Field experiments were conducted during two consecutive seasons in 2012 and 2013 at Tezpur (Central) University campus, Napaam, Sonitpur District (26˚41'N, 92˚50'E) along the Brahmaputra valley (North Bank Plain Zone) of Assam, India. The region is subtropical humid and is characterized by hot-wet summers. Daily air temperature and precipitation were recorded from April to July for both the years of experimentation from a weather station located in the University campus. Total rainfall received was 1099.8 mm in 2012 and 1062.7 mm in 2013 during the crop growth period (Figure 1). The soil of the experimental area is characterized by recent and old alluvium riverine soils with sandy-loam to silt loam texture (USDA classification). Soil samples were collected from five different points of the experimental plot prior to the application of fertilizers and residues and analyzed for initial physical and chemical properties (Table 1).

Experimental area was prepared by thoroughly ploughing and puddling 4 to 5 times followed by harrowing and laddering 20 days before transplanting. 25 days old seedlings of a popular high yielding pre-monsoon rice variety Lachit (Parent = CRM13-3241/Kalinga II; duration of the variety = 115 - 120 days) were transplanted in a well prepared field on 9^thApril 2012 and 10^th April 2013, at a row to row spacing of 20 * 15 cm in each plot of 16 m² = 4 m × 4 m. Five treatments with conventional fertilizer (NPK), and organic residues viz., Cow manure (CN¹ = 25:1), Rice straw (CN = 41:1), Poultry manure (CN = 15:1) and Sugarcane bagasse (CN = 108:1) were incorporated in the field prior to transplanting for uniform mixing. Each treatment was replicated into 5 subplots and laid in a randomized block design (RBD). Treatments selected were applied following the recommendations by the Department of Agriculture, Government of Assam, India. The recommended dose of inorganic fertilizer i.e., NPK, was applied in the form of Urea (N), Super Phosphate (P₂O₅) and Muriate of Potash (K₂O) at the rate of 40:20:20 kg∙ha⁻¹ and was taken as control (T1). The half quantity of N (urea) and whole quantity of P₂O₅ and K₂O was applied at the time of final puddling. Remaining part of urea was split in two equal halves and one part was applied at the tillering stage (30 DAT²) and other half at panicle initiation stage (56 DAT). The other treatments selected with different C and N content viz., cow manure (T2), rice straw (T3), poultry manure (T4) and sugarcane bagasse (T5) were applied @ 10 ton∙ha⁻¹ on dry weight basis (without any inorganic fertilizer) at the time of final puddling (Table 2). All management practices, such as irrigation, application of manures and fertilizers, plant protection measures, etc., were followed as recommended by the Department of Agriculture, Government of Assam, India. The crop is rain dependent (rainfed) and hence no irrigation was applied after establishment of the crop. However one irrigation for land soaking was applied before preparatory tillage and another irrigation of depth = 5 - 7 cm was applied before final puddling. At maturity crop was harvested manually and post harvest data were recorded. Other management practices such as weeding, cleaning and crop protection were uniformly maintained during the experimental periods. At maturity crop was harvested manually and post harvest data were recorded.

Gas samples were collected from the day of transplanting (0 Days after transplanting, DAT) onwards at 7 day interval, 2 times a day throughout the growth period. The closed chamber technique [20] -[22] was used to collect N₂O gas from the soil - plant system. The chambers used for gas collection were made of 6 mm thick acrylic sheets of 50 cm × 30 cm × 70 cm (length × breadth × height) and equipped with a circulating fan to homogenize the air inside the chamber. The chambers were covered with aluminum foil to minimize the affect of temperature on the flux inside the chambers. To match with the crop growth, chamber with 100 cm height was used from panicle initiation stage onward. A rectangular shaped aluminum channel (50 cm × 30 cm) was inserted in the soil after transplanting to accommodate the chamber. Gas samples were drawn from the chambers using a 50 ml airtight syringe fitted with a three-way stop-cock through a self-sealing rubber septum. The samples were drawn at fixed interval of 0, 15, 30 and 45 minutes respectively starting at 0900 hrs in the morning and again at 1400 hrs in the evening from 0 DAT onwards at weekly interval. Nitrous oxide (N₂O) concentration in the gas samples were analyzed using a gas chromatograph (Varian, 3800 GC) equipped with a ⁶³Ni electron capture detector (ECD) within 4 - 6 hours of collection. The gas chromatograph was calibrated by standard N₂O obtained

