Effects of biochar application during different periods on soil structures and water retention in seasonally frozen soil areas.

Currently, little is known about soil improvement by biochar in seasonally frozen soil areas. It is not clear whether the effects of biochar application on soil physical properties differ based on application period. Therefore, the purpose of this study was to explore the effects of biochar on soil structure and water retention in cold regions during different application periods. Three biochar applications during different periods were set up through field trials (A: in the early stage of freezing, B: in the middle period of thawing and C: addition of half of the biochar in the early stage of freezing and other half in the middle period of thawing), including four biochar application rates (3, 6, 9 and 12 kg·m), and no biochar was applied as a control (CK). The results showed that the selection of the biochar application period had a significant effect on the soil structure, but there was no significant difference in the stability of the soil structure. Biochar significantly increased the total porosity (TP) and the content of aggregates with diameters >0.25 mm in the soil, and all 9 kg·m biochar treatments showed the best structural stability index. The improvement of the soil structure led to the enhancement of the water storage capacities. The plant available water content (PAWC) increased from 0.0638 to 0.0927-0.1767 cm·cm, and this result was significantly related to the increases in soil TP and large aggregates (LAs: >2 mm). The optimum field capacity (FC = 0.372 cm·cm) was obtained when the applied amount was 9 kg·m. This was beneficial for the stable storage of soil moisture. However, compared with the CK, none of the treatments in area B increased the liquid water content in the field soil. In summary, we suggest that treatment C9 is the most suitable method for application in seasonally frozen soil areas.