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

Abstract

A field experiment was conducted on a clayey vertisol (Typic Pellustert) for 3 years (2001 -02 to 2003-04) dur- ing 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 Nlkg soil) and during the second month it was high in the plots receiving green-manure + paddy straw (301-414 mg Nlkg 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 pro- ductive 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 organ- ics 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.