Evaluation of the Regional-Scale Optimal K Rate Based on Sustainable Apple Yield and High-Efficiency K Use in Loess Plateau and Bohai Bay of China: A Meta-Analysis
Round 1
Reviewer 1 Report
Good job to the authors: the paper is well written, easy to read, and the ideas come across nicely. The subject is clearly of interest for the Journal. The manuscript is short, adequately written, and well-organized. The objectives are clearly stated, and all analyses seems correct. I have only a minor concern, about the depth of details about subsections 3.1 and 3.2.
In these two subsections I will recommend to authors to a little go deeply in the analysis of their results. Most part is focused on reiterating the results, then a short comparison is given to other studies. In general, I think the discussion could be improved by further analysis of the potential mechanisms causing the observed results. the section should provide a strong mechanistic basis for the observed results.
Author Response
Point 1: Good job to the authors: the paper is well written, easy to read, and the ideas come across nicely. The subject is clearly of interest for the Journal. The manuscript is short, adequately written, and well-organized. The objectives are clearly stated, and all analyses seems correct. I have only a minor concern, about the depth of details about subsections 3.1 and 3.2.
In these two subsections I will recommend to authors to a little go deeply in the analysis of their results. Most part is focused on reiterating the results, then a short comparison is given to other studies. In general, I think the discussion could be improved by further analysis of the potential mechanisms causing the observed results. the section should provide a strong mechanistic basis for the observed results.
The idea to question 1: For subsections 3.1, I start from the essential characteristics of the two models (L+P model and Q+P model) and conduct an in-depth analysis in combination with the actual situation of this research, revealing the essential reasons why L+P model is better than the Q+P model in our research.
Response 1: For L+P and Q+P models, a common feature of them is that there is a horizontal straight line (platform) after reaching optimal yield. However, in our research, their simulation effects were quite different, which might be due to the influence of not only various factors (e.g., soil and climatic conditions, especially rainfall) and fertilizer types, but their own characteristics. Before yield reached platform, L+P model could reflect that yield increased linearly with increasing K application, the increasing rate of yield-K application remained constant, although Q+P model also reflected yield increased with increasing K application, but the increasing rate of yield-K application decreased gradually (Figure 2). As the increasing rate of yield-K application of Q + P model was generally less than that of L + P model, it had a hysteresis effect than L+P model when reaching almost the same optimal yield (The shaded part in Figure 2), resulting in a relatively high optimal K rate.
The above paragraph (For L+P …. rate.) was placed below the last sentence (This further proved …. rate.) of the second paragraph of subsections 3.1 as the third paragraph of subsections 3.1.
The idea to question 2: For subsections 3.2, soil is the source of K absorption by apple trees and the sink of K fertilizer. Apple tree's demand for K varies over time, and people usually cannot apply K fertilizer accurately when apple trees need K. Because of this, apple trees will absorb and store sufficient K in advance, rather than request it from soil when needed. Based on physiological growth characteristics of apple trees, the abundance and deficiency of IKS affect the potential optimal yield of apples to a certain extent.
Response 2: Apple trees have the instinct to store nutrients in advance. They are pre-absorbed and stored by roots' sensitive response to changes in soil K and their own needs to prevent potential demand when K is supplied deficiency. In the soil with abundant IKS, apple trees can absorb and store full K in advance, so application of less K fertilizer can maintain soil K nutrients and be absorbed and used by apple roots, and at the same time obtain the potential optimal yield. Similarly, apple trees can only absorb and store less K in advance in soils with lacking IKS. Therefore, the potential optimal yield can be obtained only by applying more K fertilizer to supplement soil K nutrients and for apple roots to absorb and utilize.
The above paragraph (Apple trees have …. utilize.) was placed below the last sentence (Jing et al. [63] also …. species.) of the first paragraph of subsections 3.2 as the second paragraph of subsections 3.2.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Thank you very much for the invitation as a reviewer for the manuscript Agronomy- 1238146. "Evaluation of the regional-scale optimal K rate based on sustainable apple yield and high-efficiency K use efficiency in Loess Plateau and Bohai Bay of China: A meta-analysis. The article is interesting because potassium fertilizing is especially important for the yield and quality of apples. I hope that my remarks will help with this paper's improvement.
The title of the manuscript seems to be too wordy with a tautology sign: "high-efficiency K use efficiency."
The first paragraph in section 2.1 (Apple …. (Figure 1) is more suitable for the Introduction.
Some remark about Figure 2. The data spread is too large (especially in the graph (a)); can it be approximated in this way?
Figure 5: it seems that the authors need to indicate the significance of the differences
The authors need to pay more attention to "uncontrollable factors (e.g., soil and climatic conditions, especially rainfall) because they are often сcrucial for fertilizing success in specific sites.
