Soil nitrogen fractions and its effect on rice (oryza sativa) yield under integrated nutrient management in a north-western Himalayan soil

Long-term effect of chemical fertilizers and organics was studied on rice yield and nitrogen dynamics in an on-going field experimentinitiated in kharif 1991 in the research farm of Chaudhary Sarwan Kumar HP Krishi Vishvavidyalaya, Palampur, Himachal Pradeshunder AICRP on Cropping System Research. Nitrogen substitution to the tune of 25 to 50% of the recommended dose was made through different organic sources viz., Farm Yard Manure (FYM), Wheat Straw (WS) and Dhaincha as a Green Manure (GM) on irrigated rice (Oryza sativa L.) and wheat (Triticum aestivum L.) rotation on a silty clay loam soil continuously for seventeen years. Sub-optimal levels of NPK (50 and 75% of the recommended) were also applied through mineral fertilizers. Application of both the mineral fertilizers alone or in combination with organics increased all the inorganic and organic fractions of N in soil, except unidentified N. The unidentified N decreased with increasing levels of fertilizers. Inorganic and organic forms of nitrogen contributed about 4.3 and 95.7%, respectively, towards total soil nitrogen. Amino acid N and non-hydrolysable N were found to be the dominant organic fractions. Integrated use of inorganic fertilizers in conjunction with organics increased rice grain and straw yields over inorganically treated plots. Highest grain and straw yield of rice was recorded when 50% N was substituted through FYM. Pretransplant incorporation of 50% N through FYM produced 2.9 Mg ha ⁻¹ rice grain yield which was equivalent to the 2.5 Mg ha ⁻¹ obtained with 50% N substitution through wheat straw.