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.