Species variation in the fixation and transfer of nitrogen from legumes to associated grasses

Summary Three legume species (alfalfa, red clover, and birdsfoot trefoil) in combination with five grass species (timothy, bromegrass, red fescue, tall fescue, and orchardgrass) were used to study N transfer in mixtures, using the 15_N dilution technique. The advantage of grass-legume mixtures was apparent. Total herbage and protein yields of grasses in mixtures were higher than those alone, especially at the later cuts. This benefit of mixed cropping is mainly due to N transfer from legumes to associated grasses. N₂-fixation and N transfer by alfalfa rated highest, red clover intermediate, and birdsfoot trefoil lowest. The importance of each pathway of N transfer from legumes appeared to differ between species. Alfalfa and red clover excreted more N than trefoil, while the latter contributed more N from decomposition of dead nodule and root tissue. The greatest advantage from a grass-legume mixture, with respect to the utilization of N released from the legume, varied with early maturing tall fescue (Kentucky 31), orchardgrass (Juno), and bromegrass (Tempo), to intermediate timothy (Climax), and least with late maturing red fescue (Carlawn). Contribution no. 817 of the Ottawa Research Station.     

Positive interactions between nitrogen-fixing legumes and four different neighbouring species in a biodiversity experiment

The importance of facilitative processes due to the presence of nitrogen-fixing legumes in temperate grasslands is a contentious issue in biodiversity experiments. Despite a multitude of studies of fertilization effects of legumes on associated nonfixers in agricultural systems, we know little about the dynamics in more diverse systems. We hypothesised that the identity of target plant species (phytometers) and the diversity of neighbouring plant species would affect the magnitude of such positive species interactions. We therefore sampled aboveground tissues of phytometers planted into all plots of a grassland biodiversity–ecosystem functioning experiment and analysed their N concentrations, δ¹⁵N values and biomasses. The four phytometer species (Festuca pratensis, Plantago lanceolata, Knautia arvensis and Trifolium pratensis) each belonged to one of the four plant functional groups used in the experiment and allowed the effects of diversity on N dynamics in individual species to be assessed. We found significantly lower δ¹⁵N values and higher N concentrations and N contents (amount of N per plant) in phytometer species growing with legumes, indicating a facilitative role for legumes in these grassland ecosystems. Our data suggest that the main driving force behind these facilitative interactions in plots containing legumes was reduced competition for soil nitrate (“nitrate sparing”), with apparent N transfer playing a secondary role. Interestingly, species richness (and to a lesser extent functional group number) significantly decreased δ¹⁵N values, N concentrations and N content irrespective of any legume effect. Possible mechanisms behind this effect, such as increased N mineralisation and nitrate uptake in more diverse plots, now need further investigation. The magnitude of the positive interactions depended on the identity of the phytometer species. Evidence for increased N uptake in communities containing legumes was found in all three nonlegume phytometer species, with a subsequent strong increase in biomass in the grass F. pratensis across all diversity levels, and a lesser biomass gain in P. lanceolata and K. arvensis. In contrast, the legume phytometer species T. pratense was negatively affected when other legumes were present in their host communities across all diversity levels.     

Nitrogen fixation in annual and perennial legume-grass mixtures across a fertility gradient

Background and aims

The selection of legume species and species mixtures influences agroecosystem nitrogen (N) and carbon cycling. We utilized a fertility gradient to investigate the effects of plant species interactions on biological N fixation of an annual and perennial legume in response to shifting soil resource availability.

Methods

Legume N fixation of annual field pea (Pisum sativum) and perennial red clover (Trifolium pratense) grown in monoculture and mixtures with oats (Avena sativa) or orchardgrass (Dactylis glomerata) was estimated using the ¹⁵N natural abundance method across 15 farm fields and we measured six soil N pools ranging from labile to more recalcitrant.

