Madhab Paudel^1,, Shrawan Kumar Sah², Andrew McDonald³, Narendra Kumar Chaudhary⁴

Soil organic carbon plays the crucial role in maintaining soil quality. The impact and rate of SOC sequestration in CA and conventional agriculture is still in investigation in this environment. A field experiment was initiated in 2011/12 and continued up to 2012/13 to compare the organic carbon buildup in the soil due to conservation and conventional agriculture. The soil organic carbon data represents the cumulative after five crop cycle. The treatments consisted conventionally tilled direct seeded rice followed by zero tilled wheat, conventionally tilled transplanted rice followed by conventionally tilled wheat, zero tilled direct seeded rice followed by zero tilled direct seeded wheat without residue retention, zero tilled direct seeded rice followed by direct seeded wheat with residue retention, permanent raised bed of rice followed by wheat without residue retention and permanent raised bed of rice followed by wheat with residue retention in randomized complete block design with three replications. About 4 t ha-1 of standing rice and wheat crop and about 70% residue of moongbean was retained for succeeding crop in retained treatments. Soil samples from each treatments were collected from two soil depths (0-20 and 20-40cm) and analyzed. Result showed that, soil organic carbon buildup was affected significantly by tillage and residue level in upper depth of 0-20 cm but not in lower depth of 20-40 cm. Higher SOC content of 19.44 g kg^-1 of soil was found in zero tilled residue retained plots followed by 18.53 g kg^-1 in permanently raised bed with residue retained plots. Whereas, the lowest level of SOC content of 15.86 g kg^-1 of soil were found in puddled transplanted rice followed by wheat planted under conventionally tilled plots. Zero tilled residue retained plots sequestrated 0.91 g kg^-1 yr^-1 SOC in the year 2012/13 which was 22.63% higher over the conventionally tilled residue removed plots after five seasons of experimentation. Therefore, CA in rice-wheat system can help directly in building–up of soil organic carbon and improve the fertility status of soil.

soil organic carbon, CA, rice-wheat system, residue retention