Time-course of nitrogen fixation in two bambara groundnut (Vigna subterranea L. Verdc.) cultivars

TheA-value method, involving the application of a higher¹⁵N rate to a reference non-N₂-fixing plant, was used to assess the magnitude of N₂ fixation in two bambara groundnut cultivars at four growth stages [vegetative, 0–47 days after planting (DAP); early pod-filling, 47–99 DAP; mid-pod-filling, 99–120 DAP; physiological maturity, 120–148 DAP). The cultivars were Ex-Ada, a bunchy type, and CS-88-11, a slightly spreading type. They were grown on a loamy sand. Uninoculated Ex-Ada and CS-88-11 were used as reference plants to measure the N₂ fixed in the inoculated bambara groundnuts. In this greenhouse study, soil was the major source of N in bambara groundnuts during vegetative growth, and during this period it accounted for over 80% of the N accumulaed in the plants. However, N₂ fixation became the major source of plant N during reproductive growth. There were significant differences between the two cultivars in the ability to fix N₂, and at physiological maturity, almost 75% of the N in CS-88-11 was derived from the atmosphere compared to 55% in Ex-Ada. Also, the total N fixed in CS-88-11 at physiological maturity was almost double that in Ex-Ada. Our data indicate that the higher N₂ fixation in CS-88-11 was due to two factors, a higher intensity of N₂ fixation and a longer active period of N₂ fixation. The results also suggest that bambara groundnut genotypes could be selected for higher N₂ fixation in farining systems.     

Field evaluation of N2 fixation and N partitioning in climbing bean (Phaseolus vulgaris L.) using 15N

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

Effect of successive cuttings on uptake and partitioning of 15N among plant parts of Leucaena leucocephala

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

Transfer of N fixed by a legume tree to the associated grass in a tropical silvopastoral system

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

Time-lagged induction of N2O emission and its trade-off with NO emission from a nitrogen fertilized Andisol

Abstract

To understand the influence of basal application of N fertilizer on nitrification potential and N₂O and NO emissions, four soil samples were collected from an upland Andisol field just before (sample 1) and 4 (sample 2), 36 (sample 3) and 72 (sample 4) days after the basal application of N fertilizer during the Chinese cabbage growing season from 12 September to 30 November 2005. The potentials of N₂O production and nitrification of the soils were determined using a ¹⁵N tracer technique and the soils were incubated for 25 days at 25°C and 60% water-filled pore space (WFPS). The results revealed that as much as 84–97% N₂O and almost all NO were produced by nitrification. The ¹⁵N₂O emission peak occurred approximately 350 h after the beginning of incubation for samples 1 and 2, but just 48 h later in samples 3 and 4. Total ¹⁵N₂O emission during the 25-day incubation of samples 3 and 4 ranged from 190 to 198 µg N kg^?1 soil, which was significantly higher than the 99–108 µg N kg^?1 soil recorded in samples 1 and 2. Basal application of N fertilizer did not immediately increase the nitrification potential and the ratio of N₂O to N added, but did dramatically increase the nitrification potential and the ratio of N₂O to N added as (¹⁵NH₄)₂SO₄ 36–72 days after the basal N fertilizer was added. In contrast, NO emission was negatively correlated with nitrification potential and total N₂O emission. As a result, a trade-off relationship between total NO and N₂O emissions was identified. The results indicated that there was a time-lagged induction of the change of N turnover in the soil, which was possibly caused by slow population growth of the nitrifiers and/or a slow shift in the microbial community in the soil.     

