Study of carbonates dynamics in gypsum-rich soils after the adoption of irrigation. A multiscale approach.

Soil Inorganic Carbon (SIC) deposits are more dynamic than previously assumed. Studies have revealed an acceleration of SIC dynamics over just a few decades (Kim et al., 2020) driven by agricultural management of these soils (Plaza-Bonilla et al., 2015).

Most SIC reserves are in the arid and semi-arid regions of the planet, where atmospheric CO₂ can be incorporated into the soil as CaCO₃ precipitates through photosynthesis and root respiration processes (Monger et al., 2015). In these regions, irrigation is often a critical requirement for agriculture. Changes in the soil water regime due to irrigation can modify carbonate dynamics, affecting their dissolution and (re)precipitation, i.e., the formation of pedogenic carbonates.

If the source of these secondary carbonates is carbonate-rich parent material, this represents merely a redistribution of carbonates within the soil profile. However, if HCO₃⁻ originates from the mineralization of soil organic matter (SOM) or root respiration, and Ca²⁺ is derived from the weathering of non-carbonate minerals such as gypsum (CaSO₄·2H₂O) (Laudicina et al., 2021), the soil behaves as a net sink of atmospheric CO₂ (Sanderman, 2012).

This study is carried out using a multi-scale approach (“macro” focus at watershed level and “micro” focus at the root level) to quantify the extent of atmospheric C incorporation into carbonates in gypsum-rich irrigated soils.

First, monitoring sulfate and bicarbonate concentrations in a river draining a watershed in the Foral Community of Navarre (Spain) revealed elevated levels of these anions during periods of intensive irrigation. This indicates accelerated dissolution of both carbonates and gypsum, a phenomenon not detected in years prior to the adoption of irrigation.

The second phase examines the effect of gypsum content on pedogenic carbonate formation and root biocalcifications using a controlled pot experiment. Calcareous silt loam soil (30% CaCO₃) without gypsum was mixed with varying gypsum concentrations (0%, 5%, 50%, 80%) and planted with three species (Brassica oleracea, Rosmarinus officinalis, and Oxalis sp.). After the growth cycle, root samples were extracted and microscopically analyzed to identify calcified roots and associated carbonate features. The results demonstrated that higher gypsum content significantly increased root calcifications, confirming that gypsum dissolution supplies abundant Ca²⁺ for calcite precipitation within root tissues.

Future analyses, such as δ^87/86Sr ratios and Sr²⁺/Ca²⁺ proportions of the carbonates (indicators of Ca²⁺ origin), isotopic characterization of C, and micromorphological and digital quantification of calcitic soil features using thin sections, will provide a deeper understanding of these processes. This could support an innovative strategy for inorganic carbon sequestration in soils of arid and semi-arid regions.

 

References:

Kim et al. (2020), https://doi.org/10.1111/gcb.15207.

Laudicina et al. (2021), https://doi.org/10.1016/j.geoderma.2021.115115.

Monger et al. (2015), https://doi.org/10.1130/G36449.1.

Plaza-Bonilla et al. (2015), https://doi.org/10.1007/s13593-015-0326-x.

Sanderman (2012), https://doi.org/10.1016/j.agee.2012.04.015.