Changes in Organic and Inorganic Forms of Nitrogen in a Typic Haplustert under Soybean-Wheat System due to Conjoint Use of Inorganic Fertilizers and Organic Manures

Under soybean-wheat system the influence of 4-year continuous cropping with different fertilizer and organic manure inputs on the transformation of different N fractions, mineralizable N, total N, and organic C in both surface and subsurface soils was investigated in a Vertisol. Repeated applications of fertilizer N alone, N with FYM or poultry manure or urban compost, FYM alone led to a significant increase in organic C, total N, hydrolysable N (i.e., amino acid N, hydrolyzable NH₄-N, hexose amine N) and nonhydrolysable N in both surface and subsurface soils as compared to initial status. The status of various organic N fractions was higher in surface than the subsurface soils. On the other hand, continuous cropping without fertilization and manuring resulted in depletion of total hydrolysable N in control over the initial status by 8.5% in surface soils and 6.4% in the subsurface soils. About 3–6% of total N in surface soils and 2–5% of total N in subsurface soils got mineralized under waterlogged incubation conditions. The results clearly indicate that the subsurface soils in addition to surface soils also substantially contributed to the N requirement of the crops. The correlation studies revealed that the amino acid and hexose amine N fractions in surface soils and hydrolyzable NH₄-N and amino acid N fractions in subsurface soils were better indices of soil N mineralization. This was further supported by a better correlation between amino acid N in surface and hydrolyzable NH₄-N in the subsurface soils with yield of and N uptake by soybean and wheat crops.