Residual effects of nitrogen fertilization on soil nitrogen pools and corn growth

Given the dynamic nature of soil nitrogen (N), inorganic N fertilization to corn (Zea mays L.) has potential to alter N pool balance by creating an accumulation or depletion of soil N. Current corn N recommendations in the common corn-soybean rotation of Indiana strive to find the best N rate that maximizes producer profit. Increasing our understanding of soil N will inform producers if they should adjust fertilizer rates for corn to influence maintenance of organic N and Carbon. Our objective was to determine residual N effects from fertilized corn in a corn-soybean rotation by measuring (1) soil N pools post soybean, (2) soil fertility, (3) growth and yield of corn, and (4) nitrogen removal in rotated corn. Field-scale corn N trials were established in 2006 at 6 Indiana farms with corn-soybean rotations (near cities of West Lafayette, Farmland, Columbia City, Wanatah, Butlerville, and Lafayette). A randomized complete block design assigned six corn N rates (ranging from starter only to above the minimum needed to maximize yield) to each replication. The design was not re-randomized the next corn year. In 2015, two or three of the replications at each location had only starter fertilizer, thus, allowing for determination of cumulative differences in N fertilization on soil N supply. Soil composites of each plot were collected from 0-20 cm, 20-40 cm, and 40-60 cm post corn planting (\u3c9 \u3edays). Initial samples were analyzed for general fertility, inorganic N, and total N. In addition, a 50-day incubation with soils maintained at 25 °C and 33 kPa moisture was used to examine mineralization and nitrification at days 10 and 50 days. Earleaf samples were collected at VT followed by stover and grain samples at maturity; plant samples were analyzed for macro- and micro-nutrient concentrations. Grain yield and total plant N uptake were also determined. Locations were kept separate for statistical analyses. ANOVAs carried out on general soil fertility data revealed minimal N rate effects. At Lafayette, pH for the 0-20 cm soil decreased linearly beginning at N rate 3 (135 kg ha -1); the acidifying effect of the side-dressed urea-ammonium nitrate (UAN) may be responsible for the pH decrease with increasing N rate. This trend was not observed at other locations. There was no N rate effect on day 0 total inorganic N (α ≤ 0.05). As N rate increased, total N decreased linearly from 0.9 to 0.8 g kg-1 (R2 = 0.68) at Columbia City. When the soil was incubated for 50 days, total inorganic N did not vary by N rate. Generally, soil inorganic and organic N decreased with depth from 0 to 60 cm. When corn was grown and the predominate source of N was derived from the soil, no differences were noted in plant N uptake nor yield for any location. Spring 2015 had record-breaking rainfall amounts which certainly contributed to residual N loss. Furthermore, the soil’s natural N supply, location management practice, and crop N demand are probable cause for the variances noted between locations. Overall, we conclude that corn N rate has negligible effects on residual N abundance, soil fertility, uptake, and grain yield for Indiana corn-soybean rotations