Abstract
The mixed forests of the upper Rio Negro at the northern of the Amazon basin grow in oxisol soils that are extremely infertile. These areas exhibit deficiencies in several macro-nutrients, and may also be characterized by the shortage or toxic excess of some micronutrients. The overall goal of this research is to collect more comprehensive information regarding the micronutrient composition of the upper Rio Negro forests as well as discern the relationship between leaf micro- and macro-nutrients that may contribute to the homeostasis and balance of the ionome. Firstly, the nutrient composition within the oxisol soil and leaf tissues of two top canopy tree species from the mixed forests was determined. We then analyzed the relationship between leaf micronutrient composition with N and P levels of the two species and that of species inhabiting the Amazon caatinga. Extractable soil Zn, B, Mn and Cu were very low in the mixed forest. In contrast, Fe and Al levels were potentially toxic. The analysis of leaf N/P ratios revealed for the first time the co-limitation of N and P in the mixed forest. This contrasts with species from the adjacent Amazon caatinga toposequence that are characterized by strong N limitation. All micronutrients within leaves of species inhabiting the mixed forest were also found to have low concentrations. Moreover, Fe and Al were detected at concentrations well below those reported for accumulator species. This suggested that leaf ion homeostasis was maintained under potentially toxic soil Fe and Al conditions. Leaf micronutrient (Fe, Zn and B) contents mirrored that of leaf N and P contents, and comparable Fe/N, Fe/P, Zn/N, Zn/P, B/N as well as B/P ratios were found across species and forest types. Therefore, forest species exhibited the capability to maintain leaf nutrient balances under soil conditions with deficient or toxic levels of micronutrients.
Highlights
Many soils located in tropical rainforests are formed from materials that are highly weathered and strongly leached
This study focused on the nutrient composition of leaves sampled from two canopy trees species dominant in the mixed forest on oxisol soil: Caryocar glabrum (Aubl.) Pers. (Caryocaraceae) and Ocotea aciphylla
In the oxisol soils of the mixed forest, most of the organic matter is confined above the root mat, which explains the low levels of organic matter detected in this study [22]
Summary
Many soils located in tropical rainforests are formed from materials that are highly weathered and strongly leached. These areas exhibit deficiencies in several macro-nutrients, and may be characterized by the shortage or toxic excess of some micronutrients [1] [2]. This issue has been widely analyzed and discussed within the context of agronomic constrains and management of fertilization strategies [3]. A first approach would be to analyze the plant ionome and to identify possible interactions between elements in species inhabiting these environments. Physiological mechanisms that enable plants to respond to environmental conditions through the flexible modification of key regulators of cellular physiology may be highly selected in order to maintain ionic homeostasis [8]
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