Objectives The purpose of this study was to investigate the characteristics of photosynthetic carbon (C) sequestration in rhizosphere and non-rhizosphere soil aggregates under different fertilization treatments, and to quantify the contribution of photosynthetic C to organic C of soil aggregates, so as to provide theoretical basis for C sequestration and sustainable development of farmland soil.

Methods Based on the long-term experimental station of Shenyang Agricultural University, this study selected no fertilization (CK) and chemical fertilization (NPK) treatment plots to repeatedly conduct the in-situ ¹³CO₂ pulse labeling for many times from booting to tasseling stage of maize. The rhizosphere and non-rhizosphere soil samples were taken destructively at the matured period of maize, and the wet sieving method was used to separate different sizes of soil aggregates, and the organic C content and δ¹³C value in each soil aggregates were measured.

Results Rhizosphere effect significantly affected the proportions of >0.25 mm and <0.053 mm aggregates in soil, and the proportion of >0.25 mm aggregate was higher in CK treatment than that in NPK treatment, while the proportion of <0.053 mm aggregate was lower in CK treatment than that in NPK treatment. Fertilization significantly affected the content of ¹³C fixed in each size aggregate, and the contribution of ¹³C to organic C within the 0.25−0.053 mm and <0.053 mm aggregates; Rhizosphere effect significantly affected the content of fixed ¹³C and its contribution to organic C in 0.25−0.053 mm aggregate; The interaction effect between fertilization and rhizosphere effect significantly affected the content of ¹³C fixed in 0.25−0.053 mm aggregate, and the contribution of ¹³C to organic C within the 0.25−0.053 mm and <0.053 mm aggregates. Compared with CK treatment, the content of ¹³C fixed in >0.25 mm aggregates of rhizosphere soil was 41.80% lower, but that of non-rhizosphere soil was not significantly different, that in 0.25−0.053 mm aggregates of rhizosphere and non-rhizosphere soils were 20.43% and 126.19% higher in NPK treatment, respectively. Regardless of fertilization, the content of ¹³C fixed in 0.25−0.053 mm aggregate of rhizosphere soil was 69.48% higher than that of non-rhizosphere soil. Compared with CK treatment, NPK treatment significantly reduced the contribution of the fixed ¹³C to organic C within <0.053 mm aggregates by 52.17% in rhizosphere and by 20.00% in non-rhizosphere soil, and doubled that in 0.25−0.053 mm aggregates of non-rhizosphere soil. In the CK treatment, the ¹³C contribution of fixed ¹³C in <0.053 mm aggregates of rhizosphere soil was 15.00% higher than that of non-rhizosphere soil, but was 45.45% higher in NPK treatment.

Conclusions Photosynthetic C is mainly fixed in >0.25 mm soil aggregates. Fertilization reduces the fixation of photosynthetic C in >0.25 mm soil aggregate, enhances that in 0.25−0.053 mm aggregates, especially in rhizosphere soil. And fertilization significantly decreases the contribution of photosynthetic C to organic C of <0.053 mm aggregate of rhizosphere soil. Therefore, rhizosphere effect enhances photosynthetic C fixation in 0.25−0.053 mm aggregates, but weakens that in <0.053 mm aggregate.