Andreas Lüscher

Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes

Abstract The extent of the response of plant growth to atmospheric CO2 enrichment depends on the availability of resources other than CO2. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO2 response of plants. The effect of elevated CO2 (60 Pa) partial pressure (pCO2) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO2, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO2, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO2 than under ambient pCO2 in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO2 was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO2, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N2-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO2. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N2-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO2.

Potential of legume-based grassland-livestock systems in Europe: a review.

European grassland-based livestock production systems face the challenge of producing more meat and milk to meet increasing world demands and to achieve this using fewer resources. Legumes offer great potential for achieving these objectives. They have numerous features that can act together at different stages in the soil–plant–animal–atmosphere system, and these are most effective in mixed swards with a legume proportion of 30–50%. The resulting benefits include reduced dependence on fossil energy and industrial N-fertilizer, lower quantities of harmful emissions to the environment (greenhouse gases and nitrate), lower production costs, higher productivity and increased protein self-sufficiency. Some legume species offer opportunities for improving animal health with less medication, due to the presence of bioactive secondary metabolites. In addition, legumes may offer an adaptation option to rising atmospheric CO2 concentrations and climate change. Legumes generate these benefits at the level of the managed land-area unit and also at the level of the final product unit. However, legumes suffer from some limitations, and suggestions are made for future research to exploit more fully the opportunities that legumes can offer. In conclusion, the development of legume-based grassland–livestock systems undoubtedly constitutes one of the pillars for more sustainable and competitive ruminant production systems, and it can be expected that forage legumes will become more important in the future.

Long-term responsiveness to free air CO2 enrichment of functional types, species and genotypes of plants from fertile permanent grassland

Abstract To test inter- and intraspecific variability in the responsiveness to elevated CO2, 9–14 different genotypes of each of 12 perennial species from fertile permanent grassland were grown in Lolium perenne swards under ambient (35 Pa) and elevated (60 Pa) atmospheric partial pressure of CO2 (pCO2) for 3 years in a free air carbon dioxide enrichment (FACE) experiment. The plant species were grouped according to their functional types: grasses (L. perenne, L. multiflorum, Arrhenatherum elatius, Dactylis glomerata, Festuca pratensis, Holcus lanatus, Trisetum flavescens), non-legume dicots (Rumex obtusifolius, R. acetosa, Ranunculus friesianus), and legumes (Trifolium repens, T. pratense). Yield (above a cutting height of 4.5u2009cm) was measured three times per year. The results were as follow. (1) There were highly significant differences in the responsiveness to elevated pCO2 between the three functional types; legumes showed the strongest and grasses the weakest yield increase at elevated pCO2. (2) There were differences in the temporal development of responsiveness to elevated pCO2 among the functional types. The responsiveness of the legumes declined from the first to the second year, while the responsiveness of the non-legume dicots increased over the 3 years. During the growing season, the grasses and the non-legume dicots showed the strongest response to elevated pCO2 during reproductive growth in the spring. (3) There were no significant genotypic differences in responsiveness to elevated pCO2. Our results suggest that, due to interspecific differences in the responsiveness to elevated pCO2, the species proportion within fertile temperate grassland may change if the increase in pCO2 continues. Due to the temporal differences in the responsiveness to elevated pCO2 among species, complex effects of elevated pCO2 on competitive interactions in mixed swards must be expected. The existence of genotypic variability in the responsiveness to elevated pCO2, on which selection could act, was not found under our experimental conditions.

Potential of legume-based grassland-livestock systems in Europe.

