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 1530 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.