CHANGES IN SOLUTION CHEMISTRY OF EIGHT INORGANIC ELEMENTS IN A MONSOON EVERGREEN BROAD-LEAVED FOREST IN DINGHUSHAN

Abstract

The monsoon evergreen broad-leaved forest (MEBF) in Dinghushan Biosphere Reserve, Guangdong Province, China, is considered climax forest vegetation in this climatic zone. Studies of its ecological processes, including the hydrology and inorganic solution chemistry, are important for understanding ecosystem function and to direct ecological restoration efforts in this region. In order to understand the response of nutrient cycles to environmental impacts and characterize the nutrient element cycles in this ecosystem, water samples of precipitation, throughfall, soil water at 30 and 80 cm depth and runoff in MEBF were collected weekly and the concentrations of eight inorganic elements measured from February 2001 to March 2002. The elements Al 3+ , Mn 2+ , Sr 2+ and Pb 2+ were measured by ICP (ps-1000AT, USA), Ca 2+ and Mg 2+ were analyzed using Atomic Absorption Spectrometer, and K + and Na + were determined using Flame Atomic Absorption Spectrometer. The results showed that the average concentration of Al 3+ in throughfall (0.393 3 mg·L -1 ) was slightly higher than that in precipitation (0.329 mg·L -1 ). The average concentrations of Al 3+ soil solutions were much higher than in precipitation and were about 4.7 times and 3.8 times higher in soil water collected at 30 cm depth and runoff, respectively. Al 3+ in runoff was derived primarily from soil leaching. Average concentrations of Mn 2+ , K + and Mg 2+ were lowest in precipitation and Sr 2+ and Ca 2+ concentrations were lowest in runoff. The average concentrations of Mn 2+ , Sr 2+ , K +, Mg 2+ and Ca 2+ in throughfall were much higher than in precipitation as a result of strong interactions with the tree canopy. The average concentration of Pb 2+ in precipitation was as high as 0.062 mg·L -1 , indicating some Pb 2+ pollution, and it was about 5 times and 10 times higher than that in throughfall and runoff, respectively. This indicated that the Pb 2+ in precipitation could be largely absorbed by the tree canopy and soil. The average concentration of Na + gradually increased as it passed through the ecosystem from precipitation (0.320 2 mg·L -1 ) to throughfall (0.524 7 mg·L -1 ), soil water at 30 cm (0.627 3 mg·L -1 ) and 80 cm (0.639 1 mg·L -1 ) depth and finally as runoff (0.629 6 mg·L -1 ). Coefficients of variation of the elemental concentrations in the different solutions ranged from 51.1% and 236.7%. The average concentrations of those elements in precipitation, throughfall, soil water at 30 and 80 cm depth were higher during the dry season than during wet season, but their concentrations in runoff during the dry season were lower than or close to that during the wet season. The element Mg had the highest element-couple correlation as compared to all other elements. The greatest element-couple correlations were found in throughfall. Correlations among other elements were highly variable. The results show that element nutrient cycles in this forest ecosystem are stable even though under environmental stress.