Effects Of Straw Returning On Aggregate And Available Nutrients Of Shajiang Black Soil

The Shajiang black soil covers an area of about 4 million hectares,which is the most typical medium and low yield field soil in the Huanghuaihai Plain.Straw returning has played an important role in improving the properties of Shajiang black soil,improving the productivity of Shajiang black soil,and promoting crop yield.Based on the statistics of straw nutrient resources in Huanghuaihai Plain and Shajiang black soil area and their potential to replace chemical fertilizers,this paper studied different straw returning methods in wheat-maize rotation system through long-term straw returning positioning experiment（CK₁:control treatment,WMS+F:returning wheat and maize straw+conventional NPK,WS+F:returning wheat straw+conventional NPK,MS+F:returning maize straw+conventional NPK,F:conventional NPK）and straw returning to the field with nitrogen fertilizer（the nitrogen fertilizer application rates of CK₂,SN1,SN2,SN3,SN4 was 0 kg/hm²,157.5kg/hm²,180 kg/hm²,202.5 kg/hm²,225 kg/hm²,respectively）.The effect of Shajiang black soil aggregates and their available nutrients,and the fluvo-aquic soil in the same area（Huanghuaihai Plain）was selected,and a positioning experiment of straw not returning to the field+nitrogen fertilizer（the nitrogen fertilizer application rate of CK₃,FN1,FN2,FN3,FN4 was 0 kg/hm²,150 kg/hm²,190 kg/hm²,and 270 kg/hm²,respectively）,to further verify the effect of nitrogen application on soil aggregates.The research results are as follows:1.The Huanghuaihai Plain and the Shajiang black soil area are mainly planted with wheat and maize,and the straw resources of the main crops（wheat+maize）are 15836.29×10⁴ t and 2058.72×10⁴ t,respectively.The total amount of nitrogen,phosphorus and potassium nutrient resources of main crop straws in the Huanghuaihai Plain were 105.23×10⁴ t（N）,40.25×10⁴ t（P₂O₅）and 316.38×10⁴ t（K₂O）,respectively,while the wheat and maize straws in the black soil area of Shajiang black soil.The total amount of nitrogen,phosphorus and potassium nutrient resources is about 60.04×10⁴ t,including 13.68×10⁴ t（N）,5.23×10⁴ t（P₂O₅）and 41.13×10⁴ t（K₂O）.The straw returning to the field has great potential to replace chemical fertilizers,among which the potentials（N+P₂O₅+K₂O）of the main crop straws in the Huanghuaihai Plain to replace chemical fertilizers are 190.55 kg/hm² and 120.36 kg/hm²respectively,and the straws（wheat+maize）in the Shajiang black soil area.The nutrient potentials were N 16.93 kg/hm²,P₂O₅ 8.10 kg/hm² and K₂O 91.61 kg/hm²,respectively.2.Typical Shajiang black soil organic matter accumulated significantly under long-term straw returning,but soil p H was reduced;soil total nitrogen,total phosphorus,alkali-hydrolyzed nitrogen,available phosphorus,and available potassium all accumulated to varying degrees,among which the accumulation of available potassium.The most significant one showed the law of WMS+F>WS+F>MS+F>F>CK₁,which was highly consistent with the change law of soil organic matter.Total potassium was less affected by different fertilization treatments and straw returning patterns.Straw returning to the field combined with nitrogen fertilizer significantly increased the content of soil alkali-hydrolyzable nitrogen（P<0.05）,and decreased the content of soil p H,available phosphorus and available potassium.3.Typical Shajiang black soil aggregates are dominated by>2 mm aggregates,accounting for more than 30%of the soil dry weight,and the content of<0.25 mm aggregates is the lowest,which accounts for no more than 15%.After Straw returning to the field,the poor soil structure（>2 mm）was easily broken and formed aggregates<2 mm.Among them,the wheat straw returning treatment（WS+F）had the most significant effect（P<0.05）.Helps to improve soil structure.With the increase of nitrogen fertilizer application rate,the aggregates larger than 2 mm increased in a gradient,which was not conducive to the improvement of soil structure.4.In typical Shajiang black soil,the content of available nutrients in aggregates<0.25 mm was higher.Under the straw returning mode,a small amount of alkaline hydrolyzable nitrogen was accumulated along with the formation of aggregates of<1 mm;exogenous inorganic phosphorus was easy to accumulate in the<2 mm particle size,but this accumulation occurred more obviously in the straw returning mode.The content of available potassium in the aggregates with different particle sizes increased significantly under the full amount of Straw returning to the field and the balanced application of NPK.The straw returning is an important measure to maintain soil potassium balance.5.The increase of available nutrients for aggregates of various particle sizes was not obvious when straw was returned to the field and nitrogen fertilizer was applied,but showed a downward trend.The soil nutrient decline in the<2 mm aggregates was significantly lower than that in the>2 mm aggregates,especially under the condition of single nitrogen fertilizer application.Under the single application of nitrogen fertilizer,along with the formation of unfavorable structures,soil available phosphorus was mainly concentrated in aggregates>2mm.In conclusion,straw returning is beneficial to improve the soil structure of typical Shajiang black soil.Nitrogen fertilizer application is an important control measure in the process of straw returning,but excessive application will affect the returning effect.There is an obvious process of coordination and restriction between straw returning and nitrogen fertilizer regulation in the improvement of typical Shajiang black soil,and the related mechanism still needs to be further explored.