• Moll, R.H., E.J. Kamprath, and W.A. Jackson. 1982. Analysis and interpretation of factors which contribute to efficiency to nitrogen utilization. Agron. J. 74:562-564.

• Olson, R.V., and C.W. Swallow. 1984. Fate of labeled nitrogen fertilizer applied to winter wheat for five years. Soil Sci. Soc. Am. J. 48:583-586.

Olson and Swallow, 1984 (27-33% of the applied N fertilizer was removed by the grain following 5 years)

Consideration of additional parameters not discussed in Moll et al. (1982) *plant N loss 

• Maximum N accumulation has been found to occur at or near flowering in wheat and corn and not at harvest. 
• In order to estimate plant N loss without the use of labeled N forms, the stage of growth where maximum N accumulation is known to occur needs to be identified. 
• The amount of N remaining in the grain + straw or stover, is subtracted from the amount at maximum N accumulation to estimate potential plant N loss (difference method). 
• Use of difference methods for estimating plant N loss are flawed since continued uptake is known to take place beyond flowering or the point of maximum N accumulation.

Enriched 15N:

Materials with a greater than natural concentration of 15N % plant N derived from fertilizer = %15N excess in sample  % 15N excess in fertilizer
 

Depleted 15N:

Materials with a lower than natural concentration of 15N (0.003 - 0.01 atom % 15N) or (< 0.01 atom % 15N)

• Use of isotopic 14N

• Studies involving residual soil nitrogen are not practical with depleted materials due to the high dilution factor.
 

% plant N derived from the fertilizer =

(Nu - Nt)/(Nu - (Nf/n))
Nu =atom % 15N in unfertilized plants
Nt = atom % 15N in fertilized plants
Nf = atom % 15N in the fertilizer (for example 0.006%)
n = the plant discrimination factor between 14N and 15N.

If it is assumed that there is no discrimination between 14N and 15N, then n = 1.

1. Difference method

PFR = (NF)-(NC) / R

NF = total N uptake in corn from N fertilized plots
NC = total N uptake in corn from unfertilized plots
R = rate of fertilizer N applied
PFR = percent fertilizer recovery

2. Isotopic method (Depleted material)

PFR = ((NF) x (C-B)/D) / R

NF = total N uptake in corn from N fertilized plots
B = atom % 15N of plant tissue from N fertilized plots
C = atom % 15N of plant tissue from unfertilized plots (0.366%)
D = depleted atom % 15N in applied N fertilizer
R = rate of applied 15N-labeled fertilizer

3. Isotopic method (Enriched material, Sanchez et al., 1987)

F = As-Ar/Af-Ar
F= fraction of total N uptake derived from 15N enriched fertilizer
As = atom % 15N measured in the harvested plant sample
Af = atom % 15N in the enriched fertilizer
Ar = atom % 15N of the reference harvested plant material from non 15N enriched fertilizer treatments

Ef = F x total N uptake
Ef = uptake of 15N enriched fertilizer

Shearer and Legg (1975) found that d15N of wheat plants decreased as the N application rate increased.

d15N = atom % 15N (sample) - atom % 15N (standard) x 1000 / atom % 15 N (standard)

15N composition of the total N of grain and leaf samples of corn (Zea mays L.) decreased systematically as N fertilizer rates increased (Kohl et al., 1973). This result was considered to be consistent with increasing contributions of fertilizer N to plants as the rate of applied N increased.

Hauck and Bremner, 1976

percent nitrogen recovered (plant or soil) =

= 100P (c-b) / f(a-b)

P = total N in the plant part or soil in kg ha-1
f = rate of 15N fertilizer applied
a = atom percent 15N in the labeled fertilizer
b = atom percent 15N in the plant part or soil receiving no 15N
c = atom percent 15N in the plant part or soil that did receive 15N

unlabeled N uptake = (total N uptake in grain and straw) -

[N rate(% recovery of 15N in grain and straw)]

Davis, R.L., J.J. Patton, R.K. Teal, Y. Tang, M.T. Humphreys, J. Mosali, K. Girma, J.W. Lawles, S.M. Moges, A. Malapati, J.Si, H. Zhang, S. Deng, G.V. Johnson, R.W. Mullen, and W.R. Raun. 2003. Nitrogen balance in the Magruder Plots following 109 years in continuous winter wheat. J. Plant Nutr. 26(8):1561-1580.

Raun, W.R. and G.V. Johnson. 1999. Improving Nitrogen Use Efficiency for Cereal Production. Agron. J. 91:357-363

Moll, R.H., E.J. Kamprath, and W.A. Jackson. 1982. Analysis and interpretation of factors which contribute to efficiency to nitrogen utilization. Agron. J. 74:562-564.

Olson, R.V., and C.W. Swallow. 1984. Fate of labeled nitrogen fertilizer applied to winter wheat for five years. Soil Sci. Soc. Am. J. 48:583-586.

Vetsch, Jeff, and Gyles Randall. 2004. Corn production as affected by nitrogen application timing and tillage. Agron. J. 96:502-509.

Varvel, G.E., and T.A. Peterson. 1990. Nitrogen fertilizer recovery by corn in monoculture and rotation systems. Agron. J. 82:935–938.