Arbuscular mycorrhizae affect the N and C economy of nodulated Phaseolus vulgaris (L.) during NH4 nutrition

In the symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, the C and N economy can be influenced by the source of N-supply from either AM-derived NH₄⁺ uptake or nodule-derived biological nitrogen fixation (BNF). This relationship was investigated in terms of NH₄⁺ supply and BNF by the two symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were hydroponically grown with either 0 N or 1 mM NH₄⁺ supply. Plants were harvested at 30 days after emergence and measurements were taken for biomass, N₂ fixation, photosynthesis, CO₂ and O₂ root respiration, calculated C and N economy. AM roots had higher NH₄⁺ uptake and this was associated with the suppression of BNF and nodule growth. The higher NH₄⁺ uptake in AM roots occurred with lower root maintenance respiration, compared to when N was derived from BNF. There was also an increase in the below-ground sink strength of NH₄⁺ fed AM roots compared to NH₄⁺ fed non-AM roots, as evidenced by the increases in root CO₂ and O₂ respiration and photosynthetic stimulation. These results indicate that although the AM root had higher total below-ground respiratory costs during NH₄⁺ nutrition, there were lower respiratory C costs associated with N derived from AM symbionts in comparison to N from BNF.     

Incorporation of 15N into various nitrogenous compounds in intact soybean nodules after exposure to 15N2 gas

The intact nodules attached to the upper part of soybean roots were exposed to ¹⁵N₂ and the incorporation of ¹⁵N into various soluble nitrogen constituents was investigated. Results indicated that ammonia, a primary product of N₂ fixation, was located in more than two compartments. Ammonia reduced from N₂ gas seemed to be incorporated firstly into glutamine especially amido-group nitrogen. Newly fixed nitrogen was secondly incorporated into glutamic acid and alanine in this sequence. These results suggested that fixed ammonia was assimilated by glutamine synthetase/glutamate synthase pathway. Turn-over rate of allantoin plus allantoic acid and serine was relatively high, although apparently these compounds were not primary products of newly fixed ammonia. ¹⁵N content of allantoin was always higher than that of allantoic acid. ¹⁵N incorporation to aspartic acid and asparagine was relatively slow, especially in early period. In bacteroid fraction there is much amount of ammonia comparing with other compounds, while allantoin and asparagine were presented exclusively in cytosol. ¹⁵N was incorporated into nitrate within a few minutes especially in bacteroids.     

Soil N Losses by Denitrification Evaluated Using the 15N Tracer Method

Molecular nitrogen (N₂) and nitrous oxide (N₂O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N₂ and N₂O from potassium nitrate (K¹⁵NO₃) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the ¹⁵N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D⁺) and without (D^?) dextrose added. The treatments received 100 mg N kg^?1 (KNO₃ with 23.24 atom% ¹⁵N). N₂ emissions occurred in all of the treatments, but N₂O emissions only occurred in the D⁺ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D⁺ treatment exhibited the highest accumulated N emission (48.26 mg kg^?1). The ¹⁵N balance suggested trapping of the gases in the soil.     

Short-term measurements of uptake of nitrogen fixed in the rhizospheres of sorghum (Sorghum bicolor) and millet (Pennisetum americanum)

Summary In a series of short-term experiments root systems of young sorghum and millet plants inoculated with N₂-fixing bacteria were exposed to ¹⁵N₂-enriched atmospheres for 72 h. The plants were grown in a normal atmosphere for up to 22 days after the end of the exposure to allow them to take up the fixed N₂. Environmental conditions and genotypes of sorghum and millet were selected to maximise N₂-fixation in the rhizosphere. Detectable amounts of fixed N (> 16 g/plant) were rapidly incorporated into sorghum plants grown in a sand/farmyard manure medium, but measurable fixation was found on only one occasion in plants grown in soil. N₂ fixation was detectable in some experiments with soil-grown millet plants but the amounts were small (2–4 g/plant) and represented less than 1 % of plant N accumulated over the same period. In many cases there was no detectable ¹⁵N₂ incorporation despite measurable increases in ethylene concentration found during an acetylene reduction assay.Published as ICRISAT Journal Article No. JA 740     

Does labelling frequency affect N rhizodeposition assessment using the cotton-wick method?

