Effects of long-term compost and fertilizer application on stability of aggregate-associated organic carbon in an intensively cultivated sandy loam soil

The study examined the influence of compost and mineral fertilizer application on the content and stability of soil organic carbon (SOC). Soil samples collected from a long-term field experiment were separated into macroaggregate, microaggregate, and silt + clay fractions by wet-sieving. The experiment involved seven treatments: compost, half-compost N plus half-fertilizer N, fertilizer NPK, fertilizer NP, fertilizer NK, fertilizer PK, and control. The 18-year application of compost increased SOC by 70.7–121.7%, and mineral fertilizer increased by 5.4–25.5%, with no significant difference between control soil and initial soil. The C mineralization rate (rate per unit dry mass) in microaggregates was 1.52–2.87 mg C kg⁻¹ day⁻¹, significantly lower than in macroaggregate and silt + clay fractions (P < 0.05). Specific C mineralization rate (rate per unit SOC) in silt + clay fraction amounted to 0.48–0.87 mg C g⁻¹ SOC day⁻¹ and was higher than in macroaggregates and microaggregates. Our data indicate that SOC in microaggregates is more stable than in macroaggregate and silt + clay fractions. Compost and mineral fertilizer application increased C mineralization rate in all aggregates compared with control. However, compost application significantly decreased specific C mineralization rate in microaggregate and silt + clay fractions by 2.6–28.2% and 21.9–25.0%, respectively (P < 0.05). By contrast, fertilizer NPK application did not affect specific C mineralization rate in microaggregates but significantly increased that in silt + clay fractions. Carbon sequestration in compost-amended soil was therefore due to improving SOC stability in microaggregate and silt + clay fractions. In contrast, fertilizer NPK application enhanced SOC with low stability in macroaggregate and silt + clay fractions.