Effects of Excess Nitrogen (N) on Fine Root Growth in Tropical Forests of Contrasting N Status
, Frank S. Gilliam 3, Jan Mulder 4, Muneoki Yoh 5, Jiangming Mo 2,* and Xiankai Lu 2,*
Round 1
Reviewer 1 Report
I have only some minor remarks and questions:
L 132-133: Could you briefly provide more details about this indication, for the readers who are not familiar with 14C measurement?
L 208-216: For better readability suggest to present only P value, because all the parameters are presented also in the figure. Could be applied also for the further text.
L 209: DecreaseD
L 218-219: Are also sulphate deposition data available? Did sulphate depositions maybe also explain soil pH changes?
What kind of N deposition? What was the ratio of NO3 and NH3?
L 230: There is reference to Fig. 2, which is however presented several pages (and figures) later.
L 296: Seems there is no decrease in time shown on the referenced figure, but comparison only.
L 347-349: Lower fine root vitality was rather manifested by changing proportion of biomass/necromass.
L 364-365: Could you briefly describe the mentioned difference?
L 423: evidenceD?
L: 465, 467 was, not were
L: 579-582 this long sentence is hard to understand
Table A.1: What parameter is given in parentheses?
Question of curious reader: is Fe3+ concentration in fine roots somehow important, except as an indicator of Fe mobilization?
Author Response
I have only some minor remarks and questions:
L 132-133: Could you briefly provide more details about this indication, for the readers who are not familiar with 14C measurement?
Responses: Thanks for this suggestion. The following information was added into the maintext to give more details on the 14C dating technique (from lines 132): “The age of the primary forest was determined to be older than 400 years using the 14C dating technique[36]. The 14C dating technique hinged on the fact that the 14C signature within organisms after death decay at a first order reaction and the decay rate is proportional to the number of 14C atoms present. Then by comparing with the known half-life of 14C, the mean age of certain sample or the residence time of soil organic matter can be calculated.”. We believe it should be clearer now.
L 208-216: For better readability suggest to present only P value, because all the parameters are presented also in the figure. Could be applied also for the further text.
Responses: Thanks. Done as suggested.
L 209: DecreaseD
Responses: Thanks. Corrected as suggested.
L 218-219: Are also sulphate deposition data available? Did sulphate depositions maybe also explain soil pH changes?
What kind of N deposition? What was the ratio of NO3 and NH3?
Responses: Thanks. Sulphate deposition data was not available from Liu et al. 2013, but were found from other literatures on wet sulphate deposition in Zhuhai city in Guangdong province, adjacent to our study site Dinghushan (Wang et al. 2021 Research of Environmental Sciences). Wet sulphate deposition in Zhuhai city averaged 31kg ha-1 yr-1，being higher than NO3- (11.4~30.5 kg ha-1 yr-1) and NH4+ deposition (5.6~14.4 kg ha-1 yr-1) over the period of 2008-2018. Thus sulphate deposition could also be high in Dinghushan which however are not available from literature.
As a more important contributor than DIN to acid rain in Zhuhai city, we suggested that sulphate deposition could also have contributed to soil pH changes, in addition to the 17% and 15% explanation from N deposition to soil pH changes in forests in Dinghushan.
The data on N deposition in Figure 1 were from Liu et al. (2013). Bulk N deposition was reported. The ratio of NH4 to NO3 within precipitation was 6 in 1980s and decreased to 4.5 in 1990s, to 3 in 2000s. The average value in 2010 was 1.4. Thus the NH4/NO3 ratio decreased constantly from 1980s to 2010.
Wang G, Liu S, Yu X (2021) Characteristics of precipitation chemistry and wet deposition in Zhuhai, China. Research of Environmental Sciences 34(7): 1612-1620. In Chinese with English abstract
L 230: There is reference to Fig. 2, which is however presented several pages (and figures) later.
