Elevated CO<Subscript>2</Subscript> effects on nutrient competition between a C<Subscript>3</Subscript> crop (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) and a C<Subscript>4</Subscript> weed (<Emphasis Type="Italic">Echinochloa crusgalli</Emphasis> L.)

Rice (a C₃ crop) and barnyard grass (Echinochloa crusgalli L.) (a C₄ weed) were grown in a 1:1 mixture in a paddy field in ambient condition and with supplemented free air carbon dioxide enrichment (FACE, CO₂ concentration + 200 μmol mol⁻¹), in order to evaluate the impact of rising atmospheric carbon dioxide on nutrient competition between rice crop and weed. Results showed that elevated CO₂ significantly enhanced the biomass, tillers, leaf area index (LAI) and net assimilation rate (NAR) of rice, but reduced those of barnyard grass after elongation. Tissue nitrogen (N) concentrations were decreased in both competitors, but their phosphorus (P) and potassium (K) concentration were increased. The increase in tissue P concentration of rice was greater than that in barnyard grass. Furthermore, the absolute uptake of C, N, P, K by rice were increased while those of barnyard grass decreased. As a result, significant increase of the ratios of rice/barnyard grass of biomass and absolute nutrient uptake were observed under elevated CO₂. The results suggest that rising atmospheric CO₂ concentration could alter the competition between rice and barnyard grass in paddy fields in favor of rice. The ability of rice to compete more successfully for nitrogen and phosphorous under elevated CO₂ is likely an important factor underlying this response. More generally, the results suggest that elevated CO₂ may have varying implications on nutrient dynamics between different elements of overall plant biomass and the soil nutrients pool.     

Biologically Fixed N2 as a Source for N2O Production in a Grass–clover Mixture, Measured by 15N2

The contribution of biologically fixed dinitrogen (N₂) to the nitrous oxide (N₂O) production in grasslands is unknown. To assess the contribution of recently fixed N₂ as a source of N₂O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a ¹⁵N₂-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass–clover pots were sampled for N₂ fixation determination, whereas the remaining half were examined for emission of ¹⁵N labelled N₂O for another 8 days using a static chamber method. Biological N₂ fixation measured in grass–clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. The drop in N₂ fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m⁻² d⁻¹, which represented 1.7 ± 0.3% of the N accumulated in grass shoots during the labelling period. Total N₂O emission was 91, 416 and 259 μg N₂O–N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N₂ was emitted as N₂O on a daily basis, which accounted for 2.1 ± 0.5% of the total N₂O–N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N₂O. An erratum to this article is available at .     

Response of intensively grazed ryegrass dairy pastures to fertiliser phosphorus and potassium

Intensively grazed, rain-fed dairy pastures on the predominantly sandy soils in the high rainfall (>800 mm annual average) Mediterranean-type climate of south-western Australia comprise >90% ryegrass (annual ryegrass, Lolium rigidum Gaud. and Italian ryegrass, L. multiflorum Lam.). To maximise pasture use for milk production, the pastures are rotationally grazed by starting grazing when ryegrass plants have 3 leaves per tiller, and fertiliser nitrogen (N) and sulfur (S), in the ratio of 3–4 N and 1S, need to be applied after each grazing for profitable pasture dry matter (DM) production. In addition, farmers usually also apply low levels of phosphorus (P) and potassium (K) fertiliser to these pastures after each grazing, despite Colwell soil test P usually being well above critical values for pasture production, and fertilizer K being only required for clover in the traditional clover (Trifolium subterraneum L.) ryegrass pastures of the region. In field experiments undertaken May 2006–June 2010 on intensively grazed ryegrass dairy pastures in the region, no significant ryegrass DM responses to applied fertiliser P or K were obtained, regardless of level or method of P or K application. When no P was applied, soil test P declined gradually, by between 4.4 and 7.1 mg/kg per year, and remained above the critical value for the soils at 2 sites, but declined below the critical value for soil at a third site. Critical soil test P is located near the maximum yield plateau in the flat part of the relationship between yield and soil test P, particularly when, as appropriate for dairy production, the critical value is for 95% of the maximum pasture DM yield. Consequently, when no P is applied and soil test P decreases, significant pasture DM yield decreases will only occur when soil test P approaches the steeper part of the relationship, which can take some time. In addition, as occurs on farms, faeces deposited by cows while grazing supplied P to pasture even when no fertiliser P was applied. Soil K testing proved unreliable for indicating the need for fertiliser K applications to pasture in the next growing season because many soil samples collected within and between urine patches contained elevated levels of K deposited by cows while grazing. We conclude fertiliser P should only be applied to intensively grazed ryegrass dairy pastures when soil testing indicates it is required. Further research is required to assess if plant K testing is an alternative, but urine patches may also pose a problem for plant testing.     

