Competition for phosphorus among plant parts in early and medium-duration cultivars of pigeon pea (Cajanus cajan L.)

Differences in absorption and distribution of phosphorus (P) between early and medium-duration cultivars of pigeon pea grown on nutrient solution at two P concentrations were investigated. Low-P treatment (0.03 ppm) significantly reduced shoot weight, root length and root surface area in both cultivars compared to the control (1.0 ppm), but the reduction was more pronounced in the early than the medium-duration cultivar. Dry-matter accumulation in shoots was slightly higher in the medium-duration than in the early cultivar.³²P-labelled P in the whole plant, P uptake rate and P-absorption ability were higher in the medium than in the early cultivar. At the low-P, however, no differences in these parameters were observed for control plants. In shoots,³²P-labelled P tended to accumulate in the stem of the early cultivar whereas more P was distributed to the leaves and petioles in the medium cultivar. The results suggest that when P supply is limited, medium-duration cultivars accumulate more dry matter through high efficiency of P absorption and distribution P to leaves and petioles.     

Phosphorus uptake from different sources of phosphatic fertilizers by maize and wheat at two stages of growth

The results of pot experiments on wheat and maize with³²P labelled phosphatic fertilizers showed that P uptake after one month of sowing and at ear emergence in case of wheat has significant positive correlations with dry matter yield where as P uptake after one month of sowing has only significant positive correlation with dry matter yield and P uptake at tasseling has no correlation with dry matter yield in case of maize. P uptake and P concentration have significant correlation at both the stages of growth in case of wheat but P uptake from fertilizer and its respective concentration has only significant correlation at both the stages in case of maize.     

Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars

Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The¹⁵N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N₂ fixed in soybean than at 20 kg N/ha in the first year of the study. However, N₂ fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N₂ fixed as measured by the difference method. Further proof of differences in N₂ fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N₂ at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N₂ at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The¹⁵N methodology offers a unique tool to evaluate germplasms directly in the field for their N₂ fixation abilities at different N fertilizer levels.     

Native soil and fresh fertilizer phosphorus uptake as affected by rate of application and P fertilizers

³²P labelled fertilizers were used to measure native soil and fresh fertilizer phosphorus uptake byLolium perenne L. in greenhouse experiments. The P source evaluation was carried out for multiple rates of application for a standard P fertilizer (DAP) on low and medium soil P levels and for North Carolina rock phosphate (RP) at the medium soil P level only. At the low soil P level, the native P uptake increase was independent of P-DAP applied, and represented 19% of the nil P uptake. At the medium soil P level, the variability of the native soil as a nutrient P source depended on the phosphate fertilizer applied, and the rate of application. Consequently the amount of total P uptake could conceal differences in P fertilizer evaluations as the nutrient P source. Fresh P uptake increased linearly with the rates of P applied as standard or tested P fertilizer. The comparison of various P sources by means of fresh P uptake ratio (i.e. fresh P uptake from tested phosphate divided by fresh P uptake from standard phosphate) was independent of the rate of application. It was therefore suggested that various phosphorus sources be evaluated by measuring the fresh P uptake for a single rate of application.     

Minimizing the effect of mineral nitrogen on biological nitrogen fixation in common bean by increasing nutrient levels

Although common bean (Phaseolus vulgaris L.) has good potential for N₂ fixation, some additional N provided through fertilizer usually is required for a maximum yield. In this study the suppressive effect of N on nodulation and N₂ fixation was evaluated in an unfertile soil under greenhouse conditions with different levels of soil fertility (low=no P, K and S additions; medium = 50, 63 and 10 mg kg^–1 soil and high = 200, 256 and 40 mg kg^–1 soil, respectively) and combined with 5, 15, 60 and 120 mg N kg^–1 soil of ¹⁵N-labelled urea. The overall average nodule number and weight increased under high fertility levels. At low N applications, nitrogen had a synergistic effect on N₂ fixation, by stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg^–1 soil), the stimulatory effect of N fertilizer on N₂ fixation was still observed, increasing the amounts of N₂ fixed from 88 up to 375 mg N plant^–1. These results indicate that a suitable balance of soil nutrients is essential to obtain high N₂ fixation rates and yield in common beans.     

Effect of soil compaction on root growth and uptake of phosphorus

Summary Zea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m^–3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m^–3. The soil was labelled with³²P and³³P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between –20 and –100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m^–3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of –20 and –100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m³m^–3. Roots failed to grow completely through the compacted layer of soil at bulk densities 1550 kg m^–3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m^–3.     

