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.

Due to regressive pedogenesis, SAT soils show strikingly different soil properties as compared to normal soils, which pose a challenge to land resource managers on how to revive the required balance between exchangeable and water-soluble Ca²⁺ ions in the soil systems to make calcareous sodic soils resilient. Sodic soils in the north-western (NW) parts of the IGP show salt-efflorescence in the surface as an evidence of soil sodicity, and this soil characteristic is mappable using remote sensing techniques. However, such maps for non-zeolitic and non-gypsiferous Vertisols, which show poor saturated hydraulic conductivity (sHC) (<10 mm h⁻¹) even at ESP >5 < 15 and have poor crop productivity, are not available because of the absence of salt-efflorescence in the soil surface. Also, such soils do not qualify as salt-affected soils according to the US Salinity Laboratory criteria. Therefore, the reclamation technology developed by the Indian Council of Agricultural Research-Central Soil Salinity Research Institute (ICAR-CSSRI), Karnal, remained inaccessible to this kind of poorly drained and agriculturally less productive soils. Vertisols, which are impoverished in organic carbon (OC) but are rich in CaCO₃, show enough resilience under improved management (IM) system (without adding gypsum and FYM) of the International Crops Research Institute for Semi-Arid Tropics (ICRISAT). The average grain yield of the IM system over 30 years was five times more than that in the traditional management (TM) system. Due to the improvements in physical, chemical and biological properties of soils after adaptation of the IM system, the poorly drained black soil (Sodic Haplusterts) now qualifies for well-drained soils (Typic Haplusterts). Continuous release of higher amount of Ca²⁺ ions during the dissolution of CaCO₃ under the IM system, compared to slower rate of formation of CaCO₃, provides enough soluble Ca²⁺ ions to replace unfavourable Na⁺ ions on the soil exchange sites. Higher exchangeable Ca/Mg ratio in soils under IM system improved the sHC for better storage and release of soil water during the dry spell between rains. Adequate supply of soil water helped in better crop productivity and higher OC sequestration. The improvement in Vertisols’ sustainability suggests that the IM system is capable of mitigating the adverse effect of climate change. This management protocol though slow as compared to the gypsum-aided one is however cost-effective and farmer-friendly. This technology is recommendable for a large-scale impact on agricultural productivity. It also creates an interesting area of soil research that is to realize the benefit of the presence of CaCO₃ as a hidden treasure and further as an ecosystem engineer during the reclamation of sodic soils. Role of SIC as an ecosystem engineer is also evidenced during the reclamation processes of IGP sodic soils even after the addition of gypsum. Sodic soils (Natrustalfs) of NW part of the IGP, after their reclamation by gypsum, improve in terms of their morphological, physical and chemical properties so much that these soils are now reclassified as well-drained and OC-rich normal Alfisols (Haplustalfs). Such remarkable resilience could be possible even with the low amount of added gypsum, suggesting that the added gypsum does not enrich soil solution by the required amount of Ca²⁺ ions to replace Na ions on the soil exchange sites. The fulfilment of Ca saturation could be possible by the dissolution of PC during the growing of the rice crop under submerged conditions. It was observed that the rate of dissolution of PC was much higher than its rate of formation in the top 1 m soil depth. Resilience of sodic soils of NW parts of the IGP through the dissolution of PC was also realized under protection of nearly four decades long naturally occurring grassland system. The released Ca²⁺ ions by dissolving PC effected through the acidic root exudates caused a remarkable reduction in pH and ECe and enhanced more than double OC concentration in the surface horizons. In addition, growing of trees for 12 years reclaimed the IGP sodic soils and improved their biological activity, which also resulted in a decrease in CaCO₃ content. These observations suggest that the improvement in soil properties was triggered first by biological activity followed by the release of Ca ions from PC. Even after becoming normal soils as Haplustalfs and Haplusterts, both IGP and Vertisols are still calcareous. In view of its slow rate of dissolution, it is quite likely that Ca-ions-enriched chemical environment would allow neither Haplustalfs nor Haplusterts to transform to any other soil order so long CaCO₃ would continue to act as a soil modifier. Positive role of CaCO₃ in both the reclamation and sequestration of OC in SAT soils may benefit the maintenance of soil health of the farmlands, if additional financial support is made available to stakeholders of the sodic soils. The unique ecosystem services of SIC thus show enough potential to cause an increase in the present SOC stock of Indian soils beyond 30 Pg. Role of SIC in enhancing OC in SAT soils assumes a great importance in the present carbon cycle in addition to coupled carbonate weathering representing an atmospheric CO₂ sink.