Patterns of Needle Nutrient Resorption and Ecological Stoichiometry Homeostasis along a Chronosequence of Pinus massoniana Plantations
Round 1
Reviewer 1 Report (New Reviewer)
L 18-19: You did not mention soil organic carbon before, but nutrients in plant tissue. What was important for this work, plant nutrients or SOM, or both?
L24: change the word “and “ by “while”
L:100 Be more specific about the soil type. What it is? an Alfisol, Ultisol (Soil taxonomy) or Acrisol, Luvisol, Lixisol (WRB)??
Table 1: "Canopy density" is this a fraction index, m2/m2, I suppose.
Table 1. “Soil mean water content” This information is not useful. Soil water content is extremely dynamic. Indicate Water constants at field capacity or wilting point, which reflect inherent soil properties.
Table 1. Your C/N ratios are quite low. Revise. Is there any explanation?? Check this reference: Tian, H., Chen, G., Zhang, C., Melillo, J. M., & Hall, C. A. (2010). Pattern and variation of C: N: P ratios in China’s soils: a synthesis of observational data. Biogeochemistry, 98, 139-151.
Figure 3. Axis label: Resorption.
L 291-294: In your discussion you say that the variation of NRE and PRE are explained by the thinning practices in the first stage of the plantations, but you say don’t mention anything about the natural close of the canopy as the trees grew. Once the canopy is close, litter maas production and nutrient resorption may be also affected. You should discuss this point also.
L 384-387: This is no clear. I did not understand what you wanted to say. Please revise.
Other minor comments in the attached file.
Comments for author File: 
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Author Response
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Reviewer 2 Report (New Reviewer)
In the Introduction, it is desirable to indicate the rotation period for P.massoniana plantations.
L.72. It is possible that 106 is better replaced by a million.
L.255-257. “As we know, 30% of trees were removed during the 10 and 20 years of the plantations in this study area, which made the amount of litter decrease dramatically.” It would be desirable to measure the litter amount under 10, 20, 30, and 36-year-old trees.
There are 62 references in the text, but only 61 in the References.
“The previous researcher indicated that soil C concentration mainly depended on the quantity of organic matter and the size of the soil humus [37].”
37. Almeida, L.M.; Mise, K.M. Diagnosis and key of the main families and species of South American Coleoptera of forensic importance. Rev. Bras. Entomol. 2009, 53, 227–244. ???
“Pinus tabulaeformis [44], and Robinia pseudoacacia [45] along the chronosequence”.
43 Zhang, W.; Qiao, W.; Gao, D.; Dai, Y.; Deng, J.; Yang, G. Relationship between soil nutrient properties and biological activities along a restoration chronosequence of Pinus tabulaeformis plantation forests in the Ziwuling Mountains, China. Catena 2018, 485 161, 85–95.
44 Zhang, W.; Liu, W.; Xu, M.; Deng, J.; Han, X.; Yang, G.; Feng, Y.; Ren, G. Response of forest growth to C:N:P stoichiometry in plants and soils during Robinia pseudoacacia afforestation on the Loess Plateau, China. Geoderma 2019, 337, 280–289.
Here are the additional comments:
The work studies the change in the C:N:P stoichiometry in soil and in needles of Pinus massoniana on plantations of different ages (from 10 to 36 years). The study is original, since stoichiometry was studied only on young (8-year-old) plants of P. massoniana. However, an age limit of P. massoniana plantations must be specified in the Introduction.
The study showed the similarity of changes in C, N, and P concentrations in needles of P. massoniana with other conifer species, Cunninghamia lanceolata and Larix kaempferi. NRE and PRE exhibited the same change trend. The conclusions are consistent with the results.
L. 127-129. “The senesced needles were also collected from the same branches and mixed to form a litter needle sample.” Were the senesced needles collected before natural fall? It would be more correct to collect natural litter, for example, in a mesh bag on a branch.
In the captions to Tables 1 and 2, the data format must be indicated: mean ± SD or mean ± SE.
Author Response
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Reviewer 3 Report (New Reviewer)
The authors investigated the nutrient resorption efficiency and stoichiometry balance of carbon, nitrogen, and phosphorus concentrations in soil and in fresh and senesced needles along a chronosequence of Pinus plantations during 36 years with 10-year intervals. The topic is of wide international interest and is appropriate for the journal. An entire idea is discussed. This revised version is missing minor points that should be considered before acceptance.
