[Effects of Continuous Cropping on the Physiochemical Properties, Pesticide Residues, and Microbial Community in the Root Zone Soil of Lycium barbarum]

Abstract

Continuous cropping is a common obstacle limiting the high quality and yield of Lycium barbarum （wolfberry）. To clarify the response of soil characteristics of the wolfberry root zone to continuous cropping years, we systematically determined the physicochemical properties and pesticide residues of soils in the wolfberry root zone with different continuous cropping years. In addition, soil bacterial and fungal communities were characterized using high-throughput sequencing technology. The results were as follows： The content of total salt and imidacloprid in the root zone of wolfberry increased with increasing years of continuous cropping. Compared to that with 2 and 9 years, the total salt content in the root zone of wolfberry with 15 years of continuous cropping increased by 51.97% and 54.33%, respectively, and the imidacloprid content increased by 39.58% and 36.61%, respectively. Alkaline nitrogen and available potassium showed an increasing and then decreasing trend. Compared to that with 2 and 15 years, alkaline nitrogen and available potassium in the root-soil of wolfberry with 9 years of continuous cropping increased by 16.94%-28.09% and 18.31%-18.34%, respectively. The diversity and abundance of bacterial communities and the abundance of fungal communities were higher in the root-soil of wolfberry with 9 years of continuous cropping compared to that with 15 years of continuous cropping. In addition, the increase in continuous cropping years also increased the accumulation of harmful plant pathogens such as Pseudomonas, Arthrobacter, Actinomucor, and Trichoderma in the root zone of L. barbarum. Soil total salinity, organic matter, alkaline hydrolyzable nitrogen, and available potassium were the main factors influencing the distribution of bacterial communities. Soil alkaline hydrolyzable nitrogen, available potassium, and ammonium nitrogen were the main factors influencing the distribution of fungal communities. In addition, the soil bacteria in the root zone of L. barbarum were dominated by metabolic functions； in particular, amino acid metabolism, energy metabolism, and nucleotide metabolism were most abundant in the root soil of wolfberry with 9 years of continuous cropping, whereas the highest abundance of functional genes related to membrane translocation was found in the root-soil of wolfberry with 15 years of continuous cropping. The soil fungi were all dominated by saprophytic trophic types, followed by pathogenic cross-nutrients in the root zone of L. barbarum. In conclusion, long-term continuous cropping induced changes in the soil microenvironment in the root zone of L. barbarum, increased soil residues of harmful pesticides and the enrichment of plant pathogens, and reduced the diversity of soil bacterial and fungal communities. Therefore, it is necessary to control the rate of application of soil nutrients and pesticides in the management of L. barbarum and to carry out deep ploughing and deep tilling in good time, and the turnover of old plants in the cultivation of L. barbarum.

Keywords: Lycium barbarum; continuous cropping years; microbial community; pesticide residues; physicochemical properties.