Differences in total iron content at various altitudes of Amazonian Andes soil in Ecuador

Although iron is not contained by chlorophyll, it is indispensable for plants as it plays an essential role in the biosynthesis of chlorophyll. It is a component of many important plant enzyme systems, e.g. cytochrome oxidase, which is responsible for electron transport. Therefore, examining iron content of soils, particularly ionic forms of iron (Fe 2+ and Fe 3+) is important for fruit growers. In this article, we disclose the total iron content determined in soils (Hyperalic Alisol soil) at three altitudes of Amazonian rainforest in Ecuador. We examine how different altitudes impact the pH and total iron content in the selected study area. We found that total iron content significantly decreases (R2=0.966) at lower altitudes. For future studies, the authors recommend that along with Fe ion content one should determine calcium, microbial biomass, and microbial activity to better understand iron mobility and dynamics of iron uptake in the area.


Introduction
Total iron concentration of soils mainly depends on pH (Colombo et al., 2014;Jelic et al., 2010) and moisture content; and is also affected by root respiration, soil microbial activity, leaching, and erosion (Spectrum Analytic, Inc. 2020). Given that iron deficiency is a regular problem for various crops, it is essential to determine the total iron content of soils (Mengel et al., 2001), particularly in orchards (Simon & Szilágyi, 2003).
In a highly cited review paper, Bünemann et al., (2018) identify the most frequently used soil quality indicators under agricultural land use: organic matter, pH, available phosphate, and water storage. Soil quality evaluation should specify targeted soil threats, functions, and ecosystem services. The authors of the review recommend developing increasingly interactive assessment tools.
Recently, several studies have been undertaken on the effects on soil quality exerted by various minerals contained in the soil, such as ammonium lactate-soluble potassium and phosphorus content (Jakab, 2020;Li et al., 2020). Also investigated was the impact of various soil cultivation methods on some microbial soil properties (Beni et al., 2017;Sándor et al., 2020;Sándor, 2020;Veres et al., 2015).
In this article, we report the variations with altitude of the total iron content measured in intact soil in the Amazonian rainforest (in an uncultivated and uninhabited area). Considering that orchards are the most sensitive to iron deficiency, our results are aimed to support local farmers, when they select new areas for fruit plantations. An intact area was chosen as the control for soil samples, which will serve as the reference for future studies initiated in the nearby agricultural region.

Soil sampling
A total of 15 soil samples were collected from three altitude levels: 420, 1000, and 1600 m.a.s.l. (meters above sea level) near Tena, Ecuador, on December 10, 2019, from the upper layer (top 20 cm) of Hyperalic Alisol (Ultisols in US Soil Taxonomy) soil ( Table 1).
We determined total iron (all ionic forms) according to modified Blakemore 1981 method described in Singla et al., 2018. Briefly, 50 mL of ammonium oxalate monohydrate (Spectrum

Data analysis
We applied simple linear regression (Z-test) for statistical analysis, using SPSS (version 26) to reveal possible relevant differences in pH values and total iron content at different altitudes.

Results and discussion
Examined soil samples in the chosen area were strongly or moderately acidic, with pH values in the range from pH 4.95 ± 0.05 to pH 5.95 ± 0.05 (Table 1). We did not find any meaningful correlation between altitude and pH values, or between pH and total iron content. Moisture content is the highest at 1600 m.a.s.l. Allophane was detected in all samples, which supports the volcanic nature of the sampling area (Fieldes & Perrot, 1986) (Table 1).
Total iron content significantly decreases (R2=0.966) at lower altitudes ( Figure 1). No significant changes in pH were found, and we can explain this by the following: (i) vegetation at lower lying areas receive less light, so it absorbs a greater quantity of iron ions; so far, there is no relevant literature data on the effect of light intensity on the iron uptake of plants (Borowski, 2013).
(ii) there is a greater concentration of iron-reducing bacteria in the lower lying areas, which seems to be verified by a prior study (Fiedler et al., 2007). This finding is, however, unusual, because such bacteria are typically present in sea water (Bae et al., 2001) and paddy soils (Singla & Inubushi, 2013), rather than in Hyperalic Alisol soils.
High moisture content of the soil and organic matter accumulated on the soil surface can make air circulation difficult, hence, anaerobic conditions can develop in lower lying areas.
Our results (from 400 m.a.s.l. to 1000 m.a.s.l.) are comparable with a prior study performed in the same region (Singla et al., 2018), in which the authors report a decrease in iron content for lower laying areas. The main difference between our assessment relative Singla and colleagues' results is that they observed a radical decrease in iron content above 1000 m.a.s.l., while we found greater iron concentrations at this altitude. Our results are comparable in magnitude to other study findings (Fageria & Stone, 2008) carried out in South American Hyperalic Alisol soils in which high iron content was found at depths of 0-20 cm.

Conclusions
Total iron content significantly decreases (R2=0.966) at lower altitudes. Genomics studies could detect possible iron consuming bacterial strains. For future studies, we recommend that in addition to Fe2+ and Fe3+ content one should determine calcium, microbial biomass, and microbial activity. Altogether, this approach would enable a better understanding of iron mobility and dynamics of iron uptake in the area.
available as well.
Summarizes and concludes from research findings correctly. Conclusions:

Is the work clearly and accurately presented and does it cite the current literature? Yes
Is the study design appropriate and is the work technically sound? Yes

If applicable, is the statistical analysis and its interpretation appropriate? Yes
Are all the source data underlying the results available to ensure full reproducibility? Yes

Are the conclusions drawn adequately supported by the results? Yes
No competing interests were disclosed.

Competing Interests:
Reviewer Expertise: Soil Microbiology I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.

Yes
Are all the source data underlying the results available to ensure full reproducibility? Yes Are the conclusions drawn adequately supported by the results? Yes No competing interests were disclosed. Competing Interests: Reviewer Expertise: Soil biology, soil chemistry I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.

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