Does nitrogen nutrition restrict the CO 2 response of fertile grassland lacking legumes?

Abstract

The extent of the response of plant growth to atmospheric CO enrichment depends on the availability of resources other than CO. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO response of plants. The effect of elevated CO (60 Pa) partial pressure (pCO) 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 pCO, 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 pCO, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO than under ambient pCO in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO, 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 N-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO.

Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multi‐species mixtures

“…However, there was a clear and consistent dominance hierarchy among species along the N gradient, which was maintained under elevated CO # . Predominance of the effect of N over that of CO # has already been documented for two-species mixtures of grasses and legumes Schenk et al, 1997 ;Zanetti et al, 1997) and more complex communities (Zangerl & Bazzaz, 1984 ; Response to CO 2 of the biomass of species grown as isolated plants # of species are calculated as the ratio of smoothed plant dry weight under elevated CO # to that under ambient CO # ; they are plotted for all concentrations of N. Danthonia richardsonii ; filled circles ; Lotus pedunculatus ; filled triangles, apex up ; Phalaris aquatica ; filled squares ; Trifolium repens, filled triangles, base up. y l x, dotted line.…”

Carbon isotopic composition of branched tetraether membrane lipids in soils suggest a rapid turnover and a heterotrophic life style of their source organism(s)

“…Mixed with atmospheric CO 2 this resulted in a δ 13 C value of the CO 2 above the plot of −18‰, significantly lower than that of atmospheric CO 2 (−8‰). For experimental details see Zanetti et al (1997). The sampled plots received N-fertilization of ca.…”

Frequency and Diversity of Nitrate Reductase Genes among Nitrate-Dissimilating Pseudomonas in the Rhizosphere of Perennial Grasses Grown in Field Conditions

“…Indeed, higher N 2 O emissions were measured in high Nfertilized L. perenne monoculture under elevated pCO 2 [1,28]. Anyway, long-term CO 2 enrichment experiments have shown that N may become a limiting factor for the reaction of plants (including grasses) to elevated pCO 2 [14,58]. In the studied (N-limited) plots, experimental results suggested that pCO 2 treatment had no significant effect on apparent N uptake and transformations [22,48].…”