Variation Characteristics of Nutrient Elements through Hydrological Processes in Pinus densata Natural Forest of Southeast Tibet
2016, 31 (1):
Pinus densata is a tree unique to China. P. densata natural forest is an important water conservation forest in the middle reaches of Yarlung Zangbo River in Tibetan Plateau. It plays an important role in maintaining water balance and soil and water conservation. Precipitation, throughfall and stemflow were monitored in natural forest of P. densata distributed in Gongbu Nature Reserve of Southeast Tibet from April to October of 2013. Meanwhile, nutrient elements (N, P, K, Na, Ca, S, Mg, Si, Mn, Fe, Cu, and Zn) contents in the water were determined. According to allotment of water amount, input of each element in the forest was calculated. The variation characteristics of nutrient elements through the hydrological processes of the forest were analyzed. The results showed that: Since the precipitation in Tibetan Plateau was divided into the obvious dry and wet season, temporal and spatial dynamics of nutrient elements contents from throughfall and stemflow changed remarkably when precipitation pass through canopy layer and after rainfall and surface of P. densata’s interaction (needles, branches, and stem). The biggest variation coefficient of concentration value in the precipitation was 1.485 for Zn, and the smallest was 0.255 for S. The biggest in the throughfall was 0.844 for Na, and the smallest was 0.304 for S, while those in the stemflow were 0.809 for P and 0.318 for Si, respectively. There were no significant relations among the trace elements (Mn, Fe, Cu, Zn), and between the trace elements and the others, respectively. There were significant relations between the others. The average contents of Cu annually in the precipitation, throughfall and stemflow were the lowest, which were 0.023, 0.047 and 0.066 mg/L, respectively. That of N was the highest in the precipitation, which was 1.604 mg/L, and those of Na were highest in throughfall and stemflow, which were 3.401 and 4.307 mg/L, respectively. The differences of each element contents in the samples collected from the precipitation, throughfall and stemflow were significant, and the order were: stemflow>throughfall>precipitation. The monthly patterns of nutrient concentration were “∩”, “N” or “∪” types. The leaching coefficients of the throughfall and stemflow were both more than one, and the stemflow leaching coefficients were greater than the corresponding throughfall leaching coefficient. The nutrients input of precipitation, throughfall and stemflow were 64.172, 79.949 and 15.623 kg/hm2, respectively. The Na, K and Ca input of throughfall and stemflow accounted for more than 50%. The total input of throughfall was 5.12 times that of stemflow. The input of precipitation was main N element. The net leaching input was 34.057 kg/hm2 (except for N), and ranked in the order of Na>Ca>K>Mg>Si>S>P>Mn>Fe>Zn>Cu, and the input of N was -2.658 kg/hm2. The research results not only provided a theoretical basis for economic evaluation of P. densata forest ecosystem service function, but also enriched the assessment content of the national ecological security barrier in Tibet Plateau.
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