European grassland-based livestock production systems face the challenge of producing more meat and milk to meet increasing world demands and to achieve this using fewer resources. Legumes offer great potential for achieving these objectives. They have numerous features that can act together at different stages in the soil–plant–animal–atmosphere system, and these are most effective in mixed swards with a legume proportion of 30–50%. The resulting benefits include reduced dependence on fossil energy and industrial N-fertilizer, lower quantities of harmful emissions to the environment (greenhouse gases and nitrate), lower production costs, higher productivity and increased protein self-sufficiency. Some legume species offer opportunities for improving animal health with less medication, due to the presence of bioactive secondary metabolites. In addition, legumes may offer an adaptation option to rising atmospheric CO2 concentrations and climate change. Legumes generate these benefits at the level of the managed land-area unit and also at the level of the final product unit. However, legumes suffer from some limitations, and suggestions are made for future research to exploit more fully the opportunities that legumes can offer. In conclusion, the development of legume-based grassland–livestock systems undoubtedly constitutes one of the pillars for more sustainable and competitive ruminant production systems, and it can be expected that forage legumes will become more important in the future.

Due to symbiotic N2 fixation, five years of elevated atmospheric pCO2 had no effect on the N concentration of plant litter in fertile, mixed grassland

Experimental findings indicate that, in terrestrial ecosystems, nitrogen cycling changes under elevated partial pressure of atmospheric CO2 (pCO2). It was suggested that the concentration of N in plant litter as well as the amount of litter are responsible for these changes. However, for grassland ecosystems, there have been no relevant data available to support this hypothesis. Data from five years of the Swiss FACE experiment show that, under fertile soil conditions in a binary plant community consisting of Lolium perenne L. and Trifolium repens L., the concentration of litter N does not change under elevated atmospheric pCO2; this applies to harvest losses, stubble, stolons and roots as the sources of litter. This is in strong contrast to the CO2 response of L. perenne swards without associated legumes; in this case the above-ground concentration of biomass N decreased substantially. Increased symbiotic N2 fixation in T. repens nodules and a greater proportion of the N-rich T. repens in the community are regarded as the main mechanisms that buffer the increased C introduction into the ecosystem under elevated atmospheric pCO2. Our data also suggest that elevated atmospheric pCO2 results in greater amounts of litter, mainly due to increased root biomass production. This study indicates that, in a fertile grassland ecosystem with legumes, the concentration of N in plant litter is not affected by elevated atmospheric pCO2 and, thus, cannot explain CO2-induced changes in the cycling of N.

Effects of elevated atmospheric CO2 and nitrogen fertilisation on yield of Trifolium repens and Lolium perenne

Abstract Trifolium repens L. and Lolium perenne L. were grown in monocultures and bi-species mixture in a F ree A ir C arbon Dioxide E nrichment (FACE) experiment at elevated (60 Pa) and ambient (35 Pa) CO2 partial pressure (pCO2) for two years. The effects of nitrogen fertilisation (10 and 42 g N m−2 a−1 in 1993; 14 and 56 g N m−2 a−1 in 1994) on the growth response to pCO2 were investigated in frequently defoliated (7 cuts in 1993; 8 cuts in 1994) swards. The yield of Trifolium in monocultures increased by 22% when grown at elevated pCO2. In contrast, the yield of Lolium monocultures was not affected (2%) by elevated pCO2, whereas Lolium increased its root mass considerably. The consequence of these interspecific differences in the CO2 response was an increase in the proportion of Trifolium in the mixed swards from 39% at ambient to 50% at elevated pCO2. However, the proportion of the species was more strongly affected by N fertilisation than by elevated pCO2. Based on these results, we conclude that the species proportion in managed grassland may change as the CO2 concentration increases. However, an adapted management may, at least partially, counteract such CO2-induced changes in the proportion of the species.

Interspecific and intraspecific variability in the response of grasses and legumes to free air CO2 enrichment

