|The revised manuscript has addressed some of the concerns raised previously, but critical flaws remain in data interpretation and presentation. Consequently, I cannot recommend this paper for publication. |
Any description and discussion of the results needs to make it abundantly clear that differences in VOC emission behavior observed between the different experimental conditions cannot be directly compared because the same leaf litter sample was used throughout, and emissions will change over time as litter decomposes. I understand seasonal constraints on these types of experiments very well, but the interpretation of results needs to be provided within an appropriate context. These 3 different conditions can be presented as 3 entirely separate trials, highlighting that the same litter is being used throughout so the BVOC emissions were changing over time completely independent of the experimental condition imposed. The emissions at the beginning of each condition were likely very different even before the UV, O3, or UV_O3 was applied, so making comparisons about how O3, UV, or UV_O3 influence BVOC emission behavior on leaf litter that was 2 days old at the start of one condition, 9 days old at the start of another condition, and 15 days old at the start of another condition isn’t meaningful (I’m guessing on the ages because that information was not provided clearly). The interpretation and discussion of results is still very focused on making comparisons between the different conditions instead of looking at each condition as a completely separate trial using litter with very different starting characteristics at the beginning of the experiment.
PCA analysis: the authors state that the VOC emission profiles of the different conditions were “strongly different”. First, the time/decomposition component cannot be separated out from the experimental condition component. We need to see what the emissions looked like at the start of each trial BEFORE the experimental condition was imposed (essentially a comparison of the "initial emissions" for each condition). Second, PCA does not provide any measure of statistical significance and cannot be used to make any statements about the strength of difference between samples. You can use a PCA to show clustering, but statistical significance has to be measured with a different approach, such as an analysis of variance (ANOVA).
Figures 4, 5 and 7 are not publication quality at the standard generally seen in ACP.
Now that it is clear the same leaf sample was used for each condition, there are other ambiguities in the methods section that need to be clarified. In Section 2.1 on “sample collection”, how many leaves/branches were collected? From how many different plants? It says a “random sampling method” was used, which made more sense before it was clear that the same set of leaves was used throughout the experiment. What does “random sampling method” mean in this context? Were a certain number of leaves collected from a random sampling of plants? Or were X number of leaves randomly sampled from the same plant? This is still unclear.
Now that it is clear the same sample was used for each condition, Table 1 should indicate the dates for each condition. The leaves would have been decomposing over time (and their emissions changing as a result). It would be useful to have some sense for the “age” of the leaf litter during each condition.
Section 2.4 Experimental Set-up should make is explicitly clear that the same leaf litter is being used for all three experimental conditions studied. In my opinion, there is still some ambiguity about that as written.