1. Introduction

In agricultural production, there is a certain interaction between soil and plants. Soil provides carbon (C), nitrogen (N), phosphorus (P) and other nutrients for plant growth, and the growth and development of plants will influence the soil property by the return of litter (Bermejo et al., 2020, Oo et al., 2020), the discharge of root exudates (Masikane et al., 2020, Wang et al., 2020) and the maintenance of soil and water (Li et al., 2011, Abinandan et al., 2019, Vezzani et al., 2018). But how to use the stoichiometry of plants and soil as an indicator in agricultural production to improve crop yields has been rarely studied.
Firstly, the C:N:P stoichiometry changes in different environments and geographic regions. In the Tibet region of China, the contents of C, N, P in alpine steppe soil are significantly different from alpine meadow, and the ratio of C:N, C:P and N:P in steppe soil is higher (Chen et al., 2016b). Moreover, the N:P ratio of plants in China was significantly higher than the global average level caused by a greater shortage of soil P in China (Han et al., 2005). Studies conducted in the forests of Catalonia found that with the increase of latitude and precipitation, the nutrient content of the leaves increased, and the ratio of C:N in the leaves decreased, and the leaves of plants growing in the youngest soil have the lowest N:P, supporting the Soil-Age Hypothesis (Sardans et al., 2011a). Secondly, even in the same area, the stoichiometric ratio between plants and soil is also influenced by plant species. It has been found that in various regions of the world non-legume plant communities had higher soil C:N, C:P and N:P ratios, but lower leaf N:P ratio than legumes (McGroddy et al., 2004b, Reich et al., 2004, Yang et al., 2018). In addition, the stand age or lifespan of plants also affects the stoichiometry of plants and soil. in the forest ecosystem, the C:N ratio in the tree increases with the planting age, while the C:N ratio in litter, forest floor and mineral soil have no significant change with the stand age (Hooker & Compton, 2003, Yang & Luo, 2011b, Bai et al., 2019). The N:P ratio of plants is often used to reveal growth-limiting elements (Gusewell, 2004), e.g. N:P>16 indicates that plant growth is restricted by P, and N:P<14 indicates that plant growth is restricted by N (Reich & Oleksyn, 2004b). This conclusion has been widely recognized, but it is unclear whether it can be used in the cultivation of perennial grass perennial forage alfalfa, because the cutting effect in alfalfa management might affect litter return and the C:N:P stoichiometry.
Alfalfa, as a high-yield forage, has continuously regenerating and rapidly growing leaves during the growing season (Alegre et al., 2004). The age of alfalfa and harvesting times can significantly affect the N: P of alfalfa leaves, and the N, P content of alfalfa leaves has a negative correlation with N: P (Wang et al., 2015, Wang et al., 2014). Zhang et al. (2013). indicated that the N:P ratio of alfalfa generally decreases first and then increases with stand age. The greenhouse experiments showed that increasing the water supply can ameliorate the restriction of soil P limitation (Lu et al., 2019), both studies used the alfalfa N:P ratio indicating element limitation, while with the effect of alfalfa planting age on nutrient restrictions and limitation factors of growth is still unknown.
The objective of this study was to find out the relationship between the aboveground-underground biomass distribution changes of alfalfa and C:N:P stoichiometry under Mu Us Desert ecosystem. We selected 1, 2, 3, 5, 7, 9 and 10 year of alfalfa plantations from Yulin, Shaanxi, China, to collect alfalfa and soil samples. The area is all sandy soil with uniform climate and agricultural management, which provides good conditions for the experiment. Our aims were to evaluate (1) how the stoichiometric ratio of alfalfa and soil changes with stand age; (2) the relationship between alfalfa and soil stoichiometric ratio; (3) the alfalfa biomass allocation strategy to C:N:P stoichiometry.