Au cours dune experience FACE (Free Air Carbon dioxide Enrichment), nous avons cultive de neuf a quatorze genotypes de sept graminees et deux legumineuses provenant dune prairie permanente, en les soumettant deux niveaux de concentrations en CO 2 atmospherique dans les vides dune pelouse etablie a Lolium perenne. Nous avons determine la biomasse cumulee de plantes individuelles pour deux saisons de croissance. La premiere annee, lelevation du CO 2 a accru la production de biomasse chez toutes les especes. Chez Trifolium repens et T. pratense, laugmentation de biomasse due au CO 2 (159 %) est bien plus elevee que chez les graminees (27 %). La deuxieme annees la reponse a lelevation du CO 2 a ete plus faible chez les graminees (2 %, ns) et les legumineuses (73 %). Neanmoins, les differences interspecifiques restent significatives. Des differences interspecifiques dans la reponse a un CO 2 eleve apparaissent entre les 2 groupes fonctionnels (graminees et legumineuses), mais a linterieur de ces groupes, aucune difference interspecifique significative na ete trouvee. Contrairement a la variabilite interspecifique, aucune variabilite intraspecifique significative na ete detectee en reponse au CO 2 . Nos resultats suggerent que des differences interspecifiques significatives apparaissent en reponse au CO 2 . Mais aucune difference intraspecifique na ete detectee. Il semble donc peu probable que ladaptation evolutive de la reponse des especes a une elevation du CO 2 nivelle les differences interspecifiques de cette reponse.

Large Variability of Proanthocyanidin Content and Composition in Sainfoin (Onobrychis viciifolia).

Proanthocyanidins (PAs) in sainfoin (Onobrychis viciifolia Scop.) are of interest to ameliorate the sustainability of livestock production. However, sainfoin forage yield and PA concentrations, as well as their composition, require optimization. Individual plants of 27 sainfoin accessions from four continents were analyzed with LC-ESI-QqQ-MS/MS for PA concentrations and simple phenolic compounds. Large variability existed in PA concentrations (23.0–47.5 mg g–1 leaf dry matter (DM)), share of prodelphinidins (79–96%), and mean degree of polymerization (11–14) among, but also within, accessions. PAs were mainly located in leaves (26.8 mg g–1 DM), whereas stems had less PAs (7.8 mg g–1 DM). Overall, high-yielding plants had lower PA leaf concentrations (R2 = 0.16, P < 0.001) and fewer leaves (R2 = 0.66, P < 0.001). However, the results show that these two trade-offs between yield and bioactive PAs can be overcome.

Effects of elevated partial pressure of carbon dioxide and season of the year on forage quality and cyanide concentration of Trifolium repens L. from a FACE experiment

Abstract Differently managed (cutting frequency and N fertilization) Trifolium repens monocultures were grown at 60 Pa and 35 Pa of pCO2 (partial pressure of CO2) in a Free Air Carbon dioxide Enrichment (FACE) array. The concentrations of cyanide, digestible organic matter, crude protein and net energy for lactation were measured at different harvests throughout the growing season. The average cyanide concentrations differed significantly in the years and the seasons within the year; however, the concentrations were not affected by CO2. Digestible organic matter, crude protein and net energy for lactation differed significantly with the seasons of the year and cutting frequencies. While digestible organic matter and net energy for lactation were not affected by elevated pCO2, the concentration of crude protein decreased from 288 g kg−1 at ambient to 251 g kg−1 at elevated pCO2. Since the crude protein concentration in herbage from Trifolium monocultures was very high even at elevated CO2, it is suggested that this decrease in crude protein concentration does not negatively affect forage quality. We conclude that, in Trifolium herbage, the seasons of the year and management practices are more decisive for forage quality than changes in pCO2. We shall discuss how forage quality and cyanide intake by ruminants may, however, be affected by CO2-induced shifts in the proportion of species in mixed plant communities.

Benefits of sward diversity for agricultural grasslands

Abstract A pan-European experiment carried out at 28 sites across Europe showed strong benefits of sward diversity in agricultural grasslands. We systematically varied the relative abundance of four agronomic plant species (sown species evenness), and found that 4-species mixtures yielded more forage than could be expected on the basis of the monoculture yields. Mixtures generally yielded more than even the best performing monoculture (transgressive overyielding). Mixtures strongly reduced the incidence of unsown species in the sward. These diversity effects were consistent over the wide range of environmental conditions and persisted over three harvest years and in highly fertilized conditions. These results indicate a strong potential for agronomic mixtures to contribute to more sustainable agricultural systems. Agronomic diversity can improve forage yield and reduce weed invasion in intensively managed grasslands, and may also enhance the provision of other ecosystem services.