Mineral and Labile Organic Nitrogen Fractions in Soil Profile and Their Response to FYM and Inorganic Fertilization in Different Growth Stages of Rice Crop

Nitrogen is the key element among the major nutrients in crop production. The mineralizable soil organic nitrogen is the main contributors of soil N supply. A better understanding of soil organic nitrogen dynamics in agro-ecosystems is needed to improve N management. The present investigation was conducted in Regional Agricultural Research Station (RARS), Assam Agricultural University, Titabar, Jorhat during 2017-18. In this study, the impact of Farmyard Manure (FYM) and inorganic fertilization on soil mineral nitrogen (NO3−-N and NH4+-N) dynamics and labile organic nitrogen fractions viz. microbial biomass N (MBN), particulate organic N (PON) and water-extractable organic N (WEON) at three growth stages of rice viz. active tillering, flowering and physiological maturity stages and nitrogen stock were assessed. Six treatments viz. control (no fertilization), 100% NPK, 100% NPK + FYM 5 t ha-1, 50% NPK, 50% NPK + 50% N through FYM and FYM 10 t ha-1 were tested in randomized block design with four replications. The results showed that NO3−-N and NH4+-N were found to be significantly higher in 100% NPK+FYM 5 t ha-1 at the three growth stages of rice. The labile organic nitrogen fractions were significantly higher in FYM 10 t ha-1. All these variables were decreased with increase in crop growth stages. Integrated use of inorganic fertilizer and FYM recorded the highest nitrogen stock. Stepwise regression analysis indicated that NH4+-N was the main contributor to nitrogen stocks at the three growth stages of rice of the studied soils.