Atmospheric deposition and canopy exchange of anions and cations in two plantation forests under acid rain influence

Abstract

Acid deposition as a widely concerned environmental problem in China has been less studied in plantation forests compared to urban and secondary forests, albeit they constitute 1/3 of the total forested areas of the country. We measured the rainwater amount and chemistry outside and beneath the canopies of two widely distributed plantations (Acacia mangium and Dimocarpus longan) in the severe acid rain influenced Pearl River Delta region of southeastern China for two years. Our results showed that the frequency of acid rain was 96% on the basis of pH value <5.6. The volume-weighted mean (vwm) pH was 4.62 and higher in the dry (Oct.–Mar.) than in the wet (Apr.–Sep.) seasons. The major acidic anion was sulfate with vwm concentration of 140 μeq l−1 and annual deposition flux of 110.3 kg ha−1 yr−1. The major neutralizing cations were calcium (94.8 μeq l−1 and 28 kg ha−1 yr−1) and ammonium (41.2 μeq l−1 and 11.7 kg ha−1 yr−1). Over 95% of these major acidic anions and neutralizing cations were derived from anthropogenic and terrestrial sources as a result of industrial, agricultural and forestry activities. Plantation canopy had marked impacts on rainwater chemistry, with the measured anion and cation concentrations being significantly enriched in throughfall (TF) and stemflow (SF) rainwater by 1.4 (for NO3−) to 20-fold (for K+) compared to those in bulk precipitation (BP). Dry deposition generally contributed about 13–22% of the total deposition while canopy leaching mainly occurred for K+ (>88%) and NH4+ (10–38%). The two tree species showed distinct impacts on rainfall redistribution and rainwater chemistry due to their differences in canopy architecture and leaf/bark texture, suggesting that species-specific effects should not be overlooked while assessing the acid deposition in forested areas.