Widespread foliage δ¹⁵N depletion under elevated CO₂: inferences for the nitrogen cycle

Abstract

Leaf 15N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage δ15N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ15N of foliage of 27 field‐grown plant species from six free‐air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ15N of foliage produced under elevated CO2 was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ15N by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO2 and leaf δ15N. In a select number of species, we measured bulk soil δ15N at a depth of 10 cm, and found that the observed depletion of foliage δ15N in response to elevated CO2 was unrelated to changes in the soil δ15N. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ15N responses to elevated CO2 reported here resulted from a general drop in δ15N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ15N under high CO2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.

N2 fixation and cycling in Alnus glutinosa, Betula pendula and Fagus sylvatica woodland exposed to free air CO2 enrichment

“…The EF for the trees in our study was consistently lower, by an average of 0.8%, under elevated CO 2 , indicating a consistent change in the distribution of 15 N between soil and leaf. The relative leaf 15 N depletion and associated changes in the soil-plant 15 N EF for trees growing under elevated CO 2 follow the trend identified by BassiriRad et al (2003). The opposing response of soil and leaves suggests that changes in leaf d 15 N are not due to changes in bulk soil d 15 N or to internal fractionation.…”

“…Thus, changes in leaf d 15 N might reflect changes in bulk soil d 15 N, differential uptake of different forms of N (with different d 15 N signatures) or changes in fractionation during uptake. The impact of elevated CO 2 on N cycling can therefore be reflected in leaf d 15 N, with a tendency towards a decrease in d 15 N when CO 2 is elevated for both woody and herbaceous plants (BassiriRad et al 2003). …”

“…Thus, the d 15 N of a tree reflects the d 15 N of the N source(s) subject to any fractionation that occurs during movement from or to the tree, gains and losses of N and N pool mixing (Robinson 2001). As such, changes in d 15 N can indicate changes in these components of forest N cycling (Emmett et al 1998;Robinson 2001;BassiriRad et al 2003). While these changes cannot necessarily be used to quantify specific differences in the N cycle, they can be used to identify areas that might be affected by any impacts on the N cycle.…”

Patterns and controls of foliar nitrogen isotope composition on the Qinghai-Tibet Plateau, China

“…Although the effect of elevated atmospheric CO 2 concentration on soil N availability has been controversial, with some showing increasing (Taylor and Ball 1994;Zak et al 1993) and some decreasing (Gill et al 2002;Horz et al 2004;Billings et al 2002) responses, the predominant pattern has been that elevated CO 2 decreases soil N availability. Bassirirad et al (2003) found that foliar δ 15 N decreased with elevated CO 2 whereas Billings et al (2002) observed the converse response. From these contradictory results, based on the declining trend of foliar δ 15 N with increased altitude (Fig.…”

“…Altitudinal gradients provide unique opportunities to study the integrative effects of environmental factors and to determine which factors most affect plant δ 15 N. For climate factors, not only MAT and MAP but also atmospheric pressure (correspondingly atmospheric CO2 and O2 concentrations) changes with altitude, and this change is more obvious in high-altitude regions. Atmospheric CO 2 concentration has been shown to have an important role in controlling the ecosystem N cycling by influencing N availability and gaseous N loss (Gill et al 2002), and consequently plant δ 15 N (Bassirirad et al 2003).…”