Mulch N recycling in green manure leys under Scandinavian conditions

Nitrogen (N) recycling to the regrowth of mulched red clover (Trifolium pratense L.) and mulched mixed red clover/perennial ryegrass (Lolium perenne L.) leys was determined in field experiments during three consecutive years using ¹⁵N-labelled shoot material. Nitrogen recycling was greater in the pure clover stands than in the mixed stands in the beginning of the growing season, but increased successively in the mixed stands so that it was similar (14–15.5%) in both stands at the end of the season. This recycling of N from the mulch led to increased biomass accumulation but did not alter stand composition in the mixed stands. Mulch-derived N was incorporated into the soil organic N in both pure clover and mixed stands which thus contributed to building up soil fertility. An approximately similar proportion of N remained unaccounted for in mulched pure clover and mixed stand leys and presumably represented gaseous losses. To exploit the benefits of green manure leys in the humid temperate zone while minimising the negative environmental impact, these should be harvested rather than mulched.     

Nitrogen fixation of red clover interseeded with winter cereals across a management-induced fertility gradient

The incorporation of legume cover crops into annual grain rotations remains limited, despite extensive evidence that they can reduce negative environmental impacts of agroecosystems while maintaining crop yields. Diversified grain rotations that include a winter cereal have a unique niche for interseeding cover crops. To understand how management-driven soil fertility differences and inter-seeding with grains influenced red clover (Trifolium pratense) N₂ fixation, we estimated biological N₂ fixation (BNF) in 2006 and 2007, using the ¹⁵N natural abundance method across 15 farm fields characterized based on the reliance on BNF derived N inputs as a fraction of total N inputs. Plant treatments included winter grain with and without interseeded red clover, monoculture clover, monoculture orchardgrass (Dactylis glomerata), and clover-orchardgrass mixtures. Fields with a history of legume-based management had larger labile soil nitrogen pools and lower soil P levels. Orchardgrass biomass was positively correlated with the management-induced N fertility gradient, but we did not detect any relationship between soil N availability and clover N₂ fixation. Interseeding clover with a winter cereal did not alter winter grain yield, however, clover production was lower during the establishment year when interseeded with taller winter grain varieties, most likely due to competition for light. Interseeding clover increased the % N from fixation relative to the monoculture clover (72% vs. 63%, respectively) and the average total N₂ fixed at the end of the first growing season (57 vs. 47 kg N ha⁻¹, respectively). Similar principles could be applied to develop more cash crop-cover crop complementary pairings that provide both an annual grain harvest and legume cover crop benefits.     

