Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica

Intensive cropping of Italian ryegrass (Lolium multiforum L.) in pots was used to assess the contribution of non-exchangeable K to plant uptake. The soils used were: two soils high in mica (illite) developed on recent alluvium plus two smectitic (beidellitic) soils and a soil of mixed mineralogy rich in mica. Four K treatments were used (0, 28.6, 143, and 286 mg kg^-1 soil) with 8 successive monthly cuttings. A response of plant K uptake to added K was observed in all soils. Both 1.0 M NH₄0Ac and 0.2 M CaCl₂ extractable K were depleted to a minimum level specific for each soil. The minima were lower in the old upland soils compared to the young alluvial soils. Uptake of K by Italian ryegrass induced K release from the non-exchangeable K to replenish the plant available pool of K ions. The release of mica interlayer K in the alluvial and in the high K smectitic soil supplied sufficient K to plants even under intensive cropping. The rate of mobilization of interlayer K was low in the smectitic soil with lower K. The lowest release rate was in the old high mica soil. Iron coatings may have inhibited mobilization of interlayer K. The rates of mobilization cannot be predicted from mineralogical and K-extraction data only. The rates of K uptake and the rates of K release by ryegrass under intensive cropping are potential values which can be used for modelling K availability to plants in the soils studied.     

丛枝菌根真菌群落对白三叶草生长的影响

不同施肥处理影响AMF(Arbuscular mycorrhizal fungi)群体结构,然而不同AMF群体结构对植物的生长以及养分吸收的影响尚未见报道,试验利用盆栽实验研究了7种不同来源的丛枝菌根真菌(AMF)群落对白三叶草生长和N、P、K以及微量元素Cu、Zn、Mn的吸收的影响。7种AMF群落分离自长期定位施肥试验地,分别为NPK、OM、CK、1/2OM、NP、NK和PK。每年施肥量是300kg N/hm2,135kg P2O5/hm2,300kg K2O/hm2。有机肥处理的N、P、K养分量与试验地NPK处理含量相同,原料以粉碎的麦秆为主,加上适量的大豆饼和棉仁饼,有机肥经堆制发酵后施用。试验土壤采用封丘试验地土壤,经灭菌处理。试验结果表明,接种不同AMF群落均能促进三叶草的生长,对养分吸收则表现不同。分离自CK试验地的AMF群落对三叶草侵染率显著低于其它6种AMF群落。分离自1/2OM和OM试验地的AMF群落较分离自NPK、CK、NP和NK的AMF群落显著促进了三叶草对P的吸收;各种AMF群落都促进了对N和K的吸收;分离自OM、CK、1/2OM、NP、NK试验地的降低了三叶草植株N含量;分离自NPK试验地的AMF群落提高了三叶草植物K含量;对于Cu、Zn、Mn元素的吸收,不同处理存在较大的差异。AMF群落对三叶草生长以及养分吸收贡献不同,这与不同施肥管理下不同AMF群落的优势种属的侵染率、养分转化以及菌丝发育及分布有关。     

Determination of K forms in K-fertilized soils by electro-ultrafiltration

We confirmed the suitability of electro-ultrafiltration (EUF) for (a) determination of the distribution of potassium fertilizer among the various forms of potassium in soils with a predominance of micaceous minerals in their clay fraction, and (b) investigated the effects of the degree of openness of the dominant micaceous mineral and of incubation time on the kinetics of the EUF extraction of K from these soils.Samples of illitic, mixed-layer and vermiculitic soils from Galicia (N.W. Spain) were incubated at field capacity for 450 days with 0 (blank), 5,15 or 25 mg K (as KCl) per 100 g dry soil. After 1, 30, 150 and 450 days, subsamples were removed and repeatedly extracted using electro-ultrafiltration at low (20° C/200 V) and then high (80° C/400 V) temperature/voltage (6 and 10 five-minute extractions, respectively). Five different pools of K were identified: solution K (K_s), surface and internal K (collectively, K_p), slowly exchangeable K (K_e) and non-exchangeable K (K_i). The effects of increasing the incubation time depended on the dominant clay mineralogy: after 450 days, the K added to illitic soils was mostly solution K, whereas that added to vermiculitic soils was mostly internal K.For both low and high temperature/voltage EUF experiments, the extraction-time data were best fitted by the Elovich equation (extracted K=a+b ln t). The kinetic coefficient b depended on the incubation time and dominant clay mineral, and for given soil and incubation time increased linearly with the dose of added K.Abbreviations EUF Electroultrafiltration - K_s Solution potassium - K_p Easily exchangeable (surface + internal) potassium - K_e Slowly exchangeable potassium - K_i Non-exchangeable potassium     

