Effects Of Organic Amendment Rate On The Soil Structure Properties And Their Seasonal Variations In Black Soil

Soil structure controls all processes in the soil and affects the regulation ability of soil water and nutrient conservation or supply.Soil organic matter is an important component of soil structure.Long-term changes of soil organic matter and decomposition of exogenous organic matter drive the formation and evolution of soil structure.The way of returning organic matter to the field affects the initial soil structure,and then influences the decomposition process of exogenous organic matter and change the process of soil aggregation structure.Different amount or type of exogenous organic matter were incorporated to the soil,the mineralization loss of exogenous organic matter in the soil may be different.It affects the long-term change of soil organic matter and the decomposition process of exogenous organic matter,and then controls the formation and evolution of soil structure and soil fertility become different.Based on the long-term experiment in typical black soil region,this paper systematically studied the changes of soil organic carbon components and soil physical properties after long-term application of different amounts of organic fertilizers(0,7.5,15 and 22.5 Mg/ha)in black soil.Meanwhile,the changes of soil organic carbon components and soil physical properties were also investigated monthly during crop growing season to reveal the relationship between the temporal changes of soil organic matter and soil structural properties in black soil region.In addition,the seasonal variation of soil physical properties under different amount or modes of maize straw incorporation(no returning,half returning,full returning and full deep returning)was studied,and the response of straw returning quantities and modes to soil structural turnover and soil fertility characteristics was discussed.The main results are as follows:(1)After long-term application of different amounts of organic amendment,with the increase of application rate of organic amendment,the total organic carbon content of soil,the organic carbon storage in all density fraction and the organic carbon storage in > 0.25 mm aggregates also increased significantly.The results show that the soil in black soil region still has great carbon sequestration potential.As long as the organic matter input is large enough,more exogenous organic matter can be immobilized into soil by soil minerals bounds and macroaggregate conservation mechanisms.At the same time,the stability of soil aggregates,saturated hydraulic conductivity or > 1.0 mm effective porosity increased with the increasing rate of organic amendment.(2)Due to tillage and freeze-thaw effect,the stability of soil aggregates was the lowest in all treatments in May.With the decomposition of exogenous organic matter,the organic matter was fixed into > 0.25 mm aggregates,and the stability of soil aggregates increased gradually,and the aggregate stability decreased with the continued decomposition of particulate organic carbon.The saturated hydraulic conductivity and effective macroporosity also changed,and with the increase of organic fertilizer application,their dynamic change intensity became more intense.The results showed that increasing the amount of organic amendment could enhance the restoration of soil pore structure by increasing the content of soil organic carbon and promoting the transformation of organic matter into macroaggregates.These processes play an important role in regulating soil water conservation and drainage capacity,and are also critical to improving crop yields.In addition,our results show that in order to maintain the resilience of soil structure and water conservation and drainage capacity,sufficient amount of organic matter(equivalent to the total biomass of annual crop straw)should be added to the soil in the black soil region in northeast China.(3)Soybean root growth significantly increased the resilience of soil structure,the resilience of structure in crop-growth zone was significantly higher than that in crop-free zone.In addition,the application rate of organic amendment and root system had significant interaction on the turnover of soil structure.The larger amount of organic amendment,the soybean root system developed better,and the more significant recovery of soil structure.In the black soil region in Northeast China,the total soil porosity in May was significantly increased by soil tillage and freeze-thaw effect,but these pore structures were very unstable.With the passage of time and the influence of internal and external forces,these pore structures disappeared gradually.The increased amount of organic amendment and the crop growth effect improved the soil resistance by increasing the resilience of soil structure,thus reduce soil shrinkage.(4)Straw incorporation significantly reduced soil bulk density and penetration resistance,increased soil capillary pore and micropore,and also increases soil saturated hydraulic conductivity.The stability of soil aggregates increased with the increasing of straw incorporation.In addition,soil water repellency also increased with the increasing amount of straw incorporation.The physical properties in surface soil were not significantly improved in deep straw burial treatment,but in the 20-35 cm soil,the stability of soil aggregates,capillary porosity and saturated hydraulic conductivity were significantly increased.In addition,soil water retention and permeability were increased,and the penetration resistance of deep soil was significantly decreased which contributed to the root development of crops.Straw incorporation increased the content of soil macroaggregates at the beginning of the experiment,and the content of soil macroaggregates increased rapidly from May to June.The turnover of soil aggregates occurs mainly in micro-aggregates and silt-clay particles in CK treatment,and the change of large group content is small.In the straw incorporation treatments,the turnover of soil aggregates mainly occurs between macro-aggregates and micro-aggregates.Our results show that straw returning can improve soil structure by improving the content of soil macroaggregates and promoting the turnover between macroaggregates and microaggregates,thus affecting the physical properties of soil.