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

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.

Methods

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:

Figure 1. Correlation between total iron content (mg/kg) and m.a.s.l in soil samples from Amazonian Andes soil in Ecuador.

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.

Data availability