Yield responses of two ryegrass and two white clover varieties grown in contrasting mixtures with applications of phosphate at different depths and on different soils

To validate earlier findings using ³²P in upland soil and at different depths in lowland soil, ryegrass varieties S.23 or Titania were grown in mixtures with white clover varieties S.184 or Olwen. Unlabelled phosphate (0 or 43 kg P/ha) was applied to the surface or at 30 cm depth in lowland soil, and dry matter yields of grass and clover were measured. In June, on lowland soil, the greatest yield of clover was obtained by surface application. The most productive mixture was S.23/Olwen, although in August Titania outyielded S.23. By October S.23 again outyielded Titania. These results validated the isotopically discovered interactions between phosphate depth and grass/clover mixtures. On upland peat and mineral soil, 0 or 43 kg P/ha was surface applied to the same varietal mixtures. In May, phosphate application to mineral soil slightly decreased clover presence, while clover was increased by mixture with Titania rather than S.23. In July, no significant phosphate effects were found, but in October phosphate increased clover yield and, at least on mineral soil, S.23/Olwen responded most.     

Uptake of 32P labelled phosphate by clover and ryegrass growing in mixed swards with different nitrogen treatments

Lolium perenne cv. S.23 and Trifolium repens cv. Olwen were sown together in 1975, fertilised then and in 1976, and finally given nitrogen doses of either 50, 100, 200 or 400 kg/ha (as N) combined with 0.64 times as much potassium (as K₂O) in 1977. As nitrogen increased, grass yield increased, but clover decreased. Grass roots absorbed more ³²P than clover roots, and nitrogen increased this difference. Grass roots bore more mycorrhiza than clover roots. The difference in ³²P uptake between grass and clover was less in June and July than in August. Clover roots took up most phosphate from the upper layers of soil, while grass absorbed ³²P rather uniformly down to 25 cm. It was concluded that optimum fertiliser placement for clover growth was a surface dressing in the early season.     

Effect of white clover cultivar on apparent transfer of nitrogen from clover to grass and estimation of relative turnover rates of nitrogen in roots

The apparent transfer of N from clover to associated grass was evaluated over a four year period both on the basis of harvested herbage and by taking account of changes in N in stubble and root (to 10 cm depth) in swards with perennial ryegrass and three different white clover cultivars differing in leaf size. The large leaved Aran transferred 15% of its nitrogen while Huia transferred 24% and the small leaved Kent Wild White transferred 34%. When changes in stubble and root N were taken into account the percentage of N transferred was calculated to be 5% less than in harvested herbage only, as the small leaved types had proportionately more N in the roots and stolons, but the large leaved type was probably more competitive towards the grass.Loss of N from clover roots from July to October was compared to that from grass roots in a grass/white clover sward continuously stocked with steers using a method which incorporated tissue turnover and ¹⁵N dilution techniques. Less than 1 mg N m^-2 d^-1 was lost from the grass roots. In contrast 8 mg m^-2 d^-1 were estimated to be lost from clover roots while 12 mg N m^-2 d^-1 were assimilated.It is concluded that clover cultivar and competitive ability on grass have to be taken into account together with the relationship between N turnover in roots and N available for grass growth when modelling N transfer in grass/clover associations.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

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.     

Changes in growth of two C4 grasses (Schizachyrium scoparium and Paspalum plicatulum) in monoculture and mixture: influence of soil depth

The growth of Schizachyrium scoparium (Michx.) Nash (little bluestem) and Paspalum plicatulum Michx. (brownseed paspalum), two C₄ grasses, was examined in monoculture and in mixed culture in three soil depths. Effects of soil depth and competition were both significant. Growth of both species increased as soil depth increased from 30 to 180 cm in pot experiments. However, P. plicatulum dry mass increased approximately four times more than S. scoparium as soil depth increased. Schizachyrium scoparium dry mass was always greater in monoculture while P. plicatulum dry mass was always greater in mixture. Significant interspecific competition was not measured in the shallow soil treatment (30 cm) suggesting equal competition. Significant interspecific competition was measured in the deeper soil treatments (90 and 180 cm), and P. plicatulum dry mass was clearly greater in mixtures in the two deeper soils. Paspalum plicatulum does not appear to prevent the growth of S. scoparium roots in the deeper soil, but seems able to exploit deeper soil resources by proliferating roots at depths where S. scoparium roots do not seem capable of growth.     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 2. A Comparison of Monocultures and Mixed Swards

