The fate of carbon in a mature forest under carbon dioxide enrichment

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. © 2020, The Author(s), under exclusive licence to Springer Nature Limited

Interactive carbon priming, microbial response and biochar persistence in a Vertisol with varied inputs of biochar and labile organic matter

There has been great interest in biochar application to soil for long-term carbon (C) sequestration. However, the interactive priming of organic C mineralization, including shifts in microbial community structure and the persistence of biochar in a clayey soil amended with biochar and labile organic matter (LOM) over a relatively long period (i.e. years) remain poorly understood. A 2-year incubation study was carried out with δ 13 C-depleted biochars produced from Eucalyptus saligna Sm. wood biomass at 450 and 550°C. Each of the biochars (at 2%, w/w) in combination with LOM, such as sugarcane residue at input rates of 0, 1, 2 or 4% w/w, were mixed with a C 4 -dominated Vertisol. The interactive effect of biochar and LOM on the structure of the microbial community was analysed by terminal restriction fragment length polymorphism (T-RFLP). Our results showed that at the small LOM rates (0 and 1%), there was a positive priming effect of biochar on organic C in soil (i.e. native soil organic C (SOC) + LOM-C)), which shifted to being negative when the LOM input was increased to 2 or 4%. Over the 2 years, mineralization of C from the 450°C biochar (1.2–1.7%) was significantly greater than that for 550°C biochar (0.6–1.0%), and the positively primed mineralization of biochar-C by LOM was enhanced by the increasing rates of LOM input. The negative priming of native SOC + LOM-C mineralization by biochar was greater at large than small inputs of LOM, which would have been facilitated by greater shifting in fungal communities, while enhancing biochar-C mineralization and possibly soil aggregation. In conclusion, over the long term, the amount of LOM stabilized by biochar was greater than that of positively primed biochar-C mineralization by LOM, in particular at the large LOM input. Biochar can persist in soil on a centennial scale and decreases the turnover of native SOC + LOM-C over the long term, whereas LOM input can shift microbial communities, favouring LOM stabilization in the biochar-amended Vertisol. Highlights: Do C priming, biochar persistence and microbial response change 2 years after biochar–LOM inputs? Examined interactive C priming (stable-C isotope), microbial community structure and biochar MRT. Priming of SOC ± LOM-C by biochar and vice-versa increased with LOM inputs. Biochar–LOM interaction (2-year) shifted microorganisms, favouring LOM-C stabilization in a Vertisol

The fate of carbon in a mature forest under carbon dioxide enrichment

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration. While evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO2. 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 responds to four years of eCO2 exposure. We show that, although the eCO2 treatment of ambient +150 ppm (+38%) induced a 12% (+247 gCm-2yr-1) 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 contributing ~50% 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 CO2 fertilization as a driver of increased carbon sinks in standing forests and afforestation projects

Interactive carbon priming, microbial response and biochar persistence in a Vertisol with varied inputs of biochar and labile organic matter

There has been great interest in biochar application to soil for long-term carbon (C) sequestration. However, the interactive priming of organic C mineralization, including shifts in microbial community structure and the persistence of biochar in a clayey soil amended with biochar and labile organic matter (LOM) over a relatively long period (i.e. years) remain poorly understood. A 2-year incubation study was carried out with δ 13 C-depleted biochars produced from Eucalyptus saligna Sm. wood biomass at 450 and 550°C. Each of the biochars (at 2%, w/w) in combination with LOM, such as sugarcane residue at input rates of 0, 1, 2 or 4% w/w, were mixed with a C 4 -dominated Vertisol. The interactive effect of biochar and LOM on the structure of the microbial community was analysed by terminal restriction fragment length polymorphism (T-RFLP). Our results showed that at the small LOM rates (0 and 1%), there was a positive priming effect of biochar on organic C in soil (i.e. native soil organic C (SOC) + LOM-C)), which shifted to being negative when the LOM input was increased to 2 or 4%. Over the 2 years, mineralization of C from the 450°C biochar (1.2–1.7%) was significantly greater than that for 550°C biochar (0.6–1.0%), and the positively primed mineralization of biochar-C by LOM was enhanced by the increasing rates of LOM input. The negative priming of native SOC + LOM-C mineralization by biochar was greater at large than small inputs of LOM, which would have been facilitated by greater shifting in fungal communities, while enhancing biochar-C mineralization and possibly soil aggregation. In conclusion, over the long term, the amount of LOM stabilized by biochar was greater than that of positively primed biochar-C mineralization by LOM, in particular at the large LOM input. Biochar can persist in soil on a centennial scale and decreases the turnover of native SOC + LOM-C over the long term, whereas LOM input can shift microbial communities, favouring LOM stabilization in the biochar-amended Vertisol. Highlights: Do C priming, biochar persistence and microbial response change 2 years after biochar–LOM inputs? Examined interactive C priming (stable-C isotope), microbial community structure and biochar MRT. Priming of SOC ± LOM-C by biochar and vice-versa increased with LOM inputs. Biochar–LOM interaction (2-year) shifted microorganisms, favouring LOM-C stabilization in a Vertisol