Objectives Soil particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) are the main components of soil organic carbon pool. Understand the changes in their content caused by the application of biochar, their proportion in the total soil organic matter and their chemical composition characteristics. It is of great significance for understanding the dynamic changes of soil organic carbon.

Methods This swine manure biochar application amount and method positioning test was conducted on acidic and calcareous paddy soil in Fuyang District, Hangzhou City in 2019. The planting system was single-crop rice. The experiment set up 6 treatments: conventional application of chemical fertilizer (NPK), application of swine manure 4.5 t/(hm²·a) (SM) based on NPK, application of swine manure biochar 4.5 t/(hm²·a) (SBc1) , 11.25 t/(hm²·a) (SBc2), 22.5 t/(hm²·a) (SBc3), and one-time application of swine manure biochar 112.5 t/hm² (SBc4) based on NPK fertilizer. After the rice harvest in 2021 (the third year), take soil samples from the 0−20 cm plow layer and analyze their basic physical and chemical properties. And the soil samples are divided into three particle sizes: 250−2000 μm (coarse particulate organic matter), 53−250 μm (fine particulate organic matter) and <53 μm (mineral-bound organic matter), analyze the content and proportion of organic carbon in soil at each particle size, and use Fourier transform infrared spectrometer to analyze the characteristic spectrum of soil organic carbon at each particle size, and use the characteristic spectrum Peaks provide a semi-quantitative assessment of changes in organic carbon composition.

Results Compared with SM treatment, the soil organic carbon content of SBc3 and SBc4 treatments in acidic paddy soil increased significantly by 49.4% and 103.3%, and the soil organic carbon content in calcareous paddy soil increased by 42.2% and 53.0%. Compared with SM, the four swine manure biochar treatments significantly increased the coarse particulate organic carbon (C-POC) content of acid paddy soil by 137.8%−554.1%, and the fine particulate organic carbon (F-POC) content by 37.6%−85.2%. (P<0.05); SBc3 and SBc4 treatments significantly increased the C-POC content in calcareous paddy soil to 110.0% and 203.0%, and the F-POC content to 54.8% and 96.0% (P<0.05); The effect of four swine manure biochar treatment on the MAOC content of the two paddy soils was not significant (P>0.05). Fourier transform infrared spectrum characteristic results show that the application of swine manure and swine manure biochar has no significant impact on the types of main organic compounds in each particle size of the two paddy soils, but it will affect their proportions. Compared with the NPK treatment, the SBc3 treatment and the SBc4 treatment significantly increased the proportion of aromatic carbon in acidic paddy soil by 19.5% and 38.0%, and significantly reduced the proportion of phenolic alcohol carbon in acidic paddy soil by 7.8% and 17.9%. In addition, the proportion of aromatic carbon in calcareous paddy soil was reduced by 21.0% and 19.1%, but the proportion of phenolic alcohol carbon was increased by 19.1% and 33.2% (P<0.05).

Conclusions Higher application rates of swine manure biochar, regardless of one-time application or annual application, can significantly increase the total organic carbon content of paddy soil, and the increase in coarse-grained organic carbon is greater than that of fine particles, but has no significant effect on the content of mineral-bound organic carbon. The application of swine manure biochar has no significant effect on the main organic compounds of soil organic carbon, but it will increase the stability of organic carbon in acidic paddy soil and reduce the stability of organic carbon in calcareous paddy soil. At the same application rate, the effect of swine manure biochar on increasing the organic carbon content of acidic paddy soil is higher than that of calcareous paddy soil. The results of this experiment can provide technical support and theoretical basis for the scientific application of swine manure biochar in farmland.