Coupling of plant and soil C:N:P stoichiometry in black locust (Robinia pseudoacacia) plantations on the Loess Plateau, China

C:N remained constant, but C:P and N:P were higher in the 37-year-old stands. The correlations were linked by P and N:P among plant, but N:P and C:P between soil and plant. Carbon (C), nitrogen (N), and phosphorus (P) stoichiometries are critical indicators of interlinked biogeochemical cycles in terrestrial ecosystems. However, the linking manner between plant and soil in plantations remains unclear. We examined the variations in the concentrations of C, N, and P, and their ratios in plant tissues, litter and soils in an age sequence of 9-, 17-, 30- and 37-year-old black locust (Robinia pseudoacacia) plantations afforested on the Loess Plateau, China. The C:N ratios in plant tissues and soil remained relatively stable across age sequence. However, the C:P and N:P ratios were significantly higher in the 37-year-old stands than in those of the previous growth stages because of the decreased P concentrations in the plant tissues and the increased C and N concentrations in the soils. The strong positive correlations among plant tissues were tightly linked by elemental P and the N:P ratios. Moreover, the N:P and C:P ratios between soil and plant tissues were strongly correlated. The age-related increase of N:P ratios indicated that the growth of black locust plantations in the Loess Plateau becomes progressively P limited when aging. Therefore, it is necessary to enhance the forest structure and to reorganize ecological functions of plantations in the Loess Plateau when they appear to decline under nutrient limitation as a result of long-term development.