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

Abstract

~Symbiotic N, 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 CO, partial pressure (pC0,). The effect of elevated pC0, (60 Pa) on symbiotic N, fixation ('5N-isotope dilution method) was investigated using Free-Air-C0,-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 N, fixation increased significantly in T. repens under elevated pC0,. All additionally assimilated N was derived from symbiotic N, fixation, not from the soil. In the mixtures exposed to elevated pCO,, an increased amount of symbiotically fixed N (+7.8,8.2, and 6.2 g m-' a-' in 1993, 1994, and 1995, respectively) was introduced into the system. lncreased N, 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.

Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

“…In this study, we focused on L. perenne and T. repens mainly to compare our data with other studies performed on the FACE field experiment in Eschikon (Switzerland) [23,25,29,30,42,57,72,74]. Moreover, the plants have different rhizosphere structures.…”

“…These authors showed in the rhizosphere of T. repens a twofold increase in R. leguminosarum counts in samples retrieved in May and in November. Moreover, in the same FACE study site, Zanetti et al [74] showed that total N yield increased significantly in T. repens under elevated CO 2 and was assigned to an increased formation of nodules [73].…”

“…However, the symbiotic nitrogen fixation occurring in clover roots could counterbalance the mineral nutrient limitation. Indeed, the relative contribution of symbiotically fixed N to the total plant nitrogen increased with increasing pCO 2 [59,72,74].…”

“…Free-air-CO 2 -enrichment (FACE) technology [33] was used to release CO 2 into the open field rings during the photoperiod. The experiment [25,74] started at the end of May 1993 and lasted during the entire growing seasons. Monocultures of Lolium perenne cv Bastion and Trifolium repens cv Milkanova were cultivated in plots measuring 2.8 × 1.9 m and fertilized with P 2 O 5 (12 g m −2 yr −1 ), K 2 O (29 g m −2 yr −1 ), Mg (1.6 g m −2 yr −1 ), and N (14 g m −2 yr −1 ).…”

“…In the rhizosphere of T. repens, the population of Rhizobium leguminosarum biovar trifolii, determined by MPN plant infection assay of T. repens seedlings, was increased twofold under CO 2 enrichment [57]. Zanetti et al [74] reported an increase in symbiotic nitrogen fixation activity for T. repens growth under enriched CO 2 atmosphere. The elevated CO 2 had no effect on population of nitrifiers in the rhizospheres of L. perenne and T. repens [57] and Liriodendron tulipifera [43].…”