Site description and treatments:
The study was conducted in rabi season between November 2019 - February 2020 in experimental field (23°59´21.955´´N, 90°24´33.739´´E) at Soil Science Division, BARI. Initial soil samples were collected from 0 to 15 cm depth at experimental site. The soil at the site of the experiment is under chiata soil series, brown hill soil type classified as Inceptisols. The experimental site had previously been with a variety of crops such as tomato, cabbage, maize, broccoli, cauliflower for the previous five years. In this year, the experiment consisted of randomized complete block design (RCBD) with three replications of eight treatments. The treatments were: i) Native fertility; ii) 100% RDCF (STB); iii) 100% RDCF (STB)+ Rice husk biochar @ 1.5 t ha-1, iv) 100% RDCF (STB) + Rice husk biochar @ 3.0 t ha-1; v) 100% RDCF (STB) + Rice husk biochar @ 4.5 t ha-1; vi) 80% RDCF (STB) + Rice husk biochar @ 1.5 t ha-1; vii) 80% RDCF (STB) + Rice husk biochar @ 3.0 t ha-1; and viii) 80% RDCF (STB) + Rice husk biochar @ 4.5 t ha-1. Each block comprised of 8 plots and each plot was 2 m × 3 m. Blocks were 1 m apart and plots were 0.5 m apart. The exact same location and layout of the plots and treatments will be used for next season.
Soil analysis:
Eight sampling locations were selected by dividing the trial area into 3 cells (10 m × 10 m). Three sampling points were randomly selected within each of the 3 cells. At each sampling point, surface debris and litters were cleared away and three samples from a 3-m radius were collected using a manual auger with 20 cm core barrel of 6 cm internal diameter. The 3 samples were combined, making a composite sample for each cell, resulting in 3 composite samples for analysis. Pest harvest soils were collected from individual treatment plot. Soil samples collected were homogenized and ground to pass through a 2-mm sieve. Soil samples were analyzed for pH using a ratio of 2.5 ml water to 1 g soil (McLean, 1982); Available P was determined by Bray-1 extraction followed by molybdenum blue colorimetry (Frank et al., 1998). Soil organic carbon (OC) was determined by the procedure of Walkley and Black using the dichromate wet oxidation method (Nelson and Sommers, 1996). Organic matter (OM) was calculated by multiplying C by 1.724. Total N was determined by the Kjeldahl digestion method (Nelson and Sommers, 1980); exchangeable cations and CEC using ammonium acetate method (Black, 1965) at the soil and plant analysis laboratory of the Soil Science Division, BARI. The pre-planting chemical characteristics of the trial soil.
Plant analysis:
The clean plant samples were air-dried and placed in an electric oven, dried at 105ºC for 24 h, weighted for dry biomass. Again, place it in the oven at 105ºC for 2h. Cool it in a desiccator and weight it again. Repeat drying, cooling and weighing until the weight become constant. The dried plant samples were homogenized by grinding using willey mill and used for nutrient analysis. Then, the grains were ground and N, P, K, Ca, Mg, S, B and Zn contents were determined according to the method described by described by Jones and Case (2018). Atomic absorption spectrophotometer (Thermo Scientific - SOLAAR S Series AA spectrometer) was used for metal ion and spectrophotometer (Agiland Technologies, cary 60 UV-Vis) for anion analysis. The accumulation of nutrients in the grains was estimated by multiplying nutrient content by dry grain weight.
Carbon Stock analysis:
Composite soil samples were collected from soil surface at depth of 0-30 cm. Soil parameters were determined for soil bulk density using the core sampling method (Blake, 1965) and soil organic carbon (SOC) before the experiment and at the end of the three year cropping. Soil organic carbon stock (SOC stock) was estimated with the following equation by Milne (2008).
SOC stock= SOC content of soil x BD x A x D,
Carbon accumulation (t ha-1) = Final C stock (t ha-1) – Initial C stock (t ha-1)
Where: SOC stock= soil organic carbon stock (tons ha-1); SOC content of soil= soil organic carbon content of soil (%), BD=bulk density, Area= area of farm (m2) and D= soil sampling depth (m).
Determination of yield parameters:
Yield and yield contributing character samples were collected inside a quadrant area of 1.0 m2 per plot. At harvest, and various yield parameters as head weight, head height, head circumference, head diameter, dry matter production, and yield of cabbage were recorded on five randomly selected and tagged representative plants in each plot and expressed as mean values.
Statistical analyses:
A software package, statistix 10 (Analytical Software, Tallahassee, Fla, USA) was employed for the statistical analysis of data. A one-way analysis of variance was undertaken for each time interval of the incubations to determine significant differences between treatments. The significant effects for various treatments were detected using a t- test.