from CSIR―National Physical Laboratory, New Delhi, India. The injector, column and detector temperature were maintained at 80˚C, 150˚C and 300˚C respectively. Nitrogen (N₂) was used as a carrier gas with a flow rate of 15 ml∙min⁻¹. N₂O fluxes were estimated by successive linear increase of gas concentration inside the box at each sampling time (0, 15, 30, 45 min). The average of morning and evening fluxes were considered as the flux value for the day and calculated according to the equation of Wang et al. (2011) [21] .

Cumulative N₂O emission is expressed as seasonal integrated flux (E_sif) in mg N₂O-N m⁻² for the entire crop growth period was computed by the method given by [22] by using the following formula:

where Ri is the mean gas emission, Di is the number of days in the sampling interval and n is the number of sampling times.

Prior to rice cultivation soil samples were collected randomly from different locations of the experimental plot up to a depth of 0 - 10 cm with the help of a soil core (5 cm diameter, 30 cm height) and mixed thoroughly to prepare a single composite sample which was analyzed for physical and chemical parameters (Table 1). Soil texture was determined by the International Pipette Method. Bulk density, Soil moisture, available (mineralized) soil nitrogen (N), phosphorus (P) and potassium (K) content were determined following Page et al. [23] . Total soil organic carbon, total carbon, soil nitrate-N and soil ammonium-N were analyzed at weekly interval at all the sampling days. The samples were collected with a core sampler (5 cm diameter) inserted in the soil (0 - 10 cm) from 5 sections in each replicate of every treatment and mixed together to make one composite sample. Air dried soil samples were passed through 2 mm mesh sieve and analyzed for other parameters. The estimation of soil nitrate-N was done by colorimetric method after reaction with phenol disulphonic acid [24] and ammonium-N by extracting the soils with ultra pure water after sonicating the samples for 90 minutes in 70˚C through a milipore filter in ion chromatography (Compact IC 882 plus, make Metrohm, Germany). Total carbon content was estimated in CHN analyzer (TrueSpec CHN macro determinator, LECO corporation USA), total organic carbon (SOC) by the dry combustion method at 1000˚C analyzed using TOC analyzer (Multi N/C 2100S with HT 1300 module, Analytik Zena, Germany) [25] . Soil temperature was measured with a soil thermometer on every sampling date. Soil water content (%) was measured by gravimetric method [23] . Soil pH was measured in 1:2.5 ratio (soil: water) with a pH meter immediately after sample collection.

During the crop growth period, destructive sampling was done to estimate above ground and below ground biomass during vegetative, reproductive and maturation stages. The roots were separated from the shoot portion carefully and washed thoroughly to remove any soil particles under running water over a sieve. Biomass was recorded by drying the samples in an oven at 75˚C ± 2˚C till a constant weight was obtained and weighed. The grains were separated from the straw, dried and weighed for yield. Yield attributing parameters (sterility %, fertile panicle per square meter, harvest index, etc) were recorded after harvesting by standard methods. The Harvest index (HI) was calculated after Yasin [26] as in Equation (2):

The carbon equivalent emissions (CEE) (kg CO₂ eq ha⁻¹) were calculated as in Equation (3):

The global warming potential of N₂O was taken as 298 [27] .

Carbon efficiency ratio (CER) of the treatments was calculated as given Equation (4):

Both CEE and CER were estimated according to Bhatia et al. [7] .

In addition, as proposed by Zhang et al. [28] for highlighting the effect of residues over the other factors, we measured the residue effect intensity (REI, %) which allows an analysis of any change in organic residue effect on soil and crop productivity as well as nitrous oxide emission during a crop growth period. REI was calculated by the following Equation (5):

where REI is the residue effect intensity in % of a given parameter, Q_treated and Q_control is the soil quality, yield and yield attributing parameters or cumulative N₂O emission (E_sif) values under a certain treatment, respectively.

