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.