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Soil organic matter (SOM) decomposition and organic phosphorus (P) cycling may help sustain plant productivity under elevated CO2 (eCO2) and low-P conditions. Arbuscular mycorrhizal (AM) fungi and their role in P-acquisition and SOM decomposition may become more relevant in these conditions. Yet, experimental evidence of AM fungi and P availability interactive effects on soil carbon (C) cycling under eCO2 is scarce with the potential mechanisms of this control being poorly understood. We performed a pot experiment with soil and a grass from a low-P ecosystem where plant biomass and soil C cycling have been mostly unresponsive to eCO2. We manipulated AM fungi, P, and CO2 levels and assessed their impacts on soil C cycling and plant growth using continuous 13C plant labelling to isolate and measure short-term changes in total and SOM-derived fractions of respired CO2, dissolved organic C (DOC) and microbial biomass (MBC), as relevant components of the soil C cycle. Increases in SOM decomposition and microbial C use were hypothesised to support plant growth under eCO2 and low-P with AM fungi intensifying this effect. However, we did not detect simultaneous significant impacts of the three experimental factors. We observed instead increased root biomass and nutrient uptake with eCO2 and AM presence and lower SOM-derived DOC and MBC with low-P, decreasing further with AM inoculation. Taken together, our findings in this model plant-soil system suggest that, AM fungi can support root biomass growth and nutrient uptake under eCO2 and protect the SOM pool against decomposition even in low-P conditions. Contrary to reports from N-limited ecosystems, our results allow us to conclude that C and P biogeochemical cycles may not become coupled to sustain an eCO2 fertilisation effect and that the role of AM fungi protecting the SOM pool is likely driven by competitive interactions with saprotrophic communities over nutrients. [ABSTRACT FROM AUTHOR]

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO 2 effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO 2 enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO 2 effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO 2 -driven carbon sink is overestimated by models.

The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO 2 concentrations depends on soil nutrient availability 1,2 . Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO 2 (refs. 3-6 ), but uncertainty about ecosystem P cycling and its CO 2 response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change 7 . Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO 2 , we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO 2 and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO 2 fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage., (© 2024. Crown.)

Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular-level data (based on nuclear magnetic resonance and specific analysis of plant- and microbial-derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long-term soil priming. The degree of SOM decomposition was higher for bacteria-dominated sites with higher nitrogen (N) availability while lower for the N-poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular-level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers., (© 2022 John Wiley & Sons Ltd.)

Forest trees rely on ectomycorrhizal (ECM) fungi to obtain growth-limiting nutrients. While addition of nitrogen (N) has the potential to disrupt these critical relationships, there is conflicting evidence as to the mechanism by which ECM:host mutualism may be affected. We evaluated how N fertilization altered host interactions and gene transcription between Eucalyptus grandis and Pisolithus microcarpus or Pisolithus albus, two closely related ECM species that typically co-occur within the same ecosystem. Our investigation demonstrated species-specific responses to elevated N: P. microcarpus maintained its ability to transport microbially sourced N to its host but had a reduced ability to penetrate into root tissues, while P. albus maintained its colonization ability but reduced delivery of N to its host. Transcriptomic analysis suggests that regulation of different suites of N-transporters may be responsible for these species-specific differences. In addition to N-dependent responses, we were also able to define a conserved 'core' transcriptomic response of Eucalyptus grandis to mycorrhization that was independent of abiotic conditions. Our results demonstrate that even between closely related ECM species, responses to N fertilization can vary considerably, suggesting that a better understanding of the breadth and mechanisms of their responses is needed to support forest ecosystems into the future., (© 2021 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Atmospheric carbon dioxide enrichment (eCO 2 ) can enhance plant carbon uptake and growth 1-5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6 . Although evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3-5 , it is unclear whether mature forests respond to eCO 2 in a similar way. In mature trees and forest stands 7-10 , photosynthetic uptake has been found to increase under eCO 2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO 2 unclear 4,5,7-11 . Here using data from the first ecosystem-scale Free-Air CO 2 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 eCO 2 exposure. We show that, although the eCO 2 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 eCO 2 , and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO 2 fertilization as a driver of increased carbon sinks in global forests.