Fig. 4: Temporal changes in explanatory variables affecting the optimal N rates.
From: The optimum nitrogen fertilizer rate for maize in the US Midwest is increasing
Panels represent changes in grain yield (A, B), yield response to nitrogen (N) (C), N fertilizer use efficiency (D), N loss (E), soil net mineralization (F), kg N per kg grain ratio (G), grain N (H), and precipitation (I). The N loss and N mineralization were derived from APSIM simulations (see Methods section). Variables that included crop rotations represent continuous maize with red lines and maize-soybean with blue lines, while tan boxplots and a black line represent precipitation independent of crop rotation. Slopes refer to the change over time per variable, while values in parentheses represent the slope’s relative change. The full regression equations are given in Table S4. Lower, mid, and upper hinges of the box correspond to the 25th, 50th, and 75th percentiles, whiskers extend to 1.5 times the interquartile range, and the dots are outliers. Within each boxplot there were 8, 11, 11, 12, 13, 14, 14, 14, and 7 observations for the years of 1999, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, and 2009 onward respectively.
