Irrigation activates soil inorganic carbon dynamics in a calcareous mediterranean agroecosystem

The implementation of irrigation is a key management practice in arid and semi-arid regions to sustain agricultural productivity. Irrigation modifies the soil carbon cycle [1], [2] but its effects on soil inorganic carbon (SIC) have received far less attention than those on soil organic carbon. However, SIC constitutes most of the soil carbon stock in calcareous soils of these regions. Understanding how irrigation interacts with SIC dynamics, governed by carbonate dissolution and precipitation processes, is crucial to assess soil carbon stability and its response to management changes.

This study compares two contrasting management scenarios, rainfed maize and irrigated maize, and evaluated how irrigation affected the dynamics of the SIC in an experimental plot in Navarra (northern Spain) historically cultivated with rainfed wheat. We quantified SIC and SOC contents in bulk soil and in coarse (>50 µm) and fine (<50 µm) fractions of the tilled layer (0–30 cm) of a calcareous soil (⁓40% CaCO₃), together with the isotopic signatures of SOC (δ¹³C-SOC) and SIC (δ¹³C-SIC) along the first 7 years of the trial, as direct assessment of SIC isotopic signatures provides a more reliable estimate of pedogenic carbonate contributions than commonly used mixing equations, avoiding biases associated with C3–C4 crop changes [3].

After 7 years, it was found that the accumulated SOC inputs were higher in irrigated maize (24.0 Mg C ha⁻¹) than in rainfed maize (14.8 Mg C ha⁻¹).  Therefore, irrigated maize showed an increase in SOC stocks of +7.1% [4]. With regard to total SIC, ⁓24% of soil carbonates were found in the coarse fraction and ⁓16% in the fine fraction. No differences were observed between treatments, either in total SIC or in the coarse fraction, but there were differences in the fine fraction of irrigated maize compared to rainfed maize (-1%).

Clear differences in δ¹³C-SIC were however observed between treatments. In bulk soil, δ¹³C-SIC decreased from −3.80‰ under rainfed maize to −4.14‰ under irrigated maize. In the coarse fraction, the shift was more pronounced, from −3.70‰ to −4.95‰, while intermediate changes were observed in the fine fraction (from −3.94‰ to −4.20‰). These isotopic shifts indicate that irrigation, together with increased organic matter inputs, activated carbonate dissolution–precipitation cycles, thereby increasing the relative contribution of pedogenic carbonates.

Furthermore, the preferential accumulation of SIC in the coarse fraction may be related to the formation of pseudo-sands driven by carbonate cementation within aggregates [5], highlighting the need to adjust ultrasonic energy during particle-size fractionation.

Overall, our results demonstrate that irrigation triggers SIC dynamics in calcareous agricultural soils, promoting carbonate dissolution and precipitation processes even in the absence of significant changes in total SIC content, and emphasize the importance of jointly considering SOC and SIC to accurately interpret pedogenic carbonate formation under contrasting agricultural management regimes.

 

References

[1] Ball et al. (2023), https://doi.org/10.1016/j.soilbio.2023.109189

[2] de Soto et al. (2017), https://doi.org/10.1016/j.geoderma.2017.03.005

[3] de Soto et al. (2024), https://doi.org/10.1016/j.catena.2024.108362

[4] Antón et al. (2022), https://doi.org/10.3389/fsoil.2022.831775

[5] Rowley et al. (2018), https://doi.org/10.1007/s10533-017-0410-1