Submitted to: Soil and Crop Science Society of Florida Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2001
Publication Date: N/A
Interpretive Summary: Soybean is a particularly desirable crop because it fixes nitrogen from the atmosphere and does not require nitrogen fertilizer. However, the nitrogen fixation process is especially sensitive to soil drying. This sensitivity appears to be related to the activity of an enzyme in leaves that breakdown the products of nitrogen fixation. The activity of this enzyme is dependent in part on the presence of manganese in leaves. This study, involving an ARS-USDA scientist located at CMAVE, Gainesville, FL, was designed to test nitrogen fixation sensitivity to drought when manganese fertility was varied. Indeed, it was discovered that manganese levels were involved in influencing nitrogen fixation activity. Increasing manganese availability in the soil resulted in decreased inhibition of nitrogen fixation by the products of nitrogen fixation. Thus, special attention needs to be given to adequate manganese supplies to soybean, particularly when the crop is grown on high pH soils.
Technical Abstract: The sensitivity of N2 fixation to water deficit is related to an increase in leaf ureide concentration, which may be linked to the availability of Mn in the leaves. The rate of ureide degradation was measured in leaves from plants grown in pots on limed and non-limed soil. At 75 days after liming (DAL), the rate of ureide degradation in leaves from limed plants was about half of that in non-limed plants and was fully recovered when Mn was added to the degradation-assay solution. However, at 105 DAL, the rate of ureide degradation in limed plants was almost nil and Mn supply failed to recover the degradation rate of non-limed plants. Two field experiments were carried out in a low-Mn field to investigate the response of leaf ureide and Mn concentration to the application of various Mn treatments. Overall these experiments showed that while bulk leaf Mn concentrations were not influenced by soil Mn fertility, but leaf ureide degradation rates and leaf ureide concentration were responsive to soil Mn treatments.