Nitrogen Pools in Tropical Plantations of N₂-Fixing and Non-N₂-Fixing Legume Trees under Different Tree Stand Densities

Round 1

Reviewer 1 Report

The manuscript describes an interesting and exciting project in which the focus is on the measuring and characterizing the N pools of soil and biomass of reforestation plots populated by two legume species with opposite growth and N acquisition strategies. The slow-growing and symbiotic N-fixer A. peregrina and the fast-growing but supposedly non-fixer S. parahyba. The authors have produced a wealth of valuable data (trees planted at three different stand densities, nine plots in three blocks with somewhat different soil compositions), of which it is worth exploring as much as possible to draw important conclusions for future afforestation programs. It is a pity that the raw data is not accompanying the paper as supplemental material. Also, the presentation of the data comparisons and the writing of the results and discussion sessions should be improved a lot. I’m going to highlight some main issues that should be addressed and reconsider and will propose some specific changes in the text, but I also suggest a general check on the English language.

It’s very unusual that the first sentence of the result session is already the conclusion sentence with reference to Table 2 (e.g. Lines 241-242). The Materials and Method session contains many rational explanations why and how experiments were designed and carried out, the major points should be written in the results together with analyzing data and finishing with the conclusion of the given session. Materials and Methods session should contain the accurate description of the experiments and conditions in a more concise form. The authors gave detailed descriptions from most of the aspects of the circumstances and methods, however there are a few important things to revise.

- NUMBER OF MONTHS after planting the trees: Abstract: Line 21: 44-67 months; in Mat&Methods: Line 137: Soil N pools 67 months; Line 153: biomass N pools: 57 months; Line 265: Fig 1 legend: 56 months; Result Fig. legends: 56 months. Which one is true?

- In Table 1. Many attributes of the soil are listed about the B1, B2 and B3 blocks of A. peregrina and the B1, B2 and B3 blocks of S. parahyba. The two sets of B1, B2 and B3 are clearly different, therefore it can be misleading to give them exactly the same names, Either indicate B1_Ap, B1_Sp, etc., or design as B1, B2, B3 for one legume and B4, B5, B6 for the other. More importantly: these attributes were determined prior to planting the trees. Why the N pools were not determined in these “starting” samples the same way as was determined in the samples later? There is a column of OM in Table 1 that is not introduced and described at all, probably it stands for organic material, but not known how it is related to the N pool content measured in later samples.

As mentioned above, in the first Result session Table 2 is already a summary of a higher statistical evaluation of data displaying the effect of stand density, block and interaction DxB by calculated P values. We can see where were significant effects, but the reader has no idea in which direction! And how the A. peregrina and S. parahyba data are related to each other? The mean values per block on the right side of Table 2 (if 1, 2, 3, really means B1, B2, B3 hopefully) clearly shows reduced values for B3_Ap and B1_Sp but these will be mentioned much later in the text, nit here. On the other hand, Table 3 (which is never referred to in the text! and which is located much later, after Figs 1 and 2, in between Figs 2 and 3, which both show diagrams related to N-fixation estimations) contains mean data and st. deviation related to data in Table 2. So, Table 3 should be even before Table 2, and it would be also better if the authors used the same unit of measure, either kg ha-1 or Mg ha-1. Even looking data in Table 2 and 3 together, I still can’t see the block effect on the soil N values but would be interested to see e.g. how the Soil N mean values for the different stand densities are in the individual blocks.

In the 3.2. results session the Fig. 1 legend says “N accumulation in the leaves, branches, bark, wood, and root of A. peregrina and S. parahyba…”, but there is no Root data at all on the figure. In this session there are very interesting data of the N concentration of the different organs of the two legume species. Those numbers clearly show that the N-fixer trees have higher concentration in all examined parts but wood (or called stem in other sentences). It could be emphasized and discussed in more details.

The session 3.3 is a mixture of reflecting back to 3.1. session (Lines 283-291) and the estimation of the efficiency of N-fixing (before and after the indicated text). Though according to the 15N method the most efficient N-fixation is detected in the density 625 plots of A. peregrina, N accumulation data of the two legumes do not allow the calculation with the other N accretions method. The problem is that when comparing the total biomass N accumulation 625 density data, the mean value of the non-nodulating legume is higher. But these data include all data from the three blocks of each legume, But what happens if you checked B3_Ap and B1_Sp (which showed depleted soil fertility parameters prior to planting the trees) and compare those data to each other. In these two blocks Table 2. numbers clearly show much better performance of the N-fixing species. Then, when you compare data from the two other blocks with better soil condition, the N available in the soil might be enough for the trees to grow, so no extensive N-fixing is needed for A. peregrina either.

Some proposed specific changes in text:

Abstract:

Lines 15-17: We investigated the nitrogen pools on experimental plots with monocultures of two legume species widely used in reforestation in Brazil that have contrasting growth and nitrogen acquisition strategies.

Lines 20-21: The measurements followed standard methods and were carried out on 54 experimental plots between 44 and 67 months after planting of the species.

Lines 21-22: The N content of the soil was greater than 4 Mg ha-1 and was similar for both species.

Some typical mistakes: species is always species (single and plural, word “specie” does not exist); legume roots, etc. in phrases like this, legume is single.

Line 40: The Brazilian forestry sector has improved…

Line 91: being considered..

Line 120: symbioses in the field..

In the Discussion a new aspect is introduced in Lines 328-330: „A higher mortality rate has been observed for S. parahyba than for A. peregrina in the study area, related to wind-breaking, seasonal and interannual rainfall variability, and attack by pests and diseases (unpublished data). It would be good to know if this was considered/involved in the statistical evaluations (was not mentioned either in M&M or results sessions). Discussion about the two methods estimating N-fixation efficiency is too long and repetitive.

Some sentences are not quite clear what the authors wanted to say, e.g.:

Lines 358-360; 372-374; 385-389; 411-414

Author Response

We attach the answers in a pdf document.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript by Kelly et al. is reporting a challenge on N accumulation in the tissues and soils of Brazilian plantations using N2-fixing and non-fixing leguminous trees under three plant densities with a particular attention on estimates of nitrogen fixation. The well-designed experiments gave interesting results but need several cares because of lack of information and statistical significance.

General comments:

Comment 1 (lines 24 to 26): The quantitative estimation of N2 fixation by two different methods (δ15N method, N difference method) is main topics of this report (see lines 95 to 99). The authors are requested to tell the major results.

Comment 2 (Figure 1): Very important. Values for both plants showed same letter “A” in all of three densities. Are these true?

Specific comments:

Comment 3 (lines 22 to 25): Didn’t the both plants produce fruits/pods/seeds?

Comment 4 (line 56): “(N2)” should be placed after “chemical dorm”.

Comment 5 (line 120): Double “in in” should be single.

Comment 6 (line 120): δ15N value of fertilizer (NPK 06-30-06) should be reported, since that may affect on leaf δ15N (see line 205).

Comment 7 (Table 1): N contents and their δ15N values of the soils and soil layers are important information affecting on leaf δ15N (see line 205).

Comment 8 (lines 165, 373): “specie” is not “species”?

Comment 9 (lines 189): “x” is not “%”?

Comment 10 (line 210): What B value was actually selected in this experiment.

Comment 11 (line 373): “differences” is not “different”?

Comment 12 (Line 410): “intensify” is not “intensive”?

Comment 13 (line 420): “Others explanation” should be “Other explanations”.

Comment 13 (line 467): “curation” is not “collection”?

Author Response

We attach the answers in a pdf document.

Author Response File: Author Response.pdf