Spatial analysis of biomass and N accumulation of a winter wheat cover crop grown after a drought-stressed corn crop in the SE coastal plain

Spatial variability in crop yield can cause large within-field differences in fertilizer N removal. Cereal winter cover crops can trap the residual N, but their ability to do so partially depends on the soil conditions that determine growth. Our objective was to determine site-specific effects of soil variation on biomass and N accumulation by a wheat (Triticum aestivum L.) cover crop that was grown after a droughted corn (Zea maize L.) crop. In 1993, corn was grown with an applied fertilizer N rate of 129 kg ha-1 on an 8-ha field near Florence, SC. Nitrogen removed by grain at 10 locations (representing six soil types) ranged from 14 to 41 kg N ha-1. Wheat was planted in November without additional fertilizer N. Wheat biomass and N content were measured on 15 March, 15 April, and 14 May. Inorganic soil N to a depth of 90 cm was measured on 22 March 1994 and ranged from 49 to 95 kg ha-1. By mid-March, wheat accumulated 49% of its total N but only 14% of its biomass of that measured in May. After mid-April significant increases in N accumulation occurred at only two sites. Biomass accumulation by mid-May ranged from 2032 to 7914 kg ha-1 and N accumulation ranged from 19 to 52 kg ha-1. The amount of variability among sites for wheat biomass was greater than the amount of variability among sites for N. Variation for wheat biomass and N accumulation within soil map units was similar to the amount of variation among soil map units. Most of the variability was caused by differences in sites within and among soils associated with depression areas. Around these depression areas, site-specific management of N inputs appears more effective than cover crops at reducing N losses to the environment. Away from these areas, cover crops should be predictable and reliable in trapping N and increasing soil organic matter.