Atmospheric input of nitrogen to a coastal fynbos ecosystem of the south-western Cape Province, South Africa

Abstract

Results are presented of nitrogen input via bulk precipitation over a two-year period, to coastal fynbos vegetation occurring on soils of low nutrient status. The annual precipitation recorded during the study periods of 1980 – 81 and 1981 – 82 was 381 mm and 466 mm respectively which was below the 40-year mean of 577 mm recorded for the area. Total nitrogen input during 1980 – 81 was 1,99 kg N ha − 1 y − 1 . Of this 1,12 kg N ha − 1 y − 1 was inorganic nitrogen in the form of NO 3 -N and NH 4 -N. Inorganic N input occurred predominantly in the winter rainfall months of June to August. Inorganic N input during 1981 – 82 was 1,79 kg N ha − 1 y − 1 with an NH 4 -N to NO 3 -N quotient of 1. Logarithmic relationships exist between NO 3 -N and NH 4 -N concentrations in bulk precipitation and the volume of precipitation collected at weekly intervals. Bulk precipitation collectors at 1,5 m and 4 m were not suitable to determine the infuence of local recycling of dust aerosols. It is probable that although atmospheric N deposition may contribute a portion of the extra-system N, it alone is not sufficient to replenish N lost by coastal fynbos ecosystems as a result of recurrent short interval fires. Rather the low input of nitrogen to the Pella site reported gives an indication of the levels of N deposition to be expected in the fynbos region where N is derived mainly from oceanic sources and few anthropogenic activities influence precipitation composition.