Results

Evidence of complementary and facilitative species interactions was stronger for the perennial red clover-orchardgrass mixture than for the annual field pea-oat mixture (N Land Equivalency Ratios were 1.6 and 1.2, respectively). We estimated that the transfer of fixed N from red clover to orchardgrass increased aboveground N fixation estimates by 15% from 33 to 38?kg?N ha^?1. Despite a more than 2-fold range in soil organic matter levels and more than 3-fold range in labile soil N pools across field sites, the N fertility gradient was not a strong predictor of N fixation. While grass N assimilation was positively correlated with soil N pools, we found only weak, inverse correlations between legume N fixation and soil N availability. In grass-legume mixtures, soil N availability indirectly influenced N fixation through plant competition.

Conclusions

These results suggest that increasing diversity of cropping systems, particularly through the incorporation of perennial mixtures into rotations, could improve overall agroecosystem N cycling efficiency.     

Forage production, N uptake, N2 fixation, and N recovery of berseem clover grown in pure stand and in mixture with annual ryegrass under different managements

In Mediterranean countries, forage grasses and legumes are commonly grown in mixture because of their ability to increase herbage yield and quality compared with monocrop systems. However, the benefits of intercropping over a monocrop system are not always realized because the efficiency of a grass–legume mixture is strongly affected by agronomic factors. The present study evaluated productivity, N₂ fixation, N transfer, and N recovery of berseem clover (Trifolium alexandrinum) grown in pure stand and in mixture with annual ryegrass (Lolium multiflorum) under high or low defoliation frequencies and varying plant arrangements (sowing in the same row or in alternating rows). On average, the berseem–ryegrass mixtures resulted in a greater yield and N yield than the monocrops. When mixed together, ryegrass was more efficient than berseem at absorbing soil N, increasing the reliance of berseem on N₂ fixation. Both defoliation management and plant arrangement affected forage yield and the quality of the mixture, modifying the proportion of the two components, the N content of the forage, and the symbiotic N₂ fixation of the legume. Reducing the proximity between plants of the two species may benefit the weaker component of the mixture. No apparent transfer of fixed N from berseem to ryegrass was detected.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Context-dependent outcomes of subarctic grass-endophyte symbiosis

Symbiotic interactions are often context-dependent. We explored how different habitats have modified the symbiosis between the grass Festuca rubra and the endophyte Epichloë festucae. We grew endophytic and endophyte-free grasses originating from subarctic meadows and open river banks in a growth chamber with a hemiparasitic plant (Rhinanthus minor) and a competitor (Achillea millefolium), representing typical plant-plant interactions in meadows. Grasses from meadows were more adapted to plant species interactions than river bank grasses, and the presence of the endophyte strengthened this difference further. Endophyte-infected meadow grasses did not suffer from the hemiparasite, but the endophyte decreased the tolerance of the river bank grasses to the hemiparasite. Endophytic river bank grasses invested more than meadow grasses in vegetative spread. These results suggest differentiation of grass-endophyte symbiota between the habitats, and underline the context dependency of species interactions as well as the role of symbionts in the habitat adaptation of plants.     

