Hidden chemical reactions in soil complicate carbon storage in restored sandy lands

Restoring Land, Uncovering Chemistry

Vegetation restoration is a primary strategy for combating desertification and increasing carbon storage in soils. While the focus has largely been on biological decomposition, a new study from researchers at Northwest A&F University in China shows that non-biological chemical reactions play a substantial role in the soil carbon cycle. These abiotic processes, driven by reactive oxygen species ROS, can turn stored organic carbon into carbon dioxide gas, and their intensity depends on the type of vegetation being restored.

The Role of Reactive Molecules

The research team, led by Fuhao Liu, Kecheng Zhu, and Hanzhong Jia, investigated how revegetation affects the production of ROS in the soils of the Mu Us Sandy Land in China. ROS are highly reactive chemical molecules, and the team focused on one in particular: the hydroxyl radical •OH, a potent oxidant. They discovered that reintroducing vegetation, especially trees, creates conditions that favor the production of these radicals. This happens because plant litter and root activity increase the amount of reduced iron and organic matter in the soil, which in turn react to form •OH.

Trees Foster More Reactions than Shrubs

Through field monitoring and laboratory incubation experiments, the scientists compared soils from under trees like Populus alba, shrubs like Hedysarum multijugum Maxim., and bare sand. The results were clear: soils covered by trees produced significantly more hydroxyl radicals than those covered by shrubs, and both produced far more than bare sand. This suggests that the greater biomass and root exudates from trees create a more chemically reactive environment in the soil.

A Significant Source of CO₂

The study quantified the effect of these chemical reactions on carbon release. By using sterilization and chemical quenching techniques to isolate the abiotic processes, the team determined that •OH-mediated oxidation was responsible for 15.93% to 25.80% of the total carbon dioxide efflux from the sandy soils. This is a considerable portion of carbon release that is not directly linked to microbial respiration, which was previously thought to be the dominant process.

Different Carbon Forms, Different Vulnerabilities

The researchers also examined how hydroxyl radicals affect different forms of soil organic carbon. They separated the carbon into two main types: particulate organic carbon POC, which consists of less decomposed plant and animal residues, and mineral-associated organic carbon MOC, which is bonded to soil minerals. Their experiments showed that MOC was more susceptible to attack by hydroxyl radicals. This finding is important for predicting how stable different carbon pools in the soil are over time.

Implications for Climate and Land Management

This work offers a more complete picture of the carbon cycle in restored ecosystems. It shows that alongside the benefits of carbon sequestration from planting vegetation, there is a concurrent chemical process that releases some of that carbon back into the atmosphere. According to the corresponding author, Hanzhong Jia, these findings suggest that future carbon budget models should incorporate the effects of abiotic ROS reactions to accurately assess the carbon sequestration potential of land restoration projects. The choice between planting trees or shrubs could have different outcomes for long-term soil carbon storage due to these chemical dynamics.

Corresponding Author:

Hanzhong Jia

Original Source:

https://doi.org/10.1007/s44246-023-00074-0

Contributions:

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Fuhao Liu, Kecheng Zhu, Zhiqiang Wang, Jinbo Liu, Zheng Ni, Yuanyuan Ding, Chi Zhang and Hanzhong Jia. The first draft of the manuscript was written by Fuhao Liu and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.