Soil properties after 36 years of N fertilization under continuous corn and corn-soybean management

Modern agricultural systems rely on inorganic nitrogen (N) fertilization to enhance crop yields, but its overuse may negatively affect soil properties. Our objective was to investigate the effect of long-term N fertilization on key soil properties under continuous corn [Zea mays L.] (CCC) and both the corn (Cs) and soybean [Glycine max L. Merr.] (Sc) phases of a corn-soybean rotation. Research plots were established in 1981 with treatments arranged as a split-plot design in a randomized complete block design with three replications. The main plot was crop rotation (CCC, Cs, and Sc), and the subplots were N fertilizer rates of 0 kg N ha−1 (N0, controls), and 202 kg N ha−1, and 269 kg N ha−1 (N202, and N269, respectively). After 36 years and within the CCC, the yearly addition of N269 compared to unfertilized controls significantly increased cation exchange capacity (CEC, 65 % higher under N269) and acidified the top 15 cm of the soil (pH 4.8 vs. pH 6.5). Soil organic matter (SOM) and total carbon stocks (TCs) were not affected by treatments, yet water aggregate stability (WAS) decreased by 6.7 % within the soybean phase of the CS rotation compared to CCC. Soil bulk density (BD) decreased with increased fertilization by 5 % from N0 to N269. Although ammonium (NH4+) did not differ by treatments, nitrate (NO3−) increased eight-fold with N269 compared to N0, implying increased nitrification. Soils of unfertilized controls under CCC have over twice the available phosphorus level (P) and 40 % more potassium (K) than the soils of fertilized plots (N202 and N269). On average, corn yields increased 60 % with N fertilization compared to N0. Likewise, under N0, rotated corn yielded 45 % more than CCC; the addition of N (N202 and N269) decreased the crop rotation benefit to 17 %. Our results indicated that due to the increased level of corn residues returned to the soil in fertilized systems, long-term N fertilization improved WAS and BD, yet not SOM, at the cost of significant soil acidification and greater risk of N leaching and increased nitrous oxide emissions.