Organo-mineral complexes in soil colloids: Implications for carbon storage in saline-alkaline paddy soils from an eight-year field experiment

The combination of organic carbon (OC) and reactive minerals is a crucial mechanism of soil carbon (C) storage, which is regulated by the formation of organo-mineral complexes on the surface of soil colloids. The effect of organic fertilizer on the storage mechanism of OC in soil colloids was studied through an 8-year field experiment, which included four treatments: i) no fertilization (control, CK), ii) only mineral N, P, and K fertilization (NPK), iii) NPK plus a low level (450 kg C ha^-1 year^-1) of organic fertilization (NPKC1), and iv) NPK plus a high level (900 kg C ha^-1 year^-1) of organic fertilization (NPKC2). The main results indicated that organic fertilizer addition significantly increased the content of aromatic-C, which was 158.7% and 140.0% higher in soil colloids than in bulk soil in the NPKC1 and NPKC2 treatments, respectively. X-ray photoelectron spectroscopy further demonstrated that the relative proportion of C=C group on the surface of soil colloids was increased by 20.1% and 19.1% in the NPKC1 and NPKC2 treatments, respectively, compared with the CK. In addition, compared with the NPK treatment, the content of reactive minerals (such as Fe and Al oxides) significantly increased with organic fertilization, which was positively correlated with C=C group in soil colloids. This indicates that aromatic-C may be retained by the formation of aromatic-mineral complexes with reactive minerals in soil colloids. Organic fertilization also significantly increased OC storage efficiency (OCSE), which was significantly higher in the NPKC1 treatment than in the NPKC2 treatment. Therefore, a moderate amount of organic fertilizer application is a better agronomic practice to increase OCSE and OC storage in saline-alkaline paddy soils.