Nitrogen Fractions and their Relationships with Mineralizable N and its Uptake by Crops in a Long-term Fertilizer Experiment

Nitrogen fractions in a Typic Haplustept under maize-wheat-cowpea cropping sequence as affected by fertilizer treatments were studied in the long-term fertilizer experiment in progress at IARI, New Delhi since 1971. Soil test based fertilizer doses of 120 kg N, 60 kg P₂O₅ and 40 kg K₂O ha⁻¹ for maize and wheat and 20 kg N, 40 kg P₂O₅ and 20 kg K₂O ha⁻¹ for cowpea found to be optimum in 1971 formed the basis of the ten treatments. Surface soil (0–0.15 m) samples were analysed for total N, organic and inorganic N fractions and mineralisable N at the beginning of the cropping sequence in 1998. All the fertilizer treatments significantly increased hydrolyzable NH₃-N, amino acid-N, amino sugar-N, exchangeable NH₄⁺-N and mineralizable N in Soil. However, comparatively low amounts of these fractions were observed in plots receiving either 50% NPK or 100% NP or 100% N. A decline in the amount of these pools of N as compared to their earlier status recorded during 1978 was observed. All the fractions of N, except acid insoluble N and hydrolyzable unknown-N, were highly correlated among themselves and with mineralizable soil N and uptake of N by maize, wheat and cowpea and the cropping sequence as a whole.