The SPSS 16.0 software package was used to calculate the Pearson correlation of various soil parameters, plant physiological parameters and yield parameters with N₂O emission from the treatments. The datasets were subjected to analyze significant pooled difference between the treatments and least significant difference (LSD). One way Analysis of variance (ANOVA) was conducted to analyze the significance of difference of different parameters among the treatments and subsequently Duncan Multiple Range Test (DMRT) to find out the critical differences between the recorded parameters by the same statistical package.

The seasonal variations and dynamics of N₂O-N emission from the plots treated with different organic residues for 2012 and 2013 are presented in Figure 2. Data on seasonal amounts and overall magnitude of variation in emissions from the treatment plots are organized in Table 3.

Seasonal dynamics of nitrous oxide emission revealed a similar pattern between the two periods of crop growing season which was affected by daily precipitation (mm) and soil water content (%) under different organic residues applied. Reduction in N₂O emission from the plots treated with rice straw (RS, T3) over the control (NPK, T1) was clearly visible. The seasonal total of N₂O-N emission under T1, T2, T3, T4 and T5 were 2104.23, 2379.26, 1814.86, 2068.06 and 1998.16 N₂O-N∙µgm∙m⁻²∙hr⁻¹ in 2012 and 2183.30, 2124.34, 1489.82, 2025.05, and 2115.13 N₂O-N µgm∙m⁻²∙hr⁻¹ in 2013 respectively.

The seasonal cumulative emission (E_sif) was highest in T2 (399.7 kg N₂O-N ha⁻¹), followed by T1 (3.53 kg N₂O-N ha⁻¹), T4 (3.47kg N₂O-N ha⁻¹), T5 (3.35 kg N₂O-N ha⁻¹) and T3 (3.05 kg N₂O-N ha⁻¹) in 2012 and highest in T1 (3.66 kg N₂O-N ha⁻¹), followed by T2 (3.57 kg N₂O-N ha⁻¹), T5 (3.54 kg N₂O-N ha⁻¹), T4 (3.40 kg N₂O-N ha⁻¹) and T3(2.50 kg N₂O-N ha⁻¹) in 2013 (Figure 3). The impact of treatments T2, T3 and T4 on seasonal total of N₂O emission was different in the two years of experimentation with lower emissions in 2013 over 2012. As compared with the corresponding control (T1), the seasonal cumulative emission decreased by 14% and 31%, and by 1% and 7% and 5% and 3% in T3, T4 and T5 in 2012 and 2013 respectively. While T2 did not reveal consistent results in 2012 there was an increase in emission by 12% but a decrease by 2% during the second season. The seasonal cumulative emission revealed a significant variation (p < 0.001) from each other during both the years.

The N₂O emissions increased from the active tillering stage (28 DAT) up to panicle initiation (56 DAT) and to the maturation (84 DAT) of the crop after which there was a decline in emission irrespective of the treatments. In both the years of experimentation the emission at initial stage (0 - 21 DAT) was less compared to the later stages (28 - 98 DAT) of the crop growth in all the treatments. During the active vegetative stage soil is enriched with C by root exudates which act as a source of nutrients to the microbes responsible for N₂O production resulting in high emission peaks of N₂O during this period [29] [30] . During maturation of the crop, high emission peaks were observed which may be attributed to the decomposition of organic matter in the residues [31] and increased N mineralization [32] in the plots treated with organic residues. Decomposition of leaf litter and roots contribute to high substrate availability in the rice rhizosphere for N₂O production resulting in increased N₂O

N.B. T1: NPK (control), T2: Cow manure, T3: Rice straw, T4: Poultry manure, T5: Sugarcane bagasse.

emission [20] [33] and similar mechanism may result in high N₂O emission recorded during the panicle initiation (56 - 63 DAT) and post anthesis stages (84 - 98 DAT) in the present study. At conventional application of NPK (T1) the emission peaks coincided with split application of nitrogenous fertilizer (urea) doses at tillering (21 DAT) and panicle initiation stages (56 DAT) due to availability of inorganic nitrogen for microbial processes of nitrification and denitrification a mechanism suggested by [20] [30] . Irrespective of the treatments the emission of N₂O from the treated plots followed a similar trend throughout the cropping season and a decrease in N₂O emission after maturation of the crop up to harvesting stage was observed.