Author Response
Point 1: Thank you very much for the invitation as a reviewer for the manuscript Agronomy- 1238146. "Evaluation of the regional-scale optimal K rate based on sustainable apple yield and high-efficiency K use efficiency in Loess Plateau and Bohai Bay of China: A meta-analysis. The article is interesting because potassium fertilizing is especially important for the yield and quality of apples. I hope that my remarks will help with this paper's improvement.
The title of the manuscript seems to be too wordy with a tautology sign: "high-efficiency K use efficiency."
Response 1: In the title page of the manuscript, I have deleted the "Efficiency" in the title.
Point 2: The first paragraph in section 2.1 (Apple …. (Figure 1) is more suitable for the Introduction.
Response 2: In Section 2.1, the first sentence (Apple ….) in the first paragraph was split into “and widely …. environment.” (moved to the back of the first sentence of the first paragraph of introduction) and “Apple …. China” (Moved to the end of the second sentence (Although ….) of the third paragraph of introduction and use it as the first sentence of the new paragraph (the fourth paragraph)). The second sentence (However …. region.) of the first paragraph and "The old channel …. respectively." were deleted; the third sentence (Considering …. Highland (Figure 1).) and "In the …. regions." were moved to the end of the first sentence of the fourth paragraph of introduction. The last sentence (Some …. Table 1.) was revised to "The Geo-location …. respectively." and placed after the eighth sentence (The texture …. respectively.) of the first paragraph. In introduction, the eighth sentence (The Bohai Bay …. China [11]) of the first paragraph was deleted; Reference [11] was moved to the third sentence (In the four …. China, and …. system.) of the fourth paragraph after the word "China" and renumbered as [46]. References number [12-46] decreases by 1 in turn.
Point 3: Some remark about Figure 2. The data spread is too large (especially in the graph (a)); can it be approximated in this way?
Response 3: Thank you very much for this comment, which is very good. L+P and Q+P models, as classic fertilizer effect model functions, are mostly used in experimental studies at specific sites. However, based on large-scale regional studies, it seemed inevitable that the data spread was too much. In addition, the situation has already appeared in these following similar articles: Figure 4 in "Soil Fertility and Indigenous Nutrient Supply in Irrigated Rice Domains of Asia", Figure 3b and 4 in " EFFECT OF LONG LONG-TERM POTASSIUM FERTILIZER LEVELSON THE YIELD OF RAINFED CORN IN NORTHEAST CHINA ", Figure 1 in " Impact of uncertainty on the optimum nitrogen fertilization rate and agronomic, ecological and economic factors in an oilseed rape based crop rotation " and " Estimating on-farm wheat yield response to potassium and potassium uptake requirement in China ".
Point 4: Figure 5: it seems that the authors need to indicate the significance of the differences.
Response 4: Thank you very much for this comment, which is very meaningful. The data I used when drawing Figure 5 was theoretical values calculated based on a large amount of measured data and related models, rather than measured values, and it might not be possible to do a significant difference test. These data came from: Firstly, L + P model was used to calculate the region optimal K rate, and the high K and low K rates were defined as K-fertilizer application rate higher than and lower than the region optimal K rate. Then, the average values of all low K and high K treatments in different regions were calculated as representatives of low K and high K fertilizer, respectively. The corresponding yield was calculated by L+P model, and the corresponding KUE (or KAE) was calculated based on the exponential relationship between K application rates and KUE (or KAE), respectively.
Point 5: The authors need to pay more attention to "uncontrollable factors (e.g., soil and climatic conditions, especially rainfall) because they are often сcrucial for fertilizing success in specific sites.
Response 5: Jiang et al. [77] conducted a nutritional analysis of apple orchards in three soil types of brown loam, alluvial soil and brown soil in Shandong Province, China. The results showed that the available K content in brown loam was significantly higher than that in brown soil and significantly lower than that in alluvial soil. Apple tree is not only highly dependent on climatic conditions, especially precipitation, but also extremely sensitive. Drought leads to the deterioration of soil physicochemical environment, and apple trees are difficult to absorb required nutrients. When rain-waterlogging, soil nutrients are easily lost to affect fruit yield.
The above four sentences were placed after the fifth sentence (...sites.) in the first paragraph of subsections 3.5.2. References number ([77-83]) increased by 1 in turn, and references [77] was changed to " Jiang, Y, M.; Gu M, R.; Shu H. R. Nutrient diagnosis of starking delicious' apple. Acta Horticulturae Sinica 1995, 22, 215-220."
In addition, the footnote "R2" in table 2 was revised to "R2" in the self-examination of the manuscript.
Author Response File: Author Response.pdf
