Fig. 1

Illustration of experimental plots and layout (above) and an efficient stratified and clustered random sampling design within a plot (bottom; 5.5 m × 2.75 m). Twelve of 24 plots in the experiment were selected for this study by choosing two of four replicated plots under each N fertilization and crop type (i.e., P1 and P2). Each plot was divided into eight subplots (grey zone) and there was a centroid (dark solid circle) in each subplot (1.375 m × 1.375 m), where three soil sampling points (diamonds) were determined from random directions and distances from a centroid in each sampling region (grey area). The extent of an interpolation map was thus determined by the minimum and maximum values at horizontal and vertical axes, and each map can attain its extent less than or equivalent to a plot area. NN, LN and HN denote no fertilization, low and high fertilization treatments; SG: switchgrass; and GG: gamagrass.