Background: The Chinese Natural Forest Protection program has been conducted nationwide and has achieved resounding success. However, timber importation has increased; therefore, producing more domestic timber is critical to meet the demand for raw materials. Fertilization is one of the most effective silviculture practices used to improve tree and stand growth. However, determining the appropriate type and amount of elements is necessary for effective fertilization of big timber in different forest types and environmental conditions. Stoichiometric theory provides the criteria to assess nutrient limitation in plants and offers important insight into fertilizer requirements of forested ecosystems.
Methods: Nitrogen (N) and phosphorus (P) concentrations in plants' leaves, mineral soil, and litter were investigated in a mixed pine-oak stand.
Results: The big timber rate for Pinus tabuliformis, Pinus armandii and Quercus aliena var. acutesserata is 57.71%, 22.79% and 2.78% of current existing individuals respectively. Foliar N and P concentrations were 9.08 and 0.88 mg g-1, respectively. The N:P in the plants was 10.30. N concentration and N:P in mineral soil decreased from 0-30 cm soil depth. For litter, N and P concentrations were 16.89 and 1.51 mg g-1, respectively, and N:P was 11.51. Concentrations of N and P in mineral soil and litter did not significantly affect plants'leaf concentrations. Similar result was also obtained between litter and mineral soil concentrations. Nitrogen storage in mineral soil was significantly correlated with foliar N:P in the plants.
Discussion: Foliar N:P of dominant tree species and the plants, and foliar N concentration in Pinus tabuliformis and P. armandii, and foliar P concentration of P. armandii in the mixed pine-oak stand was lower than that in Chinese and other terrestrial plants. Foliar nutrients in the plants were not affected by soil nutrients. According to the criteria of nutrient limitation for plants, growth of dominant tree species was N limited; therefore, 1.49 t ha-1 pure N should be added to forest land to as fertilizer.
Keywords: Fertilizer; Mixed pine-oak stand; N:P; Nitrogen; Phosphorus; The Qinling Mountains.