Effect Of Straw Returning Methods And Nitrogen Fertilizer Application Rates On Soil Organic Carbon Profile Distribution And Activity

Long-term straw removal and single rotary tillage were adopted in southern Liaoning,which decreased soil fertility and limited the continuous high and stable yield of maize.Therefore,in 2018,our research group established a field plot positioning experiment of maize straw returning methods and nitrogen fertilizer application rate in Haicheng,Liaoning Province,and the experiment adopted split plot design,the main area: two-year no-tillage straw mulching + one-year subsoiling + rotary tillage straw mixed returning,referred to as two-free and one-loosening methods(NPT);three consecutive years of deep loosening + rotary tillage straw mixed returning,referred to as deep tillage mixed returning(PT);rototill straw mixed return(RT).Subzone: no nitrogen fertilizer(N0);80% nitrogen fertilizer(N192)and 100% nitrogen fertilizer(N240),a total of nine treatments.Soil samples of different soil layers(0 – 10 cm,10 – 20 cm,20 – 30 cm,30 – 40 cm)were collected after maize harvest in 2020 to study the effects of different combinations of straw returning methods and nitrogen fertilizer application rates on the profile distribution characteristics of soil physical and chemical properties,organic carbon and active organic carbon content under the condition of short-term(3 years)straw returning,so as to clarify the optimal combination of straw returning methods and nitrogen fertilizer application rates suitable for southern Liaoning,and provide theoretical reference for promoting the process of local conservation tillage and promoting high and stable yield of maize.The main research results are as follows:1.Effect of straw returning methods and nitrogen fertilizer application rates on soil physical and chemical propertiesUnder the short-term(3 years)different straw returning methods,the continuous-year mixed returning methods(RT and PT)can improve the physical properties of surface soil,while the two-free and oneloosening mode(NPT)can improve the physical properties of deeper soil.Compared with RT,NPT decreased soil bulk density(6.13 ~ 9.08 percentage points,except 30–40 cm),increased soil water content(2.99 ~ 6.70 percentage points),and alleviated p H(0.96 ~ 4.60 percentage points,except 20–30 cm)below surface layer(0–10 cm).Effects of nitrogen fertilizer on soil physical properties varied with field returning methods: Under RT mode,80 % nitrogen fertilizer had better effect on improving soil physical properties in surface layer(0–10 cm).Under NPT treatment,100 % nitrogen fertilizer had more obvious effect on the improvement of soil physical properties in the sub-till layer(20–40 cm).Regularity of soil nutrients in different soil layers under different returning method: Under the mixed returning method(PT and RT),the contents of soil total nitrogen,available phosphorus and available potassium decreased with the deepening of soil layer,and the contents of each nutrient increased first and then decreased with the deepening of soil layer under the NPT mode.Compared with rotary tillage mixed returning(RT),the contents of total nitrogen,available phosphorus and available potassium in the subtill layer of NPT increased by 14.29 %,80.49 % and 9.63 % on average.The NPT-N240 treatment had the highest contents of total nitrogen and available phosphorus in the subtill soil,with an average of 0.89g/kg and 7.99 mg/kg,respectively.2.Effect of straw returning methods and nitrogen fertilizer application rates on soil organic carbon profile distribution.Straw returning method is one of the main factors affecting soil organic carbon content and storage profile distribution: The changes of soil organic carbon content and storage under rotary tillage mixed returning method(RT)and deep tillage mixed returning method(PT)showed a decreasing profile distribution with the deepening of soil layer.But under the two-free and one-loosening methods(NPT),the soil organic carbon content and reserves first increased and then decreased with the deepening of soil layer.From different soil layers,the content and storage of soil organic carbon in the mixed returning method(PT and RT)were mainly concentrated in the surface soil layer,and the content and storage of organic carbon in the RT-N192 treatment with continuous year rotary tillage mixed returning and 80% nitrogen fertilizer were the largest,reaching 13.94 g/kg and 16.18 t/hm2,respectively.The two-free and one-loosening methods(NPT)significantly increased soil organic carbon content and storage in 30-40 cm soil layer,and NPT-N240 treatment(100 % nitrogen fertilizer)was the highest.The average soil organic carbon content and storage reached 9.49 g/kg and 13.43 t/hm2,respectively.3.Effect of straw returning methods and nitrogen fertilizer application rates on soil active organic carbon profile distribution.Straw returning method(T),nitrogen fertilizer application rate(F)and their interaction(T × F)had some degree impact on active organic carbon in different soil layer.Under the two-free and one-loosening methods(NPT),the content of organic carbon active components increased first and then decreased with soil depth;Under mixed returning method(PT and RT),the content of soil organic carbon fractions decreased with soil depth.The NPT could significantly increase the content of soil organic carbon active components in 20 – 30 cm soil layer,while the organic carbon active components in PT and RT were mainly concentrated in the topsoil(0–20 cm).Appropriate nitrogen fertilizer could increase DOC in surface soil,and the DOC content was the highest at 100 % nitrogen fertilizer application rate,but there was no significant difference between 80 % nitrogen fertilizer application rate and 100 % nitrogen fertilizer application rate.Nitrogen fertilizer could increase MBC content in tillage layer and sub-till layer,and the treatment with 80% nitrogen fertilizer was more significant.DOC and MBC stratification rates generally increased with soil depth;under each returning method,the DOC and MBC stratification rates of the same soil layer were significantly lower in NPT mode than in PT and RT modes.Nitrogen fertilizer could increase DOC stratification rate in deep layer(30 – 40 cm)under RT mode and MBC stratification rate in deep layer under PT mode,but decrease MBC stratification rate in deep layer under RT mode.In conclusion,under short-term(3 years)straw returning conditions,a two-year no-till straw mulching and one-year subsoiling-rotary tillage straw rotation should be used in maize production area in southern Liaoning,and the best nitrogen application rate was 240 kg/hm~2.