Abstract: Biochar has been reported for its beneficial effects on soil carbon sequestration, soil fertility improvement, and the immobilization of metal and organic contaminants in soils. Its large-scale agricultural and environmental application, however, is constrained by its high production cost in association with expensive equipment and operations and its high transportation cost of moving agricultural and forest residues to biochar production plant and delivering biochar to the end users. Exploring a technology for directed conversion from agricultural and forest residues to biochar in the field for local applications can significantly reduce the production and transportation costs of biochar, thus helping its applications. By mimicking the nature, where only agricultural and forest residues, water and fire were required for biomass carbonization and charcoal formation, a method for biochar production in the field was proposed and described in details. Briefly, this involved an aerobic process of biomass carbonization in a brick-constructed trough, and the formation of biochar by a fire-water coupled method. The carbonization process had the dual features: combustion on the surface of biomass and oxygen-limiting pyrolysis inside of the biomass. Three operational processes of aerobic carbonization and its termination were used to suit the production of biochars from different types of residues: 1) Large Salicaceae branches were ignited at one direction of the trough for aerobic carbonization, followed by a water-mist spray for immediate termination of the carbonization; 2) Medium-size cotton stalk was ignited at one direction for carbonization and then sprayed by a water column to crosscut formed biochar; 3) Small hollow reed straw was ignited at multiple directions, then water mist was sprayed layer by layer on biomass. The dislocated holes on side walls of brick trough performed as ignition points, channels for water mist and air ventilation channels. The biochars produced in the field by the proposed technology were characterized in this study. The biochars were relatively homogeneous, and the conversion rates from biomass to biochar were about 30%. Carbon content of biochar was 43.49%-60.30%, and nitrogen content was 0.52%-0.86%. The biochar also contained the abundant surface functional groups, with a carboxyl group content of 0.98-1.09 mol/kg and a phenolic hydroxyl group content of 0.53-0.59 mol/kg, and the specific surface area of the biochars varied between 16.0 and 262.2 m2/g, which underpins their use as adsorbents for cations, such as ammonium ions and some heavy metals and other extraneous ions and molecules. The flue gas generated from the burning of the Salicaceae, cotton stalk, and reed straw in the carbonization process was treated by a multiple-step process to reduce particulate matter concentrations. PM 2.5 in the treated flue gas was reduced to 56, 66 and 68 μg/m3 for Salicaceae, cotton stalk, and reed straw, respectively, and the corresponding PM 10 was reduced to 100, 114 and 128 μg/m3, which meet the national emission standard. The biochar preparation technology provided herein is simple to operate, low in cost, and highly efficient. Based on labor, fuel, and water inputs, the productivity was 1 t/d per person, and the cost was 162.5 yuan/t by farmers. This technology for producing low-cost biochar would make its agricultural and environmental applications feasible.