# Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests

**Authors:** Israel Carmi, Joel Kronfeld, Murray Moinester  
**arXiv:** [1702.05249](https://arxiv.org/abs/1702.05249)  
**Date:** 2017-02-17  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1702.05249v2/

## Key points

- Inorganic carbonates in semi-arid soils can act as a significant carbon sink
- Carbon dioxide from the atmosphere is absorbed by tree roots and hydrated by soil water, forming carbonic acid solutes
- Relict soil carbonate can dissolve, releasing calcium cations and additional bicarbonate
- Under specific conditions, these components combine to precipitate calcite while releasing CO2
- Previous claims suggested that such carbonates do not sequester substantial amounts of atmospheric CO2 due to their origin from pre-existing limestone and potential liberation of CO2 during calcite precipitation
- Field experiments conducted in Israel showed net sequestration of atmospheric CO2 under semi-arid forests through carbonate precipitation
- Most of the CO2 liberated during precipitation reactions may remain in the soil
- Ca+2 in sediment may come from sources other than pre-existing calcite
- Forestation can enhance pedogenic carbonate formation and have the potential to sequester 5-20% of the current annual anthropogenic increase in atmospheric CO2 as pedogenic carbonate.

## Summary

The existing study focuses on the potential of inorganic carbon, specifically allogenic and pedogenic carbonates found in semi-arid soils, to act as a significant carbon sink. It explores how carbon dioxide (CO2) from the atmosphere is absorbed by tree roots and hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils, resulting in the formation of carbonic acid solutes such as bicarbonate (HCO3-) and hydrogen ions (H+). These hydrogen ions can then dissolve relict soil carbonate (calcite), releasing calcium cations (Ca+2) and additional bicarbonate. Under specific conditions, these components combine to precipitate calcite while releasing CO2. However, previous claims have suggested that such carbonates do not sequester substantial amounts of atmospheric CO2 due to their origin from pre-existing limestone and the potential liberation of CO2 during calcite precipitation. It has been argued that only when the Ca+2 cation is derived from a non-carbonate source can sequestration be assumed. To test these assumptions, the authors conducted field experiments at two semi-arid sites in Israel. They found that bicarbonate originating from root exhalation is depleted as it becomes incorporated into the USZ during carbonate precipitation, indicating a net sequestration of atmospheric CO2 under semi-arid forests. Furthermore, most of the CO2 liberated during precipitation reactions may remain in the soil. Additionally, Ca+2 in sediment may come from sources other than pre-existing calcite. The study suggests that forestation can enhance pedogenic carbonate formation and proposes that worldwide semi-arid forests, both existing and future plantations, have the potential to sequester 5-20% of the current annual anthropogenic increase in atmospheric CO2 as pedogenic carbonate. In summary, this research provides evidence supporting the role of semi-arid soils as important carbon sinks through the formation of inorganic carbonates. It challenges previous claims by demonstrating that these carbonates can effectively sequester atmospheric CO2 when considering factors such as depletion of bicarbonate and retention of CO2 within soil particles. The findings suggest that forestation efforts in semi-arid regions could contribute significantly to mitigating global increases in atmospheric levels of carbon dioxide.