Effects of Partial Substitution of Chemical Fertilizers with Organic Fertilizers on Aggregate Stability and Organic Carbon Content in Sandy Soil

Soil aggregates and organic carbon are core elements in efforts to improve soil fertility, and this role is particularly pronounced in sandy soils of northwestern China. Based on a four-year consecutive field positioning experiment, this study investigated the effects of organic fertilizer application on soil aggregate stability and SOC sequestration capacity. It evaluated the impacts of different organic fertilizer substitution ratios—no fertilizer (Ctrl), 100% chemical fertilizer (T0), 30% organic fertilizer substitution (T1), 60% organic fertilizer substitution (T2), and 100% organic fertilizer substitution (T3)—on aggregate composition, stability, and SOC in the 0–20 cm and 20–40 cm soil layers. The results indicated the following: (1) Compared to the T0 treatment, the T3 treatment significantly increased the proportion of aggregates in the >2 mm and 2–0.25 mm size fractions in the 0–20 cm layer by 26.64% and 10.66%, respectively. (2) The T3 treatment significantly enhanced the Mean Weight Diameter (MWD) and Geometric Mean Diameter (GMD) of aggregates in the 0–20 cm layer, with increases ranging from 17.91% to 21.5% and from 44.01% to 50.43%, respectively, compared to T0. (3) Application of organic fertilizer significantly increased the SOC storage in >2 mm and 2–0.25 mm aggregate fractions. Moreover, MWD, GMD, and the content of macro-aggregates (R>0.25) all showed highly significant positive correlations with the SOC content in the >2 mm fraction. Organic fertilizer application reduced the organic carbon storage in the 0.25–0.053 mm and <0.053 mm aggregate fractions within the 0–20 cm layer. In conclusion, application of organic fertilizer provided conditions for the formation of macroaggregates, increased the organic carbon content in soil macroaggregates, and improved soil fertility status.