December 7, 2010

Nitrate-Induced Iron Deficiency In Soybean Varieties With Varying Iron-Stress Responses

Planting soybean [(Glycine max (L.) Merr.] into Fe chlorosis-prone soils where soybean has seldom, if ever, been grown may require special precautions to establish effective Bradyrhizonium japonicum populations, while simultaneously providing adequate levels of N for the current crop. However, adding fertilizer N likely will increase rhizosphere pH and [ OH∴ ] and, thus, promote Fe deficiency. Our objective was to determine whether varieties (Vs) that differed in Fe efficiency also differed in their response to added N fertilizer when grown on chlorosis-prone soils. Six varieties (2 Fe efficient, 2 moderately Fe efficient, and 2 Fe inefficient) and six rates of fertilizer N (0, 34, 68, 102, 136, and 170 kg N ha∴1) were evaluated during 2003, 2004, and 2005 using soils belonging to the soil subgroup, Aeric Calciaquolls. Growing conditions in 2004 were colder and wetter than either 2003 or 2005, whereas DTPA-extractable Fe was twofold greater in 2004. Extractable Fe did not necessarily reflect available Fe as relative chlorophyll concentrations (SPAD readings), seed number and weight, and grain yield were all significantly (P < 0.05) less in 2004. Nodulation decreased linearly in response to added N for all varieties, regardless of their Fe efficiency characterization or yearly growing conditions. SPAD readings differed markedly among Vs (22.1"“33.8), but showed little response to increasing nitrogen rates (NR) (27.8"“30.7). Plant height, seed number, and grain yield all decreased linearly in response to increasing NRs for Fe-inefficient Vs, whereas these responses in Fe-efficient and moderately efficient Vs changed little as NR increased. Our results strongly suggest that N should not be applied when Fe-inefficient Vs are grown on Fe chlorosis-prone soils.


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