Thomas Hebeisen

Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem.

Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.

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.

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.

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.

What is the Significance of Symbiotic N2 Fixation for Grassland Ecosystems in a CO2-Rich Word?

The significance of symbiotic N2 fixation (measured as 15N-isotope dilution) for grassland ecosystems under elevated atmospheric pCO2 (60 Pa) was investigated under field conditions using the free air carbon-dioxide enrichment (FACE) technology. There was a fundamental difference in the CO2 response of plant biomass production in ecosystems depending on the presence of Trifolium repens: Under elevated pCO2, Lolium perenne grown in monoculture showed symptoms of N limitation (1, 3, 6) in such a way that the above-ground N-yield decreased under elevated pCO2 (6). This was in contrast to L. perenne growing with T. repens or to T. repens growing in monoculture where N nutrition appeared to be adequate (1, 3, 6). An evaluation of the N-sources clearly showed that under elevated pCO2 all nitrogen that was additionally assimilated in T. repens came from symbiotic N2 fixation (4, 5). This was a consequence of a consistent increase in the relative contribution of symbiotically fixed N to the total N yield (4, 5); the amount of symbiotically fixed N increased by 60% in the grass/legume mixtures through both increased clover proportion and increased N2 fixation in each individual clover plant (3, 4, 5). This led to a simultaneously enhanced apparent transfer of N from clover to grasses (6). These data suggest that increased photosynthetic CO2 fixation under elevated pCO2, although not entirely reflected in biomass production, was counterbalanced by an appropriately increased symbiotic N2 fixation, thus maintaining the C: N ratio at the whole ecosystem level. Since inadequate N supply would restrict an increase in extra C-sequestration into the ecosystem under elevated pCO2, symbiotic N2 fixation is considered to be a crucial driving force for increased carbon sequestration in a CO2-rich world (2).