Plant sulfur isotopic compositions are altered by marine fertilizers

This study presents sulfur isotope compositions (S-34) for plants grown in a series of growth chamber and field experiments under controlled conditions. Maize, beans, and squash fertilized with a marine fertilizer (seabird guano) were significantly enriched in S-34 relative to the unfertilized control plants (by +4.0 to +7.2 parts per thousand) in the growth chamber experiments. No S-34 enrichment was detected in the plants from the field experiment, which likely relates to the recent use of ammonium sulfate fertilizer in these fields and the retention of residual sulfate with a comparatively low S-34. The field experiment provided a basis to estimate the apparent fractionation between soil and plant S associated with uptake and assimilation (S-34), which ranged between -4 and -6 parts per thousand depending on the taxon and tissue. The use of marine fertilizers has the capacity to increase plant S-34 values and complicate quantitative reconstructions of ancient diet based on bulk stable isotope data.