Soybean sulfur and nitrogen balance under varying levels of available sulfur

Protein quality of soybean [Glycine max (L.) Merr.] seed could be enhanced by increasing the concentration of S-containing amino acids. The N:S ratio of soybean seed tissue is an indicator of protein quality. The objective of this study was to compare N and S accumulation and distribution in the soybean plant under varying levels of S availability. Soil S levels were varied in two greenhouse trials and tissue S and N contents were monitored. A field experiment with 0 and 60 kg ha(-1) applied S was also conducted. In the two greenhouse trials, seed yield, rate of dry matter increase, and rate of N accrual by shoots all increased at least five-fold as S availability increased from less than 15 (zero added S) to 60 mg available S per plant, but showed little response to higher levels of S. Rate of S accrual was strongly related to S availability. In the field trial, there were no responses to added S. Leaf, pod, and seed tissue appeared to accumulate S in proteins, whereas, root and stem tissue appeared to accumulate S as sulfate. Harvest index values for S were consistently less than those for N, indicating S is not remobilized to seed as efficiently as is N. Seed S:N ratio was linearly related to the rate of S versus N accrual on a whole-plant basis. We estimate that a 50% increase in the amount of S-containing amino acids in soybean seed may require a 65 to 80% increase in S accrual by shoots to satisfy the increased demand for S.