Nutrient Dynamics in Switchgrass as a Function of Time

Round 1

Reviewer 1 Report

Interesting paper analysing the nutrient dynamics in switchgrass, nicely written and worth publishing after some major revisions. Mainly Material and Methods section needs to be revised, since there are some major issues which need to clarified before publication. The analysis in the results section is basic and a more detailed analysis would add value to the paper and the scientific discurs. I believe after improving the results section, you might want to rewrite/improve the discussion section.

My comments in detail are as follows:

• Figure 1: Statistically the shown design (harvest 1 - 6) is wrong, since the harvest times are ordered and not random! Earlier harvest might affect development of plants in later harvested plots and will thereby influence your measurements. Why did you select this design and what did you do to avoid such impacts? Did you consider this during statistical analysis?
• Figure 1: How did you make sure that earlier harvest in one year did not affect crop development in following year? Harvest the subplot in June will have a higher nutrient offtake than after senescence and also yield depression could take place if harvest was too early. If you come back one year later to the same plot this can be a serious problem!
• Figure 1: Sampling BG biomass by digging 90 cm deep, will impact the plants next to it. How did you make sure that sampling 1 plant in this subplot did not affect the development of the plant which is going to be sampled in the next year?
• line 101: plant available N after drying is mainly nitrate, since ammonium will largely vanish during drying. You might have underestimated the plant available soil nitrogen content and overfertilized your crop. Did you consider this during calculation of fertilizer demand?
• Table 2: I don't understand the meaning of the doted lines. E.g. starting regrowth on 28. Feb. 2009 should be considered within the vegetation period 2009 and not allocated to 2008...
• Table 2: Temperature (represented as AGDD) is interesting to understand crop and yield development. However, precipitation sum (from Jan until each harvest date) is missing to fully understand the yield and crop development
• Figure 2: Showing all 4 reps for each harvest time leads to a very crowded figure. I would recommend to show always the average of the 4 reps including error bars for standard deviation. This should be also done for Figure 3.
• Figure 3: It seems that the average nitrogen content in BG biomass at end of vegetation period is increasing from 2008 to 2010. Was the crop fertilized before or fertilization started with this analysis? This could show that nitrogen accumulation took place and the fertilization was higher than the offtake by harvesting the crop
• General remark about discussion: Nutrient concentration is interesting, but telling only half of the story. It is required to at least calculate the total nutrient amount per ha/potential nutrient offtake at each harvest time (yield * nutrient conc.) to understand your data. Without this calculation you cannot identify if decreasing nutrient concentrations where dilution effects (due to ongoing biomass growth) or starting relocation. This needs to be added in this paper.
• Remark 2: If available the paper would improve from additional data/analysis: leaf:stem share at each harvest time and nutrient conc. in leaves and stems would be helpful to also understand the relocation processes in the AG biomass.
• The discussion needs to be improved/rewritten after improving M&M and results section.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript presented by Massey and co-authors reports the translocation of major nutrients and micronutrient from aboveground to belowground as plant growth and matured in switchgrass. Switchgrass has been recognized as an emerging and promising bioenergy feedstock. Understanding switchgrass nutrient cycling or nutrient use strategy is very important for feedstock breeding and development. With three year's measurement on yield and nutrient contents, this study provided a strong and clear evidence that a large amount of major nutrients transported from aboveground to belowground that aiding regrowth in the following spring. The experiment also provides us new information about the nutrient management and harvesting time for different purpose. Overall, the idea here is nice and interesting, and the results are sound. The paper is generally well written and the results should be of interest to readers of Agronomy. There is, however, one criticism to be made. On several occasions in the manuscript, the numbers appear to inconsistent with that in figures and have no data source in the text. Authors must describe the results carefully. In addition, I have several questions that need to be addressed:

L10-13, Overall it is not clear what background were introduced. What is nutrient translocation, from where to where?

L15-16, as I understood, yield data of 13.7 ± 2.9 comes from L167 which is the maximum yield of 2009 (Fig.2). But what about 19.8 ± 1.2, where does it come from?

L17, here and L183. I think the numbers are not correct. Authors said 'Changes in N concentrations ranged from 12.4 to 2.1 g N kg-1 in AG biomass over all study years', but in Fig. 3(a), it should be from ~7 to ~3, and in Fig. 3(b), it should be from ~10 to ~4 and finally in Fig. 3(c), it should be from ~7 to ~3. Please check it carefully and clarify if the data source is correct.

L18, authors showed 3.3 to 13.9 here, but 2.9 to 13.9 in L183, which one is correct? As above, please check carefully if your numbers are correct for each year. Here and the following, authors must describe the results carefully and correctly.

L15-18, these numbers are very confusing, please indicated which year data you used here.

L18-19, what similar trends?

L89, please describe table 1 first in the text.

L110, Table 1, why the NO3-N in the soil showed very big different among three years?

L124, please clarify the meaning of 0.0875 and 0.0017 in Eqn 2.

L129, which part of aboveground sample (leaves or stems?) used for nutrient measurement?

Results, please check the results carefully.

L171-172, this sentence should be in methods or somewhere in the text but not here.

L173, the yield data of 2009 peaked before AGDD2000 and 2010 peaked after AGDD2500.

Panels of figures should be labelled with letters (e.g. (a), (b)) and referred to in the text.

Discussion, too long. Please shorten it.

L279-280, do you have evidence for other C4 grasses. Reference 57 also described switchgrass.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf