Plant productivity shapes the mineral-protection of surface soil carbon storage regardless of soil age

Dr. César Plaza, Instituto de Ciencias Agrarias (ICA), CSIC

Dr. Pablo García-Palacios, Instituto de Ciencias Agrarias (ICA), CSIC

Dr. Manuel Delgado-Baquerizo, Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC

Soil organic carbon is essential to soil life and function on the planet. Soil carbon supports the immense microbial diversity under our feet, and fuel multiple ecosystem services, such as climate regulation and soil fertility, that allow us to produce food and help fight and adapt to global warming. Because of this, we need to learn how to predict the changes in soil carbon across space and time.  

Importantly, soil carbon, however, is not in a single form, but in a complex mixture of plant and microbial cells and byproducts at different degrees of transformation and decomposition. The association of these organic compounds with soil minerals imposes physical barriers and chemical constraints to the soil carbon decomposer community, and thus govern the capacity of soils to retain carbon from being released back to the atmosphere. Despite the huge importance of understanding how the spatial and temporal dynamics of different fractions of soil carbon change, we are still far from understanding how soil formation and climate interact to explain the current levels of different carbon fractions in our planet. 

Figure 1. Researchers conducting a survey in a terrestrial ecosystem. Picture: Manuel Delgado Baquerizo.

To address this question, we leverage the CLIMIFUN global survey, which includes 16 long-term soil chronosequences spanning a wide range of vegetation and climatic types. We found that no matter how old soils are, climate and productivity determine the proportion of mineral-associated and free carbon in surface soils across the globe. In other words, soil development over millennia plays a far less relevant role than plant productivity in explaining soil carbon composition. In highly productive ecosystems, such as tropical and temperate forests, surface soil carbon stocks are dominated by unprotected carbon fractions during pedogenesis. Conversely, in less productive, drier and colder ecosystems, and in croplands, surface soil carbon stocks are dominated by the mineral-associated fraction. We also found that the microbial respiration rates, as well as the temperature sensitivity of soil carbon losses via microbial respiration, increase with the proportion of surface soil carbon stored in the free fraction. 

As a whole, we showed that climate and productivity control the proportion of mineral protection of surface soil carbon, and that all the free carbon being accumulated in tropical and temperate ecosystems is highly vulnerable to climate change. These results highlight the need of conserving ecosystem productivity to maintain carbon in surface soils, and thus the multiple ecosystems services relying on it. 

Reference: 

Plaza, C., García-Palacios, P., Berhe, A.A., Barquero, J., Bastida, F., Png, G.K., Rey, A., Bardgett, R.D., Delgado-Baquerizo, M., 2022. Ecosystem productivity has a stronger influence than soil age on surface soil carbon storage across. Communications Earth & Environment 3, 233. https:// doi.org/10.1038/s43247-022-00567-7