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Potentially Mineralizable Nitrogen in Soil: The Simple Exponential Model Does Not Apply for the First 12 Weeks of Incubation
Author(s) -
Molina J. A. E.,
Clapp C. E.,
Larson W. E.
Publication year - 1980
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1980.03615995004400020054x
Subject(s) - environmental science
I. C. R. Holford has suggested that the Langmiur adsorption isotherm data of Green et al. (1978) and McCallister and Logan (1978) is misinterpreted because no correction was made for native adsorbed phosphate. Holford suggests that what was measured was residual adsorption capacity and that the "checkmark" phenomenon found in our Langmuir isotherms is eliminated when correction is made for native adsorbed P. It was our intention to measure only residual adsorption capacity, and therefore, our reference to the Olsen and Watanabe (1957) procedure was misleading since their procedure did correct for surface adsorbed P by "P exchange. We were using the Langmuir isotherm to examine the relative sorption of P by soils and sediments, a process determined by the sorption characteristics of the soil or sediment, but also by the degree to which the adsorption capacity had already been saturated. I would concur with Holford that the term residual adsorption capacity be used in instances where no correction is made for "native", or more correctly, "previously" adsorbed P. Holford has also suggested that, if Fig. 1 of McCallister and Logan (1978) is corrected by adding 50 /ig P/g (Holford's estimate of native adsorbed P, and one he considers conservative), then the data is better fit to a two-phase Langmuir model than to a single line. Using the 50 /ig P/g estimate which we feel should be closer to 25 /ig P/g (based on desorption and Bray PI extractable P data for the Hoytville soil in Fig. 1 of McCallister and Logan), we find that either a singleor two-phase model is satisfactory in describing the data. The single-phase model has an r* of 0.976, while the r values for the two-phase Langmuir isotherms were 0.978 and 0.987. The two-phase model has greatest utility at low equilibrium phosphate concentrations, but at this range, the native adsorbed P has its most significant effect on net adsorption. Holford has correctly recognized the failure of Green et al. (1978) and McCallister and Logan (1978) to correct Langmuir adsorption data for native adsorbed P. We contend that our omission was not in clearly indicating our intention to measure residual adsorption capacity, and we laud the effort of Holford in bringing this to the readers' attention.