Title: Response to "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'"
Submitted to: Soil Science Society of America Journal
Publication Type: Other
Publication Acceptance Date: October 3, 2006
Publication Date: January 8, 2007
Citation: Jaynes, D.B., Olk, D.C., Colvin, T., Kaspar, T.C., Karlen, D.L. 2007. Response to "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'". Soil Science Society of America Journal. 71(1):255.
We thank Dr. Greenfield for his insightful comments and the opportunity to reiterate our understanding of our amino acid results and to correct an error in our original paper. In the limited studies completed before Dr. Marten's untimely death, the MSA extraction method coupled with ion chromatography and pulsed amperometric detection has been found to give as good or better recoveries of amino acid-nitrogen (N) from soils as hydrochloric acid (HCl) extraction (Martens and Loeffelmann, 2003). The extraction method also results in considerable ammonium (NH4)-N being released either from fixed or exchangeable NH4 or as a result of N-compound degradation during the extraction process. Thus, the resulting amino acid and amino sugar soil extractions are probably low estimates of their true value in soil because of a combination of incomplete extraction and losses during extraction. As pointed out in Martens and Loeffelmann (2003), the optimum extraction time for soils appears to depend on the clay fraction and particularly in differences in parent material. From their results, it is obvious that the optimum extraction time also depends on the specific amino compound, thus using a single extraction time to recover amino compounds for any soil represents a compromise between extraction efficiency and amino compound degradation.
Dr. Greenfield is correct in making the observation that the amino sugar-N values (as well as amino acid-N values) we reported were not corrected for extraction efficiency or degradation losses. Correction factors for compound degradation and imperfect extraction efficiency are often determined based on recovery of spiked model compounds. Such an approach is not clearly relevant in this case: soil amino sugars might exist in a number of chemical forms, ranging from undegraded chitin to short chain oligomers that have been incorporated into the soil organic matter. Rather than attempting to correct for these losses, our paper shows that the extraction and detection of these compounds using the MSA extraction method in a consistent manner can be related to the crop yield variability and contribution of soil N to yield.
We would also like to correct an error in our Table 3. The values for the amino sugars were inadvertently included when computing the total amino acid content of the soils. Several other small computational errors were made in Table 3 resulting in the average ratio of organic-N to total-N ratio reported in Table 3 decreasing from 84.6% to 83.8%