The aim of the present study was to test and improve the reliability of the ¹⁵N cotton-wick method for measuring soil N derived from plant rhizodeposition, a critical value for assessing belowground nitrogen input in field-grown legumes. The effects of the concentration of the ¹⁵N labelling solution and the feeding frequency on assessment of nitrogen rhizodeposition were studied in two greenhouse experiments using the field pea (Pisum sativum L.). Neither the method nor the feeding frequency altered plant biomass and N partitioning, and the method appeared well adapted for assessing the belowground contribution of field-grown legumes to the soil N pool. However, nitrogen rhizodeposition assessment was strongly influenced by the feeding frequency and the concentration of labelling solution. At pod-filling and maturity, despite similar root ¹⁵N enrichment, the fraction of plants' belowground nitrogen allocated to rhizodeposition in both Frisson pea and the non-nodulating isoline P2 was 20 to more than 50% higher when plants were labelled continuously than when they were labelled using fortnightly pulses. Our results suggest that when ¹⁵N root enrichment was high, nitrogen rhizodeposition was overestimated only for plants that were ¹⁵N-fed by fortnightly pulses, and not in plants ¹⁵N-fed continuously. This phenomenon was especially observed for plants that rely on symbiotic N₂ fixation for N acquisition, and it may be linked to the concentration of the labelling solution. In conclusion, the assessment of nitrogen rhizodeposition was more reliable when plants were labelled continuously with a dilute solution of ¹⁵N urea.     

Use of 15N isotope dilution for quantification of N2 fixation associated with roots of kallar grass (Leptochloa fusca (L.))

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

Partitioning of Newly Absorbed and Previously Stored Nitrogen and Sulfur Under Sulfate Deficient Nutrition

Sulfur (S) deficiency effects on nitrogen (N) and S fluxes during vegetative growth of Brassica napus was investigated by tracing ¹⁵N and ³⁴S for 9 d of S-sufficient [1.5 mM sulfate (SO₄^2-)] and S-deficient (0.05 mM SO₄^2-) condition. A significant decrease in leaf osmotic potential and chlorophyll content was apparent after 9 d of S-deficiency. Sulfur uptake during 9 d was remarkably decreased by 94.3% by S-deficiency, whereas no significant change occurred for N uptake. The N and S deriving from uptake were mainly allocated to the leaves in control plants, but the S flow into leaves was largely restricted under S-deficient condition. The remobilization of stored N and S were mainly issued only from leaves in control plants, while from leaves and petiole in S-deficient ones. The remobilization of N and S mainly issued from leaves flows into the roots both in control and S-deficient plants.     

Dinitrogen fixation and soil n uptake by soybean as affected by phosphorus availability

The effects of phosphorus supply (0, 30, and 90 mg P kg^‐1) on growth, N₂ fixation, and soil N uptake by soybean (Glycine max (L.) Merr.) were studied in a pot experiment using the ¹⁵N isotope technique. Phosphorus supply increased the top dry matter production at flowering and the dry matter production of seeds, straw, pod shells, and roots at late pod filling of inoculated soybeans. Phosphorus supply reduced the N concentration of plant tops at flowering, but increased the amount of N accumulated at both flowering and late pod filling. In inoculated soybeans total N accumulation paralleled the dry matter production. The P concentration in above‐ground plant parts of nodulated soybeans was not affected by P application. At flowering only 18 to 34% of total N was derived from N₂ fixation, whereas as much as 74% was derived from N₂ fixation at late pod filling. Only the addition of 90 mg P kg^‐1 soil significantly increased the amount of N₂ fixed at the late pod filling stage. Phosphorus supply did not influence the uptake of fertilizer or soil N in soybeans, even if the root mass was increased up to 60% by the P supply.     

Field measurements of nitrogen fixation in leguminous trees used in agroforestry systems: Influence of 15N-labeling approaches and reference trees

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

The difference in N metabolism between NAD(P)H-specific NR-deficient mutant and wild-type barley (Hordeum vulgare L.)

