Uptake of Fertilizer and Soil Nitrogen by Ryegrass, as Affected by Carbonaceous Residue

Annual ryegrass ( Lolium multiflorum ) was grown on limed Hartsells fine sandy loam and Decatur silty clay loam fertilized with 0, 120, 300, 600, and 900 mg of N as 15 N‐labeled (NH 4 ) 2 SO 4 per pot (3 kg of soil). Half of the pots also received 15 g of corn ( Zea mays ) forage residue (1.05% N) mixed with the soil and half received none. Six clippings of forage and roots were harvested after the final clipping. Total yield of dry forage and uptake of N were linear for all rates of applied N without residue and for the 300‐, 600‐, and 900‐mg rates with applied residue. As estimated by linear regression from total N uptake, apparent recovery by six clippings of ryegrass plus roots, grown without residue, was 78% from Hartsells soil and 76% from Decatur. The corresponding slightly lower recoveries of 75 and 72% of the fertilizer N applied reflect uptake of soil N. Recoveries of fertilizer N were 90% from the Hartsells and 85% from the Decatur soil‐plant systems. As estimated by difference, recoveries of fertilizer N by ryegrass increased, and recoveries from the plant‐soil systems decreased with increase in amount of applied N. Crop uptake of soil N increased as fertilizer N was depleted from the soils. Addition of the carbonaceous residue caused apparent immobilization of 141 mg of fertilizer N in Hartsells and 164 mg in Decatur soil. Percentages of fertilizer N immobilized in the soils decreased, but actual amounts increased with increasing amount applied. A portion of the fertilizer N, independent of amount applied, was immobilized in each soil.