Biological nitrogen fixation in mixed legume/grass pastures

Biological nitrogen fixation (BNF) in mixed legume/grass pastures is reviewed along with the importance of transfer of fixed nitrogen (N) to associated grasses. Estimates of BNF depend on the method of measurement and some of the advantages and limitations of the main methods are outlined. The amounts of N fixed from atmospheric N₂ in legume/grass pastures throughout the world is summarised and range from 13 to 682 kg N ha^-1 yr^-1. the corresponding range for grazed pastures, which have been assessed for white clover pastures only, is 55 to 296 kg N ha^-1 yr^-1.Biological nitrogen fixation by legumes in mixed pastures is influenced by three primary factors; legume persistence and production, soil N status, and competition with the associated grass(es). These factors and the interactions between them are discussed. Legume persistence, production and BNF is also influenced by many factors and this review centres on the important effects of soil moisture status, soil acidity, nutrition, and pests and disease.Soil N status interacts directly with BNF in the short and long term. In the short-term, increases in soil inorganic N occurs during dry conditions and where N fertiliser is used, and these will reduce BNF. In the long-term, BNF leads to accumulation of soil N, grass dominance, and reduced BNF. However, cyclical patterns of legume and grass dominance can occur due, at least in part, to temporal changes in plant-available N levels in soil. Thus, there is a dynamic relationship between legumes and grasses whereby uptake of soil N by grass reduces the inhibitory effect of soil N on BNF and competition by grasses reduces legume production and BNF. Factors affecting the competition between legumes and grasses are considered including grass species, grazing animals, and grazing or cutting management.Some fixed N is transferred from legumes to associated grasses. The amount of N transferred below-ground, predominantly through decomposition of legume roots and nodules, has been estimated at 3 to 102 kg N ha^-1 yr^-1 or 2 to 26% of BNF. In grazed pasture, N is also transferred above-ground via return in animal excreta and this can be of a similar magnitude to below-ground transfer.Increased BNF in mixed legume/grass pastures is being obtained through selection or breeding of legumes for increased productivity and/or to minimise effects of nutrient limitations, low soil moisture, soil acidity, and pests and disease. Ultimately, this will reduce the need to modify the pasture environment and increase the role of legumes in low-input, sustainable agriculture.     

Determinants of the abundance of invasive annual weeds: community structure and non-equilibrium dynamics

The dynamics of an annual pasture community are described from a five-year experimental and monitoring study. The community was dominated by two grasses (Lolium rigidum and Vulpia bromoides) and a legume (Trifolium subterraneum). Fits of population dynamic models to per capita rates of population change indicate that interactions between the grasses were generally strong, while interactions between the grasses and legumes were weaker. Most, but not all, of the net effects of competition on population growth could be attributed to interactions occurring during plant growth. Phase-plane analysis indicated that, for a constant environment, a joint equilibrium of the two grasses is unstable since interspecific competition between Lolium and Vulpia is stronger than intraspecific competition. Consequently, the community will tend to a mixture of only one or other of the grass species and T. subterraneum, depending on the founding composition of the pasture. Analysis of data taken from a year in which a drought occurred (1993-1994) demonstrated profound effects on all three species. Modelling of the long-term impacts of the effects of repeated droughts showed that disturbance of this form overrides the founder effect observed under constant conditions. Consequently, Vulpia is ultimately able to invade any mixture of the other species in environments where stochastic disturbances occur.     

Complementarity in mineral nitrogen use among dominant plant species in a subalpine community

The underlying mechanisms that enable plant species to coexist are poorly understood. Complementarity in resource use is among the major mechanisms proposed that could favor species coexistence but is insufficiently documented. In alpine soil, low temperatures are a major constraint for the supply of plant nitrogen. We carried out (15)N labeling of soil mineral N to determine to what extent four major species of a subalpine community compete for N, or develop ionic (NH(4)(+) vs. NO(3)(-)) or temporal complementarity. The Poaceae took up much more (15)N per soil area unit than the ericaceous species, and all species displayed three major strategies in exploiting (15)N: (1) uptake mainly early in the growing season (Vaccinium myrtillus), (2) uptake at a slow and similar rate throughout the growing season (Rhododendron ferrugineum), and (3) uptake at high rates over the growing season (Festuca eskia and Nardus stricta). However, while F. eskia used (15)NH(4)(+) mainly early and (15)NO(3)(-) mainly late in the growing season, the reverse was observed for N. stricta. Taking into account (15)N dilution in soil NH(4)(+) and NO(3)(-) pools, we calculated that NH(4)(+) provided more than 80% of the mineral N uptake in Ericaceae and about 60% in grasses. Together, such ionic and temporal complementarity would reduce competition between species and could be a major mechanism promoting species diversity.     

