Abstract: In order to explore the effects of different water tables (low water-table of 80 cm, high water-table of 20 cm) and fertilization treatments (high application of crop residue, common application of crop residue, and chemical fertilizer) on physically divided organic carbon fractions (free particulate organic carbon, fPOC; occluded particulate organic carbon, oPOC; and mineral combined organic carbon MOC), different organic carbon fractions of top soil were measured in a long-term field experiment initiated in 1981. The results showed that the content of particulate organic carbon (POC) was between 6.29-11.77 g·kg^-1 and increased with the amount of the application of crop residue. Moreover, the increment of POC content under the three fertilization treatments at the high watertable was greater than at the low watertable. Treatments with crop residue had a much higher percentage of POC (fPOC/TOC, oPOC/TOC) than those with chemical fertilizer at each of the two watertables, especially at the low water-table. Each organic carbon fraction had a very significant correlation with the soil total organic carbon (TOC) and the organic carbon combined with aggregates with particle size >2 mm. Applying crop residue can improve soil TOC and POC both at high and low water-tables, especially at the high water-table.

Key words: salinity, antioxidant, Ulva prolifera, 24-epibrassinolide, chlorophyll fluorescence parameters