D. S. Veresoglou

Depth of Root Activity of Coexisting Grassland Species in Relation to N and P Additions, Measured Using Nonradioactive Tracers

1 Root activity of coexisting plant species was assessed by injecting lithium, strontium and caesium at depths of 5 and 15 cm in the soil of an upland grassland to which a factorial combination of N and P additions had been applied. 2 For.each species and each tracer the ratio of the above-ground concentrations after injection at depths of 5 and 15 cm was calculated and the species ranked for each tracer in order of root activity with soil depth; Sr and Cs gave similar results, which in some cases were quite different from those with Li. 3 When Cs and Sr were injected at the depth of 5 cm, a positive correlation was evident between Sr and Ca concentrations for the various species but not for Cs and K. 4 Although root activity was higher at a depth of 5cm for all species, the relative activity with depth differed between species; Agrostis capillaris, the most abundant species, and Thymus sibthorpii had deeper root activity than Carex caryophyllea, the second most abundant species, and Teucrium chamaedrys. 5 When N and/or P were added Agrostis capillaris and Carex caryophyllea tended to become shallower rooted and their above-ground biomass increased; for these species a positive correlation was evident between relative abundance and ratios of concentrations, indicating that the most competitive species tended to become shallower rooted when the limiting nutrients were added. 6 Ranking of species in root activity with depth differed among the various nutrient treatments. Legumes tended to have the deepest root activity when N only was applied and the shallowest root activity when P was applied. 7 Although the ranking of species was influenced by the availability of nutrients, the hypothesis that the less competitive species are forced to deeper rooting was not clearly supported; the legume species Trifolium heldreichianum only tended to become deeper rooted when N was added, i.e. in the treatment in which it was less competitive.

Plant Species Abundance and Tissue Concentrations of Limiting Nutrients in Low-Nutrient Grasslands: A Test of Competition Theory

1 A 3-year study tested the hypothesis that proportions of plant species are negatively related to species concentrations of limiting nutrients using a moderately acid lowland grassland site in Northern Greece, where growth was limited by soil moisture, N and P. 2 Species proportions were assessed both by a cumulative measure (above-ground biomass duration for the whole growth period) and by yield at the peak of aboveground biomass of the community. Weighted N, P and K concentrations of species for the whole growth period were calculated by dividing above-ground N or P or K content duration by above-ground biomass duration. 3 Measurements during one growth period (seven samplings from mid March to mid June in 1991) showed that biomass was higher in roots than in shoots for most species examined. For total grasses and Rumex acetosella total N and P were higher in roots than in the shoot for the whole growth period. The dicot species Cichorium intybus, Hypochoeris radicata and Plantago lanceolata contained more N and P in shoots than in roots after mid April. 4 Weighted N, P and K concentrations of shoots were positively related to their respective concentrations of whole plants. 5 Addition of either N or P increased community productivity and changed species composition, especially in years when soil moisture was adequate. 6 Proportions of species were negatively related to weighted N concentrations of species. 7 Species with higher than average N or P concentrations showed the greatest relative increases when soil was enriched with N or P, respectively. 8 These results seem to be consistent with the regulation of species abundance by competition-driven nutrient depletion.

Vegetation in contrasting soil water sites of upland herbaceous grasslands and N:P ratios as indicators of nutrient limitation

Effects of differences in long-term water supply were examined on soil characteristics, primary production and species composition in a wet and a dry site of an upland herbaceous grassland. Also the responses of species to N and P enrichments were examined. N and P concentrations of non-legume species were positively related, indicating that biomass N:P ratios seem to be mainly determined by N:P supply ratios. Forbs had generally higher concentrations than graminoids. Intermittent water inundation of soil in the wet site resulted in greater soil N and P availability. The greater productivity of this site promoted the growth of forbs. A fertilizer experiment showed that biomass was limited by N only in the wet site, but by both nutrients in the dry one. The species with the higher N and P concentrations were favored more after N and P enrichment, respectively; however, species enhancement was not related to N:P ratios of species. This indicates that N and P concentrations of species, rather than N:P ratios of species, are better predictors of species responses to N and P enrichment. N:P ratios of whole communities were 8.73 for the wet and 11.36 for the dry site. These values in comparison with the responses of plant communities to N and P fertilization show that thresholds of N:P ratios indicative of N or P limitation are much lower than those found for European wetlands.

