Nitrogen Fractions under Long-Term Fertilizer and Manure Applications in Soybean-Wheat Rotation in a Vertisol

Abstract

The present investigation was aimed at distribution of nitrogen (N) fractions in a 44-year-old (1972-73 to 2016-17) long-term fertilizer experiment on soybean–wheat cropping system in a Vertisol. The experiment included eight treatments comprising of different doses of fertilizers viz. 50% NPK, 100% NPK, 150% NPK, 100% NP, 100% N, 100% NPK + 5 t FYM, 100% NPK(-S) and control and replicated four times in a randomized block design. The applications of 100% recommended dose of fertilizers (NPK) with organic manure (5 t FYM ha-1) led to significant increases in organic carbon, total N, hydrolyzable-N (i.e., amino acid-N, hydrolyzable NH4 +-N, hexose-amine-N) and non-hydrolyzable-N in both soil depths (0-15 and 15- 30 cm) as compared to control plot and their concentration was decreased with increasing soil depth. The result clearly indicates that both surface (0-15 cm) and sub-surface (15-30 cm) soil layers substantially contributed to the N requirement of soybean and wheat. The contents of various organic N fractions were higher in organic manure compared to untreated plots. The study revealed significant correlations of soil N mineralization with amino acid and hexose-amine-N fractions at 0-15 cm depth and hydrolyzable NH4 +-N and amino acid-N fractions at 15-30 cm. Grain yields of soybean and wheat showed better correlations between amino acid-N in surface and hydrolyzable NH4 +-N in the sub-surface soil layers with yields of soybean and wheat.