Coupling of different plant functional group, soil, and litter nutrients in a natural secondary mixed forest in the Qinling Mountains, China.

Soil and litter play important roles in ecosystem nutrient storage and cycling, which both affect plant growth and ecosystem productivity. However, the potential linkages between soil and litter nutrient characteristics and nutrient characteristics of different plant functional groups (PFGs) remain unclear. In this study, we investigated the carbon (C), nitrogen (N), and phosphorus (P) concentrations and stoichiometric ratios in different organs of three PFGs (trees, shrubs, and herbs), litter, and soil in nine natural secondary mixed forests in the Qinling Mountains. Leaves N and P concentrations and N:P ratios, varied from 15.6 to 18.97 mg·g, 1.86 to 2.01 mg·g, and 7.34 to 8.72, were highest at the organ level, whereas the C:N and C:P values were lowest in leaves. At the PFG level, N and P concentrations of herbaceous were 1.23 to 3.69 and 1.42 to 1.93 times higher than those in same organs of woody species, while the N:P ratio was significantly lower in herb leaves than in tree and shrub leaves. Tree organs had significantly higher C concentrations and C:N and C:P ratios than shrub and herb organs. The leaf N:P ratios of all PFGs were less than 14, suggesting that plant growth was limited by N in the study region. The nutrient contents and stoichiometric ratios in plant organs had different degrees of linkages with those in litter and soil. Soil nutrient characteristics mainly affected (23.9 to 56.4%) the nutrient characteristics of the different PFGs, and litter nutrient characteristics also had important contributions (4.5 to 49.7%) to the nutrient characteristics of PFGs, showing the following order: herbs > trees > shrubs. Our results indicate that the functional difference in plant organs resulted in diverse nutrient concentrations; and varied nutrient connections exist among different ecosystem components. Furthermore, nutrient characteristics of litter and soil can together affect the nutrient characteristics of PFGs.