Effect of four plant species on soil 15N-access and herbage yield in temporary agricultural grasslands

Positive plant diversity-productivity relationships have been reported for experimental semi-natural grasslands (Cardinale et al. 2006; Hector et al. 1999; Tilman et al. 1996) as well as temporary agricultural grasslands (Frankow-Lindberg et al. 2009; Kirwan et al. 2007; Nyfeler et al. 2009; Picasso et al. 2008). Generally, these relationships are explained, on the one hand, by niche differentiation and facilitation (Hector et al. 2002; Tilman et al. 2002) and, on the other hand, by greater probability of including a highly productive plant species in high diversity plots (Huston 1997). Both explanations accept that diversity is significant because species differ in characteristics, such as root architecture, nutrient acquisition and water use efficiency, to name a few, resulting in composition and diversity being important for improved productivity and resource use (Naeem et al. 1994; Tilman et al. 2002). Plant diversity is generally low in temporary agricultural grasslands grown for ruminant fodder production. Grass in pure stands is common, but requires high nitrogen (N) inputs. In terms of N input, two-species grass-legume mixtures are more sustainable than grass in pure stands and consequently dominate low N input grasslands (Crews and Peoples 2004; Nyfeler et al. 2009; Nyfeler et al. 2011). In temperate grasslands, N is often the limiting factor for productivity (Whitehead 1995). Plant available soil N is generally concentrated in the upper soil layers, but may leach to deeper layers, especially in grasslands that include legumes (Scherer-Lorenzen et al. 2003) and under conditions with surplus precipitation (Thorup-Kristensen 2006). To improve soil N use efficiency in temporary grasslands, we propose the addition of deep-rooting plant species to a mixture of perennial ryegrass and white clover, which are the most widespread forage plant species in temporary grasslands in a temperate climate (Moore 2003). Perennial ryegrass and white clover possess relatively shallow root systems (Kutschera and Lichtenegger 1982; Kutschera and Lichtenegger 1992) with effective rooting depths of <0.7 m on a silt loamy site (Pollock and Mead 2008). Grassland species, such as lucerne and chicory, grow their tap-roots into deep soil layers and exploit soil nutrients and water in soil layers that the commonly grown shallow-rooting grassland species cannot reach (Braun et al. 2010; Skinner 2008). Chicory grown as a catch crop after barley reduced the inorganic soil N down to 2.5 m depth during the growing season, while perennial ryegrass affected the inorganic soil N only down to 1 m depth (Thorup-Kristensen 2006). Further, on a Wakanui silt loam in New Zealand chicory extracted water down to 1.9 m and lucerne down to 2.3 m soil depth, which resulted in greater herbage yields compared with a perennial ryegrass-white clover mixture, especially for dryland plots (Brown et al. 2005). There is little information on both the ability of deep- and shallow-rooting grassland species to access soil N from different vertical soil layers and the relation of soil N-access and herbage yield in temporary agricultural grasslands. Therefore, the objective of the present work was to test the hypotheses 1) that a mixture comprising both shallow- and deep-rooting plant species has greater herbage yields than a shallow-rooting binary mixture and pure stands, 2) that deep-rooting plant species (chicory and lucerne) are superior in accessing soil N from 1.2 m soil depth compared with shallow-rooting plant species, 3) that shallow-rooting plant species (perennial ryegrass and white clover) are superior in accessing soil N from 0.4 m soil depth compared with deep-rooting plant species, 4) that a mixture of deep- and shallow-rooting plant species has greater access to soil N from three soil layers compared with a shallow-rooting two-species mixture and that 5) the leguminous grassland plants, lucerne and white clover, have a strong impact on grassland N acquisition, because of their ability to derive N from the soil and the atmosphere

Nitrogen transfer from forage legumes to nine neighbouring plants in a multi-species grassland

