Soil solution chemistry and element budget in the forest ecosystem in Guangzhou

In order to assess the effect of acidic deposition on the forest ecosystem,the precipitation,throughfall and soil solution have been collected and analyzed during the period of April 1998 to March 1999 from three monitoring forested sites (two sites at Baiyunshan and one site at Longdong) in Guangzhou, south China. pH values in rain water vary between 3 3 and 6 3, with an average of 4 45. The concentrations of SO 4 2- , NH 4 + and base cations (Ca 2+ , Mg 2+ , K +) in throughfall water are higher than in precipitation. The NH 4 +/NO 3 - ratio varies between 8 and 30 in precipitation and throughfall water. That suggests that ammonium nitrogen (NH 4 +-N) is the principle form of N in the water before penetrating the soil. In contrast, the NH 4 +/NO 3 - ratio becomes considerably lower in soil solution (less than 1/3), suggesting nitrate as the major form of N. Using the weight-averaged concentration, annual deposition and chemical fluxes are estimated at each interface. The annual throughfall deposition is taken as the input rate to forest ecosystem, while the flux at soil depth of 80 cm is considered as the output rate. The element budgets show that NO 3 - and Al 3+ are released from the humus layer (0～20 cm), whereas other components (SO 4 2- , Ca 2 +, Mg 2+ , K +) are accumulated in the soil to various degrees. Evidence for soil acidification: H + concentration in soil solution at the upper soil layer is significantly higher than in precipitation and throughfall. This suggests that the H + production mainly occurs at the upper soil layer and the amount of internally produced H + is more important than the external input. The lower NH 4 + content in soil solution than in throughfall water may be related to the uptake of this component by vegetation roots. However, this cannot be the primary cause in the case of Guangzhou because it fails to account for the negative correlation between NO 3 - and NH 4 +. Increasing H + along with the decreasing NH 4 +/NO 3 - ratios is consistent with the nitrification of NH 4 + in soils. The observed increase in NO 3 - in soil solution is likely the consequence of NH 4 + nitrification, which was further enhanced by leaching of NO 3 - from the soil system. The Al concentration in soil solutions is much higher than in precipitation and in throughfall water. Moreover, the H + concentration is positively correlated with Al 3+ in soil solution. It seems that the mobilization of Al is assisted and enhanced by the presence of H +, consistent with experimental results with simulated acid rain. Because of the potential toxicity of Al ions to plant roots and soil organism, the increase in Al content in soil solution would leave negative impact on plants and on the underground aquatic system. Adsorption of SO 4 2- in soil and its buffering to soil acidification: Numerous studies show that sulfuric load can lead to a loss of base cations (Ca 2+ and Mg 2+ ) through increasing leaching and is responsible for soil acidification. The annual wet deposition rate of SO 4 2- at Guangzhou is as high as 595~660 kg/(hm 2·a) when the contribution of leachates of forest canopy and litter is taken into account. However, the element budget indicates no leaching of base cations from soil at Guangzhou. In fact, the retention of SO 4 2- in soil is evident from the comparison between the annual SO 4 2- input and output (287～578 kg/(hm 2·a) versus 60～145 kg/(hm 2·a). This may be related to the strong adsorption capacity of the acid, Al-rich red soil at the area. The surface of red soil has net positive electron due to high contents of Fe and Al oxides. It can convert to be electronically negative after adsorption of SO 4 2- . This in turn causes adsorption of base cations (Ca, Mg, K) in soil because these elements are closely associated with SO 4 2- . The adsorption capacity of the soil and the large amount of inputted Ca mitigate to some degrees the negative effect of acidic deposition.