Significance of Legumes for the Distribution of Plant Species in Grassland Ecosystems at Different Altitudes in the Alps

The composition of plant species in grassland communities is determined in part by negative and positive interactions among neighbouring plants; such interactions could be dispersal or - in the case of legumes - symbiotic N2 fixation or resource acquisition. We hypothesised that small scale distribution of plant species may be in part affected by (i) the plant's narrow dispersal in the close neighbourhood, (ii) by the N input into the soil through symbiotic N2 fixation by legumes, and (iii) by small scale heterogeneity of N availability in the soil. To address these hypotheses, in four species-rich semi-natural and natural grasslands along an altitudinal gradient from 900 to 2100ma.s.l., the association of species that grew within a radius of 5cm of selected plant species (central plant, CP) was studied. The distribution of the plant species within the plant community was not random and the species composition that grew within a radius of 5cm of a CP varied with altitude. The strength of the association between a CP and its neighbouring species decreased with increasing altitude. The 5cm radius was most dominated by individuals of the same species as the CP, suggesting that narrow dispersal (either through seeds or through vegetative propagates) may be most important for the species composition within the sward. δ15N values of non leguminous species growing near legumes compared to non leguminous species that grew far from legumes indicate that N from symbiotic N2 fixation may be another factor influencing the association of plant species. Effects of small scale heterogeneity in apparent N availability in the soil were detected to be the least important for the species compositio

Soil mineral nitrogen availability was unaffected by elevated atmospheric pCO2 in a four year old field experiment (Swiss FACE)

The effect of elevated (60 Pa) atmospheric carbon dioxide partial pressure (pCO2) and N fertilisation on the availability of mineral N and on N transformation in the soil of a Lolium perenne L. monoculture was investigated in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment. The apparent availability of nitrate and ammonium for plants was estimated during a representative, vegetative re-growth period at weekly intervals from the sorption of the minerals to mixed-bed ion-exchange resin bags at a soil depth of 5 cm. N mineralisation was measured using sequential coring and in situ exposure of soil cores in the top 10 cm of the soil before and after the first cut in spring 1997. High amounts of mineral N were bound to the ion exchange resin during the first week of re-growth. This was probably the combined result of the fertiliser application, the weak demand for N by the newly cut sward and presumably high rates of root decay and exudation after cutting the sward. During the first 2 weeks after the application of fertiliser N at the first cut, there was a dramatic reduction in available N; N remained low during the subsequent weeks of re-growth in all treatments. Overall, nitrate was the predominant form of mineral N that bound to the resin for the duration of the experiment. Apparently, there was always more nitrate than ammonium available to the plants in the high N fertilisation treatment for the whole re-growth period. Apparent N availability was affected significantly by elevated pCO2 only in the first week after the cut; under high N fertilisation, elevated pCO2 increased the amount of mineral N that was apparently available to the plants. Elevated pCO2 did not affect apparent net transformation of N, loss of N or uptake of N by plants. The present data are consistent with earlier results and suggest that the amount of N available to plants from soil resources does not generally increase under elevated atmospheric pCO2. Thus, a possible limiting effect of N on primary production could become more stringent under elevated atmospheric pCO2 as the demand of the plant for N increase

Increased abundance of MTD1 and MTD2 mRNAs in nodules of decapitated Medicago truncatula.

To gain insight into the molecular processes occurring in root nodule metabolism after stress, we used a mRNA differential display (DDRT-PCR) approach to identify cDNAs corresponding to genes whose expression is enhanced in nodules of decapitated Medicago truncatula plants. Two full-length cDNAs of plant origin were isolated (MTD1 and MTD2). Sequence analysis revealed that MTD1 is identical to an EST clone (accession number AW559774) expressed in roots of M. truncatula upon infection with Phytophthora medicaginis, while MTD2 is highly homologous to an Arabidopsis thaliana gene (accession number AL133292) coding for a RNA binding-like protein. The two mRNAs started to accumulate in root nodules at 4 h after plant decapitation and reached even higher transcript levels at 24 h from the imposition of the treatment. MTD1 and MTD2 mRNAs were mainly induced in nodules, with very little induction in roots. The abundance of the two transcripts did not change in response to other perturbations known to decrease nitrogenase activity, such as nitrate and Ar/O2 treatments. Our results suggest that MTD1 and MTD2 represent transcripts that accumulate locally in nodules and may be involved in changes in nodule metabolism in response to decapitation

Evidence that P deficiency induces N feedback regulation of symbiotic N2 fixation in white clover (Trifolium repens L.)

Trifolium repens L. was grown to test the following hypotheses: when P is deficient (i) N2 fixation decreases as a result of the plant's adaptation to the low N demand, regulated by an N feedback mechanism, and (ii) the decrease in the photosynthetic capacity of the leaves does not limit N2 fixation. Severe P deficiency prevented nodulation or stopped nodule growth when the P deficiency occurred after the plants had formed nodules. At low P, the proportion of whole‐plant‐N derived from symbiotic N2 fixation decreased, whereas specific N2 fixation increased and compensated partially for poor nodulation. Leaf photosynthesis was reduced under P deficiency due to low Vc,max and Jmax. Poor growth or poor performance of the nodules was not due to C limitation, because (i) the improved photosynthetic performance at elevated pCO2 had no effect on the growth and functioning of the nodules, (ii) starch accumulated in the leaves, particularly under elevated pCO2, and (iii) the concentration of WSC in the nodules was highest under P deficiency. Under severe P deficiency, the concentrations of whole‐plant‐N and leaf‐N were the highest, indicating that the assimilation of N exceeded the amount of N required by the plant for growth. This was clearly demonstrated by a strong increase in asparagine concentrations in the roots and nodules under low P supply. This indicates that nodulation and the proportion of N derived from symbiotic N2 fixation are down‐regulated by an N feedback mechanis