Forest precipitation chemistry is a key issue for forest hydrology and forest ecology. Chemical content in precipitation changed significantly when different kinds of external chemical materials were added, removed, translocated and transformed to or in the forest ecosystem along with precipitation. The chemistry of precipitation was monitored and analyzed at a 31a Pinus tabulaeformis forest in Beijing Xishan area. Movement patterns of nutrient elements in hydrological processes could be uncovered by studying the monitored data. Moreover, the information is useful in diagnosing the function of ecosystem and evaluating the impact of environment on the ecosystem.
Samples of rainfall, throughfall and stemflow were collected on the site. In the lab, Ca²⁺ and Mg²⁺were analyzed by flame atomic absorption and K⁺and Na⁺ by flame emission. NH⁺₄-N was analyzed by indophenol blue colorimetry, and NO^－₃-N was analyzed by phenoldisulfonic acid colorimetry. The results showed：(1) The concentration gradient of nutrient elements clearly changed except for Na+: The nutrients in stemflow were significantly higher than those of the throughfall and rainfall as the precipitation pass through the P. tabulaeformis forest. The monthly patterns were distinctly different. It is indicated that a large amount of nutrients were leached from canopy, which was a critical function of intraecosystem nutrient cycling to improve the efficiency of nutrient use. (2) the concentrations of NO^－₃-N and K+were changed greater than that of other nutrient elements. The concentrations of NO^－₃-N in throughfall and stemflow were 4.4 times and 9.9 times higher than those in rainfall, respectively. The concentrations of K+in throughfall and stemflow were 4.1 times and 8.1 times higher than those in rainfall, respectively. (3)The leaching of nutrient elements from stand was an important aspect of nutrient return in the P. tabulaeformis forest, which returned a total nutrient of 54.1 kg hm^－2, with Ca²⁺ and K⁺ being much greater than other elements. In addition, K+ was the greatest in leaching intensity. (4)Nutrient input through precipitation was a main source in Beijing Xishan area and nutrient contents of 66.4 kg hm－2 were added, of which Ca²⁺ and N were much more than other nutrient elements. When precipitation passed through the P. tabulaeformis forest, there were 121 kg hm^－2 nutrient added to the forest floor. Ca²⁺ recorded the greatest nutrient increase, with 61.2 kg hm^－2, followed by N (NH⁺₄-N & NO^－₃-N), K⁺ and Mg²⁺, with 31.3, 16.5，and 8.11 kg hm－2, respectively and the least was Na+, 3.34 kg hm^－2.