Fig 7. Contour maps of root length density (left) and soil mineral N (Nmin, right) concentration of three maize varieties from China (ZD 958 and XY 335) and US (P32D79) in a soil volume of 70 cm × 30 cm to a depth of 60 cm.
The combined charts represented the distribution of root length density or soil Nmin concentration in each soil layer, and grey scale legends indicated the relative value range. The soil samples were taken using the monolith method (Böhm, 1979) with 10 cm3 soil block at silking in 2011. Each soil layer contained 21 soil blocks (7 × 3, the value of each soil block was the mean of three replicates) and each root system was harvested in 126 soil blocks.