Typification of Linnaean taxa of annual Poaceae: Poeae related to Vulpia and Desmazeria

STACE, C. A. & JARVIS, C. E., 1985. TypiHcation of Linnaean taxa of annual Poaceae: Poeae related to Vulpia and Desmazeria. The status and typification of 15 Linnaean species of annual grasses related to Vulpia and Desmazeria are discussed. Of these 15, eight are represented by holotypes or lectotypes in LINN, two by lectotypes in Herb. A. van Royen (L), and one by a neotype in LINN. One (Festuca marina) is based on a pre-Linnaean polynomial and is represented by a lectotype in Herb. Sloane (BM); one (Cynosurus durus) has no known type specimens and we have chosen a Barrelier (1714) illustration as lectotype; one (Nardus aristatus) is an illegitimate name change for Nardus incurvus Gouan, for which we have selected a Scheuchzer (1719) illustration as lectotype; and finally Festuca incrassala appeared on a cancelled page of Species Plantarum and has no nomenclatural standing.     

Transfer of nitrogen from a tropical legume tree to an associated fodder grass via root exudation and common mycelial networks

Symbiotic dinitrogen fixation by legume trees represents a substantial N input in agroforestry systems, which may benefit the associated crops. Applying ¹⁵N labelling, we studied N transfer via common mycelial networks (CMN) and root exudation from the legume tree Gliricidia sepium to the associated fodder grass Dichantium aristatum . The plants were grown in greenhouse in shared pots in full interaction (treatment FI) or with their root systems separated with a fine mesh that allowed N transfer via CMN only (treatment MY). Tree root exudation was measured separately with hydroponics. Nitrogen transfer estimates were based on the isotopic signature of N ( δ ¹⁵N) transferred from the donor. We obtained a range for estimates by calculating transfer with δ ¹⁵N of tree roots and exudates. Nitrogen transfer was 3.7–14.0 and 0.7–2.5% of grass total N in treatments FI and MY, respectively. Root δ ¹⁵N gave the lower and exudate δ ¹⁵N the higher estimates. Transfer in FI probably occurred mainly via root exudation. Transfer in MY correlated negatively with grass root N concentration, implying that it was driven by source-sink relationships between the plants. The range of transfer estimates, depending on source δ ¹⁵N applied, indicates the need of understanding the transfer mechanisms as a basis for reliable estimates.     

Competition between siratro and kleingrass for15N labelled mineralized nitrogen

Summary Isotope dilution provides a method for measuring plant competition for mineral N and transfer of biologically fixed N from a legume to a grass. A plant growth medium was enriched with¹⁵N, and used to grow Siratro (Macropitilium atropurpureum D.C. Urb.) and Kleingrass 75 (Panicum coloratum L.) in 20 liter pots for 98 days in a glasshouse. The plants were grown in pure stand and in mixtures. When grown in 50∶50 mixture the grass obtained 59% of the labelled N and the legume obtained 41%. The grass produced nearly as much root mass as the legume even though biomass of the shoots were less than half that of the legume. Reducing the proportion of either plant species in the mixture reduced the proportion of the mineralized N absorbed by that species. The shoots of the grass were significantly more enriched (1.166 atom%¹⁵N excess) than the roots (1.036). The grass received 12% of its N as biologically fixed N from the legume.     

Legume defoliation affects rhizosphere decomposers, but not the uptake of organic matter N by a neighbouring grass

