Characteristics of Organic Carbon Fractions and Carbon Dioxide Emissions of Different Size Aggregates in Rice Field Soils in Response to Long-Term Fertilization

This study aimed to investigate the dynamic characteristics of organic carbon (OC) and its fractions, carbon dioxide (CO₂) emissions in different particle size aggregates under different fertilization regimes in paddy soil, and to analyze the influencing factors, so as to provide a scientific foundation for enhancing carbon sequestration and CO₂ emission reduction in paddy soils from an aggregate perspective.

Based on a soil incubation experiment, and using dichromate volumetric method, physical separation method and headspace periodic gas collection-gas chromatography, the OC and its fractions, CO₂ emission characteristics of different particle size aggregates (<0.25 mm, 0.25-1 mm, 1-2 mm) were comparatively analyzed under different fertilization treatments (no fertilization, CK; conventional fertilization, NPK; and 50% substitution for 50% nitrogen fertilizers, HOM).

(1) The NPK treatment decreased OC content in 1-2 mm aggregates compared with the CK. In contrast, compared with NPK, the OC content of aggregates treated with HOM treatment increased by 5.8% to 29.4%, respectively. Both fertilization treatments (NPK and HOM) significantly enhanced the content of particulate organic carbon (POC) in 0.25-1 mm and 1-2 mm aggregates, with increases of 37.4%-79.6% and 4.3%-47.8% compared with CK, respectively, and HOM facilitated the turnover of mineral-associated organic carbon (MAOC) from 1-2 mm to <0.25 mm aggregates. (2) There were significant differences in the correlations between POC, MAOC, and OC across various aggregate particle sizes. For <0.25 mm aggregates, both POC and MAOC exhibited significantly positive correlations with OC content. For 0.25-1 mm aggregates, a significant positive correlation existed between MAOC and OC content. For 0.25-1 mm aggregates, a significant positive correlation existed between POC and OC content. (3) During the incubation experiment period, cumulative CO₂ emissions of <0.25 mm aggregates were the lowest, which significantly reduced by 51.0%-83.7% and 23.1%-89.9% compared with 0.25-1 mm and 1-2 mm aggregates, respectively. Compared with NPK, HOM treatment significantly reduced the cumulative CO₂ emissions in <0.25 mm and 0.25-1 mm aggregates by 146.27 and 364.27 g·kg^-1, respectively, which was related to the changes in organic carbon fractions and the increase of aggregate pH and C/N under HOM treatment.

Different fertilization regimes have altered the OC and its fractions, CO₂ emissions in different particle size aggregates. In detail, organic substitution promoted the accumulation of OC in aggregates, significantly increasing the POC content in 0.25-1 mm and 1-2 mm aggregates. Concurrently, it promoted the transfer of MAOC from 1-2 mm to <0.25 mm aggregates, while significantly reducing the CO₂ emissions in <0.25 mm and 0.25-1 mm aggregated relative to conventional fertilization. Overall, organic substitution for 50% nitrogen fertilizers was beneficial for reducing the CO₂ emissions of <0.25 mm and 0.25-1 mm aggregates in paddy field and enhancing the stability of organic carbon.