Over half of global soil organic carbon (SOC) is stored in subsurface soils (>20 cm depth), but the vulnerability of this deeper SOC to climate change has only recently been tested. Most soil warming experiments have either only warmed surface soils or only examined the response of the surface carbon dioxide flux, so the sensitivity of SOC at different soil depths to climate change is undetermined. As predictive models of terrestrial carbon storage move toward more mechanistic representations, we need to understand how the carbon cycle differs across soil depths. We present depth-explicit measurements of soil CO₂ production from six studies, including four in situ deep soil warming experiments. The experiments’ locations ranged from coniferous to hardwood temperate forests in the United States to volcanic soils in Hawaii. We have found that in temperate forests, deep soil carbon is just as vulnerable to warming-induced losses as surface soils. However, where minerals are strongly associated with organic carbon, as in Hawaii, or in degraded soils where much of the organic matter has been lost, deep soil carbon resists warming-induced losses. Thus, the response of deep soil to climate change is dependent on its availability to microbes. This conclusion is supported by a worldwide meta-analysis of radiocarbon data among soil density fractions, which found that the amount of carbon in the particulate free light fraction decreased with mean annual temperature, but the carbon in the heavy, mineral-associated fraction did not.

How to cite: Pries, C., Heckman, K., Templer, P., Frey, S., and Crow, S.: The response of deep soil carbon to climate change: From experiments to meta-analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16152, https://doi.org/10.5194/egusphere-egu21-16152, 2021.

Tropical rainforests harbour the greatest diversity of woody plant species in the world. Consequently, within any individual forest plot, replicating functional trait measurements, particularly at species level can be challenging. However, trait variation within and between species can be very large. Limited sampling opportunities in diverse forests poses a huge challenge to understanding the role both inter- and intra-specific variation play when we scale up individual trait measurements to plot or landscape averages. Using data from tropical forests within Latin America and South East Asia, we explore the potential role which inter- and intra-specific variation may play when attempting to compare functional trait values in tropical forests experiencing different environmental conditions. We demonstrate the need for renewed care considering how we construct sampling protocols within these forests for functional trait sampling. This includes considering the size and canopy position of the trees we sample across plots, alongside the number of individual within a species, and the number of species, we sample to generate results concerning how variation in environmental conditions influences plant functional traits. Considering such issues also offers considerable opportunities to advance our knowledge of the processes of acclimation and trait plasticity and how they may influences responses to environmental change. In-turn opening new prospects to better inform vegetation models, particularly individual-based models and therefore to investigate the impact of these properties at larger scales.

How to cite: Rowland, L., Bittencourt, P., Bartholomew, D., Giles, A., Oliveira, R., Mencuccini, M., da Costa, A., Banin, L., Burslem, D., and Meir, P.: The role of inter- and intra-specific variability in controlling trait measurements in tropical forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5126, https://doi.org/10.5194/egusphere-egu21-5126, 2021.

Tracing ‘bomb’ radiocarbon produced by atmospheric testing of atomic weapons through vegetation and soils provides information of the dynamics of terrestrial carbon cycling on timescales of years to centuries. Processes operating on these timescales are of interest because they regulate key functions in long-lived plants and regulate the potential for increasing soil carbon storage.  However, the multiple pathways taken by carbon transiting ecosystems from photosynthesis to respiration and decomposition complicate the quantitative interpretation of radiocarbon observations.  In the 14Constraint project, we are exploring how to optimize measurements of radiocarbon as well as to improve their interpretation by providing constraints for comparison with models.   This talk will focus on efforts to synthesize global radiocarbon measurements of mean age and transit time, and suggest ways forward to improve process-level understanding.

How to cite: Trumbore, S., Sierra, C., Hoyt, A., Hilman, B., Beem-Miller, J., Stoner, S., von Fromm, S., Shi, Z., and Randerson, J.: Radiocarbon constraints on carbon cycling in plants and soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13298, https://doi.org/10.5194/egusphere-egu21-13298, 2021.

Human activities have an unprecedented impact on the global carbon cycle.  Atmospheric CO2 concentrations have been continuously monitored since 1958, and show a 30% increase, from 315 ppm in 1958 to 412 ppm in 2020. Anthropogenic emissions, primarily from fossil fuel combustion, but also from land-use changes, are the drivers of these changes, with global emissions almost tripling over that period, from 4GtC per year in 1958 to almost 12 GtC per year at present. Although fossil fuel emission declined by about 7% in 2020 due to response to the COVID-19 pandemic, there are no long-term sign of global emissions declining yet, despite climate policies being put in places in many countries.

The atmospheric CO2 increase induces land and ocean carbon uptake, respectively driven by enhanced photosynthesis, leading to larger land biomass and soil carbon; and by enhanced air-sea CO2 exchange, leading to larger carbon content in the surface ocean and export to the deep ocean. These mechanisms are negative feedbacks in the Earth system and are removing about 50% of the CO2 emitted in the atmosphere. Without these land and ocean carbon sinks, current atmospheric CO2 would already be around 600 ppm.

However, modelling studies show that climate change reduces land and ocean carbon sinks, hence amplifying the warming. Although there is agreement that such positive feedback will develop over the course of the century, there are not yet clear evidence of a major climate driven reduction of the carbon sinks.  So far, observations and modelling studies of the historical carbon cycle do not show any sign of a tipping point in the global carbon cycle.

How to cite: Friedlingstein, P.: Human induced changes in the carbon cycle over the last 60 years, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2365, https://doi.org/10.5194/egusphere-egu21-2365, 2021.