Four ammonium:nitrate ratios were applied: (a) NH4
+:NO3
− = 0:100 (5 mM NO3
−), (b)
NH4
+:NO3
− = 10:90 (0.5 mM NH4
+ + 4.5 mM NO3
−), (c) NH4
+:NO3
− = 15:85 (0.75 mM NH4
+ + 4.25 mM
NO3
−), and (d) NH4
+:NO3
− = 25:75 (1.25 mM NH4
+ + 3.75 mM NO3
−).