Long‐Term Organic Fertilization Enhances Particulate Organic Matter Accumulation Through Macroaggregate Formation in the Loess Plateau

ABSTRACT Organic amendments are widely recommended to enhance soil organic carbon (SOC) in low‐fertility soils. Yet, SOC level remains persistently low in erosion‐prone croplands of the Eastern Loess Plateau. Here, we investigate how long‐term organic fertilization influences soil aggregation and SOC dynamics in loess soils based on a 33‐year maize field experiment with five treatments: no fertilizer, mineral nitrogen‐phosphorus (NP) fertilizer (M0), and NP fertilizers combined with three increasing rates of manure (M1‐M3). Results showed that higher manure rates treatments (M2 and M3) significantly increased the proportions of macroaggregates and free microaggregates at the expense of the free silt+clay fraction compared to M0. Concurrently, these treatments substantially enriched SOC content in macroaggregates (40%–81%) and free microaggregates (33%–41%), driving a preferential accumulation of particulate organic matter (POM‐C increased by 54%–61%) over mineral‐associated organic matter (MAOM‐C increased by 18%–24%). Consequently, the apparent carbon conversion efficiency (CCE) of exogenous organic inputs was 6.6% in bulk soil, with 5.3% in POM, 5.0% in macroaggregates, and 2.4% in free microaggregates. These results indicate that organic inputs drove the redistribution of carbon into larger aggregates, where fresh organic residues reinforced macroaggregate occlusion, protecting POM from decomposition. In contrast, MAOM stabilization was constrained by mineral surface saturation, leading to diminished efficiency under high carbon inputs. These findings suggest that promoting macroaggregate formation and POM enrichment is the key practical strategy for maximizing SOC persistence in dryland loess croplands.