Shoot Sr concentrations in relation to shoot Ca concentrations and to soil properties

This work was aimed to investigate whether shoot Sr concentrations of plant species are related to respective Ca concentrations and to soil properties and to compare the Sr-Ca observed ratios (OR), defined as the quotient of the ratios Sr/Ca in shoots and in the soil solution or in the extractable form, among species and soils. Ten pasture plant species were grown in pots (1-L volume) filled with eight soils differing in the various physicochemical characteristics. Each pot received 50 mg Sr except those of the soil with the highest cation exchange capacity (C.E.C.) that received 100 mg Sr per pot. For each soil, shoot Sr concentrations of species were linearly and positively related with the respective Ca concentrations. C.E.C, organic matter content and Ca in the soil solution or in the extractable form were the only soil properties that were related, all negatively, with shoot Sr concentrations. The ratio of extractable Sr and Ca was positively and linearly related with the ratio of Sr and Ca. in the soil solution. OR was affected by both species and soils. Most of OR values of all species in all soils ranged between 0.8 and 1.5, except for the grass Agrostis capillaris which had the highest values for most of soils. This indicates that Agrostis capillaris compared to other species, takes up proportionally more Sr than Ca.

Medium-term fertilization of grassland plant communities masks plant species-linked effects on soil microbial community structure

According to the singular hypothesis of plant diversity, different plant species are expected to make unique contributions to ecosystem functioning. Hence, individual species would support distinct microbial communities. It was hypothesized that microbial community dynamics in the respective rhizospheres of, two floristically divergent species, Agrostis capillaris and Prunella vulgaris that were dominant in a temperate, upland grassland in northern Greece, would support distinct microbial communities, in agreement to the singular hypothesis. Phospholipid lipid fatty acid (PLFA) profiles of the rhizosphere soil microbial community were obtained from the grassland which had been subjected to factorial nitrogen (N) and phosphorus (P) fertilization over five plant growth seasons. The soil cores analyzed were centered on stands of the two co-occurring target plant species, sampled from five blocks in all four factorial N and P fertilization combinations. Distinct PLFA clustering patterns following principle component analysis of PLFA concentrations revealed that, in the absence of P fertilization, soils under the two plant species supported divergent microbial communities. In the P fertilized plots, however, no such distinction could be observed. Results reveal that nutrient fertilization may mask the ability of plant species to shape their own rhizosphere microbial community.

Caesium and strontium uptake by two pasture plant species grown in organic and inorganic soils

Caesium (300 mg per pot) or strontium (190 mg per pot) were applied to pots, containing either 1200 g of inorganic or 560 g of organic soil 2, 1 and 0 months before sowing with either Lolium perenne or Trifolium repens. For inorganic soil only and the addition of Cs and Sr at the time of sowing, three levels of K (0, 83 and 166 mg per pot) or Ca (0, 100 and 200 mg per pot) were applied. Changes of extractable Cs were monitored with time in unsown pots. Extractable Cs decreased with time, especially in the inorganic soil. In shoots, harvested 5 months after sowing Cs and Sr concentrations were higher in Trifolium than in Lolium with a trend for lower concentrations when Cs or Sr was applied earlier. Concentrations of Cs were higher in organic than in inorganic soil. The reverse was evident for Sr concentrations. Addition of K tended to decrease and that of Ca to increase shoot Cs and Sr concentrations, respectively.

