Study site- The experiment was conducted at the research farm of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur, Bangladesh. The site is located at 24.09° N latitude and 90.25° E longitudes with an elevation of 8.2 m from sea level, which comes under the agro- ecological zone of Madhupur Tract. The soil belongs to Salna series and classified as Shallow Red Brown Terrace soil in Bangladesh classification and Inceptisols in USDA classification which is characterized by silty clay loam within 50 cm from the surface and is acidic in nature. The climate of the area is sub-tropical, wet and humid. Heavy rainfall occurs during June-July (269 to 370 mm) and scanty rains during November to February (0 to 55 mm). Surface soil samples were collected, at the time of sowing and after harvest of the crop, air dried, grounded to pass through 2 mm sieve and subsequently used for analysis of pH, , total N, available P, S, Zn & B and exchangeable K following standard protocols (Page et al., 1982). For analysis of OC, a subsample was sieved through 0.2 mm sieve. The experimental field soil had low OC (1.03%), N (0.09%), K (0.14 meq 100 g-1), S (6.5 mg kg-1); very low P (2.4 mg kg-1) and high Zn (2.32 mg kg-1) content (BARC, 2005). Experimentation- The study was conducted during August 2010 to May 2012 in four consecutive rice seasons of Transplanting aman (BRRI dhan49) and boro (BRRI dhan29) having two tillage operations (1) minimum tillage (MT) and (2) traditional tillage (TT) and three levels of rice straw management (1) control, (2) rice straw incorporation (RSI) and (3) rice straw mulch (RSM) in a factorial RCBD with four replications. The unit plot size was 4 m × 3 m and each plot was separated by 30 cm wide well structured and polyethylene lined levee. The experimental field was prepared by country plough maintaining a depth of 15 cm. The MT treatment received two passes of country plough while the TT treatment received 4 passes. Rice straw collected from previous season that contained approximately 17.5% moisture, 0.43% total nitrogen, 41% organic carbon with a C:N ratio 95 and was applied at the rate of 5 t ha-1 to the selected plots 15 days before transplanting. During the first year, the T. Aman rice was transplanted on 6 August, 2010 and Boro rice seedlings was transplanted on 15 January, 2011. In the second year of experimentation T. Aman rice was transplanted on 15 August, 2011 and Boro rice transplanted on 25 January 2012. Thirty days older seedlings were transplanted at 20 cm × 20 cm spacing. The experimental field received N, P, K and S at the rates of 110, 20, 70, and 15 kg ha-1, respectively from urea, triple super phosphate (TSP), muriate of potash (MoP) and gypsum, respectively in aman, while these nutrients in boro were 180, 27, 120 and 20 kg ha-1, respectively according to BARC fertilizers recommendation model (BARC, 2005). The whole amount of TSP, MoP, gypsum were applied during final land preparation and urea was applied in three equal splits at 15 days after transplanting, at maximum vegetative stage and panicle initiation stage. Inter-culture operations were carried out as and when required. Proper irrigation practices were undertaken maintaining at least 5 cm water throughout the growing season. Carbon dynamics and carbon dioxide emission- Carbon dioxide emission was measured by NaOH absorption followed by HCl titration (Jain et al., 2003). Observation was taken by every 7 days interval and continued throughout the four crop growing seasons. CO2 traps were prepared using 80 ml of 2N-NaOH into plastic bottles and placed in the plots under each treatment. Traps were covered with plastic buckets, which were inserted into soft mud to protect entrance of atmospheric CO2. An empty bucket was used as a control without soil but of alkali of same strength. After seven days of exposure, traps were removed from plots covering with screw cap and then titrated against HCl. From 80 ml of alkali solution, 2 ml was titrated adding few drops of phenolphthalein indicator against 2N- HCl. The amount of CO2 evolved from soil was calculated using the formula: Milligrams of C or CO2 = (B-V)*N*E; where, B and V are volume (ml) of acid needed to titrate trapped NaOH in the control and straw treated plots, respectively, N = normality of the acid, and E (equivalent weight) is either 6 or 12 to express data in terms of C or CO2, respectively. Data was expressed as kg CO2 ha-1 day-1 soil and also as kg C ha-1 day-1. Carbon degradation rate constant, k was calculated using the exponential kinetics, ln(C/C0) = - kt, where, C and C0 are final and initial C contents, respectively; t is time in days (Avnimelech et al., 1984). Carbon build up was measured by subtracting initial soil C from residual soil C. Carbon balance was calculated at the end of the study period by using the following equation. Carbon balance = Input – Output; Input = Inherent soil C + added C through rice straw before the four rice seasons + Rice straw C from straw and roots (remaining in the soil after first three rice seasons); Residue rice straw and rice roots were quantified from a sample area of 1 m2 and then extrapolated to kg ha-1. For collecting roots, a sample area was irrigated first to make soil soft and dig out from surrounding roots area and then roots were washed out, air dried and placed in an oven at 650C for 48-72 hours. Rice straw and roots were analyzed for carbon (Page et al., 1982). Output = C emission + Residual C in the soil at the end of experiment after crop harvest; Initial and residual soil C was calculated in the 15 cm soil depth, by using the following equation. Organic C (kg ha-1) = Conc. of soil C (%) * soil bulk density (g cc-1) * soil depth (cm) * 1000. Analysis of soil physical characters- Particle size distributions were done by hydrometer method (Black 1965). Bulk density and particle density of soils were determined by core sampler and Pycnometer method, respectively (Black, 1965). Soil porosity was computed from the relationship between bulk density (BD) and particle density (PD). Field capacity (FC) and permanent wilting point (PWP) were measured using pressure plate apparatus, where available water (d) was calculated using the equation given by Black (1965). Porosity (%) = (1 – BD/PD) x 100; d = (FC-PWP)/100 *BD (g cc-1) *soil depth (cm). Statistical analysis- SPSS version 12.0 statistical software (SPSS Inc., Chicago IL) was used to analyze the data. ANOVA and univariate analysis were performed and means were separated by LSD.