Abstract

Barley (Hordeum vulgare L.) is an important crop for cereal research. In this study, two barley genotypes the wild-type (Steptoe) and the mutant (Az12) were used. An experiment was conducted using ¹⁵N-tracing method to NADH-specific nitrate reductase (NR)-deficient mutant seedling of barley. The N-depleted seedlings were exposed to a nutrient solution containing nitrate and nitrite, and were labeled with ¹⁵N for 38?h under (14?L/10D) cycles. The two genotypes utilized ¹⁵NO₃^? and accumulated it as reduced ¹⁵N, predominately in the shoots. However, nitrate reduction in the Az12 shoots was 9% lower than that in the Steptoe shoots at 38?h. As a result, in the Az12, nitrate accumulation in shoots was 78% higher than that in the Steptoe. Accumulation of reduced ¹⁵N in the Az12 roots was nearly similar to that of the Steptoe roots, but 8% lower in the Az12 shoots than in the Steptoe shoots at the end of the experiment. Also for both genotypes, root contribution increased during L/D cycles and decreased during the subsequent light cycle. Upward transport of reduced ¹⁵N via the xylem in the Az12 was nearly two times higher than that in Steptoe during the second light period (24–38?h). In both genotypes, xylem transport of reduced ¹⁵N was far exceeded the downward phloem transport. Abbreviations Anl accumulation of reduced ¹⁵N from ¹⁵NO₃^? in non-labeled roots of split roots

Ar accumulation in roots of reduced ¹⁵N from ¹⁵NO₃^?

As accumulation in shoots of reduced ¹⁵N from ¹⁵NO₃^?

Rr ¹⁵NO₃^? reduction in roots

Rs ¹⁵NO₃^? reduction in shoots

Tp translocation to root of shoot reduced ¹⁵N from ¹⁵NO₃^? in phloem

Tx translocation to shoot of root-reduced ¹⁵N from ¹⁵NO₃^? in xylem

FW fresh weight

     

Acetylene reduction,hydrogen evolution and nodule respiration in Phaseolus vulgaris

Summary In an experiment performed under greenhouse conditions, four cultivars of Phaseolus vulgaris L. (Venezuela-350; Aroana; Moruna; Carioca) were inoculated with three Rhizobium leguminosarum biovar phaseoli strains (C-05; C-40 = CIAT 255; C-89 = CIAT 55) and were fertilized with an N-free mineral nutrient solution. The plants were harvested 25, 40, and 55 days after emergence and the following paramenters were evaluated: Nitrogenase activity of nodulated roots, H₂ evolution by the nodules; relative efficiency of nitrogenase; respiration rates of nodulated roots and detached nodules; dry weight and total N of stems, leaves, pods, roots, and nodules. Generally the bean cultivar, Rhizobium strain, had an effect and there was an interaction effect with both symbiotic partners, on all parameters. On average, nodules represented 23% of total root respiration but the best symbiotic combinations showed lower ratios of C respired to N fixed. The maximum N-assimilation rate (between 40 and 55 days after emergence) of 11.93 mg N plant^–1 day^–1 occurred with the symbiotic combination of Carioca × C-05, while the poorest rate of 0.55 mg N plant^–1 day^–1 was recorded with Venezuela-350 × C-89. The best symbiotic combinations always showed the highest relative nitrogenase efficiency, but the differences in N₂-fixation rates cannot be explained solely in terms of conservation of energy by recycling of H₂. This requires further investigation.     

Short‐Term Recovery of Ammonium‐15Nitrogen Applied to a Temperate Forest Inceptisol and Ultisol in East Tennessee,USA

Abstract

The short‐term fate and retention of ammonium (NH₄)‐¹⁵nitrogen (N) applied to two types of forest soils in east Tennessee was investigated. Four ridgetop forests, predominantly oak (Quercus spp.), were studied. Five applications of NH₄‐¹⁵N tracer were made to the forest floor at 2‐ to 4‐week intervals over a 14‐week period in 2004. Nitrogen‐15 recovery in the forest floor, fine roots (<2 mm), and the mineral soil (0–20 cm) was calculated at 6, 21, and 42 weeks after the last application. Most of the ¹⁵N was retained in the forest floor and the mineral soil, with only small amounts (≤2%) found in roots from both soil layers. Recovery of NH₄‐¹⁵N was greater in Inceptisols, which had a wider carbon (C)‐to‐N ratio than Ultisols. For both soil types, higher NH₄‐¹⁵N recoveries and long retention times (half‐lives>100 weeks) indicated the forest floor is an effective filter for atmospheric N inputs.     

