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José Leonardo de Moraes Gonçalves

Professor of Soils and Forest Nutrition at Esalq-USP


Ecological management of forest soil

In the last four decades, there have been major innovations in the areas of soil conservation and management, with important contributions to the sustainability of forestry production chains. Among them, the consolidation of minimum soil cultivation, advances in knowledge of forest nutrition and the calibration of fertilizer recommendations stand out. The great scientific and technological gap to be developed is that of ecological soil management.

When the native vegetation is removed and a monoculture is established, an intense alteration of its organisms (microbiota and fauna) occurs in the soil, with the proliferation of new predators and pathogens, which, before, were in balance with the other organisms. Some diseases and pests of the root system or the aerial part of trees, which have some stage of their life cycle in the soil, only reach levels of economic damage, largely because of this ecological imbalance.

Currently, among the diseases that cause vascular wilting, the main ones are: the fungus Ceratocystis fimbriata, the bacteria Ralstonia solanacearum and Erwinia psidii; and, among the pests, Gonipterus platensis, Costalimaita ferruginea, Iridopsis panopla, Apatellodes sericea , Glena unipennaria and Glena bipennaria , among others. In order to carry out the biological control of pests that have some stage of life in the soil, entomopathogenic nematodes have been tested. In a master's thesis defended by Luana Faria, in 2020, supervised by Silvia Wilcken, at the Faculty of Agronomic Sciences of Universidade Estadual Paulista, it was found that the nematode species Steinernema puertoricense had good efficiency in the control of pre -pupae of Gonipterus platens.

The use of silvicultural practices that benefit soil biota is essential to maintain the sustainability of plantations. Among them are 1) minimal soil cultivation, 2) practices with reduced impact on soil physical attributes, 3) multi-specific planting of forest essences and 4) maintenance of understory in stands.

By means of minimal cultivation, physical disruption of the soil is avoided, carbon inputs are maintained or increased and a microenvironment of temperature, humidity and aeration favorable to the biota in the upper soil layer is created. The use of machines with wheels of low compressive power, in implantation, maintenance and harvesting practices, also contributes to the maintenance of the physical integrity of the soil structure. Among the harvesting practices, the transfer of wood with a skidder is harmful to the biological activity of the soil and should be avoided, because this equipment sweeps the soil, dragging a large part of the plant residues to the edge of the plot. More than 50% of the soil area is uncovered, reducing carbon input to the soil and drying out the upper layer.

The multi-specific planting of forest essences is conventionally used in native vegetation conservation areas, which generates a large contribution of organic substrate from different plant sources to the soil, benefiting the proliferation of a wide range of organisms. As for the control of weeds, it is convenient to avoid their total extermination, at least after closing the treetops, as it also contributes to the diversified addition of organic substrate to the soil and, consequently, intensifies its biological activity.

Some of these plant species have the ability to fix nitrogen. For example, Brachiaria have bacteria of the genus Azospirilum associated with their rhizosphere, capable of fixing atmospheric Molecular Nitrogen. They also raise the organic matter content of the soil and improve its structure. Certainly, the amount of understory maintained must be below the level of economic damage for the forest plantation.

Some microorganisms can protect and/or promote plant growth. Among them are free-living symbiotic or saprophytic rhizobacteria that live in the rhizosphere: a thin layer of soil around the fine roots. Thus, plants provide a habitat (or niche) for these microorganisms to acquire water, nutrients and energy sources, such as carbon and amino acids, in addition to secondary metabolites.

Among the beneficial rhizobacteria, the most studied species are Pseudomonas fluorescens, Pseudomonas putida, Azospirillum brasilense, Serratia marcescens, Bacillus subtilis, Bacillus megaterium, Rhizobium, Bradyrhizobium, Arthrobacter, Enterobacter, Azotobacter, among others. Bacteria of the genus Pseudomonas are particularly noteworthy because they have a great capacity to suppress soil pathogens, have a wide natural dispersion and high population, in addition to producing a wide variety of antibiotics, siderophores and plant growth hormones.

However, for practical purposes of use in the field, bacteria of the genus Bacillus have some advantages over those of the genus Pseudomonas: they are more resistant to desiccation, form endospores and have greater survival capacity when formulated with different polymers and inerts. Among the phytohormones produced by microorganisms are auxins and cytokinins. Directly on the plant, they induce growth, cell elongation and resistance to abiotic stresses and stimulate the reproduction and colonization of beneficial microorganisms. Indirectly, they regulate the plant's immune response to pathogens.

Currently, with the risks of reduced supply and the increase in the cost of conventional fertilizers, studies and tests with alternative sources of nutrients have been intensified. Among them is the application by broadcasting or the incorporation into the soil of powder from rocks rich in Potassium (example: nepheline-syenite and syenite ) or Phosphorus (natural phosphates). Microorganisms contribute to the solubilization of nutrients contained in these products. Potassium and Phosphorus solubilizing bacteria and fungi carry out this process through the exudation of organic acids, H plus cation, exopolysaccharides, siderophores and enzymes (example: phosphatases).

They are also able to release phosphorus adsorbed on iron and aluminum oxides and mineralize organic phosphorus, making it available to plants. Among the main Phosphorus solubilizing bacteria are those of the genera Bacillus, Micrococcus, Pseudomonas, Burkholderia, Rhizobium, Agrobacterium, Azotobacter and Erwinia, and, among fungi, those of the genera Aspergillus and Penicillium.

Recently, Embrapa, in partnership with the company Bioma, launched the inoculant BiomaPhos, composed of phosphorus solubilizing microorganisms. The product developed by the team led by doctor Christiane Paiva, a researcher at Embrapa, is composed of strains BRM 119 (Bacillus megaterium) and BRM 2084 (Bacillus subtilis). It is sold in liquid form, indicated for application via jet in furrows or for seed treatment.

Monitoring soil organisms is a good ecological indicator of the level of environmental contamination by pesticides or heavy metals. Bioindicator organisms act as sentinel species of early changes in the environment. They don't die from these changes, they just respond to them through measurable behavioral or metabolic reactions.

Among them, earthworms have been used a lot, because they play an important role in the decomposition of organic matter and in the transport of microorganisms in the channels formed by their excavation. The levels of pollutants contained in their bodies are directly related to the degree of soil and litter contamination they ingest.

Finally, the implementation of effective soil ecological management practices is another important step towards the establishment of regenerative forestry. As seen, the intensification of soil biological activity recovers and/or maintains essential ecological processes, which contribute to mineral nutrition, phytosanitary protection, growth and survival of trees. This is a branch of science and technology that needs to be studied more. Among the necessary Research and Development guidelines, the following can be listed:
1) the characterization of soil and plant microbiomes under different edaphoclimatic conditions;
2) spatial and temporal evaluation of the influence of implantation, maintenance and harvesting practices on soil ecological processes;
3) the introduction of new silvicultural practices that intensify and diversify soil biological activity;
4) the development of bioinoculants capable of improving the nutrition and phytosanitary protection of trees.