NITROGEN FRACTIONS IN SOIL AND THEIR CONTRIBUTION TO MINERALISABLE NITROGEN AND ITS UPTAKE BY WHEAT IN A LONG-TERM FERTILIZER EXPERIMENT

Since an access to nitrogen (N) regulates primary production in ecosystems, it is important to estimate the size of the various pools of organic and inorganic N in soil and their relative contribution to bioavailable N and its uptake by crops in an intensive cropping system. In a long-term fertilizer experiment set up in 1971 on the farm of Indian Agricultural Research Institute, New Delhi, Studies were conducted on a Typic Haplustept with maize (Zea mays), wheat (Triticum aestivum), cowpea (Vigna unguiculata) cropping sequence. Apart from a control, treatments included optimum NPK level (100% NPK) as recommended according to initial soil test values, 50% NPK, and 150% NPK, 100% NP, 100% N, treatments involving NPK with FYM (for maize only), with Zn (for wheat only), with S and with hand weeding practice. The effects of different treatments were investigated on soil nitrogen fractions and their contribution to soil mineralisable nitrogen and nitrogen uptake by the wheat. The long-term application of 100% NPK + FYM resulted in the highest amount of total N in soil which was closely followed by 150% NPK. While application of 150% NPK resulted in the highest amount of hydrolysable NH4+-N and of NO3-N, amino acid N content was highest in soils under 100% NPK + S treatment. The amount of amino sugar N and that of exchangeable NH4+-N was highest due to 100% NPK+FYM application. Integrated use of NPK fertilizer and FYM resulted in the highest uptake of N by wheat. Path coefficient analysis indicated that among N fractions, the contribution of exchangeable NH4+-N to mineralisable N through its direct effect was the highest whereas amino sugar directly contributed more to N uptake by wheat.