Nitrogen-release pattern from organic sources of different C: N ratios and lignin content, and their contribution to irrigated rice (Oryza sativa)

A field experiment was conducted on a clayey vertisol (Typic Pellustert) for 3 years (2001–02 to 2003–04) during wet and dry seasons at Hyderabad to study the N-release pattern from three organic sources of different C:N ratios and lignin content, and their effect on rice (Oryza sativa L.) productivity. The three organic sources used were: paddy straw with C:N ratio 61.5 and lignin content 6.3%; greengram (Phaseolus aureus) with C:N ratio 19.5 and lignin content 8.7%; dhaincha (Sesbania aculeata) with C:N ratio 13.1 and lignin content 5.8%. These were used in different combinations along with inorganic fertilizers. The cumulative N released during the first month after planting was high in the plots receiving green-manure alone (106–319 mg N/kg soil) and during the second month it was high in the plots receiving green-manure + paddy straw (301–414 mg N/kg soil). Greengram with its medium C:N ratio as well as lignin content released N slowly and gradually for a longer period. Leaving a gap of 2 weeks after incorporation of green-manure like dhaincha resulted in higher release and loss of N in those 2 weeks, because of its succulence (low lignin) and very narrow C:N ratio. The grain yield was significantly higher in the treatments that received organics in place of inorganic fertilizers alone due to increase in the productive tillers (panicles) in the second and third year by 7–23%. The residual effect of organics added in the wet seson significantly increased grain yield in the second and third years in all the treatments that received organics compared with inorganic fertilizers alone. Thus, organics with moderate C:N ratio and lignin content released N slowly and gradually for longer periods and led to higher productivity in rice-rice system.