A nitrogen and phosphorus herbage nutrient index as a tool for assessing the effect of N and P supply on the dry matter yield of permanent pastures

In order to propose consistent decision rules for fertilizer supply, a study was made on the effect of additions of N and P fertilizers and of their interaction on the above-ground dry matter yield of pastures during spring. The interaction between N and P could occur through nutrient acquisition or nutrient efficiency for growth. We therefore characterised the herbage N and P status (N and P index) from previously established critical curves of herbage mineral content according to above-ground dry matter. First we studied the effect of N and P addition on herbage nutrient status. Secondly, we expressed the above-ground dry matter as a function of the herbage nutrient status. This study consisted of four treatments applied to four permanent pastures which had a low phosphorus availability. The results showed a positive effect of P supply on the herbage nitrogen status, which may be due to an increase of organic matter mineralization or root growth. The P herbage status decreased only if N was supplied without P. The dry matter yield was positively related to the herbage nitrogen status, but a low P herbage status reduced the slope of the relationship. For the pastures studied, the indirect effect of P supply on above-ground dry matter, revealed by an increase in N index, was greater than its direct effect. This methodology allows us to distinguish the direct and indirect effects of N and P addition on herbage growth at field level. It could be used to propose consistent rules to manage jointly both N and P supplies.     

Nutrient release during the decomposition of mowed perennial ryegrass and white clover and its contribution to nitrogen nutrition of grapevine

Sustainable management of mineral nutrition in vineyards, as well as in other fruit plantations, should aim at exploiting the use of internal sources of nutrients, in order to reduce the need for external nutrient inputs. In this paper we explore the potential of the grassed alleys to provide nutrients to the vines. We followed for one vegetative season the decomposition of ryegrass and clover, frequently present as floor vegetation in vineyards, using litter bags filled with ¹⁵N-enriched grass material. In addition, we quantified the amount of nitrogen (N) transferred from the decomposing litter to field-grown grapevines. Ryegrass and clover had a relatively rapid decomposition rate, with a loss of C approaching 80% in only 16 weeks. The release of nutrients was particularly fast for potassium (95% in 16 weeks) followed by nitrogen (80%), calcium (70–80%), phosphorous (65–85%), magnesium (70–75%), and sulfur (60–70%). In spite of the rapid release of N from decomposing material, the N uptake by grapevines was on average less than 4% of the initial amount of N present in the litter of ryegrass and clover. Even if N release during the decomposition of mowed perennial ryegrass and white clover little contributed to the N nutrition of grapevine in the same growing season, most N from mowed grassed was still recovered in the soil.     

The combined effect of fertiliser nitrogen and phosphorus on herbage yield and changes in soil nutrients of a grass/clover and grass-only sward

The combined effect of reduced nitrogen (N) and phosphorus (P) application on the production of grass-only and grass/clover swards was studied in a five-year cutting experiment on a marine clay soil, established on newly sown swards. Furthermore, changes in soil N, P and carbon (C) were measured. Treatments included four P (0, 35, 70 and 105 kg P ha^–1 year^–1) and three N levels (0, 190 and 380 N kg ha^–1 year^–1) and two sward types (grass-only and grass/clover). Nitrogen was the main factor determining the yield and quality of the harvested herbage. On the grass-only swards, N application increased the DM yield with 28 or 22 kg DM kg N^–1, at 190 or 380 kg N ha^–1 year^–1, respectively. The average apparent N recovery was 0.78 kg kg^–1. On the grass/clover swards, N application of 190 ha^–1 year^–1 increased grass production at the cost of white clover, which decreased from 41 to 16%. Phosphorus application increased grass yields, but did not increase clover yields. A positive interaction between N and P applications was observed. However, the consequences of this interaction for the optimal N application were only minor, and of little practical relevance. Both the P-AL-value and total soil P showed a positive response to P application and a negative response to N application. Furthermore, the positive effect of P application decreased with increasing N application. The annual changes in P-AL-value and total soil P were closely related to the soil surface surplus, which in turn was determined by the level of N and P application and their interaction. The accumulation of soil N was similar on both sward types, but within the grass-only swards soil N was positively affected by N application. The accumulation of organic C was unaffected by N or P application, but was lower under grass/clover than under grass-only.     

Effect of compost and soil properties on the availability of compost phosphate for white clover (Trifolium repens L.)

