The field experiment was conducted at the Bangladesh Agricultural Research Institute (BARI), Gazipur, Bangladesh, for the rabi season of 2017-2018. The physical characteristics and chemical status of the initial soil are shown in Tables 1 and 2, respectively. The experimental site is located at the centre of the agroecological zone of Madhupur tract (AEZ-28) at about 24° 23′ north latitude and 90° 08′ east longitude having a mean elevation of 8.4 m above mean sea level. The soil belongs to the Chhiata series of the Grey Terrace soils (AericAlbaquept) under the order Inceptisols in the USDA Soil Taxonomy (Huq & Shoaib, 2013). The morphological and taxonomical characteristics of the experimental site are shown in Table 3. The textural class was clay loam having soil pH 6.2 and the land type is medium high.
From late October to mid-March, the minimum and maximum temperatures were in the lowest range whereas the periods from October to May are virtually dry. The relative humidity (%) varied between day and night of which at day time relative humidity (%) was about 90 (%) and at night it fluctuated to a wide range from 43 to 85% in February and March, respectively. The first crop of the cropping system was wheat (Triticumaestivum L.) cv. BARI gom 26 which was collected from the Wheat Research Centre (WRC) of BARI, Gazipur. The experiment was laid out in a split plot design with four replications. The experimental design was performed as follows: strip tillage (ST: the sowing was done with strip planter developed by FMPE division, BARI), conventional tillage (CT: ploughed by power tiller maintaining depth by depth control lever upto 14–16 cm depth). The C sources used were Crop residue retention @ 30%, biochar of mungbean stover @ 10 t ha-1, cowdung @ 5 t ha-1 and farmers’ practice. Tillage practices were assigned in the main plots and C sources were assigned in the subplots.The unit plot size was 7.2 cm × 4 cm.The fertilizer doses for wheat (Sourav) were N140 P24 K51 S12 Zn2 B0.5 kg ha-1 based on higher yield goal. The fertilizer requirements were calculated on soil test basis. One third urea, whole amount of triple superphosphate (TSP) and cow dung were applied during final land preparation. The rest of the urea, MoP, gypsum, and ZnSO4 were applied in two equal splits at 25 days after sowing (DAS) and 45 DAS. Every split application of fertilizers was followed by irrigation. Another supplemental irrigation was applied 65 DAS, while other intercultural operations were done as and when necessary. Weeds in strip plots were controlled partially by spraying a post-emergence selective herbicide, Affinity (Carfentrazone ethyl + Isoproturon), @ 2.5 g L-1 water at 20 DAS and only one hand weeding was done at 28 DAS for full elimination of weeds. The soil moisture was monitored intensively with gravimetric method (Black 1965).Wheat (cv.BARI gom 26) seeds were sown on the 27 November, 2017 followed by irrigation. The row spacing maintained for wheat was 20 cm. The wheat was harvested on 29 March 2018.
Soil samples were collected at 1, 15, 30, 45, 60, 90 and 120 DAS. The collected soil samples were dried at room temperature mixed thoroughly, grinded, sieved with a 2 mm sieve and preserved in plastic containers for subsequent laboratory analysis. Both NH4-N and NO3-N were extracted from the soils during the crop growing period to determine the available N in the soil sample (Jackson (2005). Microbial respiration from the soil in different treatments was also assessed periodically by trapping emitted CO2 in NaOH (Anderson, 1982). All assays were performed in triplicate. The trapped CO2 was measured by adding 15 mL of 10% w/v BaCl2 to the NaOH to precipitate BaCO3. The remaining NaOH was then back titrated against 1M HCl to the phenolphthalein to neutralize NaOH. Finally, more HCl was added to the methyl orange to dissolve BaCO3 (Anderson, 1982). Plant samples were also collected for determining N uptake at 15, 30, 45, 60, 90 and 120 DAS which were then analysed in the lab for N content determination.
The post-harvest soil samples were then analyzed for SOM, total N and P. The SOM was determined by wet oxidation (Jackson 1973), total N by a modified Kjeldahl method (Page et al., 1989) and total P by using the SnCl2 reduction method (Black, 1965). The pH was determined through glass electrode pH meter method (Jackson 1962), K and S were determined through NH4OAC method (Hanlon and Johnson, 1984) and turbidimetric method (Sperber, 1984), respectively. Micronutrients were analyzed using atomic absorption spectrophotometer.Particle size distribution of the initial soil was analysed by the hydrometer method (Black, 1965) and the textural class was determined using the USDA texture triangle. The BD of the soil samples were determined by core sampler method (Karim et al., 1988). Moisture content was determined by gravimetric method (Black, 1965). Field capacity was measured though pressure plate method. The streamlined LCA approach, considered cradle-to-farm gate greenhouse gas emissions, following four steps: goal and scope, life cycle inventory, impact assessment, and interpretation (ISO 14040-44, 2006). Pre-farm GHG emissions include all activities for producing farm inputs (chemicals, energy and machinery) and the emissions from the transportation of inputs to the field. The GHG emissions from pre–farm stage involve the multiplication of the amount of inputs with their corresponding emission factors to determine the GHG emissions associated with the production and transportation of these inputs to a field. The GHG emissions from the production of chemicals were calculated so that the emission factors reflect the situation in Bangladesh. For the inputs having emission factors not calculated yet, EFs were developed with the combination of generic and local data. The EF of machineries used in the experiment was calculated according to Suh (2004). A global warming impact value for the 100-year time horizon was used to estimate the CO2 equivalent GHG emissions for the production of each functional unit of wheat production. The conversion factors used for converting N2O to the baseline unit, CO2, was 298 (IPCC, 2013). In addition, N fertilizer (organic or synthetic) induced N2O emission for mustard crop was estimated following IPCC (2006) recommendation that is 0.01 t N2O–N t−1 f ertiliser-N or N in organic amendment (or any source) for dry cropland (IPCC, 2006).