Organic carbon content of soil is an important regulatory factor for nitrification and denitrification. The nitrifiers and denitrifiers use organic carbon compounds as electron donors for energy and synthesis of cellular constituents [19] [34] [35] . In the present experiments, high fluxes of N₂O-N were observed when there was an increase in the organic carbon in the field. A general understanding is that soils with high levels of organic carbon content has a greater propensity for N₂O formation than soils with low levels, notably after N application [36] or residue decomposition [37] which results in stimulation of microbial growth and activity and subsequently increases the consumption of O₂ generating anaerobic conditions necessary for denitrification [38] [39] and a similar mechanism might operate resulting in high peaks of N₂O (coinciding with high organic carbon in the soil) in our study. The Pearson product of correlation exhibited a positive effect of soil organic carbon with nitrous oxide emission. N₂O emission from soils is strongly influenced by the water content. There is a general concept that rice fields due to its flooded conditions produces less amount of nitrous oxide then well aerated and dry soils. However, in moist soils, the rate of gas diffusion and aeration decreases which reduces NO to N₂O which makes N₂ the major end product of denitrification and N₂O the dominant product released to the atmosphere [40] . N₂O emissions have their optimum in the range of 40% - 60% water content in the soil depending upon its type. In our study the soil water content (%) ranged from 20% - 78%. Higher emissions were recorded when there was low field water content which may be attributed to increased rate of nitrification as evident from the high soil nitrate - N content (data not shown). The mineralized nitrogen in soils nitrate − N (kg∙ha⁻¹) and revealed a more strong correlation with N₂O-N emission than ammonium −N (kg∙ha⁻¹) irrespective of the treatments. It was observed that when the amount of content of the soil increased there was a decline in N content and vice versa which resulted in N₂O being emitted to the atmosphere either by the process of nitrification or denitrification. Our results are in agreement with [41] who reported a similar trend.

Application of organic residues resulted in a considerable variation in plant growth, biomass production and carbon utilization efficiency (CER) of the crops at maturity. Irrespective of the treatments, plant height was initially low up to 14 DAT, there after an increase in plant height in all the treatments were recorded till 84 DAT. Maximum plant height was recorded in T1 (0.85 ± 0.19 meter) and lowest in T3 (0.67 ± 0.16 meter) (Figure 4(a)).

During the study, observation of the tiller number per hill did not show noticeable difference within the treatments (Figure 4(b)). Shoot dry weight increased up-to panicle maturation stage where it reached a maximum and then gradually decreased towards the end of the season (Figure 4(c)). This was probably due to senescence of the older leaves and non bearing tillers. The root dry weight followed a similar trend with shoot dry weight which increased during the active crop growing season and decreased thereafter up to the end. Panicle initiation stage resulted in maximum dry matter accumulation in roots under T1, T3 and T4 (Figure 4(d)).

Significant variation in yield and yield attributing parameters due to application of different organic residues are presented in Table 4. In our study application of organic residues as substitutes of chemical fertilizer could not significantly affect the yield as compared to the conventional chemical fertilizer (T1, NPK) application as revealed from statistical data analysis. However, higher grain yield was recorded in T3 (2.81 t∙ha⁻¹) followed by T1 (2.69 t∙ha⁻¹), T4 (2.66 t∙ha⁻¹), T5 (2.66 t∙ha⁻¹) and lowest in T2 (2.54 t∙ha⁻¹). Harvest index recorded was also highest in T3 and lowest in T2 without much difference among the treatments. Higher grain yield may be attributed due to higher nutrient availability caused by greater immobilization of soil N compared to T1 (NPK) which may have influenced the crop C and N uptake and thus increasing the photosynthesis rate [42] and subsequently photosynthate partitioning towards the panicles and developing grains as explained by Baruah et al. [43] . Yield attributing parameters varied significantly among different treatments as shown in Table 4.

