Long-term Effects Of Organic Manure Application On Soil Aggregate Stability Of Saline-sodic Soil In Western Songnen Plain

The high sodium ion content of saline-sodic soil in the western part of the Songnen Plain tends to disperse soil particles and hinders the agglomeration of soil particles,resulting in poor soil structure and generally low nutrients in the soil,which plays a hindering role in the normal growth of plants and relatively low yields.The application of organic manure is an effective measure to improve saline-sodic soil,on the one hand,it can increase the content of soil organic matter and promote the formation of soil agglomerates;on the other hand,its can increase the Ca²⁺content in the soil,reduce the Na⁺content and reduce soil dispersion,thus promoting the agglomeration of soil agglomerates.Soil aggregates,as the basic units of soil structure,play an important role in the structural stability and fertility maintenance of soils.In this study,based on a long-term localization experiment of organic manure to improve saline-sodic soil,we focused on the effects of different years of organic manure application（7 years（7a）,12 years（12a）,15 years（15a）,18years（18a）for organic manure and control（CK）without organic manure）on the distribution of aggregates and their stability in saline-sodic soil of the Songnen Plain.The results of the study showed the following.（1）Long-term application of organic manure can lead to a decrease in the content of MSAs>0.25 mm and WSAs<0.25 mm and an increase in the content of WSAs>0.25 mm in saline-sodic soil.The mean weight diameter（MWD）and geometric mean diameter（GMD）of stability indicators of water-stable aggregates increased compared to the treatment without organic manure application,and the soil percentage of aggregate destruction decreased.In the 0-20 cm soil layer,MSAs<0.25 mm content was highest in the 18a treatment,reaching 26.62%,an increase of205.63%compared to the CK treatment.The water-stable aggregates were mainly small aggregates in the CK treatment,with 93.86%WSAs<0.25 mm.The most significant reduction was observed in the 18a treatment among the applied manure treatments,with a 51.07%reduction compared to the CK treatment.The water-stable aggregates MWD and GMD increased in different treatments compared with the CK treatment,and the soil percentage of aggregate destruction was significantly reduced compared with the CK treatment,and the soil percentage of aggregate destruction in all soil layers reached the lowest value at 18a treatment.（2）Long-term application of organic manure increased the organic matter content of mechanical stable aggregate fractions in the saline-sodic soil of the Songnen Plain and in the 0-20cm,the organic matter in mechanical stable aggregate fractions under different treatments increased significantly compared to the CK treatment（p<0.05）.The application of organic manure significantly reduced the p H and EC values of mechanical stable aggregate fractions,compared to the CK treatment（P<0.05）;and the EC values of mechanical stable aggregate fractions reached the lowest values at the 18a treatment.The application of organic manure can reduce t he content of water-soluble and exchangeable Na⁺and increase the content of water-soluble and exchangeable Ca²⁺and Mg²⁺in mechanical stable aggregate fractions of saline-sodic soil in the 0-20 cm,and the highest values of water-soluble Ca²⁺and Mg²⁺in mechanical stable aggregate fractions were reached in the 15a and 18a treatments;the lowest values of water-soluble Na⁺in mechanical stable aggregate fractions were reached in the 18a treatment.The lowest values were 59.69,62.78,68.47,64.04 and 63.32 mg/kg,respectively.（3）Long-term application of organic manure increased the organic matter content of water-stable aggregate fractions in the saline-sodic soil of the Songnen Plain,and in the 0-20 cm,the organic matter in water-stable aggregate fractions under different treatments increased significantly compared to the CK treatment（P<0.05）.Application of organic manure increased the CEC at water-stable aggregate fractions,with a significant increase in both treatments 15a and 18a compared to the CK treatment（P<0.05）.Organic manure application increased the exchangeable Ca²⁺content at water-stable aggregate fractions,and the exchangeable Ca²⁺content of WSAs>0.25mm was significantly higher than that of WSAs<0.25mm.The application of organic manure reduced the exchangeable Na⁺content of water-stable aggregate fractions,with the lowest value of0.43 cmol/kg at the 18a treatment for WSAs>2 mm;all other water-stable aggregate fractions reached the lowest values of 0.44,0.44,0.42 and 0.44 cmol/kg at the 15a treatment,respectively.（4）Application of organic manure increased the exchangeable Ca²⁺,exchangeable Mg²⁺and organic matter content and decreases the exchangeable N a⁺content in saline-sodic soil.And the MSAs>2mm and WSAs<0.25 mm content gradually decreased with increasing years of organic manure application.The content of MSAs>2 mm and WSAs<0.25 mm had the greatest effect on the soil percentage of aggregate destruction among all aggregate fractions.Soil aggregate stability indicators water-stable aggregates MWD and GMD were significantly positively correlated with the exchangeable Ca²⁺,exchangeable Mg²⁺and organic matter content,and significantly negatively correlated with the exchangeable Na⁺content（P<0.05）.Soil percentage of aggregate destruction was significantly negatively correlated with the exchangeable Ca²⁺,exchangeable Mg²⁺and organic matter content,and significantly positively correlated with the exchangeable Na⁺content（P<0.05）.In summary,long-term application of organic manure reduced the number of MSAs>2 mm and WSAs<0.25 mm in saline-sodic soil of the Songnen Plain,which in turn reduced soil percentage of aggregate destruction and increased soil structural stability.This is mainly due to the application of organic manure to increase the soil organic matter content to promote agglomeration of soil aggregates.The increase in soil exchangeable Ca²⁺content displaces Na⁺in the soil,reduced soil dispersion and promotes soil aggregates agglomeration,thus reducing the content of WSAs<0.25 mm,decreasing soil percentage of aggregate destruction and improving soil aggregate structure stability.