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| Titel |
Can observed ecosystem responses to elevated CO2 and N fertilisation be explained by optimal plant C allocation? |
| VerfasserIn |
Benjamin Stocker, I. Colin Prentice |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250124390
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| Publikation (Nr.) |
 EGU/EGU2016-3818.pdf |
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| Zusammenfassung |
| The degree to which nitrogen availability limits the terrestrial C sink under rising CO2 is
a key uncertainty in carbon cycle and climate change projections. Results from
ecosystem manipulation studies and meta-analyses suggest that plant C allocation
to roots adjusts dynamically under varying degrees of nitrogen availability and
other soil fertility parameters. In addition, the ratio of biomass production to GPP
appears to decline under nutrient scarcity. This reflects increasing plant C export
into the soil and to symbionts (Cex) with decreasing nutrient availability. Cex is
consumed by an array of soil organisms and may imply an improvement of nutrient
availability to the plant. These concepts are left unaccounted for in Earth system
models.
We present a model for the coupled cycles of C and N in grassland ecosystems to explore
optimal plant C allocation under rising CO2 and its implications for the ecosystem C balance.
The model follows a balanced growth approach, accounting for the trade-offs between leaf
versus root growth and Cex in balancing C fixation and N uptake. We further model a
plant-controlled rate of biological N fixation (BNF) by assuming that Cex is consumed by
N2-fixing processes if the ratio of Nup:Cex falls below the inverse of the C cost of
N2-fixation.
The model is applied at two temperate grassland sites (SwissFACE and BioCON),
subjected to factorial treatments of elevated CO2 (FACE) and N fertilization. Preliminary
simulation results indicate initially increased N limitation, evident by increased relative
allocation to roots and Cex. Depending on the initial state of N availability, this
implies a varying degree of aboveground growth enhancement, generally consistent
with observed responses. On a longer time scale, ecosystems are progressively
released from N limitation due tighter N cycling. Allowing for plant-controlled BNF
implies a quicker release from N limitation and an adjustment to more open N
cycling.
In both cases, optimal plant C allocation implies a sustained growth enhancement but a
decreased ratio of biomass productivity to GPP. Flexible allocation, C cost of N uptake, and
flexible N retention imply plant control on N availability. Thereby, plant control on BNF is
essential to determine the ultimate growth enhancement under elevated CO2 and whether this
implies higher N losses and N2O emissions. |
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