Wide variation in results exists in the literature on the effectiveness of composts to sustain the phosphorus (P) nutrition of crops. The aim of this work was to assess the importance of some soil and composts properties on the utilization of compost-P by white clover (Trifolium repens L.). This study was carried out with samples collected from four composts made from solid kitchen and garden wastes, and with two soil samples taken from the A horizon of a P-rich sandy acidic Dystrochrept and of a P-limited clayey calcareous Eutrochrept. Changes in the amount of inorganic P (Pi) isotopically exchangeable within 1 min (E_1min) were measured during 32 weeks in incubated soil-composts or soil-KH₂PO₄mixtures where P sources had been added at the rate of 50 mg P kg^–1 soil. Uptake of compost-P or KH₂PO₄-P by white clover was measured on the same amended soils during 16 weeks. In both soils, the application of composts resulted after 32 weeks of incubation in E_1min values ranging between those observed in the control without P and those observed in the KH₂PO₄treatment, i.e., in values ranging between 4.2 and 5.9 mg P kg^–1 in the sandy acidic soil and between from 1.6 to 4.3 mg P kg^–1 in the clayey calcareous soil. The total coefficient of utilization of compost-P (CU-P) by white clover reached values in both soils for the four composts ranging between 6.5% and 11.6% of the added P while in the presence of KH₂PO₄ the CU-P reached values ranging between 14.5% in the clayey calcareous soil and 18.5% in the sandy acidic soil. Results obtained in the sandy acidic soil suggest, that white clover initially used a fraction of the rapidly exchangeable compost P, while at a latter stage plant roots enhanced the mineralisation of compost organic P and took up a fraction of the mineralized P. These relations were not observed in the clayey calcareous soil probably because of its high sorbing capacity for P. In the sandy acidic soil, composts application increased the uptake of soil P by the plant from 31.4 mg P kg^–1 soil in the control without P to values ranging between 37.9 to 42.7 mg P kg^–1 soil in the presence of composts. This indirect effect was related to a general improvement of plant growth conditions in this soil induced by compost addition (from 9.9 g DM kg^–1 soil in the control without P to values ranging between 14.0 to 16.1 g DM kg^–1 soil in the presence of composts) and/or to the release of Al- or Fe bound soil P to the solution due to soil pH increase following compost application. Finally the total coefficient of utilization of P (CU-P) derived from KH₂PO₄ and composts was related to the total amount of N exported by white clover in the P-limited clayey calcareous soil but not in the P-rich sandy acidic soil. This suggests that in a soil where N₂ biological fixation is limited by low P availability, the CU-P of a compost by white clover is not only related to the forms of P present in the compost but also to its effect on N nutrition. However, it is not clear whether this improved N nutrition was due to compost mineralisation, or to an indirect compost effect on the N₂ biological fixation.     

Effect of K on nitrogen fixation of intercrop groundnut and the competition between intercrop groundnut and maize

Application of adequate level of K has shown to improve the competitive ability of the legume in legume/grass mixtures. However, the effect of K on the competitive ability of grain legumes in legume/cereal intercropping systems has not been adequately studied. Hence, studies were made to ascertain if the effects of K could be exploited in improving the performance of groundnut (Arachis hypogaea L.) cv. No. 45 when intercropped with maize (Zea mays L.) cv. Badra. The study was conducted at the Faculty of Agriculture, University of Ruhuna, Kamburupitiya, Sri Lanka in 1988 in basins filled with 36 kg of soil. It involved establishing maize and groundnut as monocrops and as intercrops at three K levels viz. 0, 20 and 40 mg of K kg^–1 of soil. Monocrop maize and groundnut had 2 and 5 plants/basin, respectively while the intercrop had 1 maize plant and 3 groundnut plants/basin. The soil used was Red Yellow Podzolic which was tagged by incorporating¹⁵N-labelled plant material. When grown as a monocrop, K had no effect on the percent N derived from atmosphere, amount of N₂ fixed, dry matter production, pod yield and total N content of groundnut. However, when intercropped with maize lack of K application affected the above parameters significantly which was overcome by improving K level. Thus, the optimum level of K for groundnut was greater when intercropped than monocropped. A significant interaction between K level and cropping system was evident with regard to N₂ fixation, pod yield and total dry matter production of groundnut. Intercrop maize derived 30–35% of its N content from the associated groundnut plants which amounted to 13–22 mg N/plant. The amount of N supplied by groundnut to associated maize plant was not affected by K level. It appears that there is scope for alleviating growth depression of the legume component in legume/cereal intercropping systems by developing appropriate K fertilizer practices.     

