4.1. Challenges for Implementing and Consolidating Sustainable Production Systems in the Amazon
In the broader perspective, authors such as [
19] reiterate the diffuse concept of “agroforestry”, which makes it difficult to formulate policies that support the transition from the agribusiness-based model (monoculture) to forest production models. Agroforestry is seen as a solution to environmental and social problems [
20,
21], but it is often an ambiguous and poorly defined concept, with a wide variety of agroforestry practices in different parts of the world. The authors recommend studies that address the broader political and economic aspects of these systems rather than biotechnical studies at the property level. Furthermore, it is important to consider agroforestry transitions as a system-level issue, thus, research that includes political, social equity and justice issues should be carried out [
19].
In a study evaluating the challenges for implementing agroforestry systems in the Amazon, [
22] identified labor, implementation costs, and know-how as the challenges most emphasized by producers in Mato Grosso, and in relation to the forestry element, implementation costs and marketing were the most important challenges. Other factors such as farm size, farmer resources, and cultural preferences were also cited as influencing the adoption of integrated systems. In addition, the same study highlights the importance of policies to encourage low-carbon agricultural production, such as the ABC plan. In a study focused on the perception of rural producers in the states of Rondônia, Acre, Mato Grosso, and Pará, [
23] discussed the difficulties in adopting integrated crop-livestock systems and identified several obstacles, including the lack of qualified labor, few marketing options, inadequate infrastructure, unfavorable regulatory environment, and, in some areas, poorly drained soils. Furthermore, non-income-related reasons, such as maintaining quality of life and preserving traditions, have diverted producers from the pursuit of profit maximization. A broader range of policies, beyond credit subsidies, is needed to encourage the adoption of sustainable intensification strategies. These include educational programs, compensation for ecosystem services, and improvements in transport and logistics infrastructure, which can support intensification and foster an environment conducive to innovation. Agroforestry systems have proven to be an effective option for increasing pasture productivity. Public policies should, in particular, prioritize the dissemination of sustainable agroforestry practices in the Amazon biome, where the growth of cattle herds and pastures has been most significant in recent decades [
24]. Regarding SAF as a strategy for restoring vegetation cover and ecosystem services, these systems have not proven to be efficient in a short period of time and in sandy soils. Furthermore, previous degradation resulted in high variability in plant development and carbon stocks[
25]. The promotion of sustainable systems in the Amazon motivated by government benefits for associations and cooperatives does not impact the intensity of adoption of these practices by organizations. Conditions such as particularities of the producer, the associate, financial and management characteristics, attributes of sustainable practice and psychological attributes are preponderant factors in the adoption of sustainable practices. Furthermore, agricultural policies should, in addition to economic support, consider continuous training on new technologies for the sustainable natural resources market [
26].
However, despite the use of integrated systems, conceptually, aggregating benefits between the elements of the system, for [
27] in the Amazon conditions in the state of Mato Grosso, the use of a silvopastoral system with eucalyptus did not reduce the thermal stress of animals, despite the better conditions under the tree canopy. According to [
24], in the Amazon biome, no significant impacts were identified on livestock stocking rates or total value of agricultural production in agroforestry, thus not qualifying the intensification of land use.
In intensive fish farming in the Amazon, low water renewal and high fish density are factors that compromise water quality. These conditions negatively affect zootechnical indices and animal welfare, resulting in reduced production yields [
28].
Regarding sustainable forest management, there is interest in its adoption by small farmers, but in the study by [
29] none of them systematically manage the forest. According to farmers, the recovery of degraded areas on their properties is hindered by the lack of economic incentives and high initial costs (seeds and seedlings). For about 40% of farmers, the collection of non-timber forest products (Brazil nuts and hearts of palm) for subsistence is one of the ways they use the forest, but policies to encourage the extraction of non-timber products from the forest are limited for this population.
For [
30], community forestry is dominated by external decisions, which promotes industrial-scale forestry practices at the community level or prioritizes the interests of external agents. In this study, the authors indicate that timber is only one of several livelihoods for producers, thus, goals beyond timber and more comprehensive should be considered. “Bottom-up” actions allow defining goals that are more important to beneficiaries and ensure the success of the forest management project.
Regarding sustainable forestry in the Amazon, the competition between timber from sustainable sources and areas of agricultural expansion is uneven. The domestic market for non-certified timber is more accessible to sawmills, which typically have small profit margins and follow the flow of the agricultural frontier in the Amazon. In this sense, developing means for community-based family exploitation to be more sustainable is essential to creating a basis for sustainable timber production in the Amazon. In addition, the decentralization and democratization of forest land ownership through the creation of extractive reserves and autonomy for states and municipalities to demarcate land is also essential for timber exploitation in a way that does not harm the environment [
31]. However, sustainable timber production on public lands is highly dependent on low-impact logging, which does not consider factors such as sustainable harvest cycles for forest stands and individual species, and planting and regeneration of seedlings for high-value species that occur at low densities. Furthermore, low-impact logging, when not combined with forestry, leads to volume reduction and imminent extirpation of high-value timber. Moreover, the regulatory and technical capacity to implement low-impact logging in new areas does not currently exist, and there is a disparity between demand and human resources in government agencies [
32]. This scenario hinders the monitoring of illegal logging.
