Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem (original) (raw)
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Plant Ecology, 2009
This study is the first to investigate quantitative effects of plant community composition and diversity on N 2 fixation in legumes. N 2 fixation in three perennial Trifolium species grown in field plots with varied number of neighbouring species was evaluated with the 15 N natural abundance method (two field sites, several growing seasons, no N addition) and the isotope dilution method (one site, one growing season, 5 g N m -2 ). The proportion of plant N derived from N 2 fixation, pNdfa, was generally high, but the N addition decreased pNdfa, especially in species-poor communities. Also following N addition, the presence of grasses in species-rich communities increased pNdfa in T. hybridum and T. repens L., while legume abundance had the opposite effect. In T. repens, competition for light from grasses appeared to limit growth and thereby the amount of N 2 fixed at the plant level, expressed as mg N 2 fixed per sown seed. We conclude that the occurrence of diversity effects seems to be largely context dependent, with soil N availability being a major determinant, and that species composition and functional traits are more important than species richness regarding how neighbouring plant species influence N 2 fixation in legumes.
Symbiotic N2 fixation in a high Alpine grassland: effects of four growing seasons of elevated CO2
Functional Ecology, 1999
Increasing carbon dioxide concentration (E: 680 µl CO 2 litre -1 vs ambient, A: 355 µl CO 2 litre -1 ) around late-successional Alpine sedge communities of the Swiss Central Alps (2450 m) for four growing seasons (1992)(1993)(1994)(1995) had no detectable effect on symbiotic N 2 fixation in Trifolium alpinum-the sole N 2 -fixing plant species in these communities (74 ± 30 mg N m -2 year -1 , A and E plots pooled). 2. This result is based on data collected in the fourth growing season showing that elevated CO 2 had no effect on Trifolium above-ground biomass (4·4 ± 1·7 g m -2 , A and E plots pooled, n = 24) or N content per unit land area (124 ± 51 mg N m -2 , A and E pooled), or on the percentage of N Trifolium derived from the atmosphere through symbiotic N 2 fixation (%Ndfa: 61·0 ± 4·1 across A and E plots) estimated using the 15 N dilution method.
Global Change Biology, 2003
Reduced soil N availability under elevated CO 2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N 2 -fixing system. We studied the effects of Trifolium repens (an N 2 -fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO 2 under FACE conditions. 15 Nlabeled fertilizer was applied at a rate of 140 and 560 kg N ha À 1 yr À 1 and the CO 2 concentration was increased to 60 Pa pCO 2 using 13 C-depleted CO 2 . The total soil C content was unaffected by elevated CO 2 , species and rate of 15 N fertilization. However, under elevated CO 2 , the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer-N (f N ) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO 2 , had a significant effect on f N values of the total soil N pool. The fractions of newly sequestered C (f C ) differed strongly among intra-aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO 2 , the ratio of fertilizer-N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more 15 N fertilizer than T. repens: 179 vs. 101 kg N ha À 1 for the low rate of N fertilization and 393 vs. 319 kg N ha À 1 for the high N-fertilization rate. As the loss of fertilizer-15 N contributed to the 15 N-isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N 2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non-N 2fixing L. perenne system. This suggests that N 2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.
Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years
Soil Biology and Biochemistry, 2003
Plant response to increasing atmospheric CO 2 partial pressure (pCO 2) depends on several factors, one of which is mineral nitrogen availability facilitated by the mineralisation of organic N. Gross rates of N mineralisation were examined in grassland soils exposed to ambient (36 Pa) and elevated (60 Pa) atmospheric pCO 2 for 7 years in the Swiss Free Air Carbon dioxide Enrichment experiment. It was hypothesized that increased below-ground translocation of photoassimilates at elevated pCO 2 would lead to an increase in immobilisation of N due to an excess supply of energy to the roots and rhizosphere. Intact soil cores were sampled from Lolium perenne and Trifolium repens swards in May and September, 2000. The rates of gross N mineralisation (m) and NH 4 þ consumption (c) were determined using 15 N isotopic dilution during a 51-h period of incubation. The rates of N immobilisation were estimated either as the difference between m and the net N mineralisation rate or as the amount of 15 N released from the microbial biomass after chloroform fumigation. Soil samples from both swards showed that the rates of gross N mineralisation and NH 4 þ consumption did not change significantly under elevated pCO 2 : The lack of a significant effect of elevated pCO 2 on organic N turnover was consistent with the similar size of the microbial biomass and similar immobilisation of applied 15 N in the microbial N pool under ambient and elevated pCO 2 : Rates of m and c; and microbial 15 N did not differ significantly between the two sward types although a weak (p , 0:1) pCO 2 by sward interaction occurred. A significantly larger amount of NO 3 2 was recovered at the end of the incubation in soil taken from T. repens swards compared to that from L. perenne swards. Eleven percent of the added 15 N were recovered in the roots in the cores sampled under L. perenne, while only 5% were recovered in roots of T. repens. These results demonstrate that roots remained a considerable sink despite the shoots being cut at ground level prior to incubation and suggest that the calculation of N immobilisation from gross and net rates of mineralisation in soils with a high root biomass does not reflect the actual immobilisation of N in the microbial biomass. The results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated pCO 2 : It is suggested that differences in plant N demand, rather than major changes in soil N mineralisation/immobilisation, are the long-term driving factors for N dynamics in these grassland systems.
Ecosystems, 2008
Nitrogen fixation was measured in constructed old-field ecosystems that were exposed for 3 years to different combinations of elevated atmospheric [CO 2 ] and temperature (300 ppm and 3°C above ambient, respectively), and ambient or reduced soil moisture (corresponding to 25 or 2 mm rainfall per week). The old-fields included seven planted herbaceous annual and perennial species, including two legumes (Trifolium pratense and Lespedeza cuneata). Potential asymbiotic N 2 -fixation by soils, measured in laboratory incubations, was significantly less under the ''dry'' treatment but was estimated to contribute little overall to annual ecosystem N budgets. Foliar N concentrations declined significantly under elevated [CO 2 ]. Effects of the three environmental factors on the mean (±SE) fraction of legume N derived from atmospheric N 2 (FN dfa ) varied from year-to-year, and FN dfa ranged from 0.64 ± 0.05 to 0.94 ± 0.03 depending on species and growing season. High rates of symbiotic N 2 -fixation (4.6-12 g N m )2
Global Change Biology, 2005
The impact of elevated CO 2 on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO 2 on N availability to plants and microbes. We examined the effects of atmospheric CO 2 enrichment (ambient 1 370 lmol mol À1 ) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua, respectively. The A. barbata experiment was conducted in a N-poor sandy loam and the A. fatua experiment was on a N-rich clayey loam. Plant-microbial N partitioning was examined through determining the distribution of a 15 N tracer. In the A. barbata experiment, 15 N tracer was introduced to a field labeling experiment in the previous year so that 15 N predominantly existed in nonextractable soil pools. In the A. fatua experiment, 15 N was introduced in a mineral solution [( 15 NH 4 ) 2 SO 4 solution] during the growing season of A. fatua. Results of both N budget and 15 N tracer analyses indicated that elevated CO 2 increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO 2 increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO 2 -led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO 2 enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO 2 increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO 2 may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short-term microcosm experiments can be extrapolated to field conditions. Long-term studies in less-disturbed soils are needed to determine whether CO 2 -enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO 2 environment.