter remobilization. It appeared that either more nitrogen accumulation prior to podfill, or continued nitrogen assimilation during podfill would increase nitrogen and dry matter partitioning to pods, but that increasing photosynthesis without concomitantly increasing nitrogen input may not necessarily result in enhanced seed production.

     

Weather and nodule mediated variations in delta 13C and delta 15N values in field-grown soybean (Glycine max L.) with special interest in the analyses of xylem fluids

The nodulating soybean (Enrei) and its non-nodulating mutant (EN 1282) were grown in outdoor plots for 2 years (1994: extraordinary dry, high temperature, 1995: ordinary year). Carbon and nitrogen accumulation, delta 13C and delta 15N values in plant parts and xylem fluids and delta 15N values in the water-extractable soil N were analysed throughout the growing period. Plant growth in 1994 was rapid during the early growth stages, but no pods were produced. In 1995, plant growth was normal and pods were formed. The delta 13C values of the leaves were less negative in 1994 than in 1995 and the nodulated plants showed less negative delta 13C values than non-nodulated plants in both years. The delta 13C values of the leaves during the vegetative phase were positively correlated to the leaf N concentrations. Leaf N concentrations in their turn were influenced by nodulation and weather conditions and/or soil available N. The delta 15N values in the plants and xylem fluids were lower in the nodulated soybean than in non-nodulated soybean in both years, and estimates of the contribution of N2 fixation in nodulated plants based on plant top delta 15N values were 7-14% in 1994 and 37-63% in 1995. The delta 13C values of xylem fluids did not differ between nodulated and non-nodulated plants. Thus, the expected contribution by phosphopenolpyruvate carboxylase-mediated CO2 fixation in the root nodules to plant C-incorporation could not have been significant.     

Symbiotic N2 fixation by soybean in organic and conventional cropping systems estimated by 15N dilution and 15N natural abundance

Nitrogen (N) is often the most limiting nutrient in organic cropping systems. N₂ fixing crops present an important option to improve N supply and to maintain soil fertility. In a field experiment, we investigated whether the lower N fertilization level and higher soil microbial activity in organic than conventional systems affected symbiotic N₂ fixation by soybean (Glycine max, var. Maple Arrow) growing in 2004 in plots that were since 1978 under the following systems: bio-dynamic (DYN); bio-organic (ORG); conventional with organic and mineral fertilizers (CON); CON with exclusively mineral fertilizers (MIN); non-fertilized control (NON). We estimated the percentage of legume N derived from the atmosphere (%Ndfa) by the natural abundance (NA) method. For ORG and MIN we additionally applied the enriched ¹⁵N isotope dilution method (ID) based on residual mineral and organic ¹⁵N labeled fertilizers that were applied in 2003 in microplots installed in ORG and MIN plots. These different enrichment treatments resulted in equal %Ndfa values. The %Ndfa obtained by NA for ORG and MIN was confirmed by the ID method, with similar variation. However, as plant growth was restricted by the microplot frames the NA technique provided more accurate estimates of the quantities of symbiotically fixed N₂ (Nfix). At maturity of soybean the %Ndfa ranged from 24 to 54%. It decreased in the order ORG > CON > DYN > NON > MIN, with significantly lowest value for MIN. Corresponding Nfix in above ground plant material ranged from 15 to 26 g N m^-2, with a decreasing trend in the order DYN = ORG > CON > MIN > NON. For all treatments, the N withdrawal by harvested grains was greater than Nfix. This shows that at the low to medium %Ndfa, soybeans did not improve the N supply to any system but removed significant amounts of soil N. High-soil N mineralization and/or low-soil P availability may have limited symbiotic N₂ fixation.     

Field evaluation of symbiotic nitrogen fixation by rhizobial strains using15N methodology

Summary Small differences in N₂ fixation by nodulated soybeans (Glycine max. (L.) Merr.), inoculated with various strains ofRhizobium japonicum, were assessed in field experiments using¹⁵N methodology, and compared with yields of plant dry matter and total N. Percentage of plant-N derived from atmospheric N₂ and from fertilizer, and values of %¹⁵N atom excess had lower coefficients of variation than did total N and dry matter yield. Nevertheless the precision of estimates of kg N/ha fixed were sufficient to differentiate only the extremes of the range of strains tested, and there were discrepancies between ranking of strains based on % N derived from fertilizer and on total N yield.     

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.     

