This study examined how three tillage treatments (i.e., conventional till [CT], ridge till [RT], and no till [NT]) affected the relationship between soybean yield and the N mineralization of soil organisms. Using two soil depths, the investigators sampled N mineralization and the densities of soil organisms, including soil microbes (bacteria and fungi), nematodes, mites, and springtails. The authors found that food webs in RT and NT treatments released more N than the CT treatment, which appeared to correspond to higher crop yields in the RT and NT treatments. Although the energetic food web modeling is not new (see Moore and de Ruiter 2012 for details), the ecological context is novel and sheds insights into how the soil food web can contribute to N cycling and associated crop yield. It is also useful that the investigators directly measured N mineralization in addition to the energetic modeling; thus, the modeling can be loosely compared to N mineralization rates for each treatment. The manuscript is generally easy to follow, and the replication and methodological approaches appear appropriate.
While the manuscript covers an important topic in soil and agroecology, the presentation of the analyses and results were not as effective, so the study’s novelty and impact are less apparent. More detailed comments, which I hope the authors find helpful, are below.
I think the manuscript can benefit from a discussion (either in the Introduction or Methods) on the utility and limitations of energetic food web modeling. Indeed, these methods have been used for several decades now, but they are not mainstream, and many readers interested in this study will likely be less familiar. From my understanding, energetic food webs are highly parameterized theoretical models that are meant to give crude estimates of C and N cycling rates through food webs; the results are not meant to be interpreted as absolute. Addressing these expectations/limitations upfront should help eliminate the confusion of why so many parameter values are derived from different systems while also elucidating the usefulness of this method.
Furthermore, there should be a figure that illustrates the energetic food web models (similar to Figure S1), ideally showing all three tillage treatments so that they can be compared visually (see Pressler et al. 2018 for example). The line size of each pathway should indicate the strength of the interaction between food web compartments (the ‘igraph’ package in R is one way to do this). I would also suggest eliminating (or reducing) Table 3 that shows the same information or placing that information in the appendix.
I propose adding a separate section for the Sensitivity Analysis under the ‘Statistical Analysis’ heading in the Methods. Explicitly state what was done and if the results are consistent when using different feeding preferences and/or different food web configurations (see Koltz et al. 2018 for example) -- simply stating that “a sensitivity analysis was performed by re-assigning omnivorous collembolans into fungivores and herbivores (50% each)” is not sufficient. A formal sensitivity analysis would help evaluate the robustness of the results in this ecological context. There should be text explaining how it was performed in the Methods, reported in the Results, and discussed in the Discussion.
The various types of tillage treatments need to be clearly defined in both the Introduction and Methods section. What is the difference between conservation tillage vs. conventional tillage? Which one of the experiment treatments is considered to be conservation tillage? Is the NT the control treatment? There appears to be a disconnect between the terminology used in the Introduction and Methods; the terms should be consistent and clear throughout the manuscript. Not all readers will be familiar with agricultural practices.
The hypotheses in the last paragraph of the Introduction need to be developed further. Why do you expect there to be more N release from conservation tillage from conventional tillage? Why would soil depth be pertinent for determining the influence of soil organisms regarding N mineralization?
Overall, the Discussion is long and disorganized. I suggest starting with a short summary of the results, making sure to highlight the key new insights, followed by a discussion of if the hypotheses were supported or not. The middle paragraphs should be used to explain what the findings mean within a broader context, drawing on support (or contrasting support) from the literature. I will reserve the section on study limitations and commentary on the sensitivity analyses for the section right before the Conclusions paragraph.
The Abstract is confusing. I’m not sure what results are from the energetic modeling, N mineralization measurements, or other associated analyses. For example, in Lines 46-48, is this the energetic modeling result, or is this from the biomass estimates of the trophic guilds? In general, the Abstract needs to be summarized more effectively, as the current take-home message is unclear.
The way the experiment is described is somewhat misleading, as at first read it appears that the experiment and its sampling have been going on since 2001, which was not the case. To make the experimental design clearer, I suggest stating the time interval of the tillage treatments (e.g., 2001 – 2015) and mention that this study only sampled for N mineralization and soil organisms in 2015.
The extraction of 120 hours at room temperature is not standard for Berlese-Tullgren funnel methods. What is the justification here, and how do you know that all arthropods were extracted if there wasn’t a heat gradient?
Although there is information on the various groups of soil fauna sampled in Table S1, it would have useful to include the average densities and standard errors for each taxonomic unit according to tillage treatment. Soil core extractions will usually capture springtails, mites, and nematodes – which of these groups and functional guilds were most abundant?
The assumptions of the model are acceptable and similar to previously published models (e.g., Schwarz et al. 2017, Koltz et al. 2018, Pressler et al. 2018). However, because some of the literature values are from very different regions and study systems, the authors should explain why these parameter values are appropriate for this study. Obviously, it would’ve been nice to measure and confirm each parameter value, but that is usually impossible to accomplish for these types of models.
Lines 221-223: I would recommend stating if the N mineralization equations are modified from de Ruiter et al. (1993), and if so, how?
Lines 305-314: The Results don’t do an adequate job in covering how the food web modeling relates to the crop yield. Maybe I missed this, but is there a way to loosely compare the energetic food web modeling to the crop yield data?
Koltz, A. M., A. Asmus, L. Gough, Y. Pressler, and J. C. Moore. 2018. The detritus-based microbial-invertebrate food web contributes disproportionately to carbon and nitrogen cycling in the Arctic. Polar Biology 41:1531–1545.
Moore, J. C., and P. C. de Ruiter. 2012. Energetic food webs: an analysis of real and model ecosystems. Oxford University Press, Oxford, UK.
Pressler, Y., E. J. Foster, J. C. Moore, and M. F. Cotrufo. 2017. Coupled biochar amendment and limited irrigation strategies do not affect a degraded soil food web in a maize agroecosystem, compared to the native grassland. Gcb Bioenergy 9:1344–1355.
De Ruiter, P. C., J. A. Van Veen, J. C. Moore, L. Brussaard, and H. W. Hunt. 1993. Calculation of nitrogen mineralization in soil food webs. Plant and Soil 157:263–273.
Schwarz, B., A. D. Barnes, M. P. Thakur, U. Brose, M. Ciobanu, P. B. Reich, R. L. Rich, B. Rosenbaum, A. Stefanski, and N. Eisenhauer. 2017. Warming alters energetic structure and function but not resilience of soil food webs. Nature climate change 7:895–900.