Carbon and nitrogen fractions and stocks under 41 years of chemical and organic fertilization in a sub-humid tropical rice soil

- Soil profile C&N fractions and C sequestration analyzed after 41 years of fertilization.
- Soil C and N stocks changed positively across the fertilizer and manure treatments.
- Differential accumulation pattern of SOC and N fractions in different depths observed.
- As compared to initial sequestration of C was also observed in unfertilized control.

Effects of different fertilizers and manure on dynamics of soil organic carbon (SOC) and nitrogen fractions and their stock changes remains unclear under intensively cultivated rice soil profile. Profile soil samples in six treatments viz. control, nitrogen (N), nitrogen-phosphorus-potassium (NPK), farm yard manure (FYM), N + FYM and NPK + FYM were collected from a long term fertilizer experiment continuing since 1969 at ICAR-NRRI, Cuttack, India to study the soil organic carbon and nitrogen fractions (particulate organic carbon, POC; microbial biomass carbon, MBC; particulate organic nitrogen, PON; and microbial biomass nitrogen, MBN) and SOC sequestration. Results from the study revealed that in the control treatment, where no manure or chemical fertilizer was applied,the bulk density was highest. Balanced fertilization and combined use of chemical fertilizers and manure for 41 years increased the SOC in all the plots except the unfertilized control as compared with the initial value (6.6 g/kg). Balanced fertilization (NPK) and integrated fertilization (NPK + FYM) resulted in similar increases in particulate organic carbon, carbon mineralization and microbial biomass carbon, whereas particulate organic nitrogen, nitrogen mineralization and microbial biomass nitrogen were more in integrated fertilization (NPK + FYM) compared with control treatment. Soil organic C and nitrogen stocks changed positively across the fertilizer and manure treatments over the control. In the control plot, at 0-15 cm depth the soil carbon and nitrogen stock was 15.1 and 1.77 Mg ha−1, respectively which increased to the 19.5 Mg ha−1 in NPK + FYM for carbon and 2.25 Mg ha−1 in N + FYM for nitrogen. As compared to the initial (13.7 Mg ha−1), the SOC stock in 0-15 cm depth increased under all the fertilized treatments during 41 year period in the order: NPK + FYM > N + FYM > NPK > FYM > N > control. The rate of increase in SOC (carbon sequestration) due to fertilizer application alone varied between 57 and 89 kg ha−1 yr−1, while for FYM addition the rate of increase was 61 to 138 kg ha−1 yr−1, highest being in NPK + FYM. Correlation coefficients between different soil organic C and N fractions indicate differential accumulation pattern of organic C and nitrogen in different depths. Based on crop yield and C storage, integrated fertilization of chemical fertilizers and manure proved to be the best for improving crop productivity and SOC sequestration.