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.