Deposition of nitrogen to grassland versus forested areas in the vicinity of sabie, Mpumalanga, South Africa

[摘要] Nitrogen deposition to adjacent grassland and forested areas in the vicinity of Sabie,Mpumalanga, South Africa was studied. Total deposition amounts to the forested area arecalculated to be 7l.2 kg N ha-1 yr1 and to the grassland area, 25 kg N ha· 1 yr1. Theaverage deposition amounts are similar to or at least approach nitrogen mineralisationamounts at nearby sites of 50-70 kg N ha-1 yr1. The deposition amounts are made up of21.4 kg N ha-1 yr1 dry deposition, 7.8 kg N ha-1 yr1 wet deposition and 42 kg N ha-1 yr1cloud droplet deposition for the forest. For the grassland, the amounts are 7 kg N ha-1 yr1dry deposition, 7.8 kg N ha-1 yr1 wet deposition and 10.5 kg N ha-1 yr1 cloud droplet.deposition. For both wet and cloud droplet deposition, the amount attributable to nitratewas greater than that attributable to ammonium. For wet deposition, nitrate contributed4.1 kg N ha-1 yr1 and ammonium contributed 3.7 kg N ha-1 yr1 to both forests andgrasslands. For cloud droplet deposition to forests, the amounts were 28 kg N ha-1 yr1attributable to nitrate and 14 kg N ha-1 yr1 attributable to ammonium. For grasslands theamounts were 7 and 3.5 kg N ha-1 yr1. In both forests and grasslands, the component ofdry deposition contributing the most to deposition was ammonia gas, the amounts being14.2 and 4.3 kg N ha-1 yr1 respectively. Nitric acid contributes 3.7 and 1.9 kg N ha-1 yr1respectively and is followed by the nitrogen dioxide component that contributes 1.6 and0.5 kg N ha- I yr1. Ammonium and nitrate particles contribute the least to deposition. Forthe forests the amounts are 1 and 0.9 kg N ha-1 yr1 and for the grasslands they are 0.2 and0.1 kg N ha-1 yr1. A strong seasonal variance in deposition amounts is apparent withmaximum deposition amounts occurring in Summer and minimum amounts in Winter.Intermediate amounts are deposited in Autumn and Spring, with the latter season havingslightly larger deposition amounts. The seasonal variance is strongly linked to theseasonal rainfall and cloud droplet deposition patterns. Biomass burning is indicated as apossible important factor in influencing the chemical composition of rainfall duringSpring. Of the deposition amounts obtained in this study, the deposition from clouddroplets is high compared to other studies and is probably overestimated. Furtherresearch into this area is needed.

[发布机构] University of the Witwatersrand