Soil Nitrogen Fractions in Relation to Rice-Wheat Productivity: Effects of Long-Term Application of Mineral Fertilizers and Organic Manures

Major portion of soil N, present as organic fractions (SON), plays an indirect but vital role in N nutrition of plants. The SON fractions are sensitive to management practices, including fertilization. The nature and distribution of different forms of soil N were studied under long-term use of mineral fertilizers and organic manures in a rice (Oryza sativa)-wheat (Triticum aestivum) cropping system. Long-term fertilizer treatments included control (T-1), 100% N (T-2), 100% NP (T-3), 100% NPK (T-4), 150% NPK (T-5), 100% NPK plus straw (T-6), 100% NPK plus green manure (T-7), and 100% NPK plus farmyard manure (T-8). Prolonged application of fertilizers and organic manures differentially influenced mineral N (NO3--N and NH4+-N), organic N fractions, and total N in soils. Organic N fractions constituted about 94.2% of total N as compared with 5.8% share of mineral N. Nitrate N was the dominant mineral N fraction in the soil, which was 4.4 times higher than NH4+-N fractions. Application of fertilizers improved available N in soil. Hydrolyzable-N increased with the application of organic manures. Of the total hydrolyzable-N fractions in soils, ammonia-N was 25%, amino sugar-N 9.9%, amino acid-N 25.2%, and hydrolyzable unknown-N 39.8%. A significant positive correlation was recorded between crop yield and N uptake. Amino acid-N and amino sugar-N explained 82.9% of the variation in rice yield and 73.4% of the variation in wheat yield. Integrated use of organic manures and inorganic fertilizers improved the N fractions in the soil, which should help increase rice and wheat production.