Differentiation between responses of primary productivity and phosphorus exploitation to species richness

Several studies have shown that ecosystem functioning increases with increasing species richness. Most of these studies examined the effects of species richness on primary productivity. The underlying mechanism that explains this pattern is usually the selection effect. The higher the diversity in plant communities the higher the chance in including a very productive species that dominates the community, or a legume species that brings N into the soil. Less attention has been given so far to the effects of species richness on phosphorus exploitation. The aim of this work was to investigate the effect of species richness on aboveground primary productivity and P accumulation in a plant diversity experiment. For this reason, 14 grassland plant species were grown in containers as monocultures and in mixtures of 2-, 3-, 4-, 8-, 11- and 14-species combinations. Results show that the aboveground phytomass and total P increased with increasing species richness. Complementarity effects, probably through partitioning of resources, were most apparent in the highest levels of species richness, and were observed to be greater for total P in comparison to phytomass. Selection effects generally were greater for phytomass than for total P; they were significantly positive at the 2- to 8-species combinations but close to 0 or negative in the highest levels of species richness. The increases in phytomass and total P at the highest levels of species richness appeared to be caused by the increased performance of intermediate-productive species.

Transfer factors for Sr as influenced by species Ca uptake and soil Ca availability

Strontium (Sr) and calcium (Ca) concentrations were studied in different plant species grown in five soil treatments. For either shoots or roots, a positive linear relationship was found between Sr and Ca concentrations in different plant species grown in the same soil treatment. Strontium and calcium concentrations of different species were related to the soil selectivity coefficient for Sr and Ca, defined as the ratio of CH3COONH4-extractable Sr and Ca to the ratio of Sr and Ca in the soil solution. For the species used in all soils, transfer factors (TF) for Sr, defined as the ratios of the Sr amount per g of dry plant material and the Sr amount per g of dry soil, were negatively correlated with extractable Ca of the soil. Transfer factors for Sr varied greatly among species or between roots and shoots. This variation of transfer factor was reduced when transfer factor values were divided by the shoot or root Ca concentration of each species. The proposed index TF for Sr per Ca concentration could be used to compare various soils according to their ability to supply plants with Sr when different plant species are grown in these soils.

Leaf Physiological Traits and their Importance for Species Success in a Mediterranean Grassland

We related leaf physiological traits of four grassland species (Poa pratensis, Lolium perenne, Festuca valida, and Taraxacum officinale), dominant in a Mediterranean grassland, to their origin and success at community level. From early May to mid-June 1999, four leaf samplings were done. Species originating from poor environments (P. pratensis, F. valida) had low carbon isotope discrimination (Δ), specific leaf area (SLA), leaf water and mineral contents, and net photosynthetic rate on mass basis (Pmass) but high chlorophyll content. The reverse traits were evident for the fast-growing species (L. perenne, T. officinale). Under the resource-limiting conditions (soil nitrogen and water) of the Mediterranean grassland, the physiological traits of P. pratensis and F. valida showed to be more adapted to these conditions leading to high species abundance and dominance.

Decomposition and nutrient release from wheat and fababean straw under field conditions

Abstract The rate of biomass and organic matter loss, and the release of N, P, K, Mg and Ca in decomposing wheat and fababean straw, buried in the soil, were studied for 30 months. The effect of the container type (boxes of 0.8 × 0.8 cm and nylon bags with 0.55-mm diameter holes) in which straw was placed for burial in the soil was also studied. The rate of biomass loss was constant and was not affected by either the straw species or the container type. The proportion of organic matter in the undecomposed biomass was steadily reduced with time and affected by the container type. The rate of loss of absolute quantities of organic matter was linearly reduced with time and for both container types, was higher than the rate of biomass loss in the first 24 months and lower thereafter. Both N and P concentrations in the decomposing material increased with time. Total N and P appeared to be reduced at the end of the experiments but not significantly. The C/N ratio of biomass was reduced with time and affected by the container type, but at the end of the experiments it was still higher than that of the soil. The rate of loss of K, Mg and Ca differed between the two species. For K and Mg, it was more rapid in fababean than in wheat straw at the beginning of decomposition. For Ca, the rate of loss was significant only in fababean straw and was constant.