Abstract:

To discriminate dry deposition and leaching is important because dry deposition represents mostly an external input of nutrients to the ecosystem while leaching represents mostly a within-system recirculation of nutrients previously taken up by trees from the soil. It is also important to quantify the nutrient fluxes in the development of sustainable land use systems. The experimental plot is located in the Wanglang National Nature Reserve in Pingwu, Sichuan. An automatic weather station (AWS) (Measurement Engineering Pty Ltd, Australia) was installed to measure gross rainfall, throughfall, temperature, air humidity, incoming radiation, wind speed and direction during one year in every plot. Measurements were scanned every 5 s and recorded every 15 min. The timely manual measurements of rainfall, throughfall, and stemflow were made over one growing season, and their nutrient concentrations were analyzed. The objectives of this study were to characterize nutrient concentrations and nutrient inputs via rainfall, throughfall and stemflow, and understand the factors influencing the nutrient concentrations and inputs in the subalpine forests in the western Sichuan.The results indicated that the concentrations of S and NH₄⁺-N were lower in the throughfall than in the rainfall, while those of NO₃^--N, K, Ca, Mg, C and P were the opposites, indicating that S and NH₄⁺-Nwere absorbed from precipitation by canopy. Regardless of the forest communities, the concentrations of all the measured elements were higher in the stemflow than in the throughfall and rainfall. Nutrient inputs via rainfall during 1 year were 40.57 kg·hm^-2 of C, 6.44 kg·hm^-2 of N, 38.69 kg·hm^-2 of K, 0.4 kg·hm^-2 of P, 39.92 kg·hm^-2 of Ca, 8.27 kg·hm^-2 of Mg, and 44.25 kg·hm^-2 of S. Negative fluxes were found for NH₄⁺-N, NO₃^--N, Ca, S (in the fir forest and spruce forest), and P (in the spruce forest) and positive fluxes for K, C, Mg, and P (in the birch forest and Fir forest) in the net throughfall deposition during the measured period. Linear regressions were developed for net throughfall deposition related to dry deposition (antecedent dry period) and leaching (event quantity, duration, rainfall intensity and interception, etc), showing that S and C may be influenced by foliar leaching whereas NO₃^--N, K (in the birch forest and spruce forest), NH₄⁺-N, Ca, Mg and P may be mainly influenced by intercepted dry deposition by crown, and K in the fir forest by both. The negative net throughfall fluxes of some elements resulted from the longer rainfall duration, the smaller rainfall and the extreme values of interception losses in the studied period.

Key words: subalpine forest, throughfall, stemflow, nutrient input, canopy leaching