To properly conduct a review of this kind, we refined our search in ScienceDirect, Scopus, Google Scholar, and other additional scientific browsers in order to build a sufficiently strong literature database of long-term and short-term field research, meta-analyses published in peer-reviewed journals by introducing the terms “pecan,” “nuts,” “noz pecá,” “nogales,” “nuez pecanera,” “orchard,” “carbon dioxide,” “carbon sequestration,” and “nutritional quality.” We also related the aspects of this search with regional information such as maps and statistics.

Unlike previous research efforts focused on analyzing standalone components of the pecan production system, this review adopts a novel perspective to contextualize and analyze the potential contribution of pecan to SDG 2 under the dual perspective of carbon storage and human nutrition. Particularly, the study focuses on the pecan agroecosystems in the Americas, considering the most important pecan-producing countries (the USA, Mexico, Brazil, Argentina, Uruguay, and Peru). Therefore, the American pecans are here understood as an AFS that may potentially provide several opportunities to meet SDG 2’s 2.2 and 2.4 targets. Our review is particularly relevant as the value of pecans is often measured only based on their economic and social benefits, while their function and current and potential role in climate change mitigation has been systematically overlooked.

The rest of this paper is structured as follows. Section 2 summarizes the literature on the problem of soil carbon depletion in AFSs and the role of pecan-based agroforestry as an SOC improver. Section 3 demonstrates the global need for a balanced diet and the nutritional value of pecan. Section 4 mainly exposes relevant statistics about pecan production and planted area in the Americas, with estimates about carbon stock. Section 5 puts forward the relevance of pecan orchards as a platform to integrate sustainable AFSs. Section 6 analyzes pecan orchards as a sink for carbon sequestration in aboveground and belowground biomass. Finally, Section 7 bears conclusions and recommendations.

The Agriculture, Forestry, and Other Land Use activities contributed to 13% of CO₂, 44% of CH₄, and 81% of N₂O emissions from human activities during 2007–2016 (19); however, depending on the soil management and land use, agricultural activities can be considered as a source or sink of GHGs, changing the carbon and nitrogen dynamics (19, 20) and contributing negatively or positively to the global C budget. For example, Sanderman et al. (21) estimated that, at the global level, soils accumulated a C loss of 116 Pg at 2 m depth. This loss is often associated to regions with soil degradation caused by crop and pasture mismanagement, leading to high soil respiration rates and SOC exhaustion, suggesting that these regions should be targeted in efforts to restore SOC (21). The global estimates of total degraded areas range from 1 to 6 billion ha, which represents 40% of the world’s soil (22, 23). Planting pecan orchards can contribute to mitigate SOC loss in such areas by acting over soil C respiration (15, 17) and through C sequestration (16).

Carbon sequestration (as in soil as well as in trees biomass) can significantly contribute to the stabilization of global warming levels, helping to keep the annual global temperature increases below 2°C or even 1.5°C in relation to pre-industrial levels, a target stipulated in the Paris Agreement (21st Session of the Conference of the Parties or COP-21) (23, 24). In such a sense, it has been estimated that an annual 0.4% increase in the SOC stock of the 0–40 cm layer of the soil can offset anthropogenic emissions and stabilize climate change, in addition to ensuring food security (3). Nonetheless, the latter estimate overlooked the contribution of C sequestered by tree trunks.

Likewise, although some conservationist agricultural practices (e.g., no till) to protect the soil and increase SOC are essential for maintaining the sustainability of agricultural systems and the environment (25–27), they demand more inputs every year. Therefore, the adoption of agricultural systems based purely on low-carbon agriculture technologies is a condition sine qua non to guarantee a positive C budget. Agroforestry plays a significant role to guarantee that result, with South America having a high potential to offset global emissions, estimated approximately 8.24 Pg C for the period 2016–2050 (28). As mentioned earlier, and given its suitability for agroforestry, pecans perfectly fit to meet this challenge. Thus, the assessment of the environmental impacts caused by agricultural systems has increasing focus, as well as the development of technologies and agricultural management practices to reduce and offset these impacts (29, 30). Agricultural management practices can significantly change the dynamics of GHG emission; thus, it is essential to study and monitor GHG fluxes in different climate and soil conditions (31–33).

