Ecological stoichiometry is an important aspect in the analysis of the changes in ecological system composition, structure, and function and understanding of plant adaptation in habitats. Leaf carbon (C), nitrogen (N), and phosphorus (P) concentrations in desert phreatophytes can be affected by different depths of groundwater through its effect on the adsorption and utilization of nutrient and plant biomass. We examined the biomass, soil organic C, available (mineral) N, and available P, and leaf C, N, and P concentrations of Alhagi sparsifolia grown at varying groundwater depths of 2.5, 4.5, and 11.0 m in 2015 and 2016 growing seasons in a desert-oasis ecotone in northwest China. The biomass of A. sparsifolia and the C, N, and P concentrations in soil and A. sparsifolia showed different responses to various groundwater depths. The leaf P concentration of A. sparsifolia was lower at 4.5 m than at 2.5 and 11.0 m likely because of a biomass dilution effect. By contrast, leaf C and N concentrations were generally unaffected by groundwater depth, thereby confirming that C and N accumulations in A. sparsifolia were predominantly determined by C fixation through the photosynthesis and biological fixation of atmospheric N2, respectively. Soil C, N, and P concentrations at 4.5 m were significantly lower than those at 11.0 m. Leaf P concentration was significantly and positively correlated with soil N concentration at all of the groundwater depths. The C:N and C:P mass ratios of A. sparsifolia at 4.5 m were higher than those at the other groundwater depths, suggesting a defensive life history strategy. Conversely, A. sparsifolia likely adopted a competitive strategy at 2.5 and 11.0 m as indicated by the low C:N and C:P mass ratios. To our knowledge, this study is the first to elucidate the variation in the C, N, and P stoichiometry of a desert phreatophyte at different groundwater depths in an arid ecosystem.

Present studies show that nitrate accumulation is the major factor influencing the water uptake by vegetables, but there is lack of knowledge on how the relationship between nitrate and water concentrations in different vegetable tissues. We determined nitrate accumulation and water content in vegetables of rape, Chinese cabbage, and spinach under different nitrogen levels (experiment I), 29 spinach cultivars (experiment II), and 27 rape cultivars (experiment III). The results from experiments I-III showed a highly linear relationship between nitrate accumulation and water content, and the levels thereof in different organs of the vegetables revealed that the petiole exhibited the best correlation between them compared with the root and leaf blade. These suggest that (1) vegetables with high water content have high nitrate content, (2) water content in the petiole can be used to screen cultivars with low levels of nitrate accumulation, and (3) increasing soil water content in agronomic practice is essential to decreasing nitrate accumulation in vegetables.

Soil nutrients are vital for plant growth and survival and present a crucial role in terrestrial function and productivity. However, little is known about the effect mechanism of groundwater table on soil nutrients in an arid desert ecological system. This study investigated the impacts of groundwater depth on the concentrations of soil organic carbon (C), available nitrogen (N), phosphorus (P), and potassium (K) at shallow groundwater depths (0.4, 0.8, 1.2, 1.8, and 2.2 m) and field deep groundwater depths (2.5, 4.5, and 11.0 m) in a desert-oasis ecotone in Central Asia in 2015 and 2016. Soil nitrate-N, inorganic-N, soil available P, and K concentrations were significantly affected by shallow and field deep groundwater. Groundwater depths did not alter soil ammonium-N concentration. Soil organic C concentration was influenced by field deep groundwater depth. Structural equation model showed that groundwater depth directly affected soil nitrate-N and K concentrations and indirectly altered the soil inorganic-N, soil organic C and available P concentrations in shallow groundwater. Moreover, groundwater depth directly influenced soil nitrate-N and soil organic C, available P and K concentrations and indirectly affected soil inorganic-N concentration in deep groundwater. Hence, groundwater depth should be considered one of the most critical environmental factors affecting soil nutrient variation in an arid desert. This study provides new insights into the soil nutrient variation under a declining groundwater depth in a hyper-arid ecosystem.