Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift

Abstract

Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO 2 ). The extent to which the biosphere can act as a buffer against rising atmospheric CO 2 concentration in global climate change projections remains uncertain at the present stage [1][2][3][4] . Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO 2 concentration, diminishing the CO 2 -fertilization effect on terrestrial plant productivity in unmanaged ecosystems [3][4][5][6][7] . Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO 2 -fertilization effect 8,9 . However, conclusive demonstration that added N enhances plant productivity in response to CO 2 -fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO 2 concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C 3 sedge and C 4 grasses, and is capable of responding rapidly to environmental change 10 . We found that N addition enhanced the CO 2 -stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO 2 response. But we also found that N addition strongly promotes the encroachment of C 4 plant species that respond less strongly to elevated CO 2 concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO 2 -stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO 2 concentrations and N pollution affect plant species differently 11-13, and that they may drive plant community changes 14-17 , we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO 2 concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.The progressive nitrogen limitation (PNL) hypothesis 7 suggests that N additions should enhance CO 2 effects on plant productivity. However, only a limited number of studies have provided direct experimental evidence that N addition actually sustains or enhances the CO 2 response of productivity 3,7 . In a pine forest, N addition amplified the CO 2 effect on woody tissue increment 5 . A CO 2 3 N experiment in a grassland reported that a positive CO 2 3 N interaction emerged after three years, indicating that N addition amplified the effect of elevated CO 2 on productivity 6 . In managed ryegrass swards, N addition yielded larger CO 2 responses, an effect that strengthened over time on a relative basis, but diminished in terms of absolute magnitude 18 .As originally articulated, the PNL hypothesis does not explicitly consider the effects that elevated CO 2 and added N can have on the ecosyst...

C3 and C4 Biomass Allocation Responses to Elevated CO2 and Nitrogen: Contrasting Resource Capture Strategies

“…To do this, we estimated biomass and productivity of C 3 and C 4 vegetation under factorial treatments of ambient and high soil N and atmospheric CO 2 . Previous work reported earlier treatment effects on plot-level productivity (Langley et al 2009a;Langley and Megonigal 2010); however, in this study, accounting for alteration in C 3 stem turnover allowed more accurate estimation of biomass allocation aboveground. Biomass allocation belowground is defined between the functional C 3 -C 4 groups through carbon isotope analysis, allowing us to distinguish the strategies of these two functional groups that differ dramatically in photosynthetic strategy but commonly co-occur in brackish wetlands.…”

“…Following N application, an equal volume of unamended brackish water was sprayed onto the plots to rinse the N solution from the vegetation canopy to the soil surface. The enrichment in soil N availability was confirmed by elevated porewater [N] in the root zone, averaging increase over controls by 25 % over the course of the experiment (Langley and Megonigal 2010). The ten chambers that were to receive no N enhancement were sprayed with unamended brackish water in the same volume as the N enhanced chambers.…”

Alterations in forest detritus inputs influence soil carbon concentration and soil respiration in a Central-European deciduous forest

“…Changes in temperature and precipitation patterns that are predicted under scenarios of global climate change will have profound effects on species diversity and forest productivity (Langley and Megonigal, 2010;Wang et al, 2011;Rózsa and Novák, 2011;Williams et al, 2012), resulting in alteration of the quality and quantity of detritus inputs to soils. These changes can thus influence decomposition (Trofymow et al, 2002;Callesen et al, 2003;vegetation composition can control SOM chemistry.…”

Potassium: a neglected nutrient in global change

“…Models of global carbon (C) circulation frequently do not include the role of nutrients in the establishment of C balances and the capacity of sinks to store C (Lé Quére et al, 2009). However, those that do usually include only N (Schimel et al, 2001;Mack et al, 2004;Langley & Megonigal, 2010) or more recently P (Peñuelas et al, , 2013a.…”