Sub-soil Nitrogen Content as Influenced by Long-term Manuring and Its Relationship with Nitrogen Availability and Productivity of a Rice-rice Cropping System in Eastern India

Investigation was made to study the impact of long-term fertilizer and manure application on the sub-soil nitrogen (N) fertility of an acidic Inceptisol under continuous rice-rice cropping system. For this purpose, a long-term fertilizer experiment commenced from 2005-06, rabi season in the Central Farm of Odisha University of Agriculture and Technology (OUAT), Bhubaneswar under aegis of ICAR, New Delhi was used. The experiment had 12 manurial treatments whose impact on sub-soil N fertility up to 75 cm depth and its relation with surface soil N fertility was studied. N uptake and crop yield has been assessed after 20 cropping cycles. The initial soil was acidic (pH 5.8) with low soil organic carbon (4.3 g kg -1) and CEC of 3.75 cmol (p+) kg-1, mineralizable N of 187 kg ha-1 and total N of 1280 kg ha-1. After 20 cropping cycles, the soil organic carbon (SOC) content increased in all the fertilized treatments in the surface layer and with depth it decreased sharply from 15-30 cm to 30-45 cm layer. Treatments with balanced nutrition registered higher content of total N, organic N, mineralizable N and inorganic N both in surface soil and sub-soil. Not only the surface soil but also the sub-soil contents of total N, organic N and mineralizable N are significantly influenced by long-term use of fertilizer nutrients and manure under wet land intensive rice production system. Sub-soil contents of total N, organic N, mineralizable N, inorganic N, ammonium N and nitrate N have strong positive correlation with those of surface layer. SOC, total N, mineralizable N and inorganic N are strongly correlated with each other in the same layer. Correlation of organic N and total N with nitrate N was however, significant up to 30 cm depth and beyond that there was no correlation at 30-45 cm layer. From the results, it is concluded that application of nutrient in balanced and integrated manner (NPK + FYM) not only increased grain yield and sustainability of rice but also improved various pools of N in surface as well as sub-soil up to 75 cm depth of a wet land rice-rice production system under subtropical climatic situation.