BORDER EFFECTS ON N-15 FERTILIZED WINTER-WHEAT MICROPLOTS GROWN IN THE GREAT-PLAINS

Field experiments requiring N-15 enriched fertilizer are costly, thus microplot techniques are generally used. Placing physical barriers around microplots to contain the N-15 may introduce artifacts that affect N recovery by crops, limit types and numbers of measurements, and cause other restrictions. The purpose of this experiment was to determine minimum microplot size (without the use of barriers) for accurately measuring enriched N-15 uptake into a winter wheat crop, while using normal field cultural practices. Winter wheat (Triticum aestivum L.) was seeded into KNO3 fertilized (56 and 112 kg N ha-1) plots (4.57 m by 3.05 m) on a Platner silt loam soil (fine montmorillonitic mesic Aridic Paleustol). Within each larger plot, four microplots (2.29 m by 1.83 m) were fertilized with 10 atom % N-15 enriched KNO3 at the same rate as for main plots. Nitrogen-rate treatments were replicated four times in a randomized block design. Above ground plant material was harvested (0.3 m of row) from six adjacent rows at flowering (Feeke's scale = 10.5). Three rows were harvested from inside (N-15 enriched KNO3 added) and three from outside (N-15 enriched KNO3 not added) of the microplots. Plant uptake of total N into plant tops was not significantly different across any of the six harvested rows. Dry matter yields and total-N uptake were significantly larger for the 112 than for the 56 kg N ha-1 fertilizer rate, as were the N-15 uptake and atom N-15 % values in plant material within the microplots. In rows adjacent to microplot borders, concentrations of N-15 in plant material changed rapidly; but there were no differences beyond 0.46 m inside or outside the microplots. These results indicate that minimum microplot size for studies with fall-applied N-15 on winter wheat grown in the Great Plains is 1.5 by 1.5 m.