Confined microplot size for nitrogen-15 uptake by corn plants in a corn intercrop system

The high cost of labeled N fertilizer places practical limitations on measuring the fate of applied N. Applying of the labeled N-15 to a microplot within a larger plot can reduce the cost. The objective of this study was to assess microplot dimensions required to produce reliable N-15 data and cropping system effects on corn N-15 uptake under a regime of monocropped corn (Zea mays L.) and corn intercropped with annual Italian ryegrass (Lolium multiflorum Lam.). Nitrogen was applied to large plots (15 by 75 m) at 270 kg N ha(-1) on a fine, silty, mixed, nonacid, frigid Typic Humaquept soil during the 1993 and 1994 growing seasons. Enriched N fertilizer was applied to 1.15- by 1-m(2) confined microplots located in opposite halves of I:he large plot in each year of the study. Corn plants inside the confined microplots were sampled at 0.1, 0.3, and 0.5 m from the end borders of each microplot. In 1993, monocropping resulted in increases in leaf, stalk, and total dry matter production of 22, 41, and 30%, respectively, within microplots and relative to intercropped corn, There were no cropping system effects for these parameters in 1994. Monocropping increased atom % N-15 enrichment in corn grain and stalks and increased fertilizer N recovery in all plant tissue compared with intercropping in both years. There were no differences in atom % N-15 enrichment and fertilizer N recovery for plants harvested at the three sampling positions. These results suggest that a confined microplot with an area as small as 0.4 by 1.15 m can provide a reliable measure of fertilizer N recovery for corn plants in monocrop or intercrop systems. Any or all of the five plants within the microplot could be used to supply a reliable estimation of N-15 recovery by corn plants.