Legumes play a crucial role in nitrogen supply to grass-legume mixtures for ruminant fodder. To quantify N transfer from legumes to neighbouring plants in multi-species grasslands we established a grass-legume-herb mixture on a loamy-sandy site in Denmark. White clover (Trifolium repens L.), red clover (Trifolium pratense L.) and lucerne (Medicago sativa L.) were leaf-labelled with 15N enriched urea during one growing season. N transfer to grasses (Lolium perenne L. and xfestulolium), white clover, red clover, lucerne, birdsfoot trefoil (Lotus corniculatus L.), chicory (Cichorium intybus L.), plantain (Plantago lanceolata L.), salad burnet (Sanguisorba minor L.)and caraway (Carum carvi L.) was assessed. Neighbouring plants contained greater amounts of N derived from white clover (4.8 gm-2) compared with red clover (2.2 gm-2) and lucerne (1.1 gm-2). Grasses having fibrous roots received greater amounts of N from legumes than dicotyledonous plants which generally have taproots. Slurry application mainly increased N transfer from legumes to grasses. During the growing season the three legumes transferred approximately 40 kg N ha-1 to neighbouring plants. Below-ground N transfer from legumes to neighbouring plants differed among nitrogen donors and nitrogen receivers and may depend on root characteristics and regrowth strategies of plant species in the multi-species grassland

Effect of hosts on competition among clones and evidence of differential selection between pathogenic and saprophytic phases in experimental populations of the wheat pathogen Phaeosphaeria nodorum

<p>Abstract</p> <p>Background</p> <p>Monoculture, multi-cropping and wider use of highly resistant cultivars have been proposed as mechanisms to explain the elevated rate of evolution of plant pathogens in agricultural ecosystems. We used a mark-release-recapture experiment with the wheat pathogen <it>Phaeosphaeria nodorum </it>to evaluate the impact of two of these mechanisms on the evolution of a pathogen population. Nine <it>P. nodorum </it>isolates marked with ten microsatellite markers and one minisatellite were released onto five replicated host populations to initiate epidemics of Stagonospora nodorum leaf blotch. The experiment was carried out over two consecutive host growing seasons and two pathogen collections were made during each season.</p> <p>Results</p> <p>A total of 637 pathogen isolates matching the marked inoculants were recovered from inoculated plots over two years. Genetic diversity in the host populations affected the evolution of the corresponding <it>P. nodorum </it>populations. In the cultivar mixture the relative frequencies of inoculants did not change over the course of the experiment and the pathogen exhibited a low variation in selection coefficients.</p> <p>Conclusions</p> <p>Our results support the hypothesis that increasing genetic heterogeneity in host populations may retard the rate of evolution in associated pathogen populations. Our experiment also provides indirect evidence of fitness costs associated with host specialization in <it>P. nodorum </it>as indicated by differential selection during the pathogenic and saprophytic phases.</p

Comparative study of common bean (Phaseolus vulgaris L.) landraces conserved ex situ in genebanks and in situ by farmers

Genetic diversity of populations stored ex situ or in situ can be altered due to the management practices they are subjected to. In this paper, we compare populations of two common bean (Phaseolus vulgaris L.) landraces grown on farms with material collected from the same farms and now kept in two ex situ collections (CIAT and REGEN) with the purpose to monitor any changes that have occurred due to ex situ conservation. The diversity was measured using seven bean microsatellite markers. Further phenotypic and developmental traits were registered in a field experiment. Compared with the in situ populations, the ex situ ones had a lower level of gene diversity and we suggest that this is due to the regeneration process. Most of the phenotypic traits did not differ significantly between ex situ and in situ populations, although for yield and 100-seed weight, the CIAT material showed significant lower values. We assume that these populations have gone through an adaptational change. Overall, the conservation ex situ has been successful in maintaining the majority of the adaptations found in the landraces studied, however, the probable loss of genetic diversity that we have observed, suggest that protocols for the regeneration process must be carefully worked out if the majority of alleles are to be preserved for the future. This study also highlights the complementarity of ex situ and in situ conservation methods in order to preserve landrace adaptations and to capture new, useful diversity generated in in situ populations