Re-utilization by perennial ryegrass (Lolium perenne L.) of labelled fertilizer sulphur incorporated in field-grown tops and roots of pasture plants added to soils

³⁵S-labelled gypsum fertilizer was incorporated under field conditions into pastures which were separated into white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) tops and roots. These were added to four soils from improved and unimproved pastures. The re-utilization of labelled fertilizer sulphur (S) was assessed under growth cabinet conditions (20°C day, 13°C night, daylength 16 h, light intensity 120–170 lx) by growing perennial ryegrass plants for 23 weeks.Mean recoveries of labelled fertilizer S varied from 7 to 20% depending on soil type, form amount and kind of plant residue added. Greater recovery was obtained from clover roots (9.5–16.2%) than grass roots (6.7–12.5%), and from grass tops (13.1–19.7%) than clover tops (9.7–17.9%). These results are related to contents of labelled S, total S, C/N, C/S and N/S ratios in plant residues which also accounted for their relative rates of decomposition. Ground (< 1 mm) and chopped (3 mm) roots increased labelled fertilizer S recovery by about 30% compared with whole roots. Additions of unlabelled fertilizer S influenced the recovery of labelled fertilizer S. This effect depends on the amounts of labelled grass roots and unlabelled fertilizer added.The significance of the findings is discussed with the aid of results from previous field experiments conducted on these soils.     

Comparison of white clover and ammonium nitrate as nitrogen sources for tall fescue

White clover was compared against five rates of nitrogen fertilizer (NH₄NO₃) as sources of N for tall fescue over a 3 year period. The white clover-tall fescue combination produced as much forage as tall fescue alone fertilized with 132–198 kg N ha^?1 in the first 2 years. However, in the third year the white clover-tall fescue combination only produced as much forage as tall fescue alone fertilized with 0–66 kg N ha^?1 as a result of a large decline in the stand of white clover. Tall fescue alone showed a significant response to fertilization up to the maximum rate of 264 kg N ha^?1. Tall fescue fertilized with 264 kg N ha^?1 produced significantly more forage than the white clover-tall fescue combination in all 3 years.     

Can we understand and predict the regulation of biological N<Subscript>2</Subscript> fixation in grassland ecosystems?

We discuss results from controlled environment studies including mesocosms, grazing experiments and long term field experiments which show how biological N₂ fixation in legume based systems is tightly coupled to the N demand at scales ranging from the individual plant to the grassland ecosystem. We further test the consequences of this hypothesis of a feedback regulation of biological N₂ fixation by N demand with a mechanistic model linking plant community dynamics and ecosystem functioning. Results confirm the heuristic power of this hypothesis which accounts for a number of observations concerning changes in the relative abundance and N₂ fixation rate of legumes in managed grasslands. Then we show how nitrogen and carbon fluxes are affected by plant-plant (e.g. competition and facilitation), plant-soil and plant-herbivore interactions and by climate and management changes.     

