Exploration of the reasons for yield decline in the perennial harvesting model of strawberries in Huize county of Yunnan province

[Objective] Strawberries (Fragaria × ananassa Duch.) are typically planted annually with annual disinfection treatments to suppress pests and diseases. In contrast, strawberries are cultivated perennially in Huize, Yunnan, China, fruits can be harvested for 3-5 years after planting. This strategy results in low carbon emissions and significantly reduces the labor input and the cost of nursery supplies, chemical fumigants, and plastic film. Under this cultivation system the yield and profits of strawberry decreases from the third year after planting, and the production becomes unprofitable in the fifth year. Soil degradation is suspected to be the reason for the inability to sustain stable production. This study aimed to survey the factors affecting the soil degradation in perennial cultivation (PC) area. [Methods] We examined the soil nutrient traits and the microbial structure of four strawberry fields following perennial cultivation in this area, the SOM (Soil organic matter) content, pH, and key mineral nutrient contents of the strawberry (N, P, K, Ca, Mg, Cu, Zn, and Mo) were assessed. The soil microbial community was analyzed by high-throughput amplicon sequencing based on Illumina MiSeq PE300 platform. The soil microbial functions were predicted to determine the effects of PC on soil microbial communities. The microbial function focusing on C-cycling and N-cycling processes were assessed to determine the soil productivity tendency. [Results] Our results indicated that PC resulted in significant soil acidification, salinization, and organic matter deficiency. The average soil pH was 5.58±0.67 in the first year and decreased to 4.35±0.53, 3.57±0.28, and 2.98±0.04 in the second, third, and fourth years, respectively. The average SOM content was 0.66%±0.14% in the first year and 0.27%±0.03% in the fourth year (a 25.6% decrease per year). The average soil EC was 328.5±113.3 in the first year and 923.7±158 in the fourth year (a 41.1% increase per year). The Ca content showed a generally decreasing trend at most sites, and the lowest Ca content was found in the fourth harvesting year of JC1 (Jiache Town 1, 0.878 g·kg-1, 74.4% lower than that in the first harvesting year). PC also significantly affected the soil microbial community. The Chao1 richness, Shannon, and ACE indices of the bacterial communities in the soils differed significantly among the harvesting years. The Chao1 richness decreased significantly by 13.6% from the first to the second harvesting year, 38.2% to the third harvesting year, and 41.8% to the fourth harvesting year. The greatest decline in the Chao1 richness index was observed in the fourth harvesting year of JC1 (52.2%) and the third harvesting year of JC2 (Jiache Town 2, 52.5%). The Chao1 richness and ACE indices for fungal diversity differed among the harvesting years at all sites except for XJ (Xinjie Street). The ACE index decreased by 28.7% from the first to the second year of DB (Daibu Town), 25.0% from the first to the second year of JC2, and 31.7% from the second to the third year of JC1. The fungal diversity of XJ was lower than those at the other three sites. The average ACE index of the XJ samples was 578.7, which was 37.2% lower than that of DB, 25.5% lower than that of JC1, and 33.2% lower than that of JC2. The relationship between the microbial community and environmental variables was analyzed by RDA. The RDA1 (34.33%) and RDA2 (21.96%) explained 56.29% of the total variation in bacterial community structure. For fungi, the RDA1 (26.21%) and RDA2 (12.98%) explained 39.19% of the total variation. The SOM, pH, and EC were the dominant contributors to the variation of bacterial community, accounting for 59.8%, 58.9%, and 51.8%, respectively. The soil EC and SOM were the most important contributors to the variation of fungal community, accounting for 83.3% and 82.7%, respectively. FAPROTAX was utilized to determine the functional effects of PC on the bacterial community. The relative abundance of N-cycling processes related bacteria showed a decreasing trend. The correlation analysis showed that most C-cycling and N-cycling processes were significantly positively correlated with the SOM content and negatively correlated with the EC, whereas most C-cycling processes was significantly positively correlated with pH. The FUNGuild analysis showed a significantly decreasing trend for that relative abundance of symbiotroph fungi and arbuscular mycorrhizal fungi (AMF), and the relative abundance of Fusarium, the main pathotroph of strawberry, showed a significantly increasing trend. [Conclusion] The PC of strawberries resulted in significant soil acidification, salinization, SOM deficiency, and shaped microbial community structures. Of both bacteria and fungi, the community diversity decreased by year, while symbiotroph fungi and AMF showed a significantly decreasing trend in the PC. The soil microbial function prediction suggested that PC reduced the N cycles related bacterial functions, while most C-cycling and N-cycling processes were positively correlated with the SOM, and negatively correlated with the EC. The PC of strawberries showed a significant negative impact not only on soil physicochemical properties but also on microbial community and function. Thus, for this perennial cultivation strategy of strawberries, maintaining soil physicochemical properties and soil microbial structure and function would be the key problem to solve for sustainable development.