Release of nonexchangeable potassium from different size fractions of two highly K-fertilized soils in the rhizosphere of rape (Brassica napus cv Drakkar)

The release of nonexchangeable potassium by the different particle size fractions of two soils was studied with a culture device designed to confine soil samples in the rhizosphere of rape (Brassica napus cv Drakkar). After 8 days of cropping, the contribution of nonexchangeable K to K uptake ranged from 50% in the fine clay to 80–100% in the coarser fractions. Due to their high supplying power and their relative abundance, the silt fractions provided a major part of the supply of K by these soils.     

Build up and depletion of soil phosphorus and potassium and their uptake by rice and wheat in a long-term field experiment

Summary In a field experiment initiated at the Central Soil Salinity Research Institute, Karnal in 1974 involving rice wheat cropping sequence and NPK fertilizer use on sodic soil (pH 9.2, ESP 32.0), an attempt was made to evaluate the available P and K status of the soil and their uptake by the crops during 1982–83 and 83–84.Application of P to either or both the crops significantly enhanced the yields of rice and improved available P status of the soil. Wheat yields remained unaffected. Fertilizer N reduced P content in rice but increased P uptake in crops and considerably brought down available P to a level (4.5 ppm) where rice plants showed reduced tillering and phosphorus deficiency. Application of K did not affect the yield of either crop but enhanced its available status in soil and uptake by the crops. Contribution of the non-exchangeable K towards total potassium removal was about 93% in the absence of applied K which decreased to 87% with the use of K. Application of K to both crops resulted in lesser uptake from non-exchangeable form as compared to its application to either crop. Laboratory studies carried out on soils of the experimental plots showed that cumulative K release measured after five successive extractions was higher in K-treated soils as compared to untreated ones. The major difference was only in the first extraction representing the exchangeable K after which release became independent of the available K of the soil.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

Influence of FeEDDHA and soil temperature on the growth of two soybean varieties

Bulk samples of 30 surface soils characterized in the National Cooperative Soil Survey of Louisiana were collected at the original sites and planted with sorghum-sudangrass in the greenhouse. The native K was exhausted by a total of four croppings. The available soil K was extracted with 1N NH₄OAc 7.0 and 0.1N HCl while the difficultly available-K was extracted by 0.3N NaTPB (sodium tetraphenylboron). In addition, the mica content was estimated by differential dissolution. All soils except Dundee produced the greatest dry matter yield in the first crop with steadily decreasing production in the succeeding crops. The same pattern was shown in the K uptake. It appears that the first crop removed a large portion of the exchangeable K. There was a high degree of relationship between plant uptake of K and exchangeable K in the soils (r²=0.89^**). Uptake by the subsequent crops was generally controlled by the K in the micaceous minerals (r²=0.89^**). Since there was a high coefficient of determination between mica-K and difficulty available-K (r²=0.91^**) one also finds that uptake of K by crops 2, 3 and 4 was also well correlated to difficultly available K (R²=0.94^**). The uptake of K that was not originally in the exchangeable form was also highly related to difficultly available-K (R²=0.51^**). Contribution from the Department of Agronomy, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge. Part of the M.S. theses of the senior authors. Former Graduate Students and Prof. of Agronomy, respectively.     