Responses: Thanks. Fig.2 was moved backed to the right place.
L 296: Seems there is no decrease in time shown on the referenced figure, but comparison only.
Responses: Fig.5a showed the responses of soil pH to N treatments in the three forests. There were significant differences between the control vs. high-N treatment in the primary forest, and between control vs. Low-N treatment in the planted forest. X axis stands for different levels of N treatments.
Figure 1 showed the historical change of soil pH in the three forests. Decreasing trend were observed in all three forests.
L 347-349: Lower fine root vitality was rather manifested by changing proportion of biomass/necromass.
Responses: Thanks for this suggestion. Both live fine root proportion (live/(live+dead)) and live/dead (biomass/necromass) ratio were used as indicators of fine root vitality (Persson 1995, 2002; Clemensson-Lindell et al.1995). In this study, we used live fine root proportion (biomass/(biomass+necromass)) to indicate the vitality of fine roots, it also take into account the influence of total fine root biomass (biomass+necromass).
Persson, H.; Majdi, H.; Clemensson-Lindell, A. Effects of acid deposition on tree roots. Ecological Bulletins 1995, 44, 158–167.
Persson, H.; Ahlstrom, K. Fine root response to nitrogen supply in nitrogen manipulated Norway spruce catchment areas. Forest Ecol. Manag. 2002, 168, 29–41.
Clemensson-Lindell, A.; Persson, H. The effects of nitrogen addition and removal on Norway spruce fine-root vitality and distribution in three catchment areas at Gårdsjön. Forest Ecol. Manag. 1995, 71, 123–131.
L 364-365: Could you briefly describe the mentioned difference?
Responses: Ok. I will describe briefly. In temperate/boreal forests where soil base saturation and cation exchange capacity (CEC, hundreds of mmolc kg-1) are often high, soil buffering system are dominated by soil base cations, i.e., K+, Na+, Ca2+, Mg2+, etc. During the acidification process, soils release base cations, such as Ca2+ and Mg2+, to neutralize the increase in acidity. When soil pH gets lower than 4.5, and these base cations have been depleted, Al3+ is released from the soils, often reaching toxic levels. Acid deposition had driven some forests and grasslands in Europe and North America into Al3+ buffering range.
In contrast, the soils in many tropical forests (especially lowland tropical forests, not montane tropical forests) are highly weathered soils, the cation exchange capacity are dominated by Al3+ and H+ (Fig.6 in this study). Soil base saturation was as low as 8% (Fig.6 in this study). Further N additions have driven soil exchangeable H+ increases, more Al3+ leached into soil solution in deeper soil depth (20cm), but the soil exchangeable Al3+ was not altered after N treatments. In contrast, minor but significant decreases in soil exchangeable Ca2+, Mg2+ were observed, co-occurring with higher Ca2+ content in 40cm depth soil solution than 20cm. These results were reported in Lu et al. (2015 Global Change Biology) studying the soil acidification responses to 6 years of N treatments in our primary forest). Moreover, soil exchangeable Fe3+ may increase after N treatments, co-occurring with minor Al3+ mobilization in mineral soils, as found in this study and in Lu et al. (2015 Environmental Science & Technology) focusing on soil buffering capacity in response to N treatments in the three forests.
Lu, X.; Mao, Q.; Gilliam, F.; Luo, Y.; Mo, J. Nitrogen deposition contributes to soil acidification in tropical ecosystems. Glob. Chan. Biol. 2014, 20, 3790–3801.
Lu, X.; Mao, Q.; Mo, J.; Gilliam, F.; Zhou, G.; Luo, Y.; Zhang, W.; Huang, J. Divergent responses of soil buffering capacity to long-term N deposition in three typical tropical forests with different land-use history.
Environmental Science & Technology 2015, 49, 4072–4080.
L 423: evidenceD?
Responses: Thanks, corrected as suggested.
L: 465, 467 was, not were
Responses: Thanks, corrected as suggested.