Forest management of natural populations, such as açaí, can lead to gradual impoverishment of the flora over the decades due to thinning to favor the species of interest. Therefore, multi-taxonomic studies are needed to support management plans for economic-ecological zoning in Amazonian floodplain forests managed for açaí, aiming to avoid large-scale loss of cryptic biodiversity [
33]. In addition, the use of integrated pollination, that is, the use of wild and managed pollinators, can reduce pollination by wild bees, and this can mean increased environmental and socioeconomic risks associated with the activity. Therefore, it is suggested that producers prioritize the preservation of forest areas on their properties to safeguard pollination services and the sustainability of açaí production in the Amazon [
34].
Current indications of models for predicting biomass for the exploitation of endangered species, such as rosewood (
Aniba rosaeodora Ducke) are not appropriate for measuring productivity, representing a serious obstacle to subsidizing the activity [
35].
4.2. Opportunities for Implementing and Consolidating Sustainable Production Systems in the Amazon
Agroforestry systems are promising for above-ground carbon sequestration and reestablishing nutrient cycling when compared to natural succession [
25]. According to [
36], AFSs with oil palm are equally efficient for nitrogen immobilization in soil microbial biomass as secondary forests. [
37] indicates that there is a possible difference in the microbial community of plants present between AFS (açaí and cocoa), cocoa monoculture and adjacent forest, which may allow greater ecological diversity and nutrient richness in the soil. However, further studies are needed to understand the diversity, relationships and functions of the microbial community in the system addressed.
Biodiversity maintenance can be promoted by AFS in the sense of retaining natural species; however, even with restoration planting, there may still be biodiversity loss. Re-agroforestry of degraded areas with tree and understory crops can also help in the food security of producers; however, it is important to prevent the encroachment on native forest and to intensify the system [
26].
Soil protection provided by AFS vegetation cover contributed to reducing soil vulnerability to erosion and mercury leaching processes, comparable to mature forests; however, this vegetation protection did not completely prevent leaching. The transition to this cultivation system is challenging, so it is necessary to prioritize initiatives to support them in the implementation of this agricultural model [
38].
The adoption of integrated crop-livestock-forestry systems can be stimulated by public/private partnerships to strengthen the flow of information and allow investment in infrastructure, since despite adherence, it is still a challenge to encourage them to adopt this system in the country [
39].
The application of this production model contributes to the improvement of macroaggregates and an increase in carbon and nitrogen stocks in soils [
39,
40]. However, further research is needed to better understand the driving forces and impediments to the accumulation of organic C in the soil in integrated systems, including studies on the stability of organic matter [
39].
Also considering aspects of ecological recovery capacity that integrated systems have, and their importance in the Amazon, [
41] infers that agropastoral systems can be agroecological models, with increased self-sufficiency, resilience to market shocks and reduced environmental impacts when the links between system elements (soil, crops and animals) follow agroecological principles (diversity of land use and biotic and abiotic resources, maximization of ecological and production interactions, among others), improving its performance.
The practice of livestock farming in the Amazon is a paradigm full of controversies, which is why the adoption of technologies and management supported by incentive policies for more intensive and sustainable livestock farming in the Amazon is a viable alternative for increasing productive and economic yields, and consequently reducing pressure on forests. However, this adoption by livestock farmers, especially small farmers, will require strong political will through government subsidies, such as the ABC program [
40].
Regarding the benefits arising from the use of AFS, with regard to the factor for animal production, studies linked to the synergy between the elements of the system are abundant. The production of beef cattle in integrated systems allows performance as advantageous as the monoculture of grasses, when managed correctly; in addition, the synergy between the components of the crop-livestock-forest integration indicates that this system has an even greater potential to increase cattle production in the Amazon [
42,
43], however, long-term studies are recommended [
42].
According to [
44], the shading caused by eucalyptus on Marandu grass (
Urochloa brizantha) at a distance of three meters, where the shading was longer, significantly affected the composition and characteristics of the canopy; however, the grass is resistant at greater distances from the tree rows. Pastures with Marandu grass shaded in silvopastoral systems, with a reduction of up to 30% in PAR, maintain leaf productivity similar to that of a monoculture. In the long term, pruning, thinning and east-west woodland reduce the shading effect on forage [
45]. According to [
46], if well managed, the cropping system can store carbon, resulting in benefits such as increased meat production and improved soil quality. Additionally, the inclusion of plantations and forests in these livestock systems enhances these benefits, highlighting the potential of integrated systems to offset greenhouse gas emissions.