Early growth and N2-fixation of leucaena and gliricidia at different levels of phosphorus application

The phosphorus (P) uptake and use efficiency in relation to N₂ fixation and growth of three provenances of gliricidia (Gliricidia sepium) two cultivars of leucaena (Leucaena leucocephala) and one ofL. diversifolia, all inoculated withRhizobium, were determined at five levels of P application. Increasing the P application rate resulted in a 33% increase in dry matter and total N ofL. diversifolia and an 18% dry matter increase of gliricidia provenance 13/84 over the control without P. Leucaena K8, which did not respond to P application, yielded as well with low P asL. diversifolia at the high P indicating the lower P requirement of cultivar K8. Leucaena cultivar Singapore did not yield well at either low or high P. Correlation analyses showed that differences between species and cultivars/provenances for P uptake per plant were largely related to differences in shoot growth rate and the distribution of P between roots and tops. P use efficiency was not related to dry-matter production. Genetic control of P and dry matter distribution of is probably more important than P availability in the growth media for gliricidia and leucaena provenances or cultivars. Increasing the P application rate significantly increased the number and mass of nodules in leucaena cultivars and gliricidia provenances. Significant increases in the proportion of N derived from atmospheric N₂ (% Ndfa) due to low P application (20 mg P kg^–1 soil) were observed within leucaena but not for gliricidia provenances. No significant increases in % Ndfa occurred with higher P application rates suggesting that symbiotic N₂ fixationper se is stimulated only with low rates of P.     

Can phosphorus fertilisers alone increase levels of soil nitrogen in New Zealand hill country pastures?

In New Zealand grazed pastures, nitrogen (N) fixation by clover is the traditional method of supplying N to the grasses that make up the bulk of the pasture sward. In order to stimulate satisfactory clover growth, phosphorus (P) fertilisers are applied at levels which are generally more than adequate for grass requirements. These legumes then provided N through biological nitrogen fixation. However, studies conducted in New Zealand hill country pastures have revealed that these pastures are still highly N responsive. These results draw attention to a key issue with respect to N fertility in hill country pasture and the question arises as to the value of large P fertiliser applications to overcome N deficiency through clover growth. Here we used modelling approach to evaluate the effectiveness of adding P fertilisers to stimulate clover growth for improving soil N status in hill country pastures and to explore why the hill pastures are N responsive. In addition an attempt was made to explore the potential of fertiliser N in hill pastures based on the current model outcomes and the measured values of pasture production under non-limiting N.     

Response to N-fertilizer of Italian ryegrass grown alone and in mixture with berseem clover under continental irrigated mediterranean conditions

In a field experiment over three growing seasons, the potential benefits of planting berseem clover (Trifolium alexandrinum L) with Westerwold Italian ryegrass (Lolium multiflorum Lam.) were examined under irrigated continental Mediterranean conditions.Similar N rates (0, 30, 60, 90 and 120 kg N ha^–1 cut^–1) were applied to both pure Italian ryegrass stands and mixtures, each given three successive cuts. One previously unfertilized cut was performed in late winter. Species in the mixture were established at 50:50 seed ratio but the mean proportion of berseem clover was 14%. Mean winter survival of berseem was 87% but 88% of the plants had leaves damaged by the frost. Forage production varied with both N rate and cutting sequence in both the pure stand and the mixture but differences between the two types of swards were significant only at low levels of fertilizer N. Total DM production over the four cuts in plots with N applications of 0,90, 180, 270 and 360 kg N ha^–1 a^–1 were 7.14, 9.51, 11.66, 13.91 and 14.36 t DM ha^–1 a^–1 in pure stand, respectively. Corresponding values for the mixture were 8.80, 10.94, 12.90, 14.05 and 13.64 t DM ha^–1 a^–1. The mean response of Italian ryegrass in the range of 0–360 kg N ha^–1 a^–1 was 20 kg DM per kg N applied. The corresponding value for the mixture was 13 kg DM per kg N applied. At the berseem clover proportions reached in this work, N equivalence showed values of about 80 kg N ha^–1 a^–1. As rates of N increased from 0 to 120 kg N ha^–1 cut^–1, nitrogen concentration increased by 78%. In the applied range of N fertilizers, N0₃-N was not affected.