White clover (Trifolium repens L.) and Perennial ryegrass (Loliumperenne L.) plants were grown, in Perlite, in simulated swardsas either monocultures or mixtures of equal plant numbers. Theywere supplied with a nutrient solution either high (220 µgg^–1) or low (40 µg g^–1) in ¹⁵N-labelled nitrateand grown to ceiling yield at either high (20°C day/15°Cnight) or low (10°C day/8°C night) temperature. Temperature had little effect on the maximum rates of grosscanopy photosynthesis which were similar in High-N grass andHigh-N and Low-N clover monocultures. However these maxima werereached more slowly in clover than grass, and more slowly atlow rather than high temperature. Nitrogen supply increasedphotosynthesis in grass but not in clover. Clover had higherN contents than grass in all four treatments, although in anygiven treatment its N content was lower, and contribution ofN₂-fixation relative to nitrate uptake higher, in mixture thanin monoculture. Conversely, grass had higher N contents in mixturethan monoculture, because more nitrate was available per plantand not because of transfer of biologically fixed N from clover. Under Low-N, clover outyielded grass in mixture, particularlyat high temperature. The grass plants in the Low-N mixtureshad higher N contents and higher SLA, LAR and shoot: root ratiosthan those in monoculture. It is proposed that competition forlight is the cause of the low relative yield and negative aggressivityof grass in these swards. Under High-N, grass outyielded cloverin monoculture and mixture, at both temperatures but particularlyat low temperature when grass had a high aggressivity. Nitrogenand yield component analyses shed no light on clover's apparentlylow competitive ability and evidence is drawn from the previouspaper to demonstrate that grass grew faster than clover onlyas spaced individuals during non-com petitive growth. The relativemerits of measures of competitive ability based on final harvestdata and physiological data taken over a growth period are discussed. Trifolium repens L., white clover, Lolium perenne, perennial ryegrass, competition, temperature, nitrogen     

15N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.)

The ¹⁵N isotope dilution technique and the N difference method were used to estimate N₂ fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using ¹⁵N isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha^-1. The grass in the mixture had a lower atom %¹⁵N content and a higher N content than grass in a monoculture; therefore values for N₂ fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom %¹⁵N contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N₂ fixation.     

Investigations of the host feeding preferences ofSitona weevils found commonly on white clover (Trifolium repens) in the UK

A series of laboratory and field studies were done to evaluate a range of leguminous plant species for their feeding potential by adult weevils of the genusSitona Germar. (Coleoptera: Curculionidae). Three species ofSitona, S. lineatus L.,S. flavescens Marsh. andS. hispidulus F. all of which are found commonly on white clover (Trifolium repens L.) in the UK were offered a range of 11 legume species,T. repens (white clover, cv. Olwen),T. pratense L. (red clover, cv. Marcom),T. fragiferum L. (strawberry clover, cv. Palestine),T. hybridum L. (hybrid clover, cv. Tetra),T. incarnatum L (crimson clover),T. dubium Sibth. (lesser yellow trefoil),Lotus corniculatus L. (birdsfoot-trefoil, cv. Leo),L. uliginosus Schkuhr. (large birdsfoot-trefoil),Melilotus alba Desr. (white melilot),Medicago sativa L. (lucerne, cv. Europe) andM. lupulina L. (black medick) in two laboratory experiments. The weevils were offered a choice of these legumes in one experiment whilst in the other they did not have a choice of food material. These legumes were also sown in the field and a number of measurements of damage, together with counts ofSitona spp., were made. In the laboratoryS. lineatus andS. hispidulus favoured some of the legumes to a greater or lesser extent than white clover.S. flavescens was more restricted in its feeding than the other two weevil species. In the field studyS. lineatus invaded the experimental area quickly and tended to favourMedicago spp. andMelilotus spp. Later in the yearS. flavescens dominated the sitona fauna on the experiment, with the exception of aggregations ofS. lineatus onM. sativa andM. alba. In a separate screen of 5 varieties of white clover (cvs Donna, Menna, Kersey, Olwen and Grasslands Huia), cv. Olwen appeared to be the most susceptible to sitona attack.     

The effects of mixing Italian ryegrass (Lolium multiflorum) with perennial ryegrass (L. perenne) or red clover (Trifolium pratense) on the incidence of viruses

Mixing the ryegrass mosaic virus (RMV) resistant perennial ryegrass (Lolium perenne) cv. Endura with the susceptible Italian ryegrass (L. multiflorum) cv. RvP decreased infection of RvP wth RMV from 37% when grown alone to 22% when mixed. However, Endura yielded less than RvP and there was no yield benefit from mixing the two cultivars. Mixing red clover (Trifolium pratense) cv. Hungaropoly with RvP had no detectable effect on RMV incidence in RvP but did decrease the incidence of red clover necrotic mosaic virus in Hungaropoly from 9% to 1% and of white clover mosaic virus from 53-5% to 41%. The yield of the mixture was equal to that of RvP grown alone but given nitrogen fertiliser. The numbers of eriophyid mites, including Abacarus hystrix the vector of RMV, on ryegrass leaves were similar in pure and mixed swards. It is concluded that with herbage crops, the common practice of sowing mixtures of species may help control virus diseases.     

