Effects of long-term fertilization on calcium-associated soil organic carbon: Implications for C sequestration in...

Although the contribution of calcium ion (Ca²⁺) to stabilizing organic carbon (OC) in soils has been known for years, we still have a limited understanding of the quantity and molecular composition of Ca²⁺ bound SOC (Ca-OC) evolution in response to long-term fertilization. Here we report the role of Ca²⁺ in the accumulation of OC in the topsoil (0-20 cm) from two long-term (25-37 years) fertilization experiment sites. Approximately 4.54-19.27% and 9.00-25.15% of SOC was bound with Ca²⁺ in the Ferric Acrisol and Fluvic Cambisol, respectively. The application of NPK mineral fertilizers (NPK) decreased (p < 0.05) the Ca-OC stocks from 3.40 t ha^-1 to 0.96 t ha^-1 and from 2.03 t ha^-1 to 1.17 t ha^-1 in the Ferric Acrisol and Fluvic Cambisol, respectively. Swine manure (M) addition did not change (p > 0.05) the Ca-OC stock in Ferric Acrisol, but enhanced (p < 0.05) that from 2.03 t ha^-1 to 9.75 t ha^-1 in Fluvic Cambisol. Fourier transform infrared and carbon (1s)-near X-ray absorption spectroscopies showed that Ca²⁺ was mainly bound with aromatic carbon and carboxylic carbon. Long-term M fertilization facilitated the binding of Ca²⁺ with O-alkyl C, suggesting an increment of Ca-linked polysaccharide. Calcium ion was preferentially associated with ¹³C enriched organic matter (OM). Mineral fertilization promoted the ¹³C-enriched organic compounds in the Ca-OC, while organic fertilization facilitated the binding of ¹³C-depleted organic C with Ca²⁺. This study suggests that Ca-OC may be a potentially vital and stable OC pool in arable soils, and provides direct evidence for the preferential association of OC with Ca²⁺ in edaphic environments.