Utilization of Labeled-N Fertilizer by Silage Corn Under Conventional and No-till Culture

The influence of tillage methods on the utilization of N from soil and fertilizer sources by crops has not been investigated previously by tracer methods to any great extent. The objectives of this study were to compare yields and N utilization of silage corn (Zea mays L.) in conventional and no-till culture, using a double-cropping system of corn and smooth bromegrass (Bromus inermis Leyss.). The experiment was initiated in 1974 on a Wharton-Cookport (Aquic Hapludults-Aquic Fragiudults) silt loam soil with an established stand of smooth bromegrass in West Virginia. Labeled (¹⁵N-depleted) ammonium sulfate was surface-applied annually at rates of 85, 170, and 340 kg N/ha over a 4-year period. Herbicides (2.2 kg/ha atrazine; 1.1 kg/ha paraquat) were applied annually to the grassed and plowed plots to suppress the grass sod but allow bromegrass to resume growth after corn silage was harvested. The grass was not harvested for hay, but left as a cover crop for erosion control. Yields of corn plants were determined at midseason (11 to 12 leaf stage) and at the silage stage. The proportions of total N uptake contributed by the soil and fertilizer were determined by mass spectrometry. In 1974 and 1977 when soil water regimes were most favorable, total dry matter (TDM) yields were similar under the two systems of tillage, but in the intervening years of subnormal rainfall, yields were substantially higher from no-till than from plowed plots. Amounts of available water in the surface 30 cm of soil were consistently higher in no-till than in tilled plots during the growing season, but differences were not discernible in the underlying 30 an layer. The 4-year average yields of silage TDM for all N rates were 13,400 and 11,570 kg/ha, respectively, for no tillage and conventional tillage. Because of the lower attainable yields on plowed plots in 1975 and 1976, the corresponding average amount of applied N required for near-maximum yield under conventional tillage was about one-half that required for no-tillage. Soil N uptake tended to decrease with increasing rate of N applied, generally was unaffected by method of tillage, and differences among years were relatively small. For both samplings and all N rates in 1975 and 1976, uptake and recovery of fertilizer N were substantially higher under no-tillage than under conventional tillage. In 1974 and 1977, enhanced recovery under no-tillage was evident only at the highest N rate. Four-year midseason recoveries from 85, 170, and 340 kg N/ha, respectively, were 42, 32, and 26% for no-till and 35, 27 and 16% for conventional tillage. Corresponding average annual recoveries at silage harvest were 46, 53, and 46% for no-till, and 53, 55, and 347, for conventional tillage.