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Nitrogen Dynamics in Soil as Influenced by Long-term Manuring and Fertilization under Rice Grown on Vertisol of Chhattisgarh

Effect of nutrient management practices on nitrogen (N) dynamics in rice grown on a Vertisol was studied after 16 years in an ongoing long-term experiment initiated during 1999-2000. The experiment comprised of different levels of NPK fertilizers alone and in combination with farmyard manure (FYM), blue green algae (BGA) and green manuring (GM) was laid out in split-plot design with four replications. Application of 100% NPK + FYM (5 t ha-1) significantly increased the organic N fractions viz., hydrolysable NH4-N (165 mg kg-1), amino acid-N (271 mg kg-1), amino sugar-N (36.8 mg kg-1) and hydrolysable unknown-N (127 mg kg-1). The mineral N (nitrate-N + ammonium-N) content (57.3 mg kg-1) was significantly increased with the application of 100% NPK + FYM. A decrease in N fractions was observed with advancement of crop growth stage and increase in the depth of soil from 0-15 to 15-30 cm. The contribution of inorganic and organic N fractions to total N was noted to the extent of 7 and 73%, respectively under 100% NPK + FYM, whereas comparatively lower contribution of these fractions was recorded in control treatment. The contribution of hydrolysable unknown N (HUN) to total hydrolysable N ranged between 21 to 31%, the highest contribution was noted in control (31%) and least contribution was recorded in 100% NPK + FYM. The grain yield (6.45 t ha-1) of rice was increased significantly with the combined application of manures and fertilizers. The total uptake of N by rice was increased significantly with the conjoint use of fertilizers and organic manure. The N recovery efficiency was decreased with increasing fertilizer levels. The rice grain yield was significantly correlated with various N fractions; the highest correlation coefficients among organic N fractions observed with total hydrolysable N at maximum tillering and harvest were r = 0.698** and r = 0.610** in surface and r = 0.635** and r = 0.593** in sub-surface layer of soil, respectively. The mineralizable N was significantly correlated with all fractions of N, except with HUN and non-hydrolysable N. The multiple regression analysis indicated that hydrolysable NH4-N and HUN are dominant N fractions in contributing rice grain yield and N uptake.