Studies on the methods of inorganic nutrient application in coconut

Summary Using carrier free³²P, tagged single superphosphate and⁸⁶Rb, the efficiency of different methods of plant injection and soil placement techniques for fertilizer application was examined. In the plant injection techniques the radioactivity was fed to the palms through growing roots tips, cut ends of roots, stem injection and leaf axils. The application of radioactivity through the cut ends of roots was most efficient since³²P was detected in 10 m tall palms, four hours after application. In stem, leaf axil and growing roots tips injection the³²P was detected after 8, 12 and 18 h. Out of four methods of soil application, the quickest recovery of³²P in the palms was detected after 7 days of placement when applied by the hole method. The³²P activity in the palms through circular trenches, strips and basin methods was recorded after 8, 8 and 11 days of application respectively. The accumulation of⁸⁶Rb was significantly higher than³²P. With plant injection technique the accumulation of activity was found to be significantly higher than with soil placement methods. The rate of radioactivity absorption was 10 to 60 time faster in the former technique as compared to that of the latter. The application of radioactivity through cut ends of roots and circular trench methods, were found to be better and may be recommended for nutrient application in coconut.     

The influence of phosphorus deficiency on growth and nitrogen fixation of white clover plants

The effects of P deficiency on growth, N(2)-fixation and photosynthesis in white clover (Trifolium repens L.) plants were investigated using three contrasting relative addition rates of P, or following abrupt withdrawal of the P supply. Responses to a constant below-optimum P supply rate consisted of a decline in N(2)-fixation per unit root weight and a small reduction in the efficiency with which electrons were allocated to the reduction of N(2) in nodules. Abrupt removal of P arrested nodule growth and caused a substantial decline in nitrogenase activity per unit root weight, but not per unit nodule mass. Similarly, the rate of photosynthesis per unit leaf area was unaffected by abrupt P removal, whereas CO(2) acquisition for the plant as a whole decreased due to a decline in total leaf area, leaf area per unit leaf weight and utilization of incoming radiation. These changes followed the decline in tissue P concentrations. The ratio between CO(2)-fixation and N(2)-fixation was maintained under short-term P deprivation but increased under long-term low P supply, indicating a regulatory inhibition of nodule activity following morphological and growth adjustments. It is concluded that N(2)-fixation did not limit the growth of clover plants experiencing P deficiency. A low P status induced changes in the relative growth of roots, nodules and shoots rather than changes in N and/or C uptake rates per unit mass or area of these organs.     

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.     

Phosphorus efficiency and phosphorus remobilization in two sorghum (Sorghum bicolor (L.) Moench) cultivars

With two sorghum cultivars differing in P efficiency a P uptake experiment (³²P/³³P labelling) was carried out followed by a period of P deficiency. The tendency of the total P distribution and redistribution pattern was rather similar in both sorghum cultivars. Although in the cultivar with a greater P absorbing capacity per unit root weight a higher proportion of the P was found in the inorganic P soluble fraction this is not necessarily an indication of a higher vacuolar affinity for P. Under P deficiency in both cultivars a rapid decrease of the TCA soluble P fraction in the leaves was observed. Before complete exhaustion of this fraction the TCA insoluble P fraction was also markedly reduced. In the roots the total P content was maintained fairly constant with a distinct shift in favour of the insoluble fraction occurring during the period of P deficiency. It is assumed that in the P efficient sorghum cultivar producing more dry matter per increment of P absorbed, rather inherent growth promoting factors contribute to the intraspecific P efficiency by a stimulation of the intensity of P redistribution and thus compensate for the lower P absorbing capacity of its roots.     

The effect of aluminium on growth of and nitrogen fixation in vegetable soybean germplasm

Vegetable soybeam germplasm was screened for its tolerance to 0, 50 and 100 μM Al in solution culture. Plants were inoculated with prescreened acid-Al tolerantBradyrhizobium japonicum strain USDA 110 and a localRhizobium isolate SM867. Aluminum concentrations of 0, 50, and 100 μM affected the root lengths of all germplasm lines in the first few weeks of their growth. At 100 μM, the plants had severely stunted roots throughout the growing period of 35 days, but at 50 μM the initial stunting of the roots was overcome after the third week of growth, and there were no significant differences between the root lengths of these plants and of the controls. The appearance of the first nodule was delayed for 2–3 and 4–5 days at 50 μM and 100 μM Al, respectively. There was a significant reduction in nodule numbers and acetylene reduction activity (ARA) at 100 μM Al. At 50 μM Al, even though the number of nodules was decreased significantly, nodules were larger in size, so there was no significant reduction in nodule fresh weight and ARA. No significant differences in nitrogen fixing abilities of the soybean lines were observed between the twoRhizobium strains. Germplasm line Kahala showed the greatest tolerance to 50 μM Al, and Kahala, Kim and Wolverine tolerated 100 μM Al better than other germplasm lines.     

