Showing papers by "Kerstin Huss-Danell published in 2002"

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities.

Abstract: Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and δ15N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for δ15N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and δ15N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower δ15N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.

Interspecific and spatial differences in nitrogen uptake in monocultures and two‐species mixtures in north European grasslands

Abstract: Summary 1. To study the potential for complementarity in nitrogen acquisition from different soil depths, we injected an isotope tracer (15NH4Cl) at 5 and 20 cm depths in plant communities containing Achillea millefolium L. and Festuca ovina L. or Phleum pratense L. and Trifolium pratense L. in monocultures and two-species mixtures. 2. In monoculture, Festuca and Phleum took up tracer at 5 and 20 cm depths. In contrast, Achillea and Trifolium monocultures acquired the tracer mainly from 5 cm depth. In two-species mixtures, all four species took up tracer at 5 cm depth. 3. Achillea N acquisition from 20 cm depth increased in mixture with Festuca in comparison to that in monoculture; Festuca N acquisition from 20 cm depth decreased, although not significantly. Trifolium N acquisition remained unchanged when grown in mixture with Phleum. Phleum behaved like Festuca: its N acquisition from 20 cm depth in mixture was reduced in comparison to monoculture. 4. Our data on Festuca and Achillea support spatial partitioning in resource acquisition. This was not evident in Phleum and Trifolium mixture, potentially because Trifolium relied on N2 fixation as N source. 5. These results demonstrate spatial variation among plant species and plant communities in their N acquisition in the field.

Phosphorus modifies the effects of nitrogen on nodulation in split‐root systems of Hippophaë rhamnoides

Abstract: Summary • The effects of N (ammonium nitrate), P (phosphate) and their interactions on nodulation were studied in the intercellularly infected actinorhizal plant Hippophae rhamnoides. • A split-root design, with pots receiving different concentrations of N and P, was used to determine whether the effects of N and P are local or systemic, if they are specific to nodulation or general, and whether P could counteract N inhibition. H. rhamnoides plants were grown for 6–10 wk after inoculation with Frankia. • Inhibition of nodulation by N was systemic for both nodule number and nodule biomass in H. rhamnoides. • By contrast, high P had a systemic stimulation on nodule number and biomass and P prevented systemic, but not local, N inhibition. Stimulation by P was specific to nodulation and not simply mediated via plant growth. Whether N and P alter not only nodulation but also N2-fixation in the nodules requires further investigation.

Arabinogalactan proteins are expressed at the symbiotic interface in root nodules of Alnus spp.

Abstract: Summary • We have characterized the origin and distribution of arabinogalactan proteins (AGPs) at the symbiotic interface of dinitrogen (N 2 )-fixing root nodules of Alnus spp. The interface between the host plant cell and the microsymbiont is an important zone for signaling and growth regulation during nodulation. Arabinogalactan proteins are glycoproteins that have adhesive properties, and, potentially, participate in cell wall assembly, direction of growth, and signaling cascades. These glycoproteins are expressed in several symbiotic systems in an infection-specific pattern, but their occurrence has not been examined in actinorhizal nodules. • To characterize AGP epitopes in Alnus root nodules, we have used immunogold localization with anti-AGP antibodies, correlated with other techniques. • Arabinogalactan proteins are abundant in the nodule-infected tissue. One AGP epitope (JIM4) is localized in pectin-rich cell walls, while another (JIM13) is found at the membrane-wall border along the symbiotic interface at the early infection stage, and in the host cytoplasm/vacuoles in mature, infected cells. • It is likely that AGPs play a significant role in Alnus root nodules, especially in early nodulation stages.