Legume–grass interactions have a great influence on grassland primary production and it was recently shown how defoliation of a legume can increase the transfer of fixed N to a neighbouring grass. It has also been shown that defoliation of a plant can increase soil microbial activity and lead to better soil N availability in the rhizosphere of the defoliated plant. We combined these two perspectives and tested whether defoliation of a legume (Lotus corniculatus) can enhance N nutrition of the neighbouring grass (Holcus lanatus) by increasing growth of soil decomposer biota and the availability of soil organic matter N for grass uptake. We grew mixtures of L. corniculatus and H. lanatus in grassland soil that included ¹⁵N-labelled L. corniculatus litter. In half of the systems, we subjected L. corniculatus to a defoliation treatment mimicking insect larvae feeding. At destructive harvests 1, 3, 9 and 30 days after the last defoliation event, we determined how L. corniculatus defoliation affected decomposer microbes, protozoa and nematodes and whether these changes among decomposers created a feedback on the growth and ¹⁵N uptake of the neighbouring H. lanatus. Defoliation reduced the growth and litter-N uptake, but increased shoot N concentration of L. corniculatus. Of the soil variables measured, defoliation doubled the number of bacterial-feeding protozoa, but did not affect the abundance of decomposer microbes and bacterial- and fungal-feeding nematodes. Defoliation did not have statistically significant effects on H. lanatus shoot growth, shoot N concentration or litter-N uptake. Our results demonstrate how defoliation-induced changes in legume ecophysiology can affect the growth of decomposers in soil. However, these effects did not appear to lead to a significant change in the availability of soil organic N to the neighbouring grass. It seems that when positive effects of legume defoliation on grass N nutrition are found in grassland ecosystems, these are more likely to be explained by direct transfer of fixed N rather than changes in the availability of soil organic matter N.     

The potential of legumes as a nitrogen source for the reclamation of derelicht land

Summary White clover (Trifolium repens) growing in association with grasses on colliery spoil amended with lime and complete fertilizer proved to be an effective nitrogen source for the developing ecosystem. Nitrogen transfer from the clover to an associated grass became aparent within 22 months of sowing.     

Competition between Subterranean Clover and Rygrass for uptake of 15N-labeled fertilizer

Grasses and legumes are often grown together for improving quality of forage and for better yield when soil N availability is limiting. One compatible mixture is Trifolium subterranium L., subterranean clover and Lolium multiflorum Lam, ryegrass.Experiments were conducted with plants grown in a glasshouse and plant growth chambers to determine the competitive ability of these plants for fertilizer N. Fertilizer N was enriched with ¹⁵N to measure the contribution of dinitrogen fixation and fertilizer N to the growth of clover. In pure stands, with increased fertilizer N, the legume took up similar quantities of mineral N as the grass to make up for the deficit due to less dinitrogen fixation but in mixed stands the grass by far outcompeted the legume. The growth of clover suffered due to lack of N both from less dinitrogen fixation and the inability to compete with the grass for mineral N. Increasing levels of fertilizer N reduced dinitrogen fixation by the clover. When growing with the clover the grass did not receive N from the clover. A laboratory experiment using ¹⁵N label on pure stands of the two species indicated that the grass had an inherent capability of absorbing almost twice the amount of mineral N as the legume under the same conditions even when root weight and volume was not larger for the grass. The results of this research provide insight into the often observed phenomenon that growth of clover is reduced when grown with grass in proportion to the amount of mineral N provided. This revised version was published online in June 2006 with corrections to the Cover Date.     

A trade-off between nitrogen uptake and use increases responsiveness to elevated CO2 in infrequently cut mixed C3 grasses

The aim of this study was to evaluate whether the responsiveness of mixed C3 grass species to elevated CO2 is related more to nitrogen uptake or to N-use efficiency. Nitrogen uptake and whole-plant N-use efficiency were investigated with two binary mixtures: Lolium perenne was mixed either with Festuca arundinacea or with Holcus lanatus. The swards were grown on sand with or without CO2 doubling, and subjected to two cutting frequencies. A C20 alcohol was used as a marker to determine species proportion in the total root mass of the mixtures. The mean residence time of N was calculated from that of 15N-labelled fertilizer. Lolium perenne took up significantly more N per unit root mass than its grass competitors, but its N-use efficiency was lower. Elevated CO2 significantly reduced the N uptake of the three grass species. A trade-off between N capture and use was found, as N-use efficiency and N-uptake rate were negatively correlated. A high N-use efficiency, and conversely low N uptake appeared to favour the responsiveness to elevated CO2 of the infrequently cut grasses.     