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.     

Above- and below-ground responses of C3–C4 species mixtures to elevated CO2 and soil water availability

We evaluated the influences of CO₂[Control, ～ 370 µ mol mol ^{? 1}; 200 µ mol mol ^{? 1} above ambient applied by free‐air CO₂ enrichment (FACE)] and soil water (Wet, Dry) on above‐ and below‐ground responses of C₃ (cotton, Gossypium hirsutum) and C₄ (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO₂ enrichment increased height, leaf area, above‐ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above‐ground biomass and height were generally reduced compared to monocultures, across both CO₂ and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above‐ground biomass in low‐density mixtures were similar between CO₂ treatments, but increased by 17–21% with FACE in high‐density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO₂ or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths ( > 45 cm). Sorghum roots comprised 81–85% of the total roots in the low‐density mixture and 58–73% in the high‐density mixture. CO₂‐enrichment partly offset negative effects of interspecific competition on cotton in both low‐ and high‐density mixtures by increasing above‐ground biomass, with a greater relative increase in the high‐density mixture. As a consequence, CO₂‐enrichment increased total above‐ground yield of the mixture at high density. Individual plant responses to CO₂ enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO₂‐mediated increase in C₃ growth may have on subsequent vegetation change.     

Why is Abundance of Herbaceous Legumes Low in African Savanna? A Test with Two Model Species

Although fire is frequent in African savanna ecosystems and may cause considerable loss of nitrogen (N), N₂-fixing herbaceous legumes—which could be expected to benefit from low N conditions—are usually not abundant. To investigate possible reasons for this scarcity, we conducted a pot experiment using two common plants of humid African savannas as model species, the legume Cassia mimosoides and the C₄ grass Hyperthelia dissoluta. These species were grown at different levels of water, N and phosphorus (P), both in monoculture and in competition with each other. In the monocultures, yields were significantly increased by the combined addition of N and P in pots receiving high water supply. In pots with interspecific competition, the legume grew poorly unless P was added. Foliar δ¹⁵N values of legume plants grown in mixtures were considerably lower than those in monocultures, suggesting that rates of symbiotic N-fixation were higher in the presence of the grass. Grass δ¹⁵N values, however, were also lower in mixtures, while N concentrations were higher, indicating a rapid transfer of N from the legume to the grass. We conclude that the main reason for the low abundance of C. mimosoides is not low P availability as such, but a greater ability of H. dissoluta to compete for soil N and P, and a much higher N-use efficiency. If other C₄ grasses have a similar competitive advantage, it could explain why herbaceous legumes are generally sparse in African savannas. We encourage others to test these findings using species from other types of savanna vegetation.     

Effect of different soil and nitrogen fertilizer conditions on 32P-labelled phosphate uptake by two grass species

In two years of trials, roots of ryegrass took up more ³²P-labelled phosphate than roots of fescue. Application of 672 kg N ha^-1 increased phosphate absorption compared with application of 112 kg N ha^-1. Roots in mineral soil absorbed more phosphate than those in peat soil. In both soils uptake decreased as depth of phosphate injection increased from 5 to 30 cm. An interaction occurred whereby roots in the intermediate depth (10–22-5 cm) in peat absorbed less phosphate than in mineral soil and this was apparently unrelated to the exchange or sorption properties of the soil.     

Effects of soil temperature and phosphorus supply on an annual grass and clover grown in monoculture and in mixed culture

Summary This paper describes the effects of soil temperature and phosphorus supply on Bromus rigidum Roth. and Trifolium subterraneum L. grown either in monoculture or in mixed culture. The experiment was designed as a replacement series with plant density maintained at 0.3 plants cm^–2.The results show that increasing soil temperature increases the yield of both species, and increases the competitive ability of the clover with respect to the grass. The root length of the grass increased with increasing soil temperature, but that of clover remained constant. At any given temperature clover root length was greater in mixed culture than in monoculture.The ecological implications of these results are discussed.     

Establishment of reed canary grass with perennial legumes or barley and different fertilization treatments: effects on yield,botanical composition and nitrogen fixation