Effect of salinity and inoculation on growth,nitrogen fixation and nutrient uptake ofVigna radiata (L.) Wilczek

This study reports the effect of salinity and inoculation on growth, ion uptake and nitrogen fixation byVigna radiata. A soil EC_e level of 7.5 dS m⁻¹ was quite detrimental causing about 60% decline in dry matter and grain yield of mungbean plants whereas a soil EC_e level of 10.0 dS m⁻¹ was almost toxic. In contrast most of the studied strains of Rhizobium were salt tolerant. Nevertheless, nodulation, nitrogen fixation and total nitrogen concentration in the plant was drastically affected at high salt concentration. A noticeable decline in acetylene reduction activity occurred when salinity level increased to 7.5 dS m⁻¹.     

Bioavailability of different phosphorus forms in freshwater systems

The recent literature on the bioavailability of different forms of P in freshwater systems is reviewed. Bioavailable P is defined as the sum of immediately available P and the P that can be transformed into an available form by naturally occurring processes. Methods used to estimate the bioavailable P pool, which vary between studies largely depending on the time perspective applied, are critically evaluated. Most studies on particulate P aim to determine the potentially available P pool. Potential bioavailability of particulate P is normally analysed in bioassays with algal yield determinations and the available P fraction is characterized from interpretations of results of sequential chemical extractions. NaOH-extractable P is in most studies the most algal-available P fraction. For soil samples and tributary water particulate matter, NaOH-P has often been found to be equal to algal extractable P. In other studies depletions of NaOH-P have accounted for the algal P uptake, but only a minor proportion of the fraction has been utilized. Organic P in lake water particulate matter and bed sediments of eutrophic lakes can also be algal-available to a significant extent.Studies on the bioavailability of dissolved P have often been concerned with immediate availability, or the minimum amount of available P. Such studies need other types of experimental design and normally assays with radiotracers are used. Immediately available P is frequently found to be less than P chemically assessed as dissolved reactive P (DRP) at low (< 10 µg DRP·l^-1) concentrations. However, immediate availability may also approach or exceed DRP concentrations, especially at higher concentrations. Potential bioavailability, assayed as for particulate P, may generally render higher bioavailability than P assayed as immediately available. Large fractions of dissolved P remain unutilized and are primarily found in the high molecular weight fraction of dissolved P.     

Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India

Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on ³²P recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of ³²P was monitored through radioassay of leaves. Regardless of the species, ³²P uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of ³²P from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more ³²P than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of ³²P by trees. All grass species exerted a complementary effect on ³²P absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the ³²P uptake by grass species.     

Effects of aluminium on growth and root reactions of phosphorus stressed Betula pendula seedlings

The impact of two constant non-toxic levels of Al stress (0.2 and 0.4 mM) on growth and ³²P uptake capacity on sub-optimal (P-limited) Betula pendula seedlings grown in sand culture was examined.Seedling growth was under optimum controlled conditions in a growth chamber where nutrient additions were made at a predetermined relative addition rate (R_A) of 10% day^-1. Three treatment groups of seedlings 0, 0.2 and 0.4 mM Al were harvested at 15, 29 and 42 days. The excised roots were exposed to a ³²P-labelled solution for 15 minutes to measure their capacity for P uptake. Growth was determined by weighing the roots, stems and leaves of the seedlings.Growth data showed that relative growth rate (R_G) should equal the R_A of P the most limiting nutrient, which was supplied at P/N 3% instead of an optimal 15%. Therefore, Ingestad's theory can also be used succesfully in sand culture and this may be particularly important for future studies of root and rhizosphere exudates. Low levels of Al (< 0.2 mM) in combination with low P supply significantly lowered the R_G of the birch seedlings by further reducing P supply. However, previous studies of birch seedling growth and nutrient uptake using Ingestad's solution culture technique with optimumal P supply did not show any effect of Al on growth untill the Al was in excess of 3 mM. Aluminium was not directly toxic to the plants and therefore roots could respond to the ³²P bioassay.     

