Assumptions and errors in the 15NH4+ pool dilution technique for measuring mineralization and immobilization

Soil Biol. Biochem. Vol. 28, No. 6, pp. 827-828, 1996

Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-0717/96 $15.00 + 0.00

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LETTER TO THE EDITOR ASSUMPTIONS AND ERRORS IN THE “NH,+ POOL DILUTION TECHNIQUE FOR MEASURING MINERALIZATION AND IMMOBILIZATION JOSHUA

SCHIMEL

Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, U.S.A. (Accepted

19 January

Nitrogen mineralization is critical in controlling many ecosystem processes. While net N mineralization measures the N available for plant uptake or loss, net mineralization is the difference between gross mineralization and immobilization. A full understanding of net N availability therefore requires understanding the balance between gross mineralization and gross immobilization. Measuring the rates of these processes requires using an “N isotope pool dilution approach (e.g. Schimel, 1985; Myrold and Tiedje, 1986; Nishio and Fujimoto, 1989; Schimel et al., 1989; Davidson et al., 1991; Tietema and Wessel, 1992; Hart et al., 1994). In a pool dilution approach, the product pool is labelled and its size and enrichment are measured over time [Fig. l(a)]. When examining NH,’ turnover, production supplies “‘N to the NH,+ pool, diluting the lSN, while NH,’ consumption removes both 14Nand ‘“N. This allows us to set up two simultaneous equations: d’14 + “)N/dt d”N/dt

=

=

production

-

consumption

(1)

consumption x(“N/(‘~N + “N))

(2) In these equations, we can measure all of the parameters except for the production and consumption rates. Since that leaves two equations and two unknowns we can solve for the production and consumption rates individually. There are a number of approach’:s to setting up and solving these equations but that of Kirkham and Bartholomew (1954) remains the simplest and probably the most commonly used (Hart et al.. 1994). In applying pool dilution approaches to measuring N mineralization and immobilization however, there are a number of assumptions that must be considered and met.

1996)

or fate (immobilization, Add ‘%H,’ label

,,n~,!$~etc,

Fig. 1. The pool dilution approach to measuring NH; production and consumption. Consumption is shown as a set of diverging arrows because they are measured together as ‘consumption’, though they represent all possible fates that remove NH: from the sample.

The most critical is that lSNH4+ pool dilution does not actually measure mineralization and immobilization. It measures merely the production and consumption of NHf , without regard for spec@ic processes. There are several implications for this. First, heterotrophic nitrification (converts organic N directly to NO; or NO; ) is mineralization that is not measured by NH,+ pool dilution, though this can usually be ignored without serious error. Second, NH: consumption is not immobilization since nitrification, volatilization, and other processes remove NH,’ from the sample (defined here as consumption) as well. Partitioning the fates of NH,+ requires a combination of tracer approaches (e.g. Schimel et al., 1989). Another key assumption of pool dilution approaches is that the labelled N behaves the same as and is uniformly mixed with the unlabelled N (Kirkham and Bartholomew, 1954; Davidson et al., 1991).

Table 1, Calculated rates of NH,’ production on theoretical data for a range of inorganic-N pool sizes and nitrification rates. Correct NH> production is calculated Jsing only the NH,t pool data. Incorrect NH: production is calculated by assuming that while ‘IN is added to only the NH: pool the NH; + NO; pool is analyzed as a single pool. In all calculations it is assumed that 2 pg “NH: g- ’soil was added and that initial “NO; was 0. Rates are calculated using the equations of Kirkham and Bartholomew (1954) Initial Concentrations Conditions Moderate NO, moderatl: nitrification Low NO, no nitrification Low NO; high nitrification High NO< high nitrification High NO, no nitrificaticm High NO<, nitrification, mineralization

“NH:

Final concentrations 14N0,

Initial 15N0,

Correct Incorrect

Error (%)

14NO;

14NH;

5

5

6.1

5.74

I

0.24

4.99

7.1

41.8

5 5 5 5 5

0.2 0.2 10 10 10

6.1 2 2 6.1 6.1

0.2 3 14 10 14

1 0.4 0.4 1 0.4

0 1 1.2 0

4.99 2.32 2.32 4.99 10.36

5.1 1.6 4.5 11.9 11.3

2.8 29.9 92.6 139.0 9.4

827

15NHf

Calculated rates of NH; production

I

Letter to the Editor

828

It has come to my attention that several workers have made the error of adding “NH: but measuring 15Nin NH: + NO,. This comes from thinking that you can use the definition of net mineralization - the production of inorganic N - in setting up a pool dilution analysis of gross mineralization; you cannot. Trying to do the assay this way introduces a potentially large error into the calculations. The error results from violating the assumption of a uniformly-labelled pool that all behaves the same way. When 15Nis added to NH: the NH,+ + NO< pool is not uniformly labelled, and NH.,+ and NO, do not behave identically. The range of error that can be introduced by misapplying the pool dilution method is illustrated in Table 1. These calculations indicate that low NO, concentrations minimize the error from misapplying the method, and that high nitrification rates also minimize the error. Low NO< concentrations means that the total inorganic pool is dominated by NH: and the uniform label assumption is close to accurate. Rapid nitrification brings the enrichment of the NH: and NO, pools closer together, also making the uniform labeling assumption closer to accurate. The overall problem remains that if this method is carried out incorrectly, extremely large errors are possible and the error varies with the balance of NH4+ and NO< pool sizes and with the rates of mineralization, immobilization, and nitrification. It is dangerous to use a pool dilution N turnover technique without fully understanding the assumptions behind the model used to calculate NH: production and consumption.

immobilization, and nitrification by 15N isotopic pool d&t&n,in intact soil cores. Journal of Soil Science 42, Hart S. C., Stark J. S., Davidson E. A., and Firestone M. K. (1994) Nitrogen mineralization, immobilization, and nitrification. In Methods of Soil Analysis: Part 2microbiological and biochemical properties. (Weaver, R. W., Angle: C., Bottomley, P., Bezdicek, D:, Smith, S., Tabatabai. A. and Wollum. A.. Eds.). DD.9851018. Soil Science Society of America, Madison.- 1 Kirkham D. and Bartholomew W. V. (1954) Equations for following nutrient transformations in soil, utilizing tracer data. Soil Science Society of America Proceedings 18, 33-34.

Myrold D. D. and Tiedje J. M. (1986) Simultaneous estimation of several nitrogen cycle rates using 15N: theory and application. Soil Biology & Biochemistry 18, 559-568.

Nishio T. and Fujimoto T. (1989) Mineralization of soil organic nitrogen in upland fields as determined by a “NH: dilution technique, and absorption of nitrogen by maize. Soil Biology & Biochemistry 21, 661665.

Schimel D. S. (1985) Carbon and Nitrogen turnover in adjacent grassland and cropland ecosystem. Biogeochemistry 2, 345-357.

Schimel J. P., Jackson L. E. and Firestone M. K. (1989) Spatial and temporal effects on Plant-microbial competition for inorganic nitrogen in a California annual grassland. Soil Biology & Biochemistry 21, 1059-1066.

REFERENCES

Davidson E. A., Hart S. C., Shanks C. A. and Firestone M. K. (1991) Measuring gross nitrogen mineralization,

Tietema A. and Wessel W. W. (1992) Gross nitrogen transformations in the organic layer of acid forest ecosystems subjected to increased atmospheric nitrogen input. Soil Biology & Biochemistry 24, 943-950.