In two field experiments in northern Sweden, we investigated if intercropping reed canary grass (RCG; Phalaris arundinacea L.) with nitrogen‐fixing perennial legumes could reduce N‐fertilizer requirements and also if RCG ash or sewage sludge could be used as a supplement for mineral P and K. We compared biomass production, N uptake and N‐fixation of RCG in monoculture and mixtures of RCG with alsike clover (Trifolium hybridum L.), red clover (Trifolium pratense L.), goat's rue (Galega orientalis Lam.) and kura clover (Trifolium ambiguum M. Bieb.). In one experiment, RCG was also undersown in barley (Hordeum vulgare L.). Three fertilization treatments were applied: 100 kg N ha^?1, 50 kg N ha^?1 and 50 kg N ha^?1 + RCG ash/sewage sludge. We used a delayed harvest method: cutting the biomass in late autumn, leaving it on the field during the winter and harvesting in spring. The legume biomass of the mixtures at the inland experimental site was small and did not affect RCG growth negatively. At the coastal site, competition from higher amount of clover biomass affected RCG growth and spring yield negatively. N‐fixation in red clover and alsike clover mixtures in the first production year approximately covered half of recommended N‐fertilization rate. Goat's rue and kura clover did not establish well at the costal site, but at the inland site goat's rue formed a small but vital undergrowth. RCG undersown in barley gave lower yield, both in autumn and spring, than the other treatments. The high N treatment gave a higher spring yield at the inland site than the low N treatments, but there were no differences due to fertilization treatments at the coastal site. For spring harvest, there were no yield benefits of RCG/legume intercropping compared with RCG monoculture. However, intercropping might be more beneficial in a two‐harvest system.     

The potential of carabid beetles to control slugs in grass/clover swards

Experiments conducted in sward boxes under controlled conditions showed that the carabid beetlesAbax parallelepipedus (Piller &; Mitterpacher) andPterostichus madidus F. controlled slugs in a grass/clover sward. Control was shown to be as good as that by the molluscicide, methiocarb.A. parallelepipedus proved to be more successful thanP. madidus at controlling the slugs, damage to the clover being kept to a minimum.P. madidus controlled the slugs only after some damage to the clover was sustained.     

Competition between grass and clover in spring as affected by nitrogen fertiliser

Detailed measurements of irrigated ryegrass/white clover swards growing without interruption, with or without nitrogen fertiliser in spring, showed that the relative growth rate of clover was as great as that of grass, in the + N sward, and considerably greater than grass, in the – N sward. Clover leaves were not overtopped by grass leaves. Indeed, in both swards, clover had a greater proportion of its leaf lamina area in the upper, well lit, layers of the canopy than grass did. Consequently, clover had a greater mean rate of leaf photosynthesis in situ in the sward than grass. Clover's advantage in photosynthetic rate per leaf area was offset to some extent by its smaller ratio of leaf area to total above-ground dry weight than grass. The consequences of these results for our understanding of competition between grass and clover in mixed swards are discussed.     

Effects of nitrogen on leaves,dry matter allocation and regrowth dynamics in Trifolium repens L. and Lolium perenne L. in pure and mixed swards

The effects of applied nitrogen (N) on dynamics of regrowth, dry matter (DM) allocation and leaf characteristics of grass and clover were investigated. Binary mixtures and monocultures of the diploid perennial ryegrass cultivars Barlet (erect) and Heraut (prostrate) and the white clovers cvs. Alice (large-leaved) and Gwenda (small-leaved) were established in a field experiment. Grass monocultures received three levels of N application (0, 140 or 280 kg N ha^–1), and mixtures 150 kg N ha^–1 (+N) or no N (–N). N was applied split over the season. Application of N reduced the average clover content in the DM of the mixtures from 43 to 12%. Due to defoliation, clover lost relatively more leaf area and less DM than grass, leading to a lower clover fraction in the leaf area index (LAI) of the stubble at the start of the next regrowth. In the –N mixtures, the clover fraction of the biomass and of the LAI increased within successive regrowth periods. In the +N mixtures, large-leaved Alice maintained its content during summer, mainly due to its greater petiole length which increased in response to N. The opposite was observed for Gwenda. At each harvest, the content of small-leaved Gwenda in the LAI and DM was lower than in the stubble at the start of regrowth. The allocation of DM to the petioles of Alice led to a decrease in the leaf weight ratio (LWR) in the +N mixtures, while Gwenda had a higher LWR and specific leaf area (SLA) in the +N mixtures than in the –N mixtures. There was little or no effect of ryegrass cultivar on competition with white clover.     

Interference effects in white clover genotypes grown as pure stands and binary mixtures with different grass species and varieties

Six white clover genotypes and eight grass varieties belonging to four different species were grown both in monoculture and as grass-legume binary mixtures in dense swards for two years under a mowing regime and a management including N fertilization. Dry matter yield and yield-related traits were recorded to investigate some aspects of inter-specific interference in white clover-based mixtures and to define a methodology for selecting genotypes of this clover suited to conditions of association. Clover was at a competitive disadvantage in most mixtures. Differences among grasses for aggressiveness were related more to variety vigour than to species. Clover compatibility proved specific only in relation to grass vigour. Variation among clovers for tolerance to competitive stress involved significant cross-over interactions passing from monoculture to severe stress conditions for clover yield and other traits, and was related positively to stolon density and negatively to yield and leaf gigantism traits recorded in monoculture. Clover selection for high levels of competitive stress seems possible either by genotype assessment in stress conditions or by a combination of high yield and stolon density assessed in monoculture.