Long-term application effects of chemical fertilizer and compost on soil carbon under intensive rice-rice cultivation

From a long-term fertilizer experiment on rice-rice cropping in Typic Endoaquept, established at the Central Rice Research Institute, Cuttack, India in 1969, effects of application of composted manure (5 Mg ha(-1) year(-1)) and chemical fertilizers (N, NP, NK, and NPK twice in a year), in series without compost (C-0) or with compost (C-1) on changes in soil carbon and microbial pools were examined by comparing the soils archived in 1984 and those sampled in 2004. Mean concentrations of soil organic carbon (SOC) varied between 5.5 and 7.6 g kg(-1) in 1984, and 6.8 and 10.8 g kg(-1) in 2004, respectively. Temporal increases in the total amounts of carbon, which reflect the carbon sequestration potential of the soil followed the order: C-1 + NK > C-1 + NP = C-1 + NPK > C-1 + N = C-1-control > C-0 + NP = C-0 + NK > C-0 + NPK > C-0-control > C-0 + N. Fractions of H2O-C and K2SO4-C were higher in 1984, especially in those soil treated without compost. A reverse trend was observed in case of KMnO4-C and carbohydrate-C fractions. The continuous application of compost enhanced microbial biomass carbon as well as active microbial biomass carbon in 2004. Long-term application of chemical fertilizers in combination, rather than N alone, had beneficial effects on soil carbon and microbial pools. Compost application, even once a year, invariably led to higher increments in both soil carbon and microbial pools and the combinations of chemical fertilizers with compost generally showed comparable effects in the long-term.