Organic fertilizer enhances soil aggregate stability by altering greenhouse soil content of iron oxide and organic carbon

Both soil organic carbon (SOC) and iron (Fe) oxide content, among other factors, drive the formation and stability of soil aggregates. However, the mechanism of these drivers in greenhouse soil fertilized with organic fertilizer is not well understood. In a 3-year field experiment, we aimed to investigate the factors which drive the stability of soil aggregates in greenhouse soil. To explore the impact of organic fertilizer on soil aggregates, we established four treatments: no fertilization (CK); inorganic fertilizer (CF); organic fertilizer (OF); and combined application of inorganic and organic fertilizers (COF). The application of organic fertilizer significantly enhanced the stability of aggregates, that is it enhanced the mean weight diameter, geometric mean diameter and aggregate content (%) of >0.25 mm aggregate fractions. OF and COF treatments increased the concentration of SOC, especially the aliphatic-C, aromatic-C and polysaccharide-C components of SOC, particularly in >0.25 mm aggregates. Organic fertilizer application significantly increased the content of free Fe (Fed), reactive Fe (Feo), and non-crystalline Fe in both bulk soil and aggregates. Furthermore, non-crystalline Fe showed a positive correlation with SOC content in both bulk soil and aggregates. Both non-crystalline Fe and SOC were significantly positively correlated with >2 mm mean weight diameter. Overall, we believe that the increase of SOC, aromatic-C, and non-crystalline Fe concentrations in soil after the application of organic fertilizer is the reason for improving soil aggregate stability.