Estimation of dry deposition and canopy exchange in Chinese fir plantations

Based on the data from three years of chemical monitoring for incident precipitation, throughfall and stemflow, the net throughfall flux (NTF) was analyzed to evaluate the relative importance of dry deposition and canopy exchange in two Chinese fir (Cunninghamia lanceolata) plantations with different distances from the pollution sources in the suburbs of Nanping, China. NTF analysis showed consistent canopy effects on rainfall chemistry, with H+, K+, Na+, Ca2+, Mg2+ and SO42- added to rainfall by foliage, whereas NH4+ and NO3- ions were absorbed from precipitation. Regression analysis of single-event data showed that canopy exchange (leaching and uptake) was a dominant process in controlling NTF for most solutes. However, dry deposition contributed significantly to NTF for Na+ and Ca2+ at the two sites, and for SO42- and NO3- at XQF site close to emission sources. increased precipitation acidity caused increased leaching of Ca2+. NH4+ concentration in precipitation had a significant positive effect on the retention of it in the canopy, suggesting that uptake of NH4- by canopy is an active process. The uptake of nitrogen by canopy is strongly controlled by the stand ages, with annual uptake of 2.97 kg ha(-1) at FFC site where the stand is in the fast-growing stage (16-year old), but only 0.56 kg ha(-1) in the mature stand (41-year old) at XQF. (C) 2001 Elsevier Science B.V. All rights reserved.