Natural abundance of 15N and 13C in nodulated legumes and other plants in the cerrado and neighbouring regions of Brazil

Leaves from over 1000 Brazilian native plants growing in the cerrado and neighbouring regions were sampled for C and N content. Half of these were analysed for ¹⁵N and further samples for ¹³C and ash content. Nodulated legumes from all three sub-families were included, together with two types of reference plant, non-nodulated legumes and non-legumes. Particular emphasis was placed on the large caesalpinioid genus Chamaecrista which is here for the first time reported to fix nitrogen in its native habitats. Woody and herbaceous species of this and other nodulated genera, with the exception of the mimosoid tree Stryphnodendron, showed evidence of nitrogen fixation. Amounts fixed were site-specific as was the ¹⁵N signature of reference plants. There was no evidence that nodulated legumes had higher leaf N than non-nodulated legumes: both were higher than non-legumes. Several species of Chamaecrista from section absus and species of Stryphnodendron had carbon contents of 50–55%, higher than previously reported for leaves. This was coupled with low (1–3%) ash contents. The ¹³C values of plants with 49% C were significantly more negative than those with <49% C: most species in the former group were woody and most in the latter group herbaceous. Mimosa pudica was unusual in having a wide range of percent C, percent ash and ¹³C values; these parameters were significantly correlated. It is concluded that Brazilian native legumes can fix significant amounts of nitrogen in the nutrient-poor cerrado soils. Consideration of mineral and lipid nutrition will be necessary in order fully to understand relations between ¹³C, carbon content and other physiological parameters.     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Endomycorrhizal fungal species mediate 15N transfer from soybean to maize in non-fumigated soil

The effect of mycorrhizal inoculation on ¹⁵N transfer from soybean to maize was studied in fumigated and non-fumigated soil. Three Glomus species and a non-inoculated control were compared.In spite of higher levels of root colonization and more abundant hyphae associated with plants growing in fumigated soil, mycorrhizae-enhanced ¹⁵N transfer to maize was significant only in non-fumigated plots. High ¹⁵N transfer was not only associated with high mycelium density in soil but also with low soil microbial carbon, suggesting that the effect of mycorrhizal fungi on soil microbial populations may be an important factor affecting N transfer between mycorrhizal plants.     

Comparison of two 15N labelling methods for assessing nitrogen rhizodeposition of pea

Two ¹⁵N labelling methods for assessing net rhizodeposition of nitrogen (N) in pea crop (Pisum sativum L.) were compared in the greenhouse and in the field: the cotton-wick (CW) and the split-root (SR) methods. Rhizodeposition is defined as the organic material lost from roots during their growth through the soil. CW is a method in which ¹⁵N urea was supplied to the plant in pulses via a wick threaded through the stem. In SR, the root system was divided between a hydroponic labelling compartment (LC) containing the labelling nutrient solution (1 or 5 mM ¹⁵NO₃–¹⁵NH₄) and a compartment filled with soil in which the amount of ¹⁵N rhizodeposition was assessed. The percentage of N derived from rhizodeposition (%Ndfr), was used to calculate the amount of N rhizodeposition which was obtained from the ratio of atom % ¹⁵N excess of the soil : atom % ¹⁵N excess of the roots. Above ground parts in the field accumulated markedly more dry matter and N than in the greenhouse, regardless of the labelling method. ¹⁵N enrichments of above ground parts were higher than those of roots recovered from the soil. Results indicated that amount of ¹⁵N applied to plants were lower in SR than in CW. Additionally, LC roots of SR tended to retain large amounts of ¹⁵N. As a consequence, atom % ¹⁵N excess of roots was less than 1% in SR, whereas most values varied from 1% to 4% in CW. However, relationships between enrichments of the soil and of the roots were different in SR and CW. It was not possible to compare the Ndfr:root-N ratio between the two methods, but the ratio of Ndfr:plant-N was found to be 10% higher in SR than in CW. Finally, relative to total plant-N, the total contribution of below ground parts to the N pool of the soil reached 22–25% at maturity for the two methods. From our experiments, we could not conclude that one method is better than the other for estimating either net rhizodeposition of N or the contribution of a pea plant to the soil N pool. However, CW is easier to adapt and monitor under field conditions than SR.     

Nitrogen fixation in soybean as influenced by cultivar and Rhizobium strain

Summary The¹⁵N-substratum labeling technique and other indirect methods were used to compare nitrogen (N₂) fixation in soybean varieties grown in the field in Greece and Romania. Significant variation in the amount (Ndfa) and proportion of N derived from fixation (% Ndfa) was found in different varieties. With 20 kg N/ha applied to soil, N₂ fixed ranged from 22 to 236 kg N/ha in Greece and from 17 to 132 kg N/ha in Romania. In general, varieties or treatments with higher dry matter yield supported greater fixation. Also, varieties with high Ndfa had high % Ndfa andvice versa. Breeding N₂-fixing legumes for high yields at low soil N levels therefore appears to be a reasonable strategy for enhancing N₂ fixation. Heavy applications of inorganic N fertilizer severely depressed N₂ fixation in two out of the three varieties used in Romania. One variety, F 74–412, however, derived slightly higher amounts of N₂ from fixation at 100 kg N/ha rate than when fertilized with 20 kg N/ha. In Greece, Chippewa, Williams and Amsoy-71 inoculated with a Nitragin inoculant fixed similar amounts of N₂ at both 20 and 100 kg N/ha fertilizer rates. However, when Chippewa and Williams were inoculated with amother, locally-isolated Rhizobium strain, N₂ fixation was substantially depressed at the higher N rate.     

