Planning the Post-LIDET World of Intersite Decomposition Studies

Mark E. Harmon mark.harmon@orst.edu

Participants: approximately 20

This workshop involved a discussion of the major findings of the LIDET (Long-term Intersite Decomposition Experiment Team) study and current plans for analysis of the resultant data at NCEAS. The bulk of the workshop involved a discussion of possible follow up experiments to LIDET. While LIDET has produced an unprecedented volume of data on long-tem dynamics of carbon and nitrogen during the decomposition of fine litter, it created another set of questions. Specifically, preliminary analysis of the LIDET data indicates that the stable phase of decomposition may not be as stable as anticipated (i.e., rate-constants of 0.1 to 0.3 per year). Unfortunately, the LIDET experiment was not designed to precisely measure dynamics during the stable phase of decomposition and so these rates of stable matter decomposition are only approximate. The original LIDET design assumed that the decomposition of the stable phase would be extremely low (i.e., <0.01 per year), thus the formation rate of the stable fraction could be estimated. However, given the higher rate of stable decomposition estimated, it would appear that formation and decomposition of stable material are occurring simultaneously and at about the same rates.

To gain better insight into the dynamics of this “stable” phase it was agreed that a follow on experiment would be extremely useful. The basic question would be to test the degree that the decomposition and nitrogen dynamics of stable material is controlled by substrate quality and climate. Substrate quality in this context would refer to either the nature of the original detritus (e.g., roots, leaves, wood) or the nature of the environment that formed the stable matter (e.g., tropical, temperate, mineral soil, bog). The basic approach would be to start a litterbag type experiment with stable material, which could be taken from nature or be specifically created for this project. The later would be ideal if ¹³C labeled litter was used. This would allow one to examine where the carbon is going in the soil. Methodological questions arose about the nature of the bag and how stable material might be created. With the ¹³C labeled material it might be possible to not enclose the material at all. Conversely mineral soil might be added to the bag and the stable material used to spike the confined soil. For certain systems (e.g., thick forest floors or extremely rotten wood) simply placing similar material in a bag would probably be the best solution. Given the lack of experience with this sort of material, it would probably make sense to use a variety of approaches.

Several alternative approaches were suggested including the examination of paleosols to examine the endpoints of decomposition. Another approach would be to examine the natural formation of stable material by periodically placing markers (perhaps screens) on the surface of the litter layer. This would result in a temporally defined layering that could examine the changes. This might be coupled with measures of litter input so that a quantitative analysis could be preformed.

Finally there was a discussion of the possibility of improving methods for carbon analysis. The original plan to use proximate analysis to understand changes in organic fractions as decomposition has not worked out. While proximate analysis can be useful tool to characterize initial substrate quality, it is less useful for examination of decomposed matter (although if one is interested in crude indices it might still be useful). Given the value of the LIDT samples it is less than desirable to use crude methods such as proximate analysis to determine changes in organic chemistry. Proximate analysis is especially problematic because it cannot distinguish between stable material that was in the original material versus that was formed during decomposition. As a follow on project it was agreed that ¹³C NMR would be used on a selection of samples to see if this method might give insights into the question of how this stable material is formed.

Recommended Next Steps.

After a more complete analysis of the LIDET data to determine the degree to which stable organic material formation versus decomposition can be assessed, the need for the follow on study can be assessed. In the meantime contacts with possible ¹³C labeled litter will be pursued as will methods for examining decomposition (i.e., bags, loose or confined additions) of this material. Eventually a working group should develop a proposal, but that may take several years given the pre-work required.