BIOMASS ALLOCATION AND NITROGEN DISTRIBUTION IN RYEGRASS UNDER WATER AND NITROGEN SUPPLIES

Ryegrass (Lolium perenne L.) in grassland is known to sustain with water and nitrogen (N). This study investigates biomass and N partitioning in plant organs (roots, main and the youngest tillers) under water-nitrogen interactions. Nitrogen was applied at the rates of 50 and 100 mg N kg^?1 as N₁ (low N) and N₂ (high N) treatments, respectively, with uniform irrigation until 440 growing degree-days (GDD). Thereafter, the water supply was restricted to 50 mL on a weekly basis (W₁) against 50 mL on a daily basis (W₂) and concurrently, N enriched with 1 atom% ¹⁵N isotopes. Cumulative tillers’ biomass increased linearly from 1st to 8th order, but thereafter reached a plateau with further increases in number of negligible weights. Initially tiller mass and number per plan did not differ (P < 0.05) with water and/or N applications but changed at 788 GDD with clear differences at 911 GDD with the highest under N₂W₂ and lowest under N₁W₁. Nitrogen concentration sharply decreased from 530 to 700 GDD and then levelled off with age. The decline was more pronounced in tillers than roots. The high N treatment showed elevated N-concentration under both water treatments. Watering on a daily basis promoted vegetative growth. High water and N levels significantly (P < 0.05) influenced concentration of N absorbed during ¹⁵N labeling (N_L) in all organs with relatively pronounced N_L under N₂. The additive positive effect of W₂ and N₂ was obvious on N_L as compared to N_T, which showed that plants discriminate N-uptake on mass basis. Nitrogen (mobile) was higher in young and ¹⁵N (heavier) was low in young tillers and vice versa. Accumulation of N absorbed during ¹⁵N labeling (¹⁵N_A) was significant knowing that water is a strong determining factor of N concentration in ryegrass organs.     

Evaluation of N2 fixation by applying 15N labeled plant material and ammonium sulfate

Effect of different ¹⁵N labeled sources on the estimation of N₂ fixation was investigated. The combination of ¹⁵N labeled ammonium sulfate, ¹⁵N labeled plant material, and ¹⁵N labeled ammonium sulfate with unlabeled plant material, was examined in pot experiments. Two cultivars of soybean (Glycine max) and one of mungbean (Vigna radiata) were used. No significant difference was observed among the treatments for the estimation of N₂ fixation. This was due to the homogeneity and stability of the ¹⁵N abundance in soil which resulted in a similar N uptake from the soil by the N² fixing and reference crops. The plant yield, total N uptake and amount of N₂ fixed were higher in the Yellow Soil than in the Andosol. The amount of N₂ fixed was strongly influenced by the plant growth and consequently it affected the plant yield. The slow decomposition of plant material in the Andosol resulted in a low yield in both the N₂ fixing and reference crops. Thus, the artificial decrease of the available N content in soil, by application of plant material, did not stimulate N, fixation but suppressed plant growth and N₂ fixation.     

Carbon flow in the plant-soil-microbe continuum at different growth stages of maize grown in a Mollisol

Understanding the photosynthetic carbon (C) dynamics in the plant–soil–microbe continuum is critical to the C sequestration in soils. However, such information is limited in maize (Zea mays L.) in Mollisols. Pot-grown maize was labelled with ¹³CO₂ at the 10-leaf, 15-leaf, heading, milk and dent stages to investigate the photosynthetic C flow in a maize–soil system and its contribution to soil organic carbon (SOC) in Mollisols. The majority of fixed ¹³C was recovered in shoots, ranging from 44.7% to 78.6%. The allocation of ¹³C fixed at different growth stages to belowground (roots and soil) gradually decreased over the growing period, indicating that the strength of root C sink is stronger at the early stages. However, the proportion of ¹³C in dissolved organic C and microbial biomass C to that in SOC significantly increased as the growth stages advanced. Over the entire growth period, the contribution of root-derived C to SOC was estimated to be 5461 mg C plant^?1 growth period^?1, of which approximately 79% was synthesized during the vegetative stages. Therefore, the input of photosynthetic C by maize plants into SOC mainly occurred during the younger stages of the plant, favouring the storage of SOC in Mollisols.