Nitrogen dynamics as influenced by long-term fertilization and amendments under rice-wheat system in a north-western himalayan soil

The effect of long-term addition of chemical fertilizers along with organics was studied on different forms of nitrogen, after 17 years of continuous cropping of rice-wheat in an acid Alfisol. Continuous application of 50% N through green manure (Dhaincha) in kharif followed by 100% NPK in rabi (T₁₀) recorded highest value of all the fractions of nitrogen which was significantly superior over control. Surface layer recorded higher content than sub-surface layer; however, the treatment wise effect was comparable. Different organic and inorganic fractions of nitrogen were positively and significantly correlated with organic carbon, CEC and available nitrogen status except unidentified-N. The highest degree of correlation in respect of organic carbon, CEC and available nitrogen was observed with hydrolysable ammonical-N followed by amino acid-N. Grain and straw yield of rice exhibited positive and significant correlation with all the fractions of nitrogen except unidentified-N indicating that this fraction is not contributing much towards yield. Similarly, nitrogen uptake in grain and straw of rice and total N uptake was also positively and significantly correlated with all the nitrogen fractions except unidentified-N indicating that this fraction is not contributing much towards the yield and uptake. The degree of correlation in respect of rice grain and straw yield and nitrogen uptake was highest with hydrolysable ammonical-N indicating maximum contribution of this fraction towards grain and straw yield. All the organic and inorganic fractions of nitrogen except unidentified-N were positively and significantly correlated among themselves showing interdependency of various chemical pools at both the depths. As regard the relationship of unidentified-N with other fractions, non significant relationship was observed with any of these fractions, indicating that this fraction is not contributing much towards these pools.