Long-term fertilization increases soil organic carbon and alters its chemical composition in three wheat-maize cropping sites across central and south China

Soil organic carbon (SOC) is at the core of soil fertility. Although fertilization strategies can alter SOC stocks, their effects on SOC chemical composition is less known. Using the solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy, we examined changes in the SOC chemical composition of three soils (0-20 cm depth) from an annual wheat-maize double-cropping system across central to south China. These soils had been subjected to 22 years (1990-2012) long-term fertilization. Compared with no-fertilization control, SOC stocks were significantly increased under chemical fertilization (NPK), NPK plus straw (NPKS), and NPK plus manure (NPKM). The O-alkyl C (labile C), not the alkyl C (persistent C), was consistently increased across the three fertilized treatments. Additionally, all fertilized treatments decreased the ratio of alkyl-C/O-alkyl-C (SOC decomposition index) or aliphatic-C/aromatic-C (SOC complexity index), indicating that the SOC decomposition was delayed, or SOC was converted into a more complicated structure. The soil C of NMR-determined functional groups (alkyl C, O-alkyl C, aromatic C, and carbonyl C) was positively correlated with the cumulative C input (P < 0.05). The conversion rate of functional groups was highest in O-alkyl C, indicating a largest contribution to the increase of SOC accumulation. Soil C:N ratio, pH, and clay were the main factors affecting the functional-group conversion rates, whereas annual precipitation, temperature, and accumulated temperature (> 10 degrees C) played smaller roles. In conclusion, these results can be applied to the improvement of agricultural soil C sequestration capacity through changing SOC chemical structure under long-term fertilizer managements.