Minor points:
Line 151, Correct the equation as
NuRE = (((Nug-Nus)*MLCV)/Nug)*10, use brackets to correct the equation.
Lines 152-153, correct as "[34], and Nug and Nus represented"
Figure 2, use the small letter (x) in stead of (X) in the linear equations.
Author Response
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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Experimental
An analysis along age gradient asks for a 6-10x replication of the experimental plots in space. The plots were replicated but within a very short distance which is a pseudoreplication. The gradients observed can therefore not be separated from a spatial ecological gradient. Thus, the main question asked in this paper cannot be answered with the setup of the experiment.
There is no reference to needle age. Pinus massoniana has distinctly separated needle age classes. In the paper, only “fresh” and “senescent” needles are mentioned. When the nutrient transfer between needles of different age classes is a topic the needle age class has to be assigned clearly (e.g. current year needles vs. 1 year old needles or 2 year old needles).
General comments
It seems that the soil samples were taken in fixed thicknesses, not by soil layer. Thus, the interpretation of the soil chemical analysis is awkward and a comparison of different sites is impossible.
The expression “homeostasis” means that something is kept constant. In the paper it is rather used for a missing correlation between soil analysis and leaf nutrient concentration. This is not necessarily the ability of the tree to cope with a variety of different conditions but can also be interpreted as a method of soil analysis which is not appropriate as indicator for nutrition.
The authors discuss the cycling and enrichment of N and P. They do, however, not discuss the source of these elements. How large is the atmospheric nitrogen deposition at the studied site? What is the pedogenic contribution of P? Without this knowledge the discussion on “redistribution” and “enrichment” lacks the basis. It is also not mentioned what kind of mycorrhiza the examined tree species have. On lines 258-259 the discussion suggests that the source of the nutrients is plant litter. Litter is not a primary nutrient source but just cycling.
Many references are cited without a closer explanation of the tree or plant species involved which makes the statements too general. E.g. the statements on lines 62-67 must be differentiated for the plant species or ecosystem involved. The discussion on lines 291-293 needs statements on only on plant species but also on site conditions.
Detailed comments:
Line 123-125: The branches ware taken from the middle of the crown which is not the standard procedure for nutrient analysis in trees. It should be part of the light exposed crown.
Line 153-154: A general factor for mass loss correction was used. Such a factor must be determined for the species under examination and in the examined stands.
Lines 145-146: Indicate the quality control for needle analysis.
Line 193: It is not clear what is meant with “stoichiometric ratios”. The weight by weight ratio? The molar by molar ratio?
Line 224-225: PRE is calculated from P concentration so a relation between PRE and P is not relevant.
Reviewer 2 Report
The study is generally well designed and methods used are appropriate and standard. However, there is a problem with regard to needle sampling. The paper does not indicate the needle longevity and whether fresh needles collected in August were of the first-year or also older needles if needle longevity in this species was 2-yr or more. Nutrient concentration in older fresh needles could be lower than current year fresh needles.
The data (Table 2) show a sudden decrease in P concentration from 20-yr to 30-yr of plantations. It warrants explanation. Contrary to this, N concentration increased from 20-yr to 30-yr of plantations, though less dramatically. Much of the pattern along the chronosequence is driven by these changes.
While the science part of the paper is sound, the language problems are considerable. The problem of syntax, grammar and poor selection of words are too many to be acceptable. In fact, the text is replete with mistakes. Generally, the “Discussion” part is unintelligible. I cite here some examples: ‘frequent and unnecessary use of “meanwhile” in the beginning of sentences’, phrases such as the “NRE was significantly negatively with senesced needle N concentration”, “strengthening of physiological activity of trees”, “N concentration maintained quickly increasing (lines 262, 263)”, “P increased rapidly to with plantations development further (line 273)”, “resorption significant improved (line 287)”, “which may cause by the accumulation of C (line 299)”, “nutrient releasing (line 306) were followed”, above founds (line 323), relative stable (line 342), and “may improve the transfer from N-limit to P-limit (lines 348, 349).