Evaluation of the electro-ultrafiltration technique (EUF) as a measure of the K supplying power of Quebec soils

The electro-ultrafiltration (EUF) method has been used to evaluate the short-term and long-term supplying power of soils for many essential plant nutrients. The objective of this study was to compare the capacity of EUF with other extraction techniques to predict the plant availability of soil K and K fertilizer responsiveness by 10 cuts of alfalfa (Medicago sativa L.) growing over a 366-day period. Increasingly higher average concentrations of soil K were extracted by EUF at 50 V and 20°C (29 mg kg^-1), EUF at 200 V and 20°C (48 mg kg^-1), 0.002 M SrCl₂ (55 mg kg^-1), EUF at 200 V and 80°C (85 mg kg^-1), 0.1 M HCl (105 mg kg^-1), Mehlich 3 (119 mg kg^-1), 1 M NH₄OAc (120 mg kg^-1) and boiling 1 M HNO₃ (601 mg kg^-1). The large content of vermiculitic minerals in the silt and clay fractions is responsible for EUF desorbing more K in 55 minutes than NH₄OAc in 29 out of 30 soils. The total amount of K desorbed by EUF at 80°C was as effective as Mehlich 3-extractable K in predicting K uptake for the first three cuts and was best among the extracting procedures after boiling 1 M HNO₃ in predicting the long-term K supply, the uptake of K from non-exchangeable sources and the relative yield of alfalfa over 10 cuts. The desorption of soil K with EUF provides a better evaluation of the K-supplying power of Quebec soils than the extractants currently used, especially on a long-term basis.Contribution no. 396.     

Studies of slowly available potassium in soils of New Zealand

Summary The concentration of K in the soil solution at zero change of non-exchangeable K (Co) was determined at 20°C and 50°C for six topsoils of widely different clay mineralogy. This was done with and without prior removal of the K that was exchangeable to dilute salt solutions in a two to three hour leach. Except for one case Co was greater for leached than for not leached samples, particularly at the higher temperature. This is attributed to rapid edge-weathering during leaching caused by interlayer penetration at the clay edges by calcium, sodium or hydronium ions. It is assumed that K newly exposed in this manner is not immediately exchangeable but that it exerts an influence on the equilibrium concentration of K in solution.Among soils Co was not related to the K available to ryegrass nor to the non-exchangeable K fraction of the uptake by ryegrass. However, much reduced values of Co were obtained after depletion by ryegrass in the new two soils examined.     

Potassium and phosphate uptake of cucumber plants at different ammonia supply

Summary Experiments on cucumber plants grown in nutrient solution were conducted in order to study long and short time effects of ammonia on growth, nutrient element uptake and respiration of roots.Shoot yield and potassium concentration in tissue of plants treated 18 days with varied ammonia concentration were decreased. However, it was not assumed that K deficiency caused the yield reduction. The ammonia effect on K content was more pronounced in roots than in shoots.The decreased K concentration of plant tissue was linked to a diminished ability of plant roots to absorb potassium. The maximum rate of potassium uptake was lowered by ammonia during both, long- and short-time treatment. The results indicated that the NH₃ influence on potassium uptake was due to effects on metabolism and permeability of roots because changes of K uptake rate occurred immediately after starting the NH₃ treatment. Furthermore, it is shown that ammonia inhibited respiration of roots.During the short-time treatment net potassium efflux of roots was observed at higher NH₃ concentrations. The extent of K efflux depended on K concentration of both, root tissue and nutrient solution.Pretreating the plants for 12 hours with ammonia also resulted a decline in K uptake rate. However, plant roots subjected to ammonia concentrations up to 0.09 mM completely recovered during 24 hours after removing the NH₃ treatment whereas at higher NH₃ concentrations only a partial recovery occurred.Furthermore, it was shown that ammonia also influenced P uptake by plant roots.     

