| 1. |
- Jiang, Mingkai, et al.
(författare)
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The fate of carbon in a mature forest under carbon dioxide enrichment
- 2020
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 580:7802, s. 227-231
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Tidskriftsartikel (refereegranskat)abstract
- Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1–5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3–5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7–10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO2 unclear4,5,7–11. Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO2 exposure. We show that, although the eCO2 treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO2 fertilization as a driver of increased carbon sinks in global forests.
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| 2. |
- Pihlblad, Johanna, et al.
(författare)
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Stochiometric control of SOM and plant derived soil C pools dynamics under elevated CO2
- 2021
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Ingår i: EGU General Assembly 2021. - : EGU21-General Assembly.
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Konferensbidrag (refereegranskat)abstract
- Elevated carbon dioxide in the atmosphere (eCO2) has been found to influence soil C by altering the belowground balance between the decomposition of existing soil organic matter (SOM) and the accumulation of plant-derived C inputs. Even small changes in this balance can have a potentially large effect on future climate. The relative availability of soil nutrients, particularly N and P, are crucial mediators of both decomposition and new C accumulation, but both these two processes are rarely assessed simultaneously. We asked if the effect of eCO2 on soil C decomposition was mediated by soil N and P availability, and if the effect of CO2 and soil N and P availability on soil C decomposition was dependent on C pools (existing SOM C, newly added C). We grew Eucalyptus grandis and a C3 grass (Microlaena stipoides) from seed in an experimentally manipulated atmosphere with altered δ13C signature of CO2, which allowed the separation of plant derived C, from the existing SOM C. Then we manipulated N and P relative abundance via nutrient additions. We evaluated how the existing SOM and the new plant-derived C pool, and their respiration responded to eCO2 conditions and nutrient treatments. SOM respiration significantly increased in the eucalypts when N was added but was not affected by CO2. In the grass the SOM respiration increased with eCO2 and added N and SOM respiration per unit of SOM-derived microbial was significantly higher in both the added P and added N+P nutrient treatments. The rhizosphere priming of SOM was suppressed in both the added P and added N+P nutrient treatments. The heterotrophic respiration of plant-derived C was contingent on nutrient availability rather than eCO2 and differed by species. The grass-derived respiration was significantly higher than the eucalypt and was higher in both added P and added N+P nutrient treatments. Thus, nutrient stoichiometry had similar effects on SOM and plant derived C, but e CO2 only affected SOM and only for the Eucalyptus. This study shows how species differences have large effects on rhizosphere C cycling responses to eCO2 and stoichiometric conditions.
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| 3. |
- Pihlblad, Johanna, et al.
(författare)
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The influence of elevated CO2 and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland
- 2023
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 20, s. 505-521
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Tidskriftsartikel (refereegranskat)abstract
- Abstract. Elevated carbon dioxide (eCO2) in the atmosphere increases forest biomass productivity but only where soil nutrients, particularly nitrogen (N) and phosphorus (P), are not limiting growth. eCO2, in turn, can impact rhizosphere nutrient availability. Our current understanding of nutrient cycling under eCO2 is mainly derived from surface soil, leaving mechanisms of the impact of eCO2 on rhizosphere nutrient availability at deeper depths unexplored. To investigate the influence of eCO2 on nutrient availability in soil at depth, we studied various C, N, and P pools (extractable, microbial biomass, total soil C and N, and mineral-associated P) and nutrient cycling processes (enzyme activity and gross N mineralisation) associated with C, N, and P cycling in both bulk and rhizosphere soil at different depths at the Free Air CO2 enrichment facility in a native Australian mature Eucalyptus woodland (EucFACE) on a nutrient-poor soil. We found decreasing nutrient availability and gross N mineralisation with depth; however, this depth-associated decrease was reduced under eCO2, which we suggest is due to enhanced root influence. Increases in available PO43-, adsorbed P, and the C : N and C : P ratio of enzyme activity with depth were observed. We conclude that the influences of roots and of eCO2 can affect available nutrient pools and processes well beyond the surface soil of a mature forest ecosystem. Our findings indicate a faster recycling of nutrients in the rhizosphere, rather than additional nutrients becoming available through soil organic matter (SOM) decomposition. If the plant growth response to eCO2 is reduced by the constraints of nutrient limitations, then the current results would call to question the potential for mature tree ecosystems to fix more C as biomass in response to eCO2. Future studies should address how accessible the available nutrients at depth are to deeply rooted plants and if fast recycling of nutrients is a meaningful contribution to biomass production and the accumulation of soil C in response to eCO2.
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| 4. |
- Wild, Birgit, et al.
(författare)
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Decoupling of priming and microbial N mining during a short-term soil incubation
- 2019
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 129, s. 71-79
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Tidskriftsartikel (refereegranskat)abstract
- Soil carbon (C) and nitrogen (N) availability depend on the breakdown of soil polymers such as lignin, chitin, and protein that represent the major fraction of soil C and N but are too large for immediate uptake by plants and microorganisms. Microorganisms may adjust the production of enzymes targeting different polymers to optimize the balance between C and N availability and demand, and for instance increase the depolymerization of N-rich compounds when C availability is high and N availability low (“microbial N mining”). Such a mechanism could mitigate plant N limitation but also lie behind a stimulation of soil respiration frequently observed in the vicinity of plant roots (“priming effect”). We here compared the effect of increased C and N availability on the depolymerization of native bulk soil organic matter (SOM), and of 13C-enriched lignin, chitin, and protein added to the same soil in two complementary ten day microcosm incubation experiments. A significant reduction of chitin depolymerization (described by the recovery of chitin-derived C in the sum of dissolved organic, microbial and respired C) upon N addition indicated that chitin was degraded to serve as a microbial N source under low-N conditions and replaced in the presence of an immediately available alternative. Protein and lignin depolymerization in contrast were not affected by N addition. Carbon addition enhanced microbial N demand and SOM decomposition rates, but significantly reduced lignin, chitin, and protein depolymerization. Our findings contrast the hypothesis of increased microbial N mining as a key driver behind the priming effect and rather suggest that C addition promoted the mobilization of other soil C pools that replaced lignin, chitin, and protein as microbial C sources, for instance by releasing soil compounds from mineral bonds. We conclude that SOM decomposition is interactively controlled by multiple mechanisms including the balance between C vs N availability. Disentangling these controls will be crucial for understanding C and N cycling on an ecosystem scale. © 2018 The Authors
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