Nitrogen Utilization of Corn Under Minimal Tillage and Moldboard Plow Tillage. I. Four-Year Results Using Labeled N Fertilizer on an Atlantic Coastal Plain Soil

Crop responses and N uptake of corn (Zea mays L.) grown with moldboard plow tillage (PT) and minimal tillage (MT) were measured in a 4-yr field experiment on a Mattapex silt loam soil (finesilty, mixed, mesic, Aquic Hapludult) near Salisbury, MD (Atlantic Coastal Plain region). Labeled ammonium sulfate or ammonium nitrate (¹⁵N depleted) was applied annually to the entire area (74 m²) of each plot at five N rates ranging from 0 to 180 or 270 kg N ha⁻¹. Corn dry matter yield, N content, and isotope composition were determined in the entire plant at the 11 to 12 leaf stage and at silage harvest, as well as in the corn grain at maturity. Below 135 kg N ha⁻¹, dry matter yields, total N uptake, and fertilizer N (FN) uptake were greater with PT than with MT, but with 180 kg N ha1⁻¹ there was either no difference between the tillage systems or MT was greater than PT. Fertilizer N additions had no consistent or practically significant effect on soil N (SN) uptake. At 13S kg N ha⁻¹, average crop FN recoveries were 62% for PT and 53% for MT, but this difference decreased with increasing N rate until both tillage systems averaged about 53% recovery at 180 kg N ha⁻¹. Average SN uptake was greater with PT culture than with MT culture, averaging 76 kg N ha⁻¹ and 59 kg N ha⁻¹, respectively. The crop N requirement of corn was about 17 kg N ha⁻¹ greater for MT corn than PT corn due to a 10% greater yield of total dry matter with MT. Corn grown with MT required about 68 kg more FN ha⁻¹ than PT, due to a lower uptake of SN and a greater crop N requirement. A lower uptake of N on MT plots during the second half of the growing season suggests (i) a greater net mineralization of N on PT plots and/or (ii) a greater loss of N via leaching and/or denitrification on MT plots. A nitrogen concentration of about 11 to 12 mg N g⁻¹ in the silage total dry matter, or about 15 mg N g⁻¹ in the grain, can be used as an approximate value for N sufficiency. However, these values should be considered broad guidelines since significant deviations were observed in individual years.