Biochar may improve soil hydrology by altering soil porosity, density, hydraulic conductivity, and water-holding capacity. These properties are associated with the grain size distributions of both soil and biochar, and therefore may change as biochar weathers. Here we report how freeze-thaw (F-T) cycling impacts the grain size of pine, mesquite, miscanthus, and sewage waste biochars under two drainage conditions: undrained (all biochars) and a gravity-drained experiment (mesquite biochar only). In the undrained experiment plant biochars showed a decrease in median grain size and a change in grain-size distribution consistent with the flaking off of thin layers from the biochar surface. Biochar grain size distribution changed from unimodal to bimodal, with lower peaks and wider distributions. For plant biochars the median grain size decreased by up to 45.8% and the grain aspect ratio increased by up to 22.4% after 20 F-T cycles. F-T cycling did not change the grain size or aspect ratio of sewage waste biochar. We also observed changes in the skeletal density of biochars (maximum increase of 1.3%), envelope density (maximum decrease of 12.2%), and intraporosity (porosity inside particles, maximum increase of 3.2%). In the drained experiment, mesquite biochar exhibited a decrease of median grain size (up to 4.2%) and no change of aspect ratio after 10 F-T cycles. We also document a positive relationship between grain size decrease and initial water content, suggesting that, biochar properties that increase water content, like high intraporosity and pore connectivity large intrapores, and hydrophilicity, combined with undrained conditions and frequent F-T cycles may increase biochar breakdown. The observed changes in biochar particle size and shape can be expected to alter hydrologic properties, and thus may impact both plant growth and the hydrologic cycle.

生物炭可以通过改变土壤的孔隙度，密度，水力传导率和持水能力来改善土壤水文学。这些特性与土壤和生物炭的粒度分布有关，因此可能会随着生物炭天气的变化而变化。在这里，我们报告了冻融（FT）循环如何在两种排水条件下（不排水（所有生物炭）和重力排水实验（仅豆科动物生物炭））影响松树，豆科灌木植物，猕猴和污水废生物炭的粒径。在不排水的实验中，植物生物炭显示出中值粒度减小和粒度分布变化，这与生物炭表面薄层的剥落一致。生物炭的粒度分布从单峰转变为双峰，具有较低的峰和较宽的分布。对于植物生物炭，在20个FT循环后，中值晶粒尺寸最多减少了45.8％，而晶粒长宽比则最多增加了22.4％。FT循环并没有改变污水废料生物炭的粒度或长宽比。我们还观察到生物炭的骨架密度（最大增加1.3％），包膜密度（最大减少12.2％）和孔隙内（颗粒内部的孔隙，最大增加3.2％）的变化。在排水实验中，豆科灌木生物炭在10个FT循环后显示出中值晶粒尺寸的减少（最多4.2％）并且长宽比没有变化。我们还记录了晶粒尺寸减小与初始含水量之间的正相关关系，这表明增加含水量的生物炭特性（例如高孔隙率和大孔内孔隙连通性）以及亲水性，再加上不排水的条件和频繁的FT循环，可能会增加生物炭的分解。预计观察到的生物炭颗粒大小和形状的变化会改变水文特性，因此可能会影响植物的生长和水文循环。