Organic fertilization increased soil organic carbon stability and sequestration by improving aggregate stability and iron oxide transformation in saline-alkaline soil

Purpose The poor soil structure and low stability of organic carbon limit the sustainable development of agriculture. The effect of organic fertilizer application on soil aggregates, soil organic carbon (SOC) functional groups, Fe oxides, SOC stability, and storage was assessed in saline-alkaline paddy soil. Methods A 7-year field experiment was designed with organic fertilization in saline-alkaline soil of Yellow River Delta including (i) CK (no fertilizer), (ii) NPK (255, 28, and 190 kg ha(-1) year(-1) N, P, and K mineral fertilizers, respectively), (iii) NPKC1 (NPK + 450 kg C ha(-1) year(-1)), and (iv) NPKC2 (NPK + 900 kg C ha(-1) year(-1)). Results Compared with NPK treatment, the mean weight diameter (MWD) of soil aggregates in NPKC1 and NPKC2 treatments was increased by 27.8% and 41.1%, respectively. Soil amorphous Fe oxide (Feo) content was significantly increased with organic fertilizer addition, especially in small macro-aggregates. Meanwhile, aromatic-C was primarily distributed in small macro-aggregates, which could be retained in aggregate interfaces by forming aromatic-Fe complexes with soil Feo. Furthermore, the specific C mineralization rate (SCMR) in NPKC1 treatment was lower than that in other treatments, in which the lowest SCMR was found in small macro-aggregate. Compared with NPK treatment, SOC storage in NPKC1 and NPKC2 treatments was increased by 14.4% and 20.5%, respectively, which was due to the improvement of soil structure, organo-Fe complex formation, and reduction of soil C mineralization. Conclusions The combined organic and mineral fertilizer application was an optimized management practice to improve the C stability and sequestration in coastal saline alkaline paddy soil.