Ecosystem C:N:P stoichiometry and carbon storage in plantations and a secondary forest on the Loess Plateau, China

Establishing plantations and protecting secondary forests are important measures to enhance carbon (C) sequestration in terrestrial ecosystems. We compared the C: nitrogen (N): phosphorous (P) stoichiometry of major ecosystem components and the C storage partitioning patterns in three ∼40-year-old forests of a fast-growing, N-fixing black locust plantation (Robinia pseudoacacia), an indigenous Chinese pine plantation (Pinus tabuliformis Carrière), and a secondary oak forest (Quercus liaotungensis) on the Loess Plateau, China. The concentrations of C, N, and P in tree tissues, herbs, litter and soils were measured by sampling. The tree tissues biomass was calculated using published species-specific allometric equations and the soil C stock of each soil layer is based on the soil layer depth, its bulk density, and C concentrations. Results showed that N concentrations of tree tissues, herbs, and litter in the N-fixing R. pseudoacacia stands were significantly higher than those in the P. tabuliformis and Q. liaotungensis stands. However, there were no significant differences in soil C and N concentrations between N-fixing and non-N-fixing stands. Soil P concentration was higher only in the R. pseudoacacia and P. tabuliformis plantations when compared to the secondary Q. liaotungensis forest. Leaf N:P ratios indicated soil P limitation in the R. pseudoacacia plantation and the secondary Q. liaotungensis forest (N:P ratios >16), while both N and P limitation were found in the P. tabuliformis plantation (N:P ratios = 14). No shrub vegetation was found in the plantations. Litter C storage was significantly lower in the R. pseudoacacia stands than in the P. tabuliformis and Q. liaotungensis stands. The ecosystem C storage of the secondary Q. liaotungensis forest (359 t ha−1) was higher than that of the P. tabuliformis (298 t ha−1) and R. pseudoacacia (214 t ha−1) plantations. The majority of C storage was in the soil pool, accounting for 76.75%, 60.37%, and 67.93% in the R. pseudoacacia plantation, P. tabuliformis plantation, and the secondary Q. liaotungensis forest, respectively. These results indicate that the secondary Q. liaotungensis forest and indigenous P. tabuliformis plantation enhance C storage more substantially than the fast-growing R. pseudoacacia plantation in the long-term on the Loess Plateau.