Comparative study on EUF and other methods of soil analysis for the determination of available potassium in soils from Northern Greece

Summary Twenty-one representative soils from Northern Greece could be grouped into three categories based on the EUF-K curves which displayed marked differences in the magnitude of K release by the soils employed. The cumulative K desorption by EUF within 35 min and the cumulative K-uptake values of ryegrass (10 cuts) were found to be correlated (r=0.87***). Although this correlation is rather close, the K dynamics of a soil can be better characterized by the course of the K-desorption curves. Because the quotient EUF-K-80°C/EUF-K-20°C can give information on the course of K desorption it is therefore sufficient in routine investigations to know the EUF-K-20°C contents and the numerical values of the quotients EUF-K-80°C/EUF-K-20°C.The EUF procedure does not only indicate the close relationship between K extracted and K uptake by plants, but it can also provide information on other nutrients in the same soil sample. With this extra information it was possible to explain why in some of the analysed soils K uptake was low despite high K availability, the reason being that P availability was not optimal in one of the experimental soils and that the Mn concentration of the soil solution was too high in another. At equal K availability the K uptake was also dependent on the amount of EUF-extractable N.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

Effect of lime,phosphorus and mycorrhizal fungi on growth,nodulation and nitrogen fixation by white clover (Trifolium repens) grown in UK hill soils

Summary The responses of white clover (cv NZ Grasslands Huia grown in four UK hill soil types) to additions of lime and P, to inoculation with Rhizobium and mycorrhizal fungi, and to differences in soil water status were assessed in pot and field experiments. With a deep peat soil in pots, shoot production, nodulation and N fixation by clover were increased by 160, 130 and 85% respectively following inoculation with mycorrhiza, but in the field, despite a doubling of root infection, there was no response in growth. On a brown earth soil in the field inoculation with one endophyte (Glomus mosseae L1) out of four tested depressed production of white clover shoots by 42% but enhanced that of leeks (Allium porrum) by 50%; the others were without effect. With dry peaty podzol and brown earth soils in pots, clover shoot production was highest with added P when a water holding capacity of 80% was maintained, but roots from the latter had only 2.6 compared to 68 nodules per plant from the former. Further work is required to explain poor nodulation in the brown earth soils.     

Factors affecting soil acidification under legumes I. Effect of potassium supply

This study examined the effect of K (as K₂SO₄) supply on acid production under N₂-fixing plants of lupin (Lupinus angustifolius L. cv. Gungurru) and clover (Trifolium subterraneum L. cv. Dalkeith) grown in a K-deficient soil with a low pH buffer capacity for 55 days in the glasshouse at 20/12 °C (day/night). Increasing K supply up to 240 mg K kg^-1 soil markedly increased plant growth of both species but clover growth was more responsive than lupin. Growing plants for 55 days decreased soil pH by 0.65–0.85 units under lupin and 0.45–0.83 units under clover. The amounts of H⁺ produced per kg biomass (specific acid production) were the highest at the nil K supply, generally decreased with increasing K level up to 30 mg K kg^-1 under lupin and up to 120 mg K kg^-1 soil under clover and only slightly increased with further increasing K under lupin. Increasing K₂SO₄ supply proportionally increased plant uptake of K and SO ₄ ^2- but generally decreased concentrations of Ca, Mg, Na, P and Cl. Specific acid production correlated well with concentrations of excess cations and ash alkalinity, and total acid production was strongly correlated with total excess cations and total ash alkalinity in plants. These relationships were not affected by K treatment and species. Specific acid production also correlated with plant Ca concentration but not with K concentration. In addition, lupin and clover extruded similar amounts of H⁺ per kg biomass produced. It is suggested that application of K₂SO₄ does not have a significant impact on acid production by lupin and clover.     

Regrowth and Nutrient Composition of Different Plant Organs in Grass-clover Canopies as Affected by Phosphorus and Potassium Availability