Addition of organic residues in the field significantly affected the total organic carbon (SOC) and total carbon content of the soils. An increase in organic carbon of the experimental field was observed immediately after rice transplanting (2.3% - 2.9%) with a maximum during active vegetative growth (1.5% - 2.3%) followed by a higher organic carbon percentage at reproductive stages (1.3% - 2.4%) (Figure 5). Thereafter, a declining trend was observed and the organic carbon content remained fairly constant during the rest of the crop growing season up-to harvesting of the crop. Maximum organic carbon (SOC) content was found at the T5 followed by T3 (p value = 0.000; LSD = 0.012 and 0.009) which may be the result of higher decomposition and mineralization rate of the organic residues. This may also enhance the microbial and enzyme activities of the organisms responsible for the transformation of labile C. We did not record any significant variation in the total carbon (TC) content of the soil due to the application of organics. During 2012, the plots had considerable quantity of moisture (79%) till 84 DAT primarily due to rainfall. The soil moisture content slowly decreased thereafter (30% - 36%) to a negligible level at harvest. The soil moisture was high (78%) up to 56 DAT owing to rainfall during 2013. Soil temperature under the treatments showed a significant variation (p < 0.001) which is possibly due to differential rates of reactions during transformation of organic contents in the soils. Soil pH did not show a significant difference among the treatments and ranged between 5.2 and 6.2 irrespective of the treatments.

NB: HI: harvest index, FPM⁻²: fertile panicle meter⁻², PL: panicle length, TGW: thousand grain weight, ST (%): sterility, HDG (%): high density grain.

Carbon equivalent emission (CEE) and Carbon efficiency ratio (CER) are related to each other, where positive values expressed as kg CO₂ equivalent per grain yield indicates a net source of GHGs to the atmosphere while negative values indicate a net sinks of GHG to the soils [7] . In this study the effects of organic residue amendment on the observed qualities of plant growth, productivity and N₂O emission followed a more or less proportional trend for both the years. N₂O emission was found to be higher from plots treated with cow manure (T2) resulting in higher CEE (306.47 kg CO₂ ha⁻¹) as compared to control (T1). Other treatments, T3, T4 and T5 resulted in lower emissions after two years of study (Table 5). The estimation of CEE showed that reduction in emission compared to control may provide a mitigation option from the rice paddies. Also comparing the CER it is revealed that decreased values of CEE and high value of CER would be beneficial for rice productivity and an effective option for mitigating N₂O emission from rice ecosystem. This relationship has also been explained by Das and Adhya [30] during their study on combined application of organic manures and inorganic fertilizer on both methane and nitrous oxide emission from rice.

The calculated residue effect intensity (REI, %) values for plant growth, crop productivity and N₂O-N emission are presented in Table 6. The residue amendment exerted a varied effect on plant growth, rice productivity and reducing nitrous oxide emission compared to the corresponding control treatment. A positive effect of crop residue amendment was evidenced on rice productivity and mitigating nitrous oxide emission under the influence of T3 during this study. T4 and T5 also showed decreased values N₂O emission without any positive effect on grain productivity. Plant growth parameters such as plant height and tiller number also showed a decreased effect due to organic residues as compared to control. Reports of such sustainable effect of organic residues as substitutes of chemical fertilizers in rice fields are not available from this part of the country. However, a study of organic substitution in the fertilizer schedule on wheat crop, where FYM and CR was applied at different ratio in combination with inorganic fertilizer (NPK) and revealed positive effects of organic residues on crop yield and SOC sequestration [44] . But this work did not involve the estimation of any Greenhouse gases (GHGs). Combination of organic manures and crop residues with chemical N fertilizers are reported where judicious use of the residues and management of the C:N ratio mitigates the emission of green house gas from rice fields are been addressed previously [7] [17] [19] [28] [30] . Their findings are supportive to our results of impact of organic residues on GHG emissions and reduction. The estimation REI% of soil quality was not influenced by the treatments over two years of experimentation.