Uptake of Nitrogen from Soil, Fertilizer, and Crop Residues by No-till Corn and Soybean

The relative availability of nitrogen (N) from indigenous soil organic N, immobilized fertilizer N, currently applied fertilizer N, and crop residues is largely unknown. A nonirrigated experiment was conducted on a silty clay loam at Lincoln, NE, with both continuous soybean [Glycine max (L.) Merr.] and corn (Zea mays L.), in which crops were produced with no tillage after surface application of 0, 0.5, 1.0, or 1.5 times the quantity of aboveground crop residue produced the previous year. In 1980 and 1981, depleted ¹⁵N NH₄NO₃ (at 45 kg N ha⁻¹) and crop residues were managed so that labeled isotope taken up in the 1981 crop could be separated into that from crop residues, from 1st-yr fertilizer that had become immobilized, from fertilizer N applied the 2nd yr, and from indigenous soil N (plus fixed N for soybean). Grain and straw (stover) production of both crops in 1981 were increased 2000 to 3000 kg ha⁻¹ by increasing crop residue rate. Almost all the N in soybean residues was mineralized and taken up by the second crop, with most of this uptake occurring after mid-July. Uptake by soybean of labeled N immobilized in soil organic matter the previous year increased from about 2 to 6 kg N ha⁻¹ as residue rate increased, possibly originating mainly from N temporarily immobilized in microbial biomass. Uptake by soybean of current-year fertilizer N also increased slightly with residue rate. The fraction of total N in soybean derived from indigenous soil (plus fixed) N decreased with increased residue rate from about 85% for no residues to 55% for the 1.5 residue rate. For corn, essentially none of the N immobilized in corn residues was recovered by the next crop at any residue rate. Uptake of labeled N immobilized in soil organic matter was not affected by residue rate, but uptake from 1981 fertilizer treatment increased about 6 kg ha⁻¹ with increased residue rate. Over 80% of the total N uptake by corn in 1981 came from indigenous soil N, regardless of residue rate.