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Résumé Dans un essai réalisé au cours de trois saisons agricoles, on a étudié le potentiel de 1'association du bersim avec le raygrass italien. Les cultures ont été emenées et irrigué dans des conditions continentales méditerranéennes. On a appliqué, aussi bien pour la culture pure du raygrass que pour l'association, une fertilisation azotée avec les doses suivantes (0, 30, 60, 90 et 120 kg N/ha/coupe) après chacune des trois coupes successives. Une autre coupe avant fertilisation a été faite à la fin de 1'hiver. Les proportions du bersim et du raygrass dans le mélange de graines étaient de 50:50. Cependant, dans la culture en association, les plantes du bersim n'étaient préentés qu'avec un 14 pourcent. 87% des plantes du bersim ont pu survivre en hiver, dont 88% avaient des feuilles endommagées par les gelées. La production d'herbe a été proportionnelle aux doses de fertilisation pour la culture pure et l'association. Néanmoins, différence entre les rendements de chacune de ces dernières était d'autant plus nette que les doses d'azote incorporées dans le sol étaient faibles. La production de la MS pour les quatre coupes dans les parcelles avec les applications de 0, 90, 180, 270 et 360 kg N ha^–1 a^–1 étaient de 7.14, 9.51, 11.66, 13.51, 14.36 tMS ha^–1 a^–1. Le rendement moyen du raygrass italien dans un intervalle de 0-360 kg N ha^–1 a^–1 a été de 20 kg MS par kg de N de fertilisation. Concernant le bersim, les valeurs équivalentes de N étaient de 1'ordre de 80 kg N ha^–1 a^–1. Au fur et à mesure que les doses de fertilisation azotée augmente de 0 à 120 kg ha^–1 coupe, la concentration en azote augmente de 78%. Dans l'intervalle de la fertilisation azotée appliqué NO₃ -N n'a pas été affectée.

     

An assessment of granular urea/ammonium sulphate and urea/potassium nitrate fertilizers on nitrogen recovery by ryegrass

The comparative effects of ammonium sulphate (AS), potassium nitrate (KNO₃), urea (U) or combined 1:1 (w/w) U/KNO₃, U/AS granular products were investigated on dry matter (DM) yield and¹⁵N utilisation by perennial ryegrass grown under controlled environmental conditions.The DM yield and apparent N recovery with the single N sources was in the order KNO₃ > AS > U. The¹⁵N budget in shoots, roots and soil indicated that only 55% of the urea N was recovered at the end of the experiment compared with 87% and 86% for AS and KNO₃ respectively. The DM yield and apparent N recovery from the combined U/AS source was significantly higher than would be expected (P < 0.01) based on the proportions of each N source in the mixture. Differentially labelling the U and AS with¹⁵N indicated that AS enhanced the shoot % utilisation of urea by 38% (P < 0.001) whereas urea reduced the shoot % utilisation of AS by 14% (P < 0.01). These results indicate that an interaction occurred between U and AS when combined in a 1:1 (w/w) ratio in the same pellet.     

Effectiveness of alfalfa in reducing fertilizer N input for optimum forage yield, protein concentration, returns and energy performance of bromegrass-alfalfa mixtures

