TRAFFIC AND RESIDUE MANAGEMENT-SYSTEMS - EFFECTS ON FATE OF FERTILIZER N IN CORN

Soil compaction has been recognized as a problem limiting crop product ion, especially in the Southern Coastal Plain of the USA. Development of tillage and residue management systems is needed to alleviate soil compaction problems in these soils. Fertilizer nitrogen (N) management is also an important factor in these management systems. In 1988, a s tudy was initiated with a wide-frame (6.3 m) vehicle to determine the interactive effects of traffic, deep tillage, and surface residue mana gement on the fate of fertilizer N applied to corn (Zea mays L.) grown on a Norfork loamy sand (fine-loamy, siliceous, Thermic, Typic Kandiu dults). Corn was planted into a winter cover crop of 'Tibbee' crimson clover (Trifolium incarnatum L.). Treatments included: traffic (conven tional equipment or no traffic); deep tillage (no deep tillage, annual in-row subsoiling, or one-time only complete disruption); residue man agement (no surface tillage or disk and field cultivation). The one-ti me only complete disruption was accomplished by subsoiling at a depth of 43 cm on 25 cm centers in spring 1988, In 1990-1991, fertilizer app lications were made as N-15-depleted NH4NO3 to microplots inside each treatment plot. The 1990 and 1991 data are reported here. In 1990 an e xtreme drought resulted in an average grain yield of 1.8 Mg grain ha(- 1), whereas abundant rainfall in 1991 resulted in 9.4 Mg grain ha(-1). Deep tillage increased corn dry matter production in both years. In 1 991, grain yields indicated that corn was susceptible to recompaction of soil owing to traffic when residues were incorporated with surface tillage. In the dry year, plant N uptake was increased 27% with deep t illage and decreased 10% with traffic. In the wet year, a surface till age X deep tillage X traffic interaction was observed for total N upta ke, fertilizer N uptake, and total fertilizer N recovery in the plant- soil system. When combined with traffic, plant N uptake was reduced wi th the highest intensity tillage treatment (135 kg N ha(-1)) because o f root-restricting soil compaction, and with the lowest intensity till age treatment (129 kg N ha(-1)) because of increased N losses. In thes e soils, leaving residues on the soil surface can reduce the detriment al effect of traffic on corn production, but if no surface tillage is performed, deep tillage is needed.