These authors evaluated the use and limitations of the isotope-pool dilution technique when studying nitrogen dynamics in soil. Because addition of inorganic nitrogen compounds (NH4+, NO3-) can stimulate microbial activity in N-limited systems such as most soils, estimating the rate of these processes by tracking 15N through a system will almost certainly overestimate these rates. The isotope-pool dilution method, on the other hand, measures the dilution of enrichment in the nitrogen pool at the end of a particular process, relying on the assumption that additional product of metabolism will have negligible effects on the magnitude of that metabolism. In this study, immobilization of nitrogen was the main focus of investigation, comparing 15N isotope dilution in pools of either 15NH4+ or 15NO3-.
There are three key assumptions for the isotope-pool dilution method in this context. 1. Microorganisms do not discriminate between 15N and 14N; 2. rates of processes measured remain constant over the incubation period; 3. 15N assimilated during the incubation period is not remineralized. Previously, these assumptions had been evaluated for well-mixed soils, but not for unmixed field-collected soil samples. While fractionation by biological processes certainly does result in discrimination between isotopes of nitrogen, it is of negligible importance when injected solutions are very highly enriched and incubation periods are relatively short; in this case, injections were more than 90% 15N and incubations ran for 24 hours. Rates of measured processes will change if the population and / or activity of microorganisms changes, but again, over a 24-hour incubation period under controlled conditions this is unlikely. Highly enriched injections allow the use of small injection volumes, limiting the impact of nutrient enrichment. These authors were able to measure the remineralization of immobilized 15N, and estimated that between 1.0 and 1.6% of injected 15NH4+ appeared in the 15NO3- pool after 24 hours; they consider this an insignificant amount, but caution that longer incubations would almost certainly result in much more problematic amounts of remineralization.
This paper is clearly a major part of the basis of the project I am currently engaged in with Katherine from our 2009 field season at Alexandra Fjord. I probably should have read this paper long ago. The three main conclusions stated by these authors at the end of their paper I think can be quoted verbatim as justification for both why I (should have earlier) read this paper, and as a reminder to myself to include this paper in the methods & materials section of the eventual manuscript.
"Three points should be considered when applying the isotope dilution method.
1. Accurate estimation of both 14N and 15N initial pool sizes is important. Abiotic consumption of label, such as by clay fixation, can cause significant errors. A subset of intact cores may need to be destructively sampled directly after adding 15N for estimation of initial pool sizes.
2. Homogeneity of 15N enrichment throughout a soil sample is not possible, and perfectly uniform distribution of added label is not necessary. However, significant errors can arise from a bias in 15N distribution that is concurrent with a non-random
distribution of microbial processes. Distribution of label should, therefore, be as uniform as possible.
3. In situ gross immobilization rates may be overestimated by isotope dilution methods and underestimated by chloroform fumigation methods, depending on which (if any) kN factor is applied to the latter. Gross mineralization and gross nitrification estimates from isotope dilution are more reliable because these rates should not be affected by addition of 15N label in the form of the process products."
No comments:
Post a Comment