Grasses, when grown in association with legumes, may utilize some N fixed by the legumes resulting in improved forage dry matter and protein yield. Field experiments were conducted at Lacombe and Eckville, Alberta, Canada to determine the effectiveness of alfalfa (Medicago sativaLeyss) in reducing fertilizer N requirements for optimum forage dry matter yield (DMY), protein concentration (PC), net margins (returns above N fertilization and forage harvesting costs) and non-renewable energy performance of bromegrass (Bromus inermis Leyss)-alfalfa mixtures. Ammonium nitrate was applied in early spring of 1993 to 1995 at 0, 50, 100, 150 and 200 kg N ha⁻¹ to five bromegrass-alfalfa compositions (pure bromegrass; 2:1, 1:1 and 1:2 ratio of bromegrass:alfalfa; and pure alfalfa) seeded in the summer of 1992. In the zero-N treatment, DMY was lowest in pure bromegrass stands, and increased substantially when alfalfa was grown in association with bromegrass. There was a marked increase in DMY from the application of N fertilizer in pure bromegrass stands, but the increase was much less in the mixed stands. There was a significant increase in PC in forage when bromegrass was grown in a mixture with alfalfa compared to bromegrass alone. Net margins were much greater from mixed stands than from pure bromegrass. In pure bromegrass stands, net margins increased with increasing N rates up to 200 kg N ha⁻¹, but equivalent net margins were usually attained without fertilizer N in bromegrass-alfalfa mixtures as low as 2:1. Energy performance of pure bromegrass stands was substantially improved by including alfalfa in the stands, whereas application of N fertilizer caused a strong and steady decline in energy use efficiency. Our findings indicate that seeding alfalfa in mixed stands with bromegrass can generate savings in N fertilizer (for pure bromegrass stands) equivalent to about 100 kg N ha⁻¹ or more, without any detrimental effect on forage yield, forage quality or net earnings. However, the short-lived nature of alfalfa in bromegrass-alfalfa mixtures remains a cautionary concern. Thus, producers should also adopt management practices that enhance longevity of alfalfa to maximize long-term benefits of using grass-legume mixtures. This revised version was published online in November 2006 with corrections to the Cover Date.     

Potassium uptake and requirement in organic grassland farming

Use of mineral fertilizers is restricted in organic farming. The aim of the present paper was therefore to study whether potassium (K) limits yields in Norwegian organic grasslands. The K status in soil and herbage on 26 organic farms was investigated, and the response to K application in six fertilization experiments was explored. Further, the relationship between soil K analyses and K release from soil was examined. K application to grassland on the investigated farms was generally low, giving negative field K balances on 23 of the farms. The soils were classified as low or intermediate in readily available K (K_AL) on 23 of the farms. The mean K concentration for herbage samples from the first cut on these farms was 18.0 g K kg⁻¹ dry matter. In fertilization experiments, K application increased the K concentration in herbage. However, there was no significant effect on yield, even when K concentration in herbage on plots without K application was low. The lack of significant yield response to K application can be explained by low amounts of crop-available nitrogen (N). There was a tendency for increased plant uptake from reserve K with increasing values of acid soluble K (K–HNO₃) in soil. Separate K analyses of timothy (Phleum pratense) and red clover (Trifolium pratense) revealed that red clover showed better competitiveness for K than timothy in leys where N supply was limited.     

Kinetics and mechanism of the formation of hollandites in the BaO(Cs<Subscript>2</Subscript>O)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system from initial mixtures prepared by different methods

The mechanism and kinetics of formation of solid solutions based on hollandite in the BaO(Cs₂O)-Al₂O₃-TiO₂ system are investigated using the initial mixtures prepared by two methods: (i) mechanical grinding and mixing of the initial components and (ii) coprecipitation from aqueous solutions of the salts. It is established that the mechanism of formation of hollandite in the system under investigation depends on the degree of dispersion of the initial mixtures used in the synthesis. When the synthesis is performed with the initial mixture prepared by mechanical grinding and mixing of the initial reactants (the particle size is equal to 50–300 nm), hollandite is formed at temperatures in the range 1100–1250°C in the presence of the accompanying phase Cs₂Al₂Ti₂O₈. When the synthesis is performed with the initial mixture prepared by coprecipitation from aqueous solutions of salts (the particle size is equal to 10–12 nm), hollandite is formed at temperatures in the range 850–1050°C. The investigation into the kinetics of formation of the hollandite phase from the above mixtures made it possible to determine the temperature-time conditions for the synthesis of this titanate in the form of a powder with a particle size of approximately 50 nm or in the form of a dense ceramic material with a particle size of ～200 nm.