Agroforestry systems seek sustainable agricultural production by associating the different agricultural, livestock, and forestry production systems, which can be done in intercropping, in succession, or in rotation, mutually benefiting the components (34, 35). The adoption of less aggressive management practices, such as the agroforestry system replacing the conventional system of cultivation, contributes to the mitigation of CO₂ and stores greater amounts of C in the soil (36–38). That system increases the plant residues in the soil surface and the soil organic matter content, reducing soil erosion and improving their resilience and their capacity to store carbon (34, 39). For example, after 4 years of conversion of degraded pasture to an agrosilvopastoral system in Brazil, there was an increase in soil organic carbon stocks from 52.6 to 66.5 Mg ha^-1, indicating that it is a sustainable alternative and option for the recovery of degraded soils (37). In addition to the recovery of degraded areas, the agrosilvopastoral system preserves forest areas due to the land spare effect and brings economic and social impacts, promoting positive effects throughout society (40, 41). Similarly, a nut tree–based agroforestry can lead to counterbalance SOC loss and soil GHG emissions through C sequestration (11).

Soil C erosion is driven by a weakened soil structure as result of poor agricultural practices applied in susceptible soils, which impacts long-term soil sustainability (21, 27). This loss of C also reflects the exhaustion of other nutrients as it has been demonstrated in the Argentina`s case (42). Alike the soil GHG emissions problem, several farming practices have been identified to minimize or restore SOC loss by reducing soil erosion and improving soil quality (43). One of the most widespread practices is the use of cover crops, which have been demonstrated to enhance SOC sequestration when applied to pecan-based agroforestry systems (44).

Overall, soil C sequestration depends upon the weather (especially temperature and rainfall) and soil characteristics (45) that affect the activity of the soil microorganisms involved in degrading organic residues (46) and the amount and quality of the C input (47). For that, indicators such as the precipitation/temperature ratio are essential for understanding cover crop residue decomposition in different environments (48). In addition, soil texture is another key factor that may modify soil C fixation (49) by regulating soil aeration and, consequently, SOM decomposition and mineralization (27). In fine-textured soils, the smaller average pore size and the presence of organic–mineral complexes reduce SOM exposure to microbes for mineralization compared with the large pore sizes typical of coarse-textured soils (50). Thus, cover crop residues decompose differently according to the location and the system they are included in. For example, the inclusion of cover crops in cropping systems showed average SOC sequestration rates of 0.45 t C ha^-1 yr^-1 in the Pampa Region (51–53), fertilizer use (urea, 46% N) showed an increment of approximately 0.18 t C ha yr^-1 (54, 55), and the inclusion of cycles with perennial pastures in crop rotations showed average SOC sequestration rates of 0.76 t C ha^-1yr^-1 (56, 57). Then, with proper management, a cover crop growing upon a pecan-based agroforestry system may boost soil C sequestration, adding up to soil biomass produced by the association between pecan and ectomycorrhizal fungus (15).

It worth mentioning that current area meant for agroforestry could sequester up to 2.2 Pg C above- and belowground in the next 50 years, but estimations on the global land area occupied by agroforestry systems are particularly uncertain. Global areas under tree intercropping, multistrata systems, protective systems, silvopasture, and tree woodlots are estimated at 700, 100, 300, 450, and 50 M ha, respectively. The SOC storage in agroforestry systems is also uncertain and may amount to up to 300 Mg C ha⁻¹ to 1 m depth (58). In China, for example, a meta-analysis provided evidence that existing agroforestry systems, particularly shelterbelts and agrosilvicultural systems, increase SOC stocks due to not only SOC sequestration but also the effect of protection against wind and water C dispersal. Moreover, Mayer et al. (59) observed that the use of agroforestry systems increases SOC compared to cropping systems in a temperate climate. In either case, pecan-based agroforestry integrated in the green belts of the cities may bring substantial benefits by increasing and conserving SOC stocks.

Under the context described above, pecan orchards and pecan-based agroforestry have interesting implications on soil health and particularly on SOC. In such a sense, pecan trees growing in their native region (the USA and Mexico) will successfully prevail in well-drained, OM-rich soils, which, in addition to being subjected to C enrichment via the shedding of pecan leaves, shucks, sticks, bark, and other debris, self-generate high-quality conditions (60). In a well-detailed study performed during 2020 and 2021, Slade and Wells (61) found that the values of active carbon, aggregate stability, organic matter, total N, and Solvita CO₂-burst (a test for measuring soil microbiological activity) for different aged pecan orchards amply overpassed those of row crops, with the most aged orchards having better soil quality. In another study, Dold et al. (44) marked the relevance of the contribution of the leafy material to carbon sequestration and N supply to soil in a 17-year-old pecan-based agroforestry system, estimating a mean contribution of 17 and 0.68 kg N tree^-1 for falling pecan leaves during 2016. Finally, Idowu et al. (62) proved that the addition of pecan husks to arid soils boosted P, K, and wet aggregate stability for sandy clay and sandy loam textured soils. Therefore, this positive feedback between what is produced aboveground and the soil ensures soil with high standards of quality, offering a unique AFS to cope with soil C emissions and soil erosion. Moreover, soil quality is intrinsically associated with food quality and with the agricultural practices leading to maintain such quality. In