Integrated crop-livestock systems can act as pathways for the accumulation and release of C, depending on the management and level of pasture degradation. Succession with soybean as the main crop without soil disturbance results in carbon accumulation, depending on the crop introduced, soil and climate conditions and time of use of the system [
47].
The application of nutritional management strategies for conventional pasture fertilization (urea and ammonium sulfate) can contribute to mitigating greenhouse gas emissions, improving forage accumulation and animal production. Furthermore, it enables sustainable forage intensification, avoiding the opening of new areas. Microorganisms such as
Azospirillum brasilense can be recommended for supplying nutrients to pastures in the Amazon, but further studies to evaluate this technique are needed [
48]. Although only one study on animal supplementation in silvopastoral systems in the Amazon was observed in the databases evaluated, the study by [
49] observed that supplementation reduces the emission of methane and volatile fatty acids in vitro, the main source of energy for ruminants.
The use of spontaneous capoeira grass pastures by small farmers in the Bragantina region of the state of Pará is a strategy for producers who want to maintain the recovery capacity of their lands and already have a tradition of production in fallow areas after human impact and fires. However, this system has a lower performance than grass monoculture. These results should be considered with caution, since the study time for such conclusions is relatively short (3 years). Furthermore, these systems are not suitable for intensive livestock farming, but are an alternative for small producers who cannot afford the management and high initial investment of legume pastures [
50,
51].
Agricultural production in the Amazon presents alternative models for areas replacing traditional slash-and-burn, focusing on improving soil quality. In the northeastern region of Pará, [
52] observed an improvement in the physical-chemical quality of soils after the application of the SHIFT-Tipitamba cultivation system (cutting and shredding of cover vegetation) when compared to the slash-and-burn system. For [
53], slash-and-shred provides intermediate and variable values for most of the physical and hydraulic properties of the soil, in a certain way improving physical quality. Thus, these techniques can enable the viable intensification of agriculture in the Amazon. Thus, sustainable animal production in the Amazon can present specific conformations for the biome, which consider particularities of the regional historical evolution of agricultural/animal production [
50].
The forest element in silvopastoral systems in the Amazon is considered an essential factor for the adaptation of these systems in the region. Agroforestry systems have the advantage of being able to sell timber, in addition to agricultural and animal production, in the same area. According to [
54], the production of Paricá (
Schizolobium amazonicum) in AFS is promoted when soil management practices are not carried out. However, when intercropped with soybeans (1 year) and corn (2 years), subsoiling, fertilization and inoculation of growth-promoting microorganisms are recommended. The quality of freijó (
Cordia goeldiana) wood produced in AFS was similar to that obtained in monoculture or native forest [
55]. The use of
Ficus insipida in monoculture or AFS in floodplain areas can help to reduce pressure on the few remaining areas of intact floodplain forests. Furthermore, an additional benefit would be the improvement of the economic situation of the local riverside population in a relatively short period of time [
56]
Sustainable aquaculture in the Amazon, through multitrophic integration, can be considered an important provider of ecosystem services to mitigate the eutrophication of receiving water bodies and sequester carbon dioxide from the atmosphere, revealing potential for intensification of this system when compared to conventional aquaculture. This model can be applied with cultivation in the biofloc system and can be an alternative to promote good conditions for intensive tambaqui cultivation, with minimal water exchange compared to breeding in clear water, as it maintains good zootechnical and animal welfare indices [
28].
Commercial management and sequential management of biomass through pruning of endangered forest species, such as rosewood (
A. rosaeodora), substantially reduces the volume of macro and micronutrients exported compared to cutting down entire trees, making it a satisfactory alternative for forest management [
35].
Predictive equation models to estimate mass and volume can be an alternative for forest management in productive plantations under different conditions and management options, and can be applied by the legal devices that regulate the activity [
57]. The application of the Weibull growth estimation model presents good performance to estimate the annual increase and diameter of
Manilkara elata, allowing its use in forest production planning and growth and production prognosis in short-period remediation plantations of a given species or for species that do not form annual growth rings for dendrochronological studies [
58]. Measures to assist in monitoring commercialized wood, such as analysis of wood anatomy and its availability in databases, allow the use of this information by government agencies to control illicit trade [
59].
Forest management for non-timber forest products enables integrated crop pollination (ICP), i.e. the use of wild and managed pollinators, which contributes to conserving pollinator diversity while simultaneously ensuring effective pollination services and increasing crop yields [
34].