Effects Of Reclamation Years And Different Planting Patterns On Soil Quality And Its Mechanism In Abandoned Salt Pans

Coastal salt pans emerged with the rapid development of economy and society,and gradually abandoned with the transformation of economic system.The implementation of comprehensive treatment of abandoned salt pans and giving priority to reclamation into cropland is a specific action of positive response to the major national strategy of "ecological protection and high-quality development of the Yellow River Basin",which can play an important role in alleviating the contradiction between people and land and increasing the effective cultivated land area in China.However,due to the unique production environment of salt pans,although a small amount of salt tolerant and barren tolerant crops can be planted after reclaimed into farmland through a series of engineering measures and flood irrigation and salt-leaching,there are still a lot of problems to improve the quality of cultivated layer and land output in a short time.Facing the current situation of unclear change and mechanism of reclaimed soil quality,it is urgent to explore the process,influencing factors and driving mechanism of reclaimed soil quality change.Therefore,a series of studies should be carried out around the core issue of "improvement of cultivated land soil quality" after reclamation.Firstly,it is necessary to clarify the changes of physical structure,chemical properties and biological properties of reclaimed soil with the increase of reclamation years,find out the key factors limiting the improvement of soil quality and the leading factors affecting the change of soil bacterial community under the existing planting patterns,so as to formulate targeted control measures,and predict the time when the reclaimed soil returns to the local farmland soil quality.Secondly,how to shorten the restoration time of reclaimed soil and accelerate the improvement of soil quality has become the focus of the second stage.Based on the current situation of perennial monoculture sorghum and one season planting in the study area,according to the specific environmental conditions of salt field,introduce suitable crops such as grain,cash and forage,and combine planting and land cultivation,explore a new ecological adaptive planting patterns of rotation,intercropping and coupling,improve land use efficiency,promote improve soil quality,and clarify the soil microbial mechanism under different planting patterns.The main results of this study are as follows:(1)The salt content and bulk density of reclaimed soil decreased year by year,while the contents of soil organic matter,nitrogen,phosphorus and potassium increased gradually with the increase of reclamation years(1 a—9 a).Although soil fertility improved,the content of carbon and nitrogen was still lower than the national average.Comparing the soil nutrient content of all reclamation years,it was found that the highest value of nutrient content appeared at 8 years of reclamation.From the perspective of reclamation years,the C/N of cultivated layer soil(0—40 cm)increased first and then decreased,and C/P continued to increase after 7 years of reclamation.From the perspective of soil vertical space,except for the soil reclaimed for one year(RT1),C/N gradually increases with the deepening of soil layer,while N/P gradually decreases.Besides,the restoration process of reclaimed soil nutrients was predicted.The contents of reclaimed soil organic matter,total nitrogen,available phosphorus and available potassium needed 8.75 a,8.45 a,8.24 a,and 8.41 a to reach the local farmland level,respectively.(2)The number of bacteria and actinomycetes in reclaimed soil first increased and then decreased,while the number of fungi increased year by year with the increase of reclamation years(1 a—9 a).The activities of four soil enzymes(urease,sucrase,alkaline phosphatase and catalase)increased with the increase of reclamation years,and decreased with the increase of soil depth.Sucrase,catalase,organic matter and bacteria are the main factors affecting the physico-chemical properties and biological characteristics of reclaimed soil.The bacterial ?-diversity increased first and then decreased with the increasing of reclamation years,soil bacteria ?-diversity reached the maximum in 3 years of reclamation(RT3).After reclamation,the relative abundance of dominant bacteria in soil such as Acidobacteria,Chloroflexi,and Actinobacteria increased,while the relative abundance of Proteobacteria,Gemmatimonadetes,and Bacteroidetes decreased.Compared with RTBL,the soil nutrients and bacterial community structure in 1 year(RT1),2 years(RT2),3 years(RT3),and 4 years(RT4)of reclamation showed a significant difference.After 8—9 years(RT8 and RT9)of reclamation,the soil nutrients and bacterial community structure in reclaimed abandoned salt pans showed no significant difference with farmland(RTBL).Therefore,reclamation time is a vital driving force for restoring soil physicochemical properties and bacterial communities in abandoned salt pans.Redundancy analysis showed that the dominant factor limiting the change of bacterial community in reclaimed soil was soil salt content,total nitrogen,Cu and available potassium.(3)Ten treatments were set,i.e.,monocropped sorghum(S),monocropped wild soybean(WS),sorghum and triticale rotation(S+TS),sorghum and sweet potato rotation(S+V),rowcropped sorghum each row with one row of wild soybean(S1WS1),row cropped sorghumeach row with two rows of wild soybean(S1WS2),and mixed intercropping of sorghum and wild soybean(SWS),S1WS1 and triticale rotation(S1WS1+TS),S1WS2 and triticale rotation(S1WS2+TS),SWS and triticale rotation(SWS+TS).Compared with S,Under the S+TS,S+V,S1WS1,S1WS2,SWS,S1WS1+TS,S1WS2+TS and SWS+TS patterns,SOM,NPK content increased,and soil p H and salinity decreased,SWS + TS SWS+ TS had the best effect on improving soil nutrients.After two years of planting,the yield increase rates under S1WS1,S1WS2,SWS,S1WS1+TS,S1WS2+TS and SWS+TS patterns were 12.99%,12.61%,41.25%,16.55%,18.03% and 44.68%,respectively.The land equivalent ratios(LER)and relative crowding coefficient above 1 reflected the superiority of intercropping.S1WS1,S1WS2,SWS,S1WS1+TS,S1WS2+TS and SWS+TS patterns were all improved soil bacterial diversity,The Ace,Chao indices peaked under the SWS+TS pattern,reaching 1.25,1.25 times as high as those under monocropping.The dominant phyla of soil bacteria with relative abundance > 5% included Actinobacteria,Chlorofexi,Proteobacteria,and Acidobacteria.SOM was the main environmental factors affecting bacterial community structure.SWS + TS had the largest profit among all planting patterns,an increase of 30.41% compared with S.In conclusion,under the experimental conditions,SWS + TS had the highest crop yield increase rate,the largest land equivalent ratio and the most obvious intercropping advantage.The content of soil organic matter and nitrogen,phosphorus and potassium contents all increased and the above indexes peaked which appeared in the SWS+TS,the effect of soil improvement was the best,and it was most conducive to increasing farmers' income.The above research results showed that after the reclamation of abandoned salt pans,the soil nutrient content increased with the increase of reclamation years,and the soil structure gradually improved.However,due to the imbalance of soil micro-ecology caused by perennial continuous cropping of sorghum,it affected the change of soil bacterial community,and the monoculture sorghum had a slow effect on the improvement of soil quality,timely rotation and intercropping can effectively alleviate the obstacles of land output caused by soil micro-ecological imbalance caused by continuous cropping,achieve the purpose of making full use of land resources and increasing land output,and accelerate the transformation of reclaimed land to high standard cropland.