Table I.

Calculation of NH₄⁺ fluxes into and out of maize roots

Measurement	Volume	[NH4+]	Atom % 15N	I 14NH4+	II 15NH4+	Rate
	L	mm	%	μmol 160 L−1		μmol plant−1 10 h−1
A. Initial (9 pm)	160	0.223	96.10	1392	34,288
B. Injection	0.09756	113	99.80	22	11,002
C. Final (7 am)	160	0.231	93.55	2384	34,576
D. Change (C − A − B)				+970	−10,714
E. Mean concentration (A + C)/2				1888	34,432
F. Net 15NH4+ uptake (DII/227 plants)						47.2
G. 14NH4+ uptakea(F[EI/EII])						2.6
H. Total NH4+ uptake (F + G)						49.8
I. Net 14NH4+ release (DI/227 plants)						4.3
J. “True” 14NH4+ releaseb(I + G)						6.9

Rates of ¹⁵NH₄⁺ uptake, ¹⁴NH₄⁺ uptake, and ¹⁴NH₄⁺ release by 227 maize seedlings were calculated from [¹⁵NH₄⁺] and [¹⁴NH₄⁺] in the nutrient solution at the beginning (9 pm) and end (7 am) of the first 10-h dark period, during which 97.56 mL of 113 mm ¹⁵NH₄⁺ (99.8 atom % ¹⁵N) was injected to maintain the total [NH₄⁺] close to its initial level (0.223 mm).

^a14NH₄⁺ uptake can be estimated from net ¹⁵NH₄⁺ uptake because the two isotopic species are taken up in proportion to their mean molar concentrations in the nutrient solution during the uptake period. 

^bNet ¹⁴NH₄⁺ release = true ¹⁴NH₄⁺ release − ¹⁴NH₄⁺ uptake. Therefore, true ¹⁴NH₄⁺ release = net ¹⁴NH₄⁺ release + ¹⁴NH₄⁺ uptake.