Published online 1 January 2007
Published in Soil Sci Soc Am J 71:254 (2007)
DOI: 10.2136/sssaj2006.0286l
© 2007 Soil Science Society of America
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COMMENTS & LETTERS TO THE EDITORS
Comments on "Need for a Soil-based Approach in Managing Nitrogen Fertilizers for Profitable Corn Production" and "Soil Organic Nitrogen Enrichment Following Soybean in an Iowa Corn-Soybean Rotation"
Laurence Greenfield*
School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
* Corresponding author (laurence.greenfield{at}canterbury.ac.nz).
Both of these interesting papers unfortunately contain information that is at variance with current knowledge about the chemistry of soil organic N and require comment.
From Tables 1, 2, and 3 of Mulvaney et al. (2006), one can calculate that the % value of Illinois soil N test (ISNT) N level/Soil total N level in nonresponsive and responsive soils is 19% (range 13–39, n = 33) and 18% (range 8–41, n = 69), respectively. In several cases in Table 1 where lower and upper total soils N values are given if upper rather than lower levels are used then the corresponding % values become 15 (range 9–39) and 12 (range 7–41).
The nature of this alkali-labile N fraction, which is substantial, is believed to be largely amino sugar N (Khan et al., 2001, Mulvaney et al., 2001), but this is unlikely for the following reasons. Enough is known about the chemistry and biology of soil organic N (Bremner, 1965; Stevenson, 1982; Greenfield, 2001) to suggest that (a) correction factors need to be applied to correct for amino sugar losses during acid hydrolysis; (b) the bulk of the amino sugars in soil originate largely from polymeric, for example, chitin rather than monomeric, amino sugars occurring in fungi and invertebrates. Chitin remains insoluble during hot alkali, but not acid hydrolysis (Foster and Webber, 1960; Bremner, 1965; Stevenson, 1982; Greenfield, 2001).
The ISNT was developed without due attention to these points and although reducing environmental and fiscal damage is a sensible goal in land management it may be wise to better ascertain the chemical basis of the ISNT before widespread adoption.
From Table 3 of Martens et al. (2006) one can calculate that approximately 35% (range 15–57, n = 50) and 6% (range 2–10, n = 50) of the total soil N occurred as hydrolyzable NH4–N and amino sugar-N, respectively. The NH4–N values particularly those in excess of 35% considerably exceed the "normal" values of 20–25% found in temperate soils (Bremner, 1965; Stevenson, 1982) and in the absence of data concerning preexisting levels of exchangeable and nonexchangeable NH4–N in the soils used must be treated with caution. The amino sugar-N values reported in this paper, like those in Martens and Loeffleman (2003), have not been corrected for hydrolytic losses. A similar case applies to the uncorrected amino sugar N values reported by Khan et al. (2001) and Mulvaney et al. (2001).
Received for publication February 27, 2006.
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REFERENCES
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- Bremner, J.M. 1965. Organic nitrogen in soils. p. 93–132. In W.V. Bartholomew and F.E. Clark (ed.) Soil nitrogen. Agron. Monogr. 10. ASA, Madison, WI.
- Foster, A.B., and J.M. Webber. 1960. Chitin. Adv. Carbohydrate Chem. 15:371–393.
- Greenfield, L.G. 2001. The origin and nature of organic nitrogen in soil as assessed by acidic and alkaline hydrolysis. Eur. J. Soil Sci. 52:575–583.[CrossRef]
- Khan, S.A., R.L. Mulvaney, and R.G. Hoeft. 2001. A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. p. 29–46. In R.G. Hoeft (ed.) Illinois Fertilizer Conference 2001. Proc. Coop. Ext. Serv., Univ. of Illinois, Urbana-Champaign.
- Martens, D.A., and K.L. Loeffleman. 2003. Soil amino acid composition quantified by acid hydrolysis and anion chromatography-pulsed amperometry. J. Agric. Food Chem. 51:6521–6529.[CrossRef][ISI][Medline]
- Martens, D.A., D.B. Jaynes, T.S. Colvin, T.C. Kasper, and D.L. Karlen. 2006. Soil organic nitrogen enrichment following soybean in an Iowan corn-soybean rotation. Soil Sci. Soc. Am. J. 70:382–392.[Abstract/Free Full Text]
- Mulvaney, R.L., S.A. Khan, R.G. Hoeft, and H.M. Brown. 2001. A soil organic nitrogen fraction that reduces the need for nitrogen fertilization. Soil Sci. Soc. Am. J. 65:1164–1172.[Abstract/Free Full Text]
- Mulvaney, R.L., S.A. Khan, and T.R. Ellsworth. 2006. Need for a soil-based approach in managing nitrogen fertilizers for profitable corn production. Soil Sci. Soc. Am. J. 70:172–182.[CrossRef][ISI]
- Stevenson, F.J. 1982. Organic forms of soil nitrogen. p. 67–114. In F.J. Stevenson (ed.) Nitrogen in agricultural soils. Agron. Monogr. 22. ASA and SSSA, Madison, WI.
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