Nitrogen fixation in perennial forage legumes in the field

Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N₂ fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N₂-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N₂ fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N₂ fixation in above-ground plant tissues were in the range of up to 373 kg N ha^–1 year^–1 in red clover (Trifolium pratense L.), 545 kg N ha^–1 year^–1 in white clover (T. repens L.) and 350 kg N ha^–1 year^–1 in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N₂ fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N₂ fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N₂ fixation per ha and year in northern areas (around 60°N) as in areas with a milder climate (around 40°N). Analyzing whole plants or just above-ground plant parts influenced the estimate of N₂ fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N₂ fixation (Nfix; kg N per ha and year) was significantly (P<0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026DM + 7 for T. pratense, Nfix = 0.031DM + 24 for T. repens, and Nfix = 0.021DM + 17 for M. sativa.     

Growth,ionic balance,proton excretion,and nitrate reductase activity in Alnus and Hippophaë supplied with different sources of nitrogen

Summary Pre-cultivated, nodulated and non-nodulated plants of black alder (Alnus glutinosa) and sea buckthorn (Hippophaë rhamnoides ssp.rhamnoides) were grown on different N sources, with and without acidity control. Dry matter yields were lowest when plants were supplied with only NO ₃ ^– and were much greater when NH ₄ ⁺ was supplied either alone or in combination with NO ₃ ^– as long as the external pH was controlled; the final yields of the N₂-fixing plants were relatively low, especially withH. rhamnoides. Without acidity control, yields were greatly reduced in the presence of NH ₄ ⁺ .Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, agreed well with measured excretion values. Without pH adjustment, the total proton efflux into the external solution was greater inA. glutinosa than inH. rhamnoides.Both species, but particularlyA. glutinosa, displayed the highest nitrate reductase activity in the roots.     

Probable site of allantoin formation in nodulating soybean plants

Nodulated soybean plants contain high concentration of allantoin in all parts. Excision of nodules from the roots brought about a marked decrease in allantoin. To examine the function of nodules in allantoin production, nodulated and nodule-detached soybeans were fed with ¹⁵NH₃ for 1 week. High abundance of ¹⁵N was found in the amino acid-N fraction of both plants. In the root and stem of the nodulated plants, ca 80% of the nitrogen in this fraction was derived from the NH₃ added in the medium. Excess ¹⁵N was detected also in allantoin-N fraction, but the ¹⁵N content was very low in contrast to that in amino acid-N fraction. The site involved in the allantoin formation and the possible significance of its synthesis are discussed in relation to symbiotic nitrogen fixation.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Nitrogen transfer from nodulating soybean to maize or to nonnodulating soybean in intercrops: the 15N dilution method

In 1985, 1986 and 1988, maize (Zea mays L.) was monocropped or intercropped with nodulating or nonnodulating soybean (Glycine max [L.] Merr.). In addition, nodulating soybean and nonnodulating soybean were each monocropped and grown as a mixture. In 1985 and 1986, treatments were grown at 0 and 60 kg N ha^–1 and in 1988, the treatments were grown without N fertilizer, on N-depeted soil and on non-N-depleted soil. ¹⁵N enriched N was applied to soil in all the aforementioned treatments to test for N transfer from nodulating soybean to non-N₂-fixing crops by the ¹⁵N dilution method.The ¹⁵N dilution method did not show the occurrence of N transfer in 1985 and 1986, but the N sparing effect was evident from the total N uptake of nonnodulating soybean, dwarf maize and tall maize, in 1986. In 1988, maize and nonnodulating soybean seed yields and seed N yields were higher on non-N-depleted soil than on N-depleted soil. On N-depleted soil, the ¹⁵N dilution method indicated N transfer from nodulating soybean to maize and to nonndulating soybean. At a population ratio of 67% nodulating soybean to 33% nonnodulating soybean, N transfer was also seen on non-N-depleted soil in 1988.     

Estimation of biological nitrogen fixation associated withBrachiaria andPaspalum grasses using15N labelled organic matter and fertilizer

Summary Six pasture grasses,Paspalum notatum cv batatais,P. notatum cv pensacola,Brachiaria radicans, B. ruziziensis, B. decumbens andB. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of¹⁵N labelled organic matter or frequent small (2 kg N. ha^–1) additions of¹⁵N enriched (NH₄)₂SO₄. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in¹⁵N enrichment in plant-available nitrogen (NO ₃ ^– +NH ₄ ⁺ ) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different¹⁵N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the¹⁵N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in¹⁵N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same¹⁵N enrichment in soil derived N. Data from the final harvests of this treatment indicated thatB. humidicola andB. decumbens obtained 30 and 40% respectively of their nitrogen from N₂ fixation amounting to an input of 30 and 45 kg N.ha^–1 year^–1 respectively.