Distribution and stability of soil aggregates and carbon sequestration in purple paddy soil under long-term fertilization

【Objectives】The objectives of this study were to investigate influences of long-term different fertilizer treatments on distribution and stability (in terms of mean weight diameter, MWD) of soil aggregates and responses of carbon maintained in soil aggregates to carbon input in a 30-year's field fertilizer experiment. 【Methods】 Soil samples were separated into four aggregate-size classes (2 mm, 0.25-2 mm, 0.053-0.25 mm, and 0.053 mm) by the wet sieving method, and distribution of soil aggregates and relationship between carbon sequestration rate and carbon input were analyzed. 【Results】 Compared with CK, all the fertilizer treatments (N, NP, NPK, M, MN, MNP and MNPK) significantly improve the proportions of 2 mm soil aggregates by 9.6%-36.1%, but reduce 0.25-2 mm aggregates by 6.7%-26.3%, while there are no significant impact on aggregates within the range of 0.053-0.25 mm or0.053 mm. Application of mere chemical fertilizer(s) (N, NP and NPK) does not significantly change stability of soil aggregates, but the combined application of manure with chemical fertilizers (MNP and MNPK) does significantly enhance stability of soil aggregates. Compared with CK, all fertilizer treatments significantly increase the total soil organic carbon content and that in 2 mm fraction, and the combination of chemical fertilizers and manure achieves even better results than the application of chemical fertilizer(s). After the 30-year experiment, the soil organic carbon contents in CK and N treatments are reduced, and the decomposition rates in these two treatments are 0.08 t/(hm2a) and 0.02 t/(hm2a), respectively. The averaged soil carbon sequestration rates in the treatments of chemical fertilizers (NP and NPK), M and combined application (MN, MNP and MNPK) are 0.14, 0.10 and 0.17 t/ (hm2a), respectively. There was a significant positive linear correlation between carbon sequestration and carbon input (R2 =0.531, P0.05), and the conversion rate of carbon input to soil organic carbon is 3.3%. With the increase of carbon input, the carbon contents in different aggregates are increased gradually. However, the carbon increase is significantly higher in 2 mm fraction than those in other fractions. 【Conclusions】 Changes of stability of soil aggregates were more evident in the combined application of manure with chemical fertilizers treatments than in the application of chemical fertilizer(s) treatments. The soil organic carbon does not reach the saturation point in the purple soil even after receiving 30-year fertilization and there is still considerable potential to sequester more carbon. The organic carbon is mainly sequestrated in 2 mm size fraction, which is the key component for soil carbon sequestration.