Effect of moisture content on carbon dioxide emissions in calcareous soils of the Comarca Lagunera, Mexico

Calcareous soils are characterized by containing a greater amount inorganic carbon (SIC) than organic carbon (SOC), and both contribute to CO₂ emissions to the atmosphere. SOC mineralization and SIC dissolution are related to soil moisture content, but their effect on CO2 emissions from calcareous soils is unclear. This investigation aimed to evaluate the effect of moisture content on CO₂ emission of a calcareous soil in the Comarca Lagunera, Mexico.

Calcareous soil samples were taken from a cropland and shrubland of Comarca Lagunera, Mexico and their physical and chemical properties were determined. For a 30-day period, 100g of soil were incubated in closed-jars and two moisture treatments, related to field capacity (FC) and permanent wilting point (PWP) values were applied. The CO₂ emission assessment was performed every two days using an infrared gas analyzer (IRGA, PP Systems, UK).

For cropland, the FC, PWP, SIC and SOC values were 27.2 %, 14.6 %, 7.3 % (140.4 Mg ha^-1) and 0.23 % (4.4 Mg ha^-1), while for shrubland, the values were 27 %, 11 %, 7.6 % (152.8 Mg ha^-1) and 0.08 % (1.6 Mg ha^-1), respectively. Average emission of CO₂, every two days, from cropland soil was 2.1 g CO₂ m^-2 h^-1 for moisture at FC, while to PWP was 1.7 g CO₂ m^-2 h^-1, and for shrubland soil was 1.8 g CO₂ m^-2 h^-1 for moisture at FC, while to PWP was 1.6 g CO₂ m^-2 h^-1.

In both cases, cumulative CO₂ emissions were significantly higher in FC compared to PWP. For cropland, cumulative CO₂ emissions were 23.4 g CO₂ m^-2 h^-1 and 29.4 g CO₂ m^-2 h^-1, but for shrubland were 21.7 g CO₂ m^-2 h^-1 and 25.3 g CO₂ m^-2 h^-1. Cumulative CO₂ emissions for moisture content at FC were equivalent to a soil carbon (C) loss of 1.9 Mg ha^-1 and 1.7 Mg ha^-1 for cropland and shrubland, respectively. This result implies the loss of 43.2% (1.9 Mg C ha^-1 / 4.4 Mg SOC ha^-1) of the SOC content in the cropland, but for the shrubland it suggests the total loss of the SOC (1.6 Mg C ha^-1 / 1.6 Mg SOC ha^-1) and a part of the SIC content (0.1 Mg C ha^-1 / 152.8 Mg SIC ha^-1).

Our study shows that soil moisture content has a significant effect on CO₂ emissions from calcareous soils, such as Comarca Lagunera, where an increase in soil moisture corresponds to increases in CO₂ emissions into the atmosphere, where SIC and SOC reserves are involved.