Abstract: 【Objective】 Based on investigation of organic carbon and total nitrogen distribution in aggregates of red paddy soil under different fertilization treatments, in this study, soil organic carbon mineralization in aggregate fractions was further investigated, and the influence factors of organic carbon mineralization were also analyzed. The research results could help to better understand the mechanisms of influence of fertilization on soil fertility and organic carbon mineralization.【Method】Soil samples were collected from a long-term field experiment conducted in red paddy soil, which included nine fertilization treatments, namely no fertilizer (CK), organic cycling (C), N fertilizer (N), N fertilizer plus organic cycling (NC), N and P fertilizer (NP), N, P and K fertilizer (NPK), N, P and K fertilizer plus organic cycling (NPKC), N and K fertilizer (NK), N, P and K fertilizer plus 1/2 rice straw incorporation (NPKS). Those Soil samples were separated into five aggregate-size classes by wet sieving, as ＞2 mm, 1-2 mm, 0.25-1 mm, 0.053-0.25 mm and ＜0.053 mm. The dynamics of organic carbon mineralization in aggregates and bulk soils were detected, and microbial biomass carbon contents and invertase activity in aggregate fractions were mearsured.【Result】The mineralization rate of organic carbon in bulk soils and ＞1 mm aggregates decreased rapidly during the early stage of incubation, then decreased gradually and reached a stable state. However, the mineralization rate of organic carbon in ＜1 mm size classes, especially in 0.053-0.25 mm, showed a lesser decrease during the early stage of incubation and reached stability faster. The cumulative amounts of organic carbon mineralization were highest in ＞2 mm and 1-2 mm aggregates while those were lowest in 0.053-0.25 mm ones. Compared with CK, P fertilizer application (NP and NPK) and organic manure application (C, NC and NPKC) increased the cumulative amounts of organic carbon mineralization in aggregates by average 17.0%-62.1% and 25.0%-80.5%, respectively. Aggregates of ＞2 mm and 0.25-1 mm contributed most to cumulative amount of soil organic carbon mineralization, accounting for 21.0%-42.5% and 20.6%-32.7%, respectively. Microbial biomass carbon contents and invertase activity were higher in ＞0.25 mm macro-aggregates than those in ＜0.25 mm micro-aggregates. Microbial biomass carbon contents in aggregate fractions under P fertilizer and organic manure application treatments were 73.4%-92.0% and 60.8%-99.6% higher than those in aggregate fractions of CK. The application of P fertilizer and organic manure both significantly increased invertase activity in ＞0.25 mm macro-aggregates compared with CK. The invertase activity in macro-aggregates of NC, which was 46.0%-135.0% higher than those of CK, was highest among all the treatments. The cumulative amounts of organic carbon mineralization in aggregate fractions were significantly and linearly correlated with contents of organic carbon, total nitrogen and microbial biomass carbon, as well as with invertase activity. Yet, organic carbon contents had maximum relationship with the cumulative amount of organic carbon mineralization in aggregates【Conclusion】Macro-aggregates played the leading role in soil organic carbon mineralization. Organic carbon content was the most important factor affecting organic carbon mineralization in aggregate fractions. The application of P fertilizer and organic manure promoted organic carbon mineralization in aggregates of red paddy soil, being beneficial in enhancing the capacity of soil nutrient supplying.

Key words: soil aggregates, organic carbon mineralization, microbial biomass carbon, invertase activity, red paddy soil, long-term fertilization