SULFUR, NITROGEN, AND PH LEVELS IN WISCONSIN PRECIPITATION

Precipitation is an important source of S and to a lesser extent N for agricultural crops and natural ecosystems. Reductions in S and N emissions have occurred and additional revisions in air quality standards will be expected to further reduce the amounts of nutrients such as S and N in precipitation. This study was conducted to determine current nutrient deposition rates and precipitation pH levels at 10 sites in important agricultural areas of Wisconsin. Bulk precipitation samples were collected at 10-d intervals for a 2-yr period from Nov. 1985 to Oct. 1987. Average annual SO4-S deposition was 11 kg S ha-1 in northwestern Wisconsin and 23 kg S ha-1 in the south. These S deposition rates are approximately 42% less than the amounts found in a Wisconsin study conducted from 1969 to 1971. Despite this apparent decrease in S deposition, most of the S requirement for crop production is still likely to be provided by S in precipitation in most areas of the state. Annual average NO3-N deposition also differed geographically with 4.2 kg N ha-1 deposited in the northwest and 8.4 kg N ha-1 in the south. Average annual NH4-N deposition was 8.7 kg N ha-1 but showed no geographical trend, and variations among sites were probably influenced by point sources. For most sites, annual deposition amounts of nutrients were similar both years, although concentrations were lower during a year with above-normal precipitation. The annual volume-weighted mean pH ranged from 5.1 to 5.9 in a year with above-normal precipitation and from 3.4 to 4.5 in a year with slightly below-normal precipitation.