Response of water use efficiency and plant-soil C:N:P stoichiometry to stand quality in Robinia pseudoacacia on the Loess Plateau of China

The evaluation of stand quality is of great significance for forest management and sustainable development. Water use efficiency (WUE) and the carbon (C), nitrogen (N), and phosphorus (P) stoichiometric characteristics of plants and soil are important indexes used to evaluate plant water and nutrients adaptation strategies. However, little is known about the effects of stand quality on WUE and plant soil C:N:P stoichiometry. In this study, the growth characteristics of Robinia pseudoacacia plantations and understory plants in an age series of 8-, 15-, 25-, and 35-year-old stands were investigated, and the leaf-level WUE and C:N:P stoichiometry in plants and soil were measured on the Loess Plateau. The results showed that the species richness, Shannon-Weiner diversity index, and evenness index reached their peaks in the 35-year-old stands with increasing afforestation age. WUE reached its highest value in the 35-year-old stands but was only significantly higher than that in the 8-year-old stands. Stand age had different effects on C, N, and P nutrients and stoichiometry in leaves, fine roots, and soil. The most obvious trend was that soil total P decreased significantly with increasing afforestation age, and the leaf N:P ratio was greater than 16 in the four stand ages, which indicated that P was the main limiting factor in R. pseudoacacia plantations. Leaf nutrients and stoichiometry are closely related to forest growth characteristics, while root nutrients and stoichiometry are more related to understory plant composition and diversity. Importantly, the WUE had no significant change with the increase in the stand quality index, and the response of soil stoichiometry to stand quality was stronger than that of plant stoichiometry. These results help us further understand coordinated plant-soil restoration after afforestation.