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
−
).