Is Soil Inorganic Carbon (CaCO3, SIC) Sequestration a Bane or a Hidden Treasure in Soil Ecosystem Services?

Abstract

Various estimates of the world storage of SOC are available, but there is very little effort to estimate the carbon stored in inorganic form, primarily as CaCO3. The soils that store large quantities of carbonates play an important role in global carbon cycle. Calcareous soils of India occupy an area of 228.8 m ha and cover 69.4% of the total geographical area (TGA) and spread over 38 out of 60 agro-ecological subregions, and the SIC stock is around 33.94Pg in the first 0–1.5 m depth. The SIC stock is mainly due to pedogenic formation of CaCO3 (PC) in semi-arid tropical (SAT) soils, which reduces crop productivity. The SOC stock (in the first 0–0.3 m depth) is more than double (9.55Pg) than SIC (4.14Pg). This suggests that the shallow-rooted agricultural crops enhance OC concentration in the rooting zones by preventing the rapid formation of PC, which is an important factor in rainfed agriculture in SAT environments. Current productivity of farmers’ fields in the rainfed tropics is two- to fourfold lower than the achievable crop yields. The arid and semi-arid environments cover more than 50% of the total geographical area of India, and the soils under such areas are, by and large, used for rainfed agriculture. In dry climates the primary pedogenic process is the calcification, which deprives soils of Ca ions both in solution and on exchange complex. Due to the lack of adequate Ca ions, both physical and chemical properties of soils are modified and cause reduction in crop yield. However, in view of the present soil productivity of calcareous SAT soils as evidenced from their support in India’s growing self-sufficiency in food production and food stocks since independence, a critical review is warranted to establish whether SIC is a useless C reserve and has no role in soil ecosystem services during the cultural practices of agricultural crops, under forest and natural grassland cover. The presence of PC is common in major soil types of semi-arid tropical (SAT) in India (alluvial soils of the Indo-Gangetic Plains, IGP, red ferruginous soils and shrink-swell soils). The formation of PC in the arid climate enhances the pH and also the relative abundance of Na+ ions on soil exchange sites and in the solution; and the Na+ ions in turn cause dispersion of the fine clay particles. The dispersed fine clays translocate in major soil types of India as the formation of PC creates a Na+-enriched chemical environment conducive for the deflocculation of clay particles and their subsequent movement downward. Therefore, the formation of PC and the clay illuviation are two concurrent and contemporary pedogenetic events, resulting in an increase in relative proportion of sodium, causing increased sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) and pH values with depth. These pedogenetic processes continue to represent a pedogenic threshold during the dry climates of the Holocene. Thus, the formation of PC is a basic natural degradation process, induced by tectonic-climate-linked events, which exhibits the regressive pedogenesis by capturing atmospheric CO2 and also immobilizes C in unavailable form in soils. This pedogenic process results in substantial retardation of the emission of CO2 from both cultivated and noncultivated SAT soils and also supports the recent world views on no positive role of anthropogenic CO2 increase in the global warming phenomenon.