L: 579-582 this long sentence is hard to understand
Responses: Thanks. It was divided into two sentences now to make it more readable, as “Previous study in the same N addition experiment suggested a 9% reduction in soil respiration would be enough to explain the observed accumulation of soil C if dissolved C losses keep unaltered [32]. In fact, 14% decrease in soil respiration was detected in high-N plots after 2 years of N addition in the primary forest [9], which could have contributed at least partly to the observed C accruement in this forest [72]. ”.
Table A.1: What parameter is given in parentheses?
Responses: They are SE in parentheses. These values in Table A1 are converted from the measured fine root N, P, K, Ca, Mg concentrations. The concentration of N were set as 100, then other nutrients were calculated proportionally to that of N. These values gave us more direct comparison with the optimum weight proportions of N to other nutrients for Picea abies seedlings and gave information on the ampleness or shortage of certain nutrients compared to N.
Question of curious reader: is Fe3+ concentration in fine roots somehow important, except as an indicator of Fe mobilization?
Responses: Thanks. I am not an expert on iron nutrient to plants, so I can only tell what I knew and found recently. We also found it interesting and exciting to observe fine root Fe accumulation in response to 8 years of N additions, which corresponded with soil Fe3+ mobilization in the primary forest. Iron toxicity to plant growth are mostly investigated in rice or vegetable studies, which examine the toxicity level of Fe in different forms to rice or vegetable growth, or investigate the mechanisms of different plant species in coping with iron deficiency and iron-deficiency induced chlorosis. Such studies often adopt hydroponic experiments using solution to investigate.
As the plant organ in direct contact with soil matrix and soil solution, plant roots can function as the receptor of environmental stresses of either shortage or excess of iron availability. There is evidence that roots can sense fluctuations in the external availability of iron in a cell- or meristem-specific manner, although the mechanism underlying the perception of iron is unclear. In addition to a sensor that monitors the availability and distribution in the vicinity of the roots, several lines of evidence suggest the presence of an iron sensor in the shoot that responds to overall iron needs and regulates the production of a systemic signal that controls physiological root responses (Schmidt 2003). For more details in this aspect, please refer to the review paper by Wolfgang Schmidt (2003) in TRENDS in Plant Science.
Schmidt W (2003) Iron solutions: acquisition strategies and signaling pathways in plants. Trends in Plant Science, 8(4): 188-193. doi:10.1016/S1360-1385(03)00048-7
Author Response File: 
Author Response.docx
Reviewer 2 Report
Reviewer
MDPI - Forests
Manuscript Number: forests-1857177
Title: « Effects of excess nitrogen (N) on fine root growth in tropical forests of contrasting N status».
Line 119: Figure 1 should be in the Results section or the Methods section, but not in the introduction.
Figure captions should be below the figure, not above the figure.
Same remark for figures 2-7 (line 114, 237, 262, 291, 315, 340).
Line 432: figure 2 again? Although there should be a figure 8!!
Author Response
Comments and Suggestions for Authors
Reviewer 2
Manuscript Number: forests-1857177
Title: « Effects of excess nitrogen (N) on fine root growth in tropical forests of contrasting N status».
Line 119: Figure 1 should be in the Results section or the Methods section, but not in the introduction.
Responses: Thanks for this suggestion. Now figure 1 was moved to Results section, right after the description of it (under line 223). Please check the revised word version.
Figure captions should be below the figure, not above the figure.
Same remark for figures 2-7 (line 114, 237, 262, 291, 315, 340).
Responses: Thanks. All figure captions were moved below each figure now, including figure 1.
Line 432: figure 2 again? Although there should be a figure 8!!
Responses: Thanks. Figure 2 used to be in the right place in my submission (at or under line 230). It appeared that it was misplaced during the editing process by the journal. I have moved it back to lines 230 where it should appear.
Author Response File: Author Response.docx