Regrowth after cutting and the distribution of nitrogen (N),phosphorus (P) and potassium (K) in different plant organs ofwhite clover and perennial ryegrass growing in pure or mixedswards were investigated under field conditions in a soil witha low-to-moderate availability of P and K. In all treatments,white clover constituted more than 70% of the above-ground biomassin the mixed swards. The petioles were the dominant pool ofdry matter throughout regrowth and contained the greatest amountsof N, P and K. Increased supply of P and K increased the growthof ryegrass, but not that of white clover in the mixed swards.The increased competition from ryegrass led to a decline inthe yield of white clover laminae as well as in the N contentper unit of dry matter in laminae, petioles and stolons. TheP content of all white clover organs also declined followingP application to the mixed swards, whereas K application increasedtheir K contents. In the pure swards of ryegrass and white clover,yields and contents of N, P and K in the dry matter were eithernot affected or increased following P and K application. Itwas concluded that commonly-used defoliation heights may remove80% or more of the nutrient and dry matter pools located inthe petioles but the remaining quantities of dry matter andnutrients in the petioles will normally exceed the correspondingquantities in the stolons. Copyright 2001 Annals of Botany Company Coexistence, competition, phosphorus, potassium, regrowth, ryegrass, white clover     

Water deficit and plant competition effects on growth and water-use efficiency of white clover ( Trifolium repens,L.) and ryegrass (Lolium perenne,L.)

The combined effects of soil water deficit and above and below ground interspecific plant competition on the growth, water-use efficiency (WUE), and measured carbon isotopic composition (δ¹³C) values of white clover and ryegrass were studied. White clover and ryegrass were grown in specially designed crates 1) individually; 2) in shoot competition; or 3) in shoot + root competition and either well-watered or at a moderate or severe soil water deficit. The effects of shoot + root competition on shoot dry matter growth were substantial and benefited both white clover and ryegrass when well-watered or at a moderate soil water deficit, while severely reducing white clover shoot dry matter growth at severe soil water deficit. Plant competition did not affect the WUE of white clover or ryegrass. As soil water deficit increased, the WUE of white clover did not change whereas the WUE of ryegrass increased and was greater than that of white clover. This was attributed to the lower leaf water conductance of ryegrass which conserved water and maintained growth longer compared to white clover. A stronger correlation existed between soil water deficit and measured δ¹³C values for ryegrass at each plant competition level (P<0.001) than existed for white clover (individual: P<0.01; shoot + root: P<0.001; shoot: P<0.10). Unlike white clover, the relationship between measured δ¹³C values and shoot dry matter growth indicated that C assimilation for ryegrass was dependent on type of plant competition. That WUE remained constant for white clover while measured δ¹³C values increased as soil water deficit increased, suggests that the role below ground respiration rate played in determining δ¹³C values increased. The WUE of white clover appears to be independent of the nature of the competition between plants and the soil water deficit level at which it is grown, whereas for ryegrass, the addition of root competition to shoot competition should lead to increases in its WUE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of some fungicides used against cereal pathogens on the growth of Rhizobium trifolii and its capacity to fix nitrogen in white clover

Fungicide residues in soils may affect nitrogen fixation by legumes. Effects of nine systemic fungicides, used against cereal pathogens, on Rhizobium trifolii and white clover were measured. Fenarimol and oxycarboxin inhibited growth of R. trifolii in vitro. The weight of clover plants after 10 weeks' growth in soil containing carboxin, oxycarboxin, benodanil, tridemorph and pyracarbolid was reduced. Symbiotic nitrogen fixation was decreased by carboxin, oxycarboxin and tridemorph present in soil at concentrations somewhat greater than that likely to result from several applications of the fungicide. Only tridemorph reduced nodulation. None of the compounds seems likely to affect nitrogen fixation in the field if applied at the recommended rate.     

