A Nitrogen‐15 Microplot Design for Measuring Plant and Soil Recovery of Fertilizer Nitrogen Applied to Corn1

Abstract

AbstractApplication of the stable isotope I5N to a small area, or microplot, within a larger field plot reduces the cost of measuring plant uptake and soil recovery of fertilizer‐derived N (FN). The objective of this study was to evaluate a specific microplot design for corn (Zea mays L.), including determining maximum plant sampling area. Nitrogen as (NH4)2SO4 was applied to 6‐ by 9‐m plots of corn at rates of 200 and 150 kg N ha−1 on a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) and a Mt. Carroll silt loam (fine‐silty, mixed, mesic Mollic Hapludalf), respectively, in 1982 and 1983. Enriched 15N was applied to 1.52‐ by 2.29‐m microplots located in opposite halves of the large plot in each year of the study. This allowed separate measurement of plant and soil recovery of FN for the two application years as well as for residual years. Each microplot was centered lengthwise on one row and bordered by the two adjacent rows, so that the center row was 0.76 m from each side of the microplot and the adjacent rows were on the microplot edge. To assure incorporation and uniform distribution, labeled N solution was injected with a syringe to a depth of 0.08 m on a grid pattern with 0.19‐m spacings. Grain and stover samples and soil samples to a 0.4‐m depth were taken at maturity from various positions within and outside the microplots, and percentage of N derived from fertilizer was determined. Results showed that this microplot design provided reliable FN recovery data for corn in 0.76‐m rows when plant samples were taken from the center row at least 0.38 m from the end of the microplot and when soil was sampled from a 0.76‐ by 1.14‐m area in the center of the microplot. Measurement of FN in plants from the two edge rows indicated that use of a microplot 0.76 m in width would not always provide reliable FN data.