Changes Of Soil Environmental Quality And Improvement Measurements Under Yam Continuous Cropping

As a vegetable that can provide both nutrition and health benefit,yam has been popular among consumers in recent years.However,the widespread continuous cropping,excessive application of fertilizers and pesticides has led to soil quality declining,micro-ecological imbalance and serious soil borne diseases and many other problems in yam production,which substantially limit the healthy and green development of yam industry.To shed light on this issue,taking the main yam production area as research area,this paper analyzed the physical and chemical properties,the microbial community structure and diversity of soils sampled in different planting years to study the changes in soil environmental quality and the obstructive factors that restrict yam growth under the continuous cropping.Moreover,the effects of different cover modes on soil quality and yam growth were studied,aiming to provide potential strategies for improvement,and theoretical and technical basis for improving soil quality of yam field.The main results were as follows:(1)The analysis of soil fertility properties of yam fields showed that,with increasing years of continuous cropping,decreasing trends were found in soil pH and organic matter content,while increasing trends were found in soil EC value,contents of soil nitrate,available phosphorus,potassium,iron,copper and zinc.The activity of soil urease,invertase,alkaline phosphatase and catalase reached the maximum value at the third year into continuous cropping,and then decreased afterwards.(2)Compared with the control soil in grain field,the microbial structure and composition of soil flora in yam fields planted for one year had no obvious changes,while the richness and diversity of soil microbe were significantly lower in soils with 3 years of continuous cropping.Compared with the soil planted for one year,the disease-free soil and diseased soil planted for 3 years had relatively higher abundances of Gemmatimonadetes,GAL 15,fungi Ascomycota,unclassified_f_chaetomiaceae and gymnoascaceae,and relatively lower abundances of Bacteroidetes and Microvirga.The soil available potassium and EC showed a significant positive correlation with the changes of microbial flora in diseased soil with 3 years of continuous cropping,which implied the over-fertilization of potassium and an elevated degree of soil secondary salinization further aggravated the occurrence of soil borne diseases.(3)In term of spatial variation,the nitrate accumulation in 0-200 cm soil layer increased with increasing years of continuous cropping,and two accumulation peaks were observed at 40-80 cm and 120-160 cm,respectively.Compared with the soil in grain field,the total nitrate-N accumulation in the yam field planted for 1,3,4 and 6 years of continuous cropping increased by 1.21,3.95,4.81 and 6.87 times,respectively,and the total nitrate-N accumulation in 100-200 cm soil layer accounted for about 50%of the total amount in 0-200 cm soil layer.Meanwhile,the accumulation of phosphorus and potassium in 0-100 cm soil layer also increased significantly,which suggested that phosphorus and potassium in 0-100 cm soil layer and nitrate in 0-200 cm soil layer had been accumulated with the extension of continuous cropping,and had a high risk of nitrate leaching.(4)Compared with conventional management,the dry matter biomass of yam stems and leaves were decreased significantly in treatment with ground cloth covering.But the dry matter biomass of yam tube,the ratio of root to shoot,and the total yield and second-class yield of yam were all significantly increased.In the treatment with planting clover,the dry matter of yam tuber,leaves and bulbil were all decreased significantly,while the ration of root to shoot was increased,and the yield had no significant changes.(5)Compared with conventional management,covering significantly increased the water content in 0-40 cm soil layer at the seedling stage and in 0-60 cm soil layer at harvest of yam significantly.Covering ground cloth increased soil EC value significantly at the harvest stage due to the fact that it effectively prevented the leaching of soil nitrate,phosphorus and potassium.But planting clover significantly reduced soil available phosphorus and potassium content in the surface layer because of the nutrients loss by clover absorption.In summary,the comprehensive soil fertility level is significantly reduced,and the soil microbial flora changed significantly after 3-4 years of continuous cropping.The strategy of covering ground cloth is helpful for reducing evaporation of water and nutrients leaching,increasing nutrients of surface soil,and ultimately increasing yam yield.