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To clarify the effects of co-application of rice-straw biochar and different chemical fertilizer doses on paddy soil properties and japonica rice production in Northeast China, a located-field experiment was adopted for 5 years. The treatments included fertilization (F, no biochar, control) and rice-straw biochar (7.5 t ha−1) combined with different chemical fertilizer doses (100, 80, and 60% that of F) set as CF, CF1, and CF2, respectively. The results showed that the co-application of biochar and chemical fertilizers after 5 years reduced soil bulk density, increased soil total and capillary porosity, changed the soil solid–liquid–gas ratio (increased the liquid and air ratios and decreased the solid ratio), and increased soil macroaggregates (>0.25 mm) and aggregate stability, which enhanced the soil’s physical properties. Furthermore, the co-application improved soil pH and soil enzyme activity (urease, sucrase, and catalase) and had a significant effect on promoting SOM. The soil total N, P, and K of CF and CF1, were 23.48, %, 47.28%, and 26.59% and 9.47%, 43.98%, and 27.87% higher than those of the control, respectively. The soil available nutrients (N, P, and K) increased in the co-application treatment, shown as the nitrogen was promoted higher in the early growth of rice, while phosphorus and potassium were exhibited greater in the late growth. With the soil amendment, the co-application increased the rice plant height and dry matter accumulation, and the CF and CF1 increased yields by 5.78 and 2.41%, respectively, by increasing the rice effective panicles, grains per panicle, and seed-setting rate. The co-application of biochar and chemical fertilizers could significantly amend soil properties and reduce chemical fertilizer use to ensure rice–grain output, which has an important significance for enhancing sustainable soil and crop productivity.

Under cold climate conditions, the temporal and spatial distribution of heat and the influencing factors in different layers of Albic soil remain unclear. Here, we aimed to clarify the influence of biochar on soil heat storage capacity and identify approaches to regulate temporal-spatial variations in Albic soil temperature. To this end, we studied thermal properties and temporal-spatial temperature variations in different soil layers (10 and 20 cm) amended with biochar at different concentrations (0, 10, 20, 30, 40 g·kg-1) in the key growth stages of soybeans through pot experiments for two years. We found that the daily mean soil temperature and accumulated soil temperature improved during the 2 years in both soil layers after biochar application. Compared with the control, the biochar treatments increased soil daily minimum temperature, decreased soil daily maximum temperature and soil temperature fluctuations, postponed the soil heating stage by 1 h, and caused a "warming effect" during the soil cooling stage. Furthermore, biochar treatment enhanced the thermal capacity of the Albic soil by 7.49% and 6.53% compared with the control in 2016 and 2017, respectively, whereas thermal conductivity and thermal diffusivity were decreased compared with the control. Biochar regulated the temporal-spatial response variation in temperature of the Albic soil as follows: biochar distributed heat in the Albic soil more uniformly over time by changing the soil temperature cycle and reducing the temperature difference, and heat tended to move downwards into the soil layers by increasing the heat capacity and reducing soil thermal conductivity and thermal diffusivity. Biochar changed the heat storage capacity of the Albic soil and temporal-spatial distribution of heat. These changes provide sufficient, timely, and effective heat supply for crop growth. Furthermore, biochar treatments had a significant and positive effect on soil physical properties and crop yield. In cold areas, these changes would help crops resist temperature changes caused by extreme weather and reduce the rate of cold damage. The results of this study have important practical relevance for enriching the basic scientific theory of biochar application.