Herbert Blum

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.

Quantification of soil carbon inputs under elevated CO2: C3 plants in a C4 soil

The objective of this investigation was to quantify the differences in soil carbon stores after exposure of birch seedlings (Betula pendula Roth.) over one growing season to ambient and elevated carbon dioxide concentrations. One-year-old seedling of birch were transplanted to pots containing ‘C4 soil’ derived from beneath a maize crop, and placed in ambient (350 μL L−1) and elevated (600 μL L−1) plots in a free-air carbon dioxide enrichment (FACE) experiment. After 186 days the plants and soils were destructively sampled, and analysed for differences in root and stem biomass, total plant tissue and soil C contents and δ13C values. The trees showed a significant increase (+50%) in root biomass, but stem and leaf biomasses were not significantly affected by treatment. C isotope analyses of leaves and fine roots showed that the isotopic signal from the ambient and elevated CO2 supply was sufficiently distinct from that of the ‘C4 soil’ to enable quantification of net root C input to the soil under both ambient and elevated CO2. After 186 days, the pots under ambient conditions contained 3.5 g of C as intact root material, and had gained an additional 0.6 g C added to the soil through root exudation/turnover; comparable figures for the pots under elevated CO2 were 5.9 g C and 1.5 g C, respectively. These data confirm the importance of soils as an enhanced sink for C under elevated atmospheric CO2 concentrations. We propose the use of ‘C4 soils’ in elevated CO2 experiments as an important technique for the quantification of root net C inputs under both ambient and elevated CO2 treatments.

The decomposition of Lolium perenne in soils exposed to elevated CO2: comparisons of mass loss of litter with soil respiration and soil microbial biomass.

Abstract Two key questions regarding the effects of elevated atmospheric CO2 on soil microbial biomass are, (a) will future levels of elevated CO2 affect the amount of microbial biomass in soil? and (b) how will any observed changes impact on C-flux from soils? These questions were addressed by examining soil microbial biomass, and in situ estimations of soil respiration in grassland soils exposed to free air carbon dioxide enrichment (60 Pa). Corresponding measurements of plant litter mass loss were taken using litter bags, ensuring that ambient litter was decomposed in ambient soil, and elevated CO2 grown litter was decomposed in soils exposed to elevated CO2. Significantly greater levels of microbial biomass (p

Simulation of climate change with infrared heaters reduces the productivity of Lolium perenne L. in summer

Abstract Field-grown perennial ryegrass was subjected to climate warming and elevated CO2 concentration during summer in free air conditions (no enclosure of the vegetation). Increased foliage temperature (2.5°C above fluctuating ambient) was induced by heating the stand with infrared radiation sources, modulated by an electronic control device (FATI, Free Air Temperature Increase). Enhanced CO2 was produced by a FACE system (Free Air CO2 Enrichment). Exposure to simulated climate warming drastically reduced above-ground harvestable dry matter (52% loss). The nitrogen allocated to the leaf fraction was thus concentrated into less dry matter, which enhanced the nitrogen concentration on a mass basis (+17%) but also per unit leaf area (+47%). As a consequence, CO2 assimilation rates were not affected in these slower growing plants in the +2.5°C treatment, and the photochemical efficiency of non-cyclic electron transport of photosystem II was also unaffected. Although the plants were grown in the field without root restrictions, long-term exposure to elevated CO2 concentration induced noticeable acclimation of the photosynthetic apparatus (40% loss of fixation potential), which largely outweighed the direct stimulation in this summer period. Part of the reduced rates could be attributed to lower N concentration on a leaf area basis. The results are compared with responses of this species in sunlit conditioned greenhouses, which indicates that experiments in enclosures may underestimate effects in the field. This also emphasizes the need to validate other plant responses to climate warming and CO2 enrichment in free air conditions.

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.

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 increases.

Nitrogen plays a major role in leaves when source-sink relations change: C and N metabolism in Lolium perenne growing under free air CO2 enrichment

Swards of Lolium perenne L. were grown in the field in a long-term free air CO2 enrichment (FACE) facility. The CO2 treatment was combined with two levels of N fertilization and regular defoliation, which resulted in plants with a wide range of source-sink relations. C and N metabolism were investigated to assess the role of carbohydrate and nitrogenous compounds in leaves in indicating source-sink relations. Sucrose exhibited the largest changes in contents during the day-night cycle; therefore, it was identified as the main short-term storage compound for night-time export. Fructan accumulation indicated the degree of surplus C supply in the source compared to C use in sinks. Nitrate content depended mainly on N fertilization, and was reduced under elevated pCO2. Nitrate appeared to indicate a current surplus of available N relative to the need for growth. Amino acid content responded strongly to N fertilization but decreased only slightly under elevated pCO2. Protein content, however, decreased significantly under elevated pCO2. The patterns of diurnal changes of C or N compounds did not differ between CO2 treatments. Down-regulation of photosynthesis appeared to occur when plants were extremely N-limited as under elevated pCO2, low N and at a late regrowth stage.

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).