Objectives To characterize organic carbon fractions within macroaggregates in arable soils and understand mechanisms of soil organic carbon sequestration and optimize the fertilizer management.

Methods Soil samples were collected from 0–10 cm and 10–20 cm layers of a 35-year long term fertilizer experiment in an Anthrosol in Yangling, Shaanxi Province, and organic carbon (OC) contents were analyzed within the macroaggregate size fractions (coarse particulate organic carbon referred to as cPOC, fine particulate organic carbon referred to as fPOC, intra-microaggregate particulate organic carbon referred to as iPOC and mineral associated organic carbon referred to as MOC). The fertilization treatments were 1) Control, receiving no fertilizers; 2) NP, receiving chemical nitrogen and phosphorus fertilizers; 3) M, receiving organic manure alone, and 4) MNP, receiving the combined application of chemical NP fertilizers and organic manure at the same rates as treatments NP and MNP.

Results The long-term application of chemical fertilizers (NP) had no effects on the OC contents of any macroaggregate size fractions. However, application of manure alone or combined with chemical fertilizers greatly enhanced cPOC contents by 174%–338% (0–10 cm) and 215%–245% (10–20 cm), iPOC content by 127%–241% (0–10 cm) and 106%–130% (10–20 cm), and MOC content by 28.9%–34.6% in the 0–10 cm soil layer. Moreover, application of manure plus NP significantly increased fPOC content in the 0–10 cm layer by 482%. In addition, OC contents of all macroaggreagte fractions were significantly and positively correlated with cumulative C input, especially for the iPOC content. These results indicated that the differences among fertilization regimes in OC sequestration occurred primarily in the physically protected OC fractions contained in macroaggregates.

Conclusions Application of NP showed no effect on OC contents within macroaggregates, while application of M treatment could increase OC contents within macroaggregates, and MNP treatment significantly enhanced OC contents within macroaggregates, especially for iPOC content. Amendment of organic manure, alone or in combination with chemical fertilizers, could increase soil organic carbon (SOC) storage. Our data indicated that the combination of organic and chemical fertilizer is the most effective practice for increasing SOC storage in an Anthrosol.