Response of forest growth to C:N:P stoichiometry in plants and soils during Robinia pseudoacacia afforestation on the Loess Plateau, China

Abstract

C:N:P stoichiometry in the plants and soils is an important indicator of biogeochemical cycles and functioning in ecosystems; however, the response of plant growth to ecological stoichiometry following afforestation remains unclear. To illustrate the C:N:P stoichiometry connections between the plants and soils and their effects on the growth of plants, soil and plant samples were collected from one farmland site and four Robinia pseudoacacia (RP) forests that were planted for 10, 15, 25, and 35 years. Plant community characteristics, growth of overstory trees and understory plants, physical and chemical properties of the soil, and C:N:P stoichiometry in plants and soil were measured. The results revealed that soil organic C, total N, total P, dissolved organic N, C:P and N:P ratios, and water content significantly increased, whereas soil bulk density, available P, and pH decreased with afforestation age. Leaf C:P and N:P ratios in RP were significantly higher than that in understory biomass because P concentrations gradually decreased in RP leaves and increased in understory biomass. The C, N, and P contents and stoichiometry in the soils and plants were significantly correlated, particularly for N:P ratio that can be used to reveal close coupling between plant and soil nutrients. The N:P ratio in RP leaf and understory biomass increased with afforestation age and varied from 14.44 to 20.81 and 7.59 to 10.96, respectively, suggesting that P limitation gradually increased in RP, and N limitation gradually declined in understory biomass. Furthermore, the responses of overstory trees and understory plant growth to N:P ratios were contradictory, because of their different nutrient acquisition patterns and nutrient-use efficiencies. Therefore, this finding provides evidence that the strong correlation between the plants and soils were tightly coupled to N and P concentrations and N:P ratios, and therefore, have the potential to influence the growth of forests on the Loess Plateau.