Study Of Mechanical Properties Of Black Soil After Simulated Straw Incorporation

The black soil area in northeastern China is one of national main grain production areas.The organic matter content gradually decreases and the clay content gradually increases with prolonging the reclamation years.Mechanical tillage especially the universal use of large machinery leads a more serious soil compaction and soil erosion in this area.It is recognized that straw returning is of interest to not only support sustainable agriculture and conserve soil fertility,but also relieve the mechanical compaction and reduce the soil erosion when applying conservation tillage in the black soil area.The mechanism of alleviating soil compaction or erosion belongs to the research field of soil mechanics referring to consolidation and shear.Based on the theory of soil mechanics,typical uncultivated black soil(high organic matter and low clay content)and tillage soil for 40 years(low organic matter and high clay content)were selected as research objects in this study;through combining the indoor oedometer test and triaxial test,the Compression index,Rebound index,Cohesion and Internal friction were obtained and analyzed by simulating different lengths of chopped straw(1,5,15,25 mm,soil treatment without straw),dif-ferent amount proportions of returning straw(25%,50%,75%,100%,soil treatment without straw),different positions of straw(upper returning?central returning?lower returning/whole returning and soil treatment without straw);then their effects on soil compression,rebound and shearing properties were investigated.Results showed as follows:(1)With the chopped straw length increasing,the compression index and rebound index was negatively and positively correlated with straw length,respectively;especially when straw length was 15 mm,the compression index decreased to the minimum value,and at the same time rebound index significantly increased to the maximum value;or black soil is hard to be compressed,and most easily rebound to the natural state after compaction.Soil cohesion significantly increased with straw length increasing,leading to higher shear strength,but the increase amplitude was decreasing gradually.(2)When amount of straw returning was 100%,the black soil showed a strongest ability both to resist compression and to recovery after compaction.However,shear strength of the black soil was biggest when amount of straw returning was 50%.Considering the comprcehension of compression,rebounding,shearing behaviors,the straw returning can effectively improve the mechanical properties and structure of black soil,especially when amount ratio of straw returning is 50-100%,in which black soil has better potential to resist compression and erosion(3)When straw was returning in upper position,the black soil showed a strongest ability both to resist compression and to recovery after compaction.However,shear strength of the black soil was biggest when straw was returning in the middle position.In collusion,when positions of straw returning were upper and central,in which black soil has better potential to resist compression and erosion.In summary,the inherent shear strength of uncultivated black soil is stronger than tillage soil.Straw returning could effectively improve the mechanical properties of black soil.The improved effect of straw returning on low organic matter high clay soil samples which due to long term tillage were more significant.When the length of returning straw and amount of straw returning was 15 mm and 50%-100%,respectively,and the straw was returning in the middle and upper positions,the resistance to compression and erosion is most significant.At the same time,the ability of recovery after compaction is strongest.The further researches need to combine with the field straw incorporation process combing with the straw decomposition degree and duration.The findings above could provide theoretical basis for compaction and restoration,erosion and protection of black soil;at the same time lay the foundation for enriching interdisciplinary methods of traditional soil science and soil mechanics.