Effects Of Dry-Wet Alternation And Freeze-Thaw Cycles On Aggregate Turnover And Soil Organic Carbon In Mollisols

The Northeast black soil area is the most important food production base and occupies an important position in the sustainable development of China’s crop economy.However,in recent years,it has suffered from frequent natural disturbances,resulting in the deterioration of soil aggregate stability and soil organic carbon content.In view of this,it is important to study the effects of wet-dry alternations and freeze-thaw cycles on soil aggregate turnover and organic carbon which could provide a theoretical basis for exploring aggregate turnover and organic carbon accumulation in mollisol.In this paper,5-2 mm（Nd₂O₃）,2-0.25 mm（La₂O₃）,0.25-0.053 mm（Sm₂O₃）and<0.053 mm（Gd₂O₃）aggregates were labeled by using rare earth oxidation（REOs）tracer method to quantify the aggregate turnover processes through indoor simulated wet-dry alternations and freeze-thaw cycles experiments.The dry-wet alternation experiment was conducted with two dry-wet intensities（80%field holding capacity dried to60%field holding capacity（Q1）vs.80%field holding capacity dried to 40%field holding capacity（Q2））and five different dry-wet alternations（0,1,2,4,and 8）.The freeze-thaw cycle experiment was conducted with two initial moisture contents（50%field holding capacity（T50）vs.100%field holding capacity（T100））and five different freeze-thaw cycles（0,3,6,12and 20）.The experiments were conducted to investigate the effects of dry-wet alternations and freeze-thaw cycles on aggregates size distribution,mean weight diameter（MWD）,aggregates turnover path,organic carbon（SOC）,particulate organic carbon（POC）and mineral-associated organic carbon（MOC）.The main results of this study were as follows:（1）With the same dry-wet alternation intensity,dry-wet alternation significantly increased the content of 5-2 mm aggregates and decreased the content of 0.25-0.053 mm aggregates,and MWD gradually increased with the increase of dry-wet alternation number.Under the same dry-wet alternation number,MWD was significantly higher in Q2 treatment than in Q1 treatment（P<0.05）.The dry-wet alternation promoted the formation of 2-0.25 mm aggregates and the fragmentation of 0.25-0.053 mm aggregates.With the number of wet and dry alternations increased,the turnover time of>0.25 mm aggregates first increased and then decreased,and the turnover time of<0.25 mm aggregates gradually decreased.（2）With the same dry-wet alternation intensity,the dry-wet alternation promoted the increase of SOC content compared to the control（0 times）.With the increase of the number of dry-wet alternations,the POC content gradually decreased,while the MOC content gradually increased.Under the same number of drying alternations,the POC content of Q1 dry-wet alternation was significantly greater than that of Q2 dry-wet alternation treatment（P<0.05）at the pre-dry-wet alternation period（2 times）,while the MOC content was not related to dry-wet alternations intensity.（3）With the same initial moisture content,freeze-thaw cycles significantly reduced the content of>0.25 mm aggregates and increased the content of 0.25-0.053 mm aggregates,and MWD gradually decreased with the increase of freeze-thaw cycles.After six freeze-thaw cycles,MWD was significantly higher in T50 treatment than in T100 treatment（P<0.05）.The freeze-thaw cycles promoted the fragmentation of>0.25 mm aggregates and the formation of0.25-0.053 mm aggregates.With the increase of the number of freeze-thaw cycles,the turnover time of each particle size aggregates increased significantly（P<0.05）.（4）At the same initial moisture content,freeze-thaw cycles reduced the SOC content compared with the control（0 cycles）.With the increase of the number of freeze-thaw cycles,the POC content gradually decreased,while there was no significant difference in the MOC content.At the same number of freeze-thaw cycles,the POC content of T100 was significantly greater than that of T50 treatment at the first 12 freeze-thaw cycles（P<0.05）,while the MOC content of T100 was significantly greater than that of T50 treatment at the first 3 freeze-thaw cycles（P<0.05）.In summary,dry-wet alternations and freeze-thaw cycles affect soil aggregate turnover and the changes in organic carbon fraction.This study provides a theoretical basis for quantifying the dynamic process of aggregate turnover path,which is important for improving soil tillage quality.