1 INTRODUCTION
Quinoa (Chenopodium quinoa Willd) is an annual herbaceous plant native to the Andes Mountains of South America. As the main traditional food of the Inca indigenous people, quinoa has been cultivated for more than 5000 years. The introduction of quinoa in China was relatively recent, with experimental research by the Tibetan Institute of Agriculture and Animal Husbandry and the Tibetan Academy of Agricultural Sciences in 1987, and small-scale trials in Tibet in 1992 and 1993. At present, small-scale plantings are done in Shanxi, Qinghai, Gansu, and Yunnan. Quinoa is a plant with strong environmental tolerance and can grow well under a variety of harsh conditions because it shows tolerance to cold, salinity and drought (Jacobsen et al., 2003). It also can grow on poorly fertile sandy and calcareous soils.
Quinoa grain is a rich source of a wide range of minerals, vitamins, fatty acids (e.g. linoleate and linolenate), natural antioxidants (Kozioł, 1992; Repo-Carrasco, 2003), and high-quality protein (with ample amounts of sulfur-containing amino acids) (Kozioł, 1992). Because of providing rich and balanced nutrition, quinoa has been ranked as one of the top 10 nutritious foods in the world (Wang et al., 2019). It is the only food considered by the Food and Agriculture Organization of the United Nations (FAO) to meet the basic nutritional needs of the human body from a single plant source (Ogungbenle, 2003).
The composition and activity of the soil microbial community largely determine biogeochemical cycles, organic matter turnover processes, and soil fertility and quality (Zelles, 1999). Rhizosphere microbiome influences plant growth and community succession (Herbert, 2009; Lambers et al., 2009). Plants interact with microorganisms, with plant residues and root secretions providing carbon and energy sources to soil microorganisms, and microorganisms decomposing organic compounds into inorganic nutrients for plant uptake and use (Hartmann et al., 2008; Marschner and Timonen, 2005). Therefore, understanding the composition and activity of soil microorganisms in interaction with plants can help us improve soil management and crop cultivation.
Metabolomics aims to identify and quantify the range of primary and secondary metabolites (generally <1800 kDa) involved in biological processes (Llanesa et al., 2018). Current studies on the metabolomics of quinoa are mainly in the breeding of quinoa varieties (Song et al., 2020) and nutritional composition studies (Liu et al., 2020). Metabolomic studies related to the technical aspects of quinoa cultivation are virtually non-existent.
The research on quinoa is mainly on the nutritional value and physiological characteristics (Wright et al., 2002; Ferreira et al., 2015). In addition, the screening of quinoa germplasm resources (Zurita-Silva et al., 2014) and work on quinoa pests and diseases, and genetic diversity were carried out (Hinojosa et al., 2021). Regarding agronomy, the recommended cultivation density is 67,500 plants per hectare in high altitude and cool regions, 97,500 plants per hectare in arid, semi-arid and irrigated regions, and 120,000 plants per hectare in medium altitude and arid regions (Iglesias-Puig et al., 2015). The recommended planting density varies among quinoa varieties, but studies on quinoa planting density as related to soil microbiome and root metabolome are rare or non-existent. In this study, we grew quinoa in the field at two planting densities followed by sequencing microorganisms in the rhizosphere and non-rhizosphere soils and determining quinoa root metabolome to provide a theoretical basis for the effects of planting density and soil microbiome on quinoa yield.