Stability Of Soil Aggregate And Its Distribution Characteristics Of Organic Carbon And Nitrogen Under Different Grass Species And Abandoned Years In Jujube Orchard

The effects of different grass species and abandoned years on the stability of soil aggregates,the organic carbon and nitrogen distribution and the composition of functional groups in jujube orchard could provide a theoretical basis for improving soil structure and soil quality in jujube orchard.This study is divided into two parts: Artificial grass and Natural abandoned restoration.The first is to select the jujube garden soil covered by artificial ryegrass(RG),alfalfa(AL)and natural grass(NG)in Lin County,Shanxi Province for 3 years as the research object,and take the clear tillage mode soil of the jujube garden community as the control;The second is the soil of jujube orchard(AL0)in Lvliang Mountain on the Loess Plateau was selected as the object.The soil of jujube orchard(AL0)in the adjacent area was taken as the control.To study the composition and stability parameters of soil aggregates in 0-20 cm soil layer under different artificial grass species and abandonment years;The variation of 0.25 mm aggregate content(R0.25),average mass diameter(MWD),geometric mean diameter(GMD),the total organic carbon(TOC),total nitrogen,particle organic carbon(POC),mineral bound organic carbon(MOC)and infrared spectral structure of soil and aggregates were studied.The main research results are as follows:(1)Soil bulk density of ryegrass,natural grass and alfalfa under three mulching modes was significantly decreased;The total porosity and field water capacity of the three grass mulching patterns were significantly increased by 22.06%-32.89% and 9.28%-14.6%,respectively,compared with that of the clean tillage soil.Ryegrass had the best effect,followed by natural grass and alfalfa.After natural abandonment,soil bulk density increased first and then decreased with the increase of abandonment years,which was larger in the early period of abandonment(0-3 years),and significantly decreased after 3 years of abandonment.Compared with the control,total soil porosity and field water capacity increased significantly after abandonment,with an increase range of1.37%-15.06% and 2.04%-6.10%,respectively.(2)After grass mulching in jujube orchard,the contents of macro-aggregates in soil mechanical stability and water stability increased significantly,by 1.82% to 9.05% and 2.08% to 9.05%,respectively.The MWD and GMD values of soil aggregates under the three grass species were as follows: Ryegrass > Natural grass > Alfalfa > Clear tillage,ryegrass had the best effect on the stability of aggregates.After abandon,the contents of large aggregates of soil mechanical stability and water stability and the MWD and GMD values of aggregates increased significantly except for 3years,and the R0.25 of 15 and 20 years of abandonment was significantly higher than that of the other years.(3)Artificial grass mulching significantly increased the SOC content in soil and macro-aggregate,and the SOC content was mainly concentrated in 1-0.25 mm diameter.The total organic carbon content of soil and aggregates in all treatments decreased from large to small as Ryegrass > Natural grass > Alfalfa > Clean tillage,the contribution rates of soil water-stable aggregate to total organic carbon increased by 5.06% and 0.77%,respectively,under ryegrass and alfalfa treatments,while the contribution rates of natural grasses decreased by 0.35%.Compared with clear tillage,the contribution rates of organic carbon of water-stable macro-aggregates under rye grass and alfalfa treatments were significantly increased by 13.13% and 2.25%,respectively,and were mainly concentrated in GT.5 mm and 2-1 mm in particle size;However,the contribution rate of organic carbon of macro-aggregates decreased significantly under the natural grass pattern.Soil total organic carbon and aggregate organic carbon(SOC)contents decreased first and then increased with the prolonging of abandonment years,and the TOC reached a maximum value of 7.88 g/kg under AL20.The aggregate organic carbon content increased first and then decreased with the decrease of particle size,mainly concentrated in the range of 1-0.25 mm.The contribution rate of organic carbon of mechanically stable aggregates to soil total organic carbon in different abandonment years was54.3%-82.2%,which was 25.0%-52.9% higher than that of ALl0.The contribution rate of water-stable aggregates to soil total organic carbon accounted for 17.7%-71.8%,which was higher than that of Al0(21.1%)in all the plots except for the abandoned soils of 1 and 3 years.(4)The contents of POC and MOC in soil and aggregates increased to varying degrees after abandonment.The POC content in soil fluctuated and decreased with the increase of abandonment years,while the MOC content in soil first decreased and then increased.The contents of POC and MOC in soil aggregates of natural abandonment tended to increase after 1 to 2 years of abandonment,with Al3 reaching the lowest value,and Al20 reaching the highest value after 3 years of abandonment.The POC and MOC contents of ryegrass and natural grass were significantly increased by 38.5%,19.2% and 30.1%,11.2%,respectively,compared with those of clean tillage under artificial grass planting.However,alfalfa significantly reduced soil POC and MOC contents by 15.3% and 20.3%,respectively.The content of POC and MOC in soil aggregates with different grain-covering species and years was lower than that of < 0.25 mm aggregates.The contents of POC and MOC in micro-aggregates were the highest,while the contents of POC and MOC in macro-aggregates were significantly increased by grass mulching.In addition,MOC content in soil and aggregates of each grain size were all higher than POC content,which was related to the turnover speed and stability characteristics of POC and MOC in soil.(5)After the restoration of artificial grass and natural abandoned cropland,the contents of polysaccharides,alcohols,carboxylic acids and lipids in soil organic carbon increased,which was conducive to the absorption of nutrients during the growth and development of crops.Among them,the absorbance at 1029.7 cm-1 treated with ryegrass,alfalfa and natural grass increased by 57.4%,51.2% and 15.0%,respectively,compared with CK.Compared with the years of natural abandonment,the contents of easily decomposed carbon components such as polysaccharides,alcohols,carboxylic acids and lipids in soil organic carbon increased during 1-5 years of abandonment,which is beneficial to the growth and development of plants.After 5 years of abandonment,the easily decomposed carbon components decreased,which was beneficial to the storage of soil organic carbon and increased the stability of soil organic carbon.(6)The total nitrogen content in soil and aggregates increased significantly after artificial grass,the increase range of total nitrogen content was 0.037-0.192 g/kg and 0.003-0.015 g/kg,respectively in the total nitrogen content of large aggregates(> 0.25 mm)and micro-aggregates(< 0.25 mm).Ryegrass had the best effect,followed by alfalfa and natural grass.The total nitrogen content of soil and aggregates under natural abandonment decreased first and then increased gradually,the total nitrogen content of soil and aggregates reached the lowest value under AL3,and the total nitrogen content increased steadily from 5 to 20 years after abandonment.Therefore,both artificial grass and natural abandonment can enhance soil stability and nutrient content of jujube orchard to some extent.After 3 years of natural abandonment,soil aggregate structure and organic carbon content became better and better with the extension of years.Mulching ryegrass with artificial grass had the best effect on increasing the contents of organic carbon and nitrogen in soil and aggregates,POC and MOC in macro-aggregates,and improving soil stability and carbon sequegregation capacity,which could be popularized and applied in the study area.