Partitioning of carbon an SO₂-sulfur in a native grassland

The seasonal assimilation and within-plant partitioning of ¹⁴CO₂-carbon and ³⁵SO₂-sulfur in field plots of mixed-grass prairie was investigated, as was the dry deposition of ³⁵SO₂ onto surfaces of dead leaves, litter, and soil, and possible effects of continuous low-level SO₂ fumigation on these processes. The proportion of total net-assimilated carbon found below-ground was 45% in May, 51% in July, and 17% in September. As the season progressed, greater proportions of assimilate were partitioned to 5-20 cm depths and less to the 0-5 cm depth. Rhizomes and crowns received greater proportions in late season. Significant fractions of total ³⁴SO₂-deposited sulfur were recovered on dead leaf surfaces as well as litter and soil, suggesting estimates of SO₂ removal based on stomatal resistance alone are inadequate. Only 4% to 7% of total deposited sulfur was translocated belowground, with most going to 0-5 cm roots. In July much greater proportions of the total translocated SO₂-sulfur were found in deeper depths than in September. On SO₂-fumigated plots roots had lower total sulfur concentrations than controls. Furthermore, while on control plots total sulfur in roots at 5-20 cm increased from May to July and decreased from July to September, on fumigated plots there was a decrease followed by an increase suggesting that SO₂ uptake by shoots interferes with the normal pattern of root sulfur uptake and redistribution within the plant. Continuous SO₂ fumigation also seemed to stimulate root growth in July, possibly through a stimulation of photosynthesis.