Response of forest species to C:N:P in the plant-litter-soil system and stoichiometric homeostasis of plant tissues during afforestation on the Loess Plateau, China

As the "Grain for Green" project goes on in the Loess Plateau, some drawbacks, such as the low survival rate, the appearances of dry soil layers and little old trees are appearing, especially for introduced tree species. Ecological stoichiometry, which indicates the balance of elements and the flow of energy, plays a vital role in exploring biogeochemical cycling in ecosystems. To investigate the carbon (C), nitrogen (N) and phosphorus (P) balance and flow of energy, the stoichiometry characteristics and stoichiometry homeostasis for introduced and natural forests on the Loess Plateau, four typical forests were selected, including three introduced forests composed of Robinta pseudoacacia (R. pseudoacacia), Pinus tabuliforrnis (P. tabuliforinis) and Platycladus orientalis (P. orientalis) and a natural forest composed of Quercus wutaishansea (Q. wutaishansea) and the C, N and P in the plant-litter-soil system were measured. The results showed that the C, N and P contents and C:N:P in leaves, branches, fruits, litters and soils varied widely and were influenced strongly by forest species. The N content in R. pseudoacacia leaves, branches, fruits and litters and the soil C, N and P contents in Quercus wutaishansea were higher than those in other forest species. The soil C:N:P stoichiometry in Quercus wutaishansea was higher than that in introduced forest species, while the reverse was found for plant tissues and litter C:N:P. According to the leaf N:P, with the exception of R. pseudoacacia, growth of the other three forest species was limited by N. The N, P and N:P homeostasis exist in trees, especially in Quercus wutaishansea, forest species and plant tissues had significant effects on the strength of homeostasis. Overall, the results of C, N and P content and stoichiometric homeostasis revealed that the natural forest was more adaptable to the arid environment in the Loess Plateau compared to the introduced forests.