Fall to Winter Soil Nitrate Retention and Profile Distribution In Winter Wheat: Effects of Starter and Preceding Corn Nitrogen Applications.

Winter carryover of residual nitrate (NO₃-N) can contribute to the spring nitrogen (N) requirements of winter wheat (Triticum aestivum, L.).  Nitrogen management in the preceding corn (Zea mays, L.) crop and fall starter N applied at wheat planting are potential sources of such carryover NO₃-N.  This study examined the effects of preceding corn N and wheat fall starter N applications on mid- to-late winter soil NO₃-N pools and NO₃-N soil profile distribution.  Replicated field studies using a randomized complete block split-plot design were established at nine site years on the Maryland Coastal Plain and Piedmont regions.  Corn plots that received corn N rates (the main plot factor) of 0, 135, 202, and 269 kg ha^-1 were established in the spring.  Following harvest, soil residual NO₃-N to 60 cm depth was measured, and wheat was planted.  Wheat starter N rates (the split-plot factor) of 0 and 34 kg ha^-1 were applied, and winter soil samples to 60 cm depth were collected to assess NO₃-N retention.  Corn N fertilization was found to have a significant impact on soil NO₃-N at both fall and winter sampling dates.  However, soil NO₃-N levels diminished between fall and late winter, particularly at sandy textured sites.  Typically, application of 34 kg ha^-1 fall starter N did not significantly increase soil NO₃-N levels in the surface 60 cm.  Where significantly increased winter NO₃-N was detected, it was concentrated at the deeper sampling increment (30 to 60 cm), with the surface 15 cm being NO₃-N depleted at all sites.  These data indicate potential for carryover NO₃-N contribution to spring wheat growth where fall soil NO₃-N levels are high and winter precipitation is below average; however, fall starter N should not be applied with the expectation of a significant spring carryover benefit.