| Long-term tillage and straw returning can change the soil physical structure characteristics and affect the stability of soil aggregates.However,there are few studies on the response of straw returning depth to the stability of aggregates at different depths of dryland meadow soil in Northeast plain.In this experiment,no-tillage(NT),shallow tilling(ST)and deep tilling(DT)were set.Six treatments of no-tillage mulch returning(NTS),shallow tillage returning(STS)and deep tillage returning(DTS)were used to determine soil related indexes after five consecutive years of field positioning experiments,and to investigate the effects of different soil tillage methods combined with different straw returning methods on soil structure,aggregate stability and soil organic carbon.The test results are as follows:1.The soil three-phase instrument was used to measure the soil physical properties under different soil layers under different treatments.The analysis of the test results showed that:Compared with NT,ST and DT treatments,NTS,STS and DTS all increased soil aeration and soil porosity,and reduced soil bulk density,and all treatments had significant or extremely significant negative correlation with soil bulk density and total porosity in different soil layers.2.The wet screen classification method was used to analyze the distribution of water-stable aggregates at different levels under different treatments.The results showed that STS and DTS increased the proportion of 53～250 μm micro-aggregates and < 53 μm sticky particles compared with ST and DT.Compared with NTS treatment and ST and DT treatment with NT treatment,STS and DTS were beneficial to increase the proportion of clay particles in 10-20 and 20-30 cm soil layer.The density grouping method was used to group the organic carbon of aggregate particles.The analysis results showed that: The content of mineral-bound organic carbon(MOC)in large aggregates was the highest,followed by that in micro-aggregates,and the content of MOC in clay particles was the lowest.The content of fine organic carbon(f POC)in large aggregates was also slightly higher than that in micro-aggregates.3.The fluorescence characteristics of soil water-soluble organic carbon(WSOC)in different treatments were analyzed using 3DEEM-PARAFAC as the main analysis method.The results showed that four fluorescence components were determined by soil WSOC,namely two humic substances C1(310,240/410 nm)and C2(270/465 nm)and two protein-like substances C3(280,230/340 nm)and C4(220/300 nm).Compared with DT treatment,the fluorescence intensity of humic components(C1 and C2)in WSOC in all soil layers was decreased by DTS,with ranges of 3.86%-4.91% and 1.85%-2.91%,respectively.Meanwhile,the fluorescence intensity of tyrosine proteins(C4)was enhanced by 6.43%-7.72%.It can be seen that straw deep turning can promote the conversion of soluble organic carbon of humus to soluble organic carbon of tyrosine protein which is easy to be oxidized and decomposed.The test results showed that ST and DT treatment decreased the stability of soil aggregates in 0-20 cm soil layer compared with NT treatment,and STS and DTS treatment decreased the stability of soil aggregates in 0-20 cm soil layer,possibly because continuous soil disturbance for many years was not conducive to the improvement of soil aggregate stability in this soil layer.Straw returning increased the content of soil particulate organic carbon to some extent,especially the coarse particulate organic carbon in large aggregates.NTS significantly increased the content of soil particulate organic carbon,but NT significantly reduced it.Moreover,straw returning and deep turning can promote the mineralization and decomposition of organic matter with complex structure and high aromatic property in deep soil WSOC.It is suggested to use conservation tillage such as wheel tillage to adjust straw returning depth in future production,so as to improve soil structure and properties,improve aggregate stability and fertilize soil. |
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