Effect Of Freeze-thaw Process On Aggregate Stability Of Black Soil In Northeast China

Freeze-thaw is one of the limiting factors affecting the stability of soil aggregates,which can change the size of aggregates,affect the physical and chemical properties of soil,destroy the mechanical structure of soil and then fail to recover naturally,and increase the erodibility of soil.Black soil region in northeast China is located in seasonal frozen soil region.Due to the characteristics of black soil and the particularity of natural environment,black soil always suffers from repeated freezing and thawing,which affected the formation process and particle size distribution of air-dried aggregates and water-stable aggregates.Previous studies showed that freeze-thaw increased the stability of black soil aggregates,while other studies showed that freeze-thaw decreased the stability of black soil aggregates.The black soil collected from Binxian,Keshan County and Nenjiang County in Heilongjiang Province was selected as experimental materials in the present study.The effects of freeze-thaw cycle,initial moisture content and clay content on the stability of soil aggregates and glomalin-related soil protein(GRSP)in black soil were investigated to elucidate the mechanism of freeze-thaw on the stability of soil aggregates.The results derived from the present research will provide scientific basis and technical support for the protection and sustainable utilization of black soil and regional food security.The main results of the presemt study are as follows:(1)The present study found that the size distribution of air-dried aggregates of the three black soils changed greatly after the freeze-thaw under the same initial moisture content and the ratio of aggregates of > 2 mm group apparently decreased.The proportion of small size fractions of 0.5 mm increased significantly(p < 0.05),showing a tendency to promote agglomeration,while the sizes of 1 ? 2 mm and 0.5 ? 1 mm were affected by the type of black soil and the initial moisture content.The higher the content of clay particle was,the higher the percentage of aggregates and mean weight diameter(MWD)of each particle size were.With increase of the initial moisture content,MWD of the air-dried aggregates increased significantly,indicating that appropriate initial moisture content in the soil during the freezing and thawing process can enhance the stability of the aggregates.With freeze-thaw cycle increasing,the MWD first decreased and then increased.The black soil possessed weak anti-erosion ability in the early period of freeze-thaw,and subsequent freeze-thaw will improve the anti-erosion ability.(2)The present study revealed the damage mechanism of water-stable aggregates of black soil in Northeast China and the spatial distribution characteristics of water-stable aggregates during freezing and thawing based on LB method.The present study also found that the main damage mechanism of black soil aggregates in Keshan,Nenjiang and Binxian was rapid wetting to cause the dissipation of "air burst".The order of the agglomerate destruction mechanism was FW(Fast Wetting)? WS(Wetting Stirring)?SW(Slow Wetting).(3)The effects of freezing and thawing process on water-stable aggregates and GRSP in the black soil and their distribution characteristics were also studied in the present study.Compared with no freeze-thaw(0 times),the MWD of the water-stable aggregates in Keshan,Nenjiang,and Binxian after 30 circles showed a significant decreasing trend(p < 0.05)under TC treatment.Under TC50 and DW treatment,the MWD of water-stable aggregates in Keshan,Nenjiang and Binxian counties showed a significant(p < 0.05)increase.The water-stable aggregates showed a trend of increasing content gradually as the size decreases.After freezing and thawing,the concentrations of EE-GRSP and DE-GRSP in the black soil.With the increase of freezing and thawing cycle,DOC concentrations in the black soil showed a decreasing trend.With t increase of the initial water content,DOC concentrations in the black soil showed an overall increasing trend.Under TC treatment,EE-GRSP was negatively correlated with MWD of water-stable aggregates,but did not reach significance,while there was a significant(p < 0.01)positive correlation under TC50 and DW treatment,while DE-GRSP only.In addition,there is a significant positive correlation betweeen DE-GRSP and MWD of water-stable aggregates under TC and DW.In summary,with the increase of freeze-thaw cycles,the larger size groups(> 2 mm)of the air-dried aggregates and water-stable aggregates of black soil in northeast China showed a decreasing trend,while the smaller size groups(< 0.5 mm)showed an increasing trend.The stability of air-dried aggregates and water-stable aggregates was significantly affected by initial soil water content during freeze-thaw process.The appropriate soil water content(TC50)during freeze-thaw process can increase the stability of air-dried aggregates and water-stable aggregates in northeast black soil.