Responses of SOC, labile SOC fractions, and amino sugars to different organic amendments in a coastal saline-alkali soil

Organic amendment combined with mineral fertilizer is known as an effective management practice to improve soil carbon (C) sequestration in cropland. We evaluated the effects of three organic amendments combined with mineral fertilizer on soil organic carbon (SOC), labile SOC fractions, amino sugars, C mineralization, and C-cycle enzyme activities in a coastal saline-alkali soil. The three amendments, including straw (C/N of 75.5:1), straw biochar (C/N of 43:1), and soybean cake (C/N of 7.4:1), were continuously added into the soil at an equal C amount of 2250 kg C ha−1 for four years. Compared to mineral fertilizer alone, the contents of SOC in plots receiving straw, straw biochar, and soybean cake were increased by 7.6%, 17.3%, and 12.7%, respectively. The amendment of straw biochar mainly increased easily oxidizable carbon (EOC), particulate organic carbon (POC), and fungal-derived glucosamine, whereas straw and soybean cake mainly increased dissolved organic carbon (DOC), microbial biomass carbon (MBC), and bacteria-derived muramic acid. Meanwhile, the activities of C-cycle enzymes and C mineralization were higher in straw and soybean cake treatments than in straw biochar treatment. EOC and POC, but not DOC and MBC, were strongly positively correlated with fungal-derived glucosamine and SOC, suggesting that EOC or POC could be used as appropriate indicators to predict SOC sequestration potential. In addition, the C/N ratio of organic material did not affect SOC and its components. In conclusion, straw biochar promotes the maximum accumulation of SOC through increasing EOC, POC, and fungal necromass.