Availability of fluoride to plants grown in contaminated soils

Two pot experiments were carried out to study uptake of fluoride (F) in clover and grasses from soil. Fluoride concentrations in t Trifolium repens (white clover) and t Lolium multiflorium (ryegrass) were highly correlated with the amounts of H₂O– and 0.01 t M CaCl₂–extractable F in soil when increasing amounts of NaF were added to two uncontaminated soils (r=0.95–0.98, t p<0.001). The amounts of H₂O– or 0.01 t M CaCl₂–extractable F did not explain the F concentrations to a similar extent in t Agrostis capillaris (common bent) grown in 12 soils (Cambic Arenosols) collected from areas around the Al smelters at Å: rdal and Sunndal in Western Norway (r=0.68–0.78). This may be due to variation in soil pH and other soil properties in the 12 soils. Soil extraction with 1 t M HCl did not estimate plant–available F in the soil as well as extraction with H₂O or 0.01 t M CaCl₂. Fluoride and Al concentrations in the plant material were positively correlated in most cases. Fluoride and Ca concentrations in the plant material were negatively correlated in the first experiment. No consistent effects were found on the K or Mg concentrations in the plant material. The F accumulation in clover was higher than in the grasses. The uptake from soil by grasses was relatively low compared to the possible uptake from air around the Al smelters. The uptake of F in common bent did not exceed the recommended limit for F contents in pasture grass (30 mg kg^–1) from soil with 0.5–28 mg F(H₂O) kg^–1 soil. The concentration in ryegrass was about 50 mg F kg^–1 when grown in a highly polluted soil (28 mg F(H₂O) kg^–1 soil). Concentrations in clover exceeded 30 mg F kg^–1 even in moderately polluted soil (1.3–7 mg F(H₂O) kg^–1 soil). Liming resulted in slightly lower F concentrations in the plant material.     

Influence of arbuscular mycorrhiza on clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons

 The effect of arbuscular mycorrhiza (AM) on white clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons (PAH) was assessed in a pot experiment. The soil was spiked with 500 mg kg^–1 anthracene, 500 mg kg^–1 chrysene and 50 mg kg^–1 dibenz(a,h)anthracene, representing common PAH compounds with three, four and five aromatic rings, respectively. Three treatments and two harvest times (8 and 16 weeks) were imposed on plants grown in spiked soil: no mycorrhizal inoculation, mycorrhizal inoculation (Glomus mosseae P2, BEG 69) and mycorrhizal inoculation and surfactant addition (Triton X-100). Pots without PAH were also included as a control of plant growth and mycorrhizal colonization as affected by PAH additions. The competitive ability of clover vis-à-vis ryegrass regarding shoot and root growth was enhanced by AM, but reduced by PAH and the added surfactant. This was reflected by mycorrhizal root colonization which was moderate for clover (20–40% of total root length) and very low for ryegrass (0.5–5% of total root length). Colonization of either plant was similar in spiked soil with and without the added surfactant, but the PAH reduced colonization of clover to half that in non-spiked soil. P uptake was maintained in mycorrhizal clover when PAH were added, but was reduced in non-mycorrhizal clover and in mycorrhizal clover that received surfactant. Similar effects were not observed on ryegrass. These results are discussed in the context of the natural attenuation of organic pollutants in soils. Accepted: 12 June 2000     

Influence of dissimilarities in temporal and spatial N-uptake patterns on15N-based estimates of fixation and transfer of N in ryegrass-clover mixtures

The temporal N-uptake patterns of white clover (Trifolium repens L.) mixed with perennial ryegrass (Lolium perenne L.) and of red clover (Trifolium pratense L.) mixed with Italian ryegrass (Lolium multiflorum Lam.) were determined in successive harvests of herbage within the growth cycles of a ley established near Zürich (Switzerland). Rooting patterns were examined by injecting¹⁵N-fertilizer at soil depths ranging from 10 to 40 cm. The results were analyzed to determine the effect of variations in time and depth of N-uptake on the¹⁵N-based measurement of N from symbiosis (N_sym) and N from transfer (N_trans).Grasses in mixture appeared to have deeper rooting systems than grass monocultures, which led to an overestimation of N transfer from white clover to perennial ryegrass if¹⁵N was spread on the soil surface.White clover generally lagged behind grass in soil N- uptake. Soil N-uptake of red clover slowed down before that of the grass because % N_sym almost reached 100% during the second half of each growth cycle. However, the effect of these dissimilarities on the seasonal average of %N_sym did not exceed 2%.It is concluded that at the observed high levels of N₂ fixation, failure to account for the N-uptake patterns of the test and reference crops only slightly affected the estimates of % N_sym and % N_trans, and did not invalidate the observed differences between species.