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

摘要

Although the contribution of calcium ion (Ca~(2+)) 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 Ca~(2+) bound SOC (Ca-OC) evolution in response to long-term fertilization. Here we report the role of Ca~(2+) 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 Ca~(2+) 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 0.96t ha~(-1) and from 2.03 t ha~(-1) to 1.17t 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 2.03t ha~(-1) to 9.75t ha~(-1) in Fluvic Cambisol. Fourier transform infrared and carbon (1s)-near X-ray absorption spectroscopies showed that Ca~(2+) was mainly bound with aromatic carbon and carboxylic carbon. Long-term M fertilization facilitated the binding of Ca~(2+) with O-alkyl C, suggesting an increment of Ca-linked polysaccha-ride. Calcium ion was preferentially associated with ~(13)C enriched organic matter (OM). Mineral fertilization promoted the ~(13)C-enriched organic compounds in the Ca-OC, while organic fertilization facilitated the binding of ~(13)C-depleted organic C with Ca~(2+). 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 Ca~(2+) in edaphic environments.