Molecular and phenotypic diversity of common bean landraces from Nicaragua

The knowledge and understanding of the genetic structure of bean (Phaseolus vulgaris L.) landraces is important for the implementation of measures addressed to their management and conservation. The purpose of this paper was to study the pattern of genetic variation in nine red-seeded landraces currently grown by farmers with molecular and phenotypic markers. Twelve individuals per landrace were genotyped with seven bean microsatellite markers. Fourteen phenotypic traits were additionally measured in a field study in two localities. An important finding of this study was the complementary information obtained with both kinds of markers. Most of the variation at the molecular level was explained by differences within or among landraces but not among agroecological zones, while at the phenotypic level most of the variation was attributed to differences among agroecological zones. This suggests that molecular differentiation of landraces [coancestry coefficient (F ST) = 0.34] was due to founder effect while phenotypic differentiation was due to the effect of adaptation. Within landraces, an average of 5.7 alleles per locus was identified, with a range from 2 to 13 alleles depending on the individual microsatellite. The average gene diversity within landraces and total gene diversity was 0.35 and 0.51, respectively. Implications of the findings in planning future collections of genetic resources of common bean as well as the effect of sites on uncovering adaptive traits are also discussed

Effect of deep-rooted plant species on 15Nitrogen uptake and herbage yield in temporary agricultural grasslands

Aims: Increase of plant diversity has been suggested to enhance grassland productivity and resource use efficiency. Most studies on agricultural grasslands have focused on functional diversity of mixtures comprising legumes and non-legumes, but there is little knowledge of plant nutrient acquisition from deep- and shallow-rooted grassland plant species. To investigate whether deep-rooted (chicory: Cichorium intybus L.; Lucerne: Medicago sativa L.) and shallow-rooted (perennial ryegrass: Lolium perenne L.; white clover: Trifolium repens L.) grassland plant species differ in herbage yield and depth dependent soil N-access, we investigated in the field if 1) a mixture comprising shallow- and deep-rooted grassland plant species has greater herbage yields than a shallow-rooted binary mixture and pure stands, 2) deep-rooted grassland plant species (chicory and lucerne) are superior in terms of accessing soil N from 1.2 m soil depth compared with shallow-rooted plant species, 3) shallow-rooted grassland plant species (perennial ryegrass and white clover) are superior in terms of accessing soil N from 0.4 m soil depth compared with deep-rooted plant species and 4) a mixture of deep- and shallow-rooted plant species has access to greater amounts of soil N compared with a shallow-rooted binary mixture. Method: A 15N tracer methodology with 15N enriched ammonium-sulphate placed at three different soil depths (0.4, 0.8 and 1.2 m) was applied to determine the depth dependent soil N-access measured as plant 15N-uptake in pure stands, two-species and four-species grassland plant communities. Important findings: The study showed that herbage yield of the four-species mixture including deep- and shallow rooted grassland plant species was generally greater than both the pure stands and the two-species mixture, besides for lucerne in pure stand. This positive plant diversity effect in the four-species mixture on above-ground herbage yield could not be explained by complementary soil 15N uptake from 0.4, 0.8 and 1.2 m soil depths, even though chicory indicated deep soil 15N uptake. Perennial ryegrass demonstrated relatively deep soil 15N uptake when grown in pure stand, but showed increasing shallow 15N uptake from grown in a two-species to a four-species mixture. Total soil 15N uptake from three soil depths of a mixture 51 comprising two deep-rooted and two shallow-rooted plant species was not greater compared with a shallow-rooted two-species mixture. 15Nitrogen uptake from 1.2 m may have been too small to determine any differences. Legumes stimulated perennial ryegrass in 15N uptake from shallow soil layers, which indicated greater total 15N uptake of mixtures compared with pure stands