Effect of freeze-thaw cycling on grain size of biochar

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（biochar）可通过改变土壤孔隙度、容重、导水率与持水能力，改善土壤水文特性。上述特性与土壤及生物炭的粒径分布紧密关联，故会随生物炭的风化进程发生改变。本研究系统探究了冻融（F-T）循环在两种排水条件下对松木、牧豆树、芒草及污泥生物炭粒径的影响：不排水组（涵盖全部四类生物炭）与重力排水组（仅包含牧豆树生物炭）。在不排水组实验中，植物源生物炭的中值粒径出现下降，粒径分布特征与生物炭表面薄层剥落的现象相符：其粒径分布由单峰型转变为双峰型，峰值更低且分布区间更广。经20次冻融循环后，植物源生物炭的中值粒径最大降幅达45.8%，颗粒长径比最大提升22.4%；而污泥源生物炭的粒径与长径比均未受冻融循环影响。本研究同时观测到生物炭的骨架密度出现变化（最大增幅1.3%）、表观密度最大下降12.2%，以及颗粒内孔隙率（颗粒内部孔隙度）最大提升3.2%。在重力排水组实验中，经10次冻融循环后，牧豆树生物炭的中值粒径最大下降4.2%，长径比未发生变化。本研究还证实，生物炭粒径降幅与初始含水量呈正相关关系，这表明：具备提升含水量特性的生物炭（如高颗粒内孔隙率、孔隙连通性佳的大孔隙及亲水性），若结合不排水条件与频繁冻融循环，可能会加速生物炭的破碎降解。观测到的生物炭粒径与形貌变化，将改变其水文特性，进而可能对植物生长及水文循环产生影响。