The effect of rhizoctonia root disease and applied nitrogen on growth,nitrogen uptake and nutrient concentrations in spring wheat

Root disease caused by Rhizoctonia solani is a common problem of spring wheat in South Australia. There are reports that nitrogen applications can reduce the incidence and severity of the disease. A glasshouse trail in pots examined the effects of disease and of applied nitrogen on wheat growth, and evaluated the utility of the basal stem nitrate concentration in diagnosing deficiency in plants with and without root disease. Plants were harvested at the mid-tillering stage. Shoot growth was increased by applied nitrogen until a maximum yield was attained, after which additional N had no effect on shoot yield. Root growth, however, responded positively only to low levels of applied N, after which it declined, and in the highest N treatment root mass was less than in the plants without applied N. Root disease caused severe reductions in plant growth, and both root and shoot mass were affected similarly. Even though growth of diseased plants responded positively to applied nitrogen the response was less than that of disease-free plants. The critical concentration of basal stem nitrate-N did not appear to be affected by root disease, and was estimated at 1200 mg kg^-1, consistent with other glasshouse data. The basal stem nitrate-N concentration, either in fresh or dried tissue, appeared a better diagnostic tool of N stress than did total shoot N concentration or content, because of sharper definition of critical concentrations. Concentrations of other nutrients in shoot tissue were affected differentially by both applied nitrogen and root disease, but generally did not reach critical levels, although phosphorus and magnesium appeared deficient in very disease-stressed plants.     

Quantities of fixed N and effects of grain legumes on following maize,and N and P status of soil as indicated by isotopes

Summary Two consecutive field experiments, using¹⁵N and³²P, were conducted at the National Corn and Sorghum Research Center, Thailand, to quantify N₂ fixed by mungbean, soybean and peanut and to examine effects of the legumes on the yields of succeeding maize and on status of N and P in soils during the following season. An early sorghum, non-nodulating soybean and maize which were used as standard crops in quantifying N₂ fixed by mungbean, soybean, and peanut, respectively, gave statistically comparable A-values for soil N though sorghum tended to give lower value than the other crops did. Amounts of fixed N₂ were 37.5, 119.0 and 150 kg/ha for mungbean, soybean and peanut, respectively. Plots previously grew legumes yielded higher grain and stover weights and higher N and P uptake of maize than those previously grew maize. There were no significant differences among plots previously grew different legumes. A-values, in most cases, did not differentiate the effects of previous legumes from those of previous maize. However, changes in N and P status of soil, in most cases, were too small to produce A-values changes that were large enough to outrun the experimental errors.     

The effect of varying nitrogen and phosphorus availability on nutrient use by Larrea tridentata,a desert evergreen shrub

Summary In a phytotron study of the effects of nitrogen and phosphorus supply ratio on nutrient uptake and use by Larrea tridentata, seedlings responded to increases in N and P availability with increases in leaf size, total biomass, and leaf nutrient concentration, and with decreases in root: shoot ratio. N and P use efficiency decreased with increasing N and P availability, respectively, but increased with increasing availability of the other nutrient, suggesting that Larrea responds both to the absolute and to the relative availability of limiting nutrients. Absolute amounts of N and P resorption, as well as N and P resorption efficiencies did not demonstrate a significant trend with nutrient availability, and there was no evidence of significant interactions between the two nutrients. More studies of the effects of nutrient interactions in the cycling and use of nutrients by different plant species are needed before more general conclusions can be drawn.     

Transformations between organic and inorganic sediment phosphorus in Lake Balaton

In order to estimate microbial P content and biological P uptake in sediments, the tungstate precipitation method of Orrett & Karl (1987) was used in sediment extracts. This method allows a simple and rapid separation of organic and inorganic ³²P radioactivity. Either inorganic ³²P (as carrierfree H₃ ³²PO₄) or organic ³²P (as ³²P-labelled algal material) was added to surface sediment suspensions of shallow Lake Balaton. Inorganic ³²P was rapidly transformed into organic ³²P, and this process was completely inhibited by formaline. P content of living benthic microorganisms was estimated from steady state distribution of the radioactivity. Transformation of algal organic P into inorganic P could also be detected.In extremely P limited Lake Balaton benthic microorganisms were shown to supplement their high P requirements by inorganic P uptake. The velocity of the inorganic into organic P transformation, i.e. the rate of microbial P uptake, was comparable to P uptake in the water column. Microbial P uptake contributed significantly to total P fixation by sediments, particularly at low ( 100 µg P l^–1) phosphate additions.     

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

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