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

Lowman, Guy Russell Pollock
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University of the Witwatersrand
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 are calculated to be 7l.2 kg N ha-1 yr1 and to the grassland area, 25 kg N ha· 1 yr1. The average deposition amounts are similar to or at least approach nitrogen mineralisation amounts at nearby sites of 50-70 kg N ha-1 yr1. The deposition amounts are made up of 21.4 kg N ha-1 yr1 dry deposition, 7.8 kg N ha-1 yr1 wet deposition and 42 kg N ha-1 yr1 cloud droplet deposition for the forest. For the grassland, the amounts are 7 kg N ha-1 yr1 dry 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 nitrate was greater than that attributable to ammonium. For wet deposition, nitrate contributed 4.1 kg N ha-1 yr1 and ammonium contributed 3.7 kg N ha-1 yr1 to both forests and grasslands. For cloud droplet deposition to forests, the amounts were 28 kg N ha-1 yr1 attributable to nitrate and 14 kg N ha-1 yr1 attributable to ammonium. For grasslands the amounts were 7 and 3.5 kg N ha-1 yr1. In both forests and grasslands, the component of dry deposition contributing the most to deposition was ammonia gas, the amounts being 14.2 and 4.3 kg N ha-1 yr1 respectively. Nitric acid contributes 3.7 and 1.9 kg N ha-1 yr1 respectively and is followed by the nitrogen dioxide component that contributes 1.6 and 0.5 kg N ha- I yr1. Ammonium and nitrate particles contribute the least to deposition. For the forests the amounts are 1 and 0.9 kg N ha-1 yr1 and for the grasslands they are 0.2 and 0.1 kg N ha-1 yr1. A strong seasonal variance in deposition amounts is apparent with maximum deposition amounts occurring in Summer and minimum amounts in Winter. Intermediate amounts are deposited in Autumn and Spring, with the latter season having slightly larger deposition amounts. The seasonal variance is strongly linked to the seasonal rainfall and cloud droplet deposition patterns. Biomass burning is indicated as a possible important factor in influencing the chemical composition of rainfall during Spring. Of the deposition amounts obtained in this study, the deposition from cloud droplets is high compared to other studies and is probably overestimated. Further research into this area is needed.
Grassland ecology, Nitrogen cycle, Forest ecology