基于稀土氧化物示踪法探究冻融循环对黑土团聚体周转的影响

为区分土壤团聚体形成和破碎过程，阐明冻融循环对黑土土壤结构的影响，本文利用稀土氧化物（REOs）示踪技术，通过室内模拟实验，探究不同初始含水量（50 %田间持水量（T50） vs. 100 %田间持水量（T100））和冻融循环次数（0次、3次、6次、12次和20次）对团聚体粒径分布、平均质量直径（MWD）以及团聚体周转过程的影响。研究结果表明：同一初始含水量下，随着冻融循环次数的增加，MWD、>0.25 mm和<0.053 mm团聚体含量显著降低，0.25～0.053 mm团聚体含量显著增加（P < 0.05）。6次冻融循环后，T50处理下的MWD显著高于T100处理（P < 0.05），5～2 mm和<0.25 mm团聚体含量无显著差异。除5～2 mm团聚体外，相邻粒级团聚体之间周转更为激烈；在同一冻融循环次数下，5～2 mm团聚体向0.25～0.053 mm团聚体的破碎量在T100处理下显著高于T50处理（P < 0.05）。冻融循环促进了>0.25 mm团聚体的破碎和＜0.053mm的团聚，表现为0.25-0.053mm团聚体的累积，该变化与土壤初始含水量无关。冻融循环过程中，MWD与各粒径团聚体相对形成量呈显著正相关，与其相对破碎量呈显著负相关（P < 0.05）。随着冻融循环次数的增加，各粒径团聚体周转时间显著增加（P < 0.05）。同一冻融循环次数下，>0.25 mm团聚体的周转时间高于<0.25 mm团聚体，T100处理下的团聚体周转时间显著高于T50处理（P < 0.05）。综上所述，冻融循环次数和土壤初始含水量通过影响团聚体形成和破碎过程改变土壤结构的稳定性。本研究结果可为进一步探究冻融循环下黑土土壤结构变化提供理论依据。

Effects of Freeze-thaw Cycles on Soil Aggregates Turnover in Mollisols Using Rare Earth Oxides as Tracers

【Objective】In order to elucidate the effects of freeze-thaw cycles and initial water content on soil aggregates structure, rare earth oxides (REOs) were used as tracers to separate soil aggregates formation and breakdown processes. 【Method】REOs-labelled soil was reformed and investigated herein. Two initial water contents (50 % field water holding capacity (T50) vs. 100 % field water holding capacity (T100)) and five freeze-thaw cycles (0, 3, 6, 12 and 20 cycles) were involved in the simulation experiments. Soil aggregates distribution, mean weight diameter (MWD), the aggregate turnover process were measured accordingly. 【Result】The results showed that freeze-thaw cycles significantly reduced MWD, the contents of > 0.25 mm aggregates and <0.053 mm aggregates but increased the contents of 0.25~0.053 mm aggregates under the same initial water content. After 6 freeze-thaw cycles, MWD was significantly higher under T50 compared with that under T100(P < 0.05), there was no significant difference between the aggregate content of 5~2 mm and <0.25 mm. Except for 5~2 mm aggregates, the intensive transformations between neighboring size aggregates were observed during the whole simulation experiments. For the same freeze-thaw cycles, the fragmentation amount of 5~2 mm aggregate to 0.25~0.053 mm aggregate was significantly higher at T100 compared with T50 treatment(P < 0.05).The freeze-thaw cycles promoted the breakdown of >0.25 mm aggregates and the formation of 0.25～0.053 mm aggregates both under T50 and T100 treatments. MWD was significantly positively correlated with the relative formation of soil aggregates and negatively related with the relative breakdown of soil aggregates(P < 0.05). The turnover time of soil aggregate significantly increased with freeze-thaw cycles(P < 0.05). The aggregate turnover time of > 0.25 mm aggregates was higher than that for <0.25 mm aggregates. The aggregate turnover time was significantly higher under T100 than that under T50 with the same freeze-thaw cycle(P < 0.05). 【Conclusion】In conclusion, the number of freeze-thaw cycles and the initial water content of the soil significantly affect the aggregate turnover, which changes the stability of the soil structure by affecting the aggregate formation and fragmentation process. The results can provide a theoretical basis for further exploration of the structural changes of black soil under the freeze-thaw cycle.