e., the field water capacity of the soil). The unamended control was also subject to disruption of mixing. The incubated pots were placed in a room at 28 °C and
weighed every 5 d to maintain a constant moisture content. All treatments were carried out in triplicate. The incubation time was 105 d in total, and soils were analyzed at 21 d, 42 d, 63 d, 84 d, and 105 d to determine their physical and chemical properties. Soil samples were air dried and ground to pass through a 2-mm sieve for subsequent analysis. The particle size distribution was determined by the pipette method (Gee and Bauder, 1986). Soil pH was determined by a ratio of soil to water find more of 1:2.5 (McLean, 1982). Total soil C and N contents were measured with a Fisons NA1500 elemental analyzer (Thermo Electron Corporation, Waltham, Massachusetts, USA). Soil organic carbon (SOC) was determined BMN 673 by wet oxidation method (Nelson and Sommers, 1982). Each extracted fraction was analyzed for total organic C (O.I. Analytical 1010) using the heat-persulfate oxidation method. The cation exchange capacity (CEC) and exchangeable bases were measured using the ammonium acetate (pH = 7) method (Thomas, 1982). Bulk density was determined by the core method (Blake and Hartge, 1986). Saturated hydraulic conductivity (Ksat) was measured in saturated soil packed in 100 cm3 columns. The Ksat was determined in the laboratory using
the Klute and Dirksen (1986) falling-head method with distilled water. Modified fast-wetting in water, as proposed by Le Bissonnais (1996), was used to measure the aggregate through stability of 2-mm air-dried aggregates (35 g). Four cm amplitude was applied for 5 min vertical movement to a nest of sieves (> 2000, 1000–2000,
500–1000, 250–500, 250–106, < 106 mm) immersed in a container of tap water (101 mS/cm). The material that remained after wet-shaking in each sieve was carefully removed, and the mean weight diameter (MWD) of the aggregate size was calculated using equation(1) MWD=∑i=1nxiwiwhere n is the number of sieves, and x and w are diameter and weight, respectively. The specific surface areas of soil and biochar were determined by N adsorption isotherms at 77.3 K interpreted by the BET equation (Brunauer et al., 1938) (PMI Automated BET Sorptometer BET-202A). Soil microbial biomass carbon (MBC) was determined via fumigation and extraction (Brookes et al., 1985 and Vance et al., 1987). The MBC was only determined at 0, 21, 63 and 105 days during the incubation period. Fifteen grams of subsample of the incubated soil was fumigated with ethanol-free chloroform for 24 h at 25 °C. After chloroform removal, the subsample was extracted with 200 ml 0.5 M K2SO4 solution for 30 min. Organic carbon in the extract was measured by wet digestion with dichromate and titration with FeSO4. Fourier-transform infrared (FTIR) analysis was performed to test the quality of the study biochar. Ground biochar (0.3–0.