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.     

Soil physical properties and carbon/nitrogen relationships in stable aggregates under legume and grass fallow

Short-season fallow with legumes and/or grasses can restore the soil organic C and nitrogen (N) and improve soil structure. In this study, we accessed the effects of 2-season legume and grass fallow on structural properties and C/N relationships in aggregates of a sandy loam soil. Two legumes (Calopogonium mucunoides and Centrosema pubescens), and two grasses (Guinea grass (Panicum maximum) and goose grass (Eleusine indica) were used. Results showed that Calopogonium and Centrosema increased soil total porosity and reduced soil bulk densities, while goose grass increased bulk density and reduced total porosity of the soils at 0–15 and 15–30?cm depths. Guinea grass significantly increased the saturated hydraulic conductivity (50.4?cm?h^?1) and water holding capacity of the soils. Aggregates, 4.75 to 0.5?mm were greater in Guinea grass and least in goose grass fallowed soils. Calopogonium increased macro-aggregates at 0–15?cm soils by 48%, and mean weight diameter (MWD) by 44%. Organic carbon in 0.5–0.25?mm and <0.25?mm aggregate sizes was higher in Guinea grass soils. Generally, grasses had 4-fold increases of C:N contents in dry aggregates. In conclusion, short-season fallow with Guinea grass, Calopogonium and Centrosema, increased soil C and N and protected them from losses in stable aggregates.     

外来杂草加拿大一枝黄花对入侵地植物的化感效应

采用人工气候箱培养试验,初步研究了入侵杂草加拿大一枝黄花对本地植物的化感作用.测定了根系和根状茎的不同浓度浸提液对白三叶发芽率和幼苗生长的影响;不同播种密度下根系1:60提取浓度对白三叶种子萌发和幼苗生长的影响;根状茎1:60提取浓度对白三叶、红三叶、天蓝苜蓿、黑麦草、北美车前、鸡眼草、苇状羊茅、胜红蓟、马齿苋、碱蓬和刺苋的种子萌发和幼苗生长的影响.结果表明,加拿大一枝黄花根系和根状茎的化感抑制作用均随浓度的升高而增强,且根状茎提取物的抑制效应高于根系提取物的效应;在1:60低浓度根系浸提液作用下,不同密度的白三叶幼苗萌发率和幼根、幼芽长度差异不显著;1:60低浓度根状茎提取物对11个物种的种子萌发均有显著抑制作用,但对不同物种的抑制程度有差异,对禾本科植物的抑制作用大于非禾本科和豆科;1:60低浓度根状茎提取物对10个物种的幼根影响与对萌发率的影响相似,但在对幼芽的影响上,仅对鸡眼草、刺苋和苇状羊茅起抑制作用,而对其他植物无显著抑制(黑麦草、北美车前、胜红蓟、马齿苋和刺苋)或起促进作用(白三叶、红三叶、天蓝苜蓿).     

Estimation of nitrogen fixation and transfer from alfalfa to associated grasses in mixed swards under field conditions

Fixation and transfer of nitrogen (N) from alfalfa (Medicago sativa L.) to different grass species including timothy (Phleum pratense L.) and bromegrass (Bromus inermis Leyss) were studied under field conditions, using the¹⁵N dilution technique.The percentage of alfalfa N derived from fixation (%NF) increased throughout the growing seasons and ranged from 62 to 83%. Nitrogen transfer (NT) from alfalfa to associated grasses was evident and contributed 26,46 and 38% of the total annual N yield of associated grasses or represented absolute amounts of 5, 20 and 19 kg N ha^–1 during the first, second and third year, respectively. The gradual and consistent percentage of NT that occurred before first harvest indicated that this transfer is a result of a direct excretion of N compounds from alfalfa root systems. Decomposition of root and nodule debris seems to contribute to the NT from alfalfa to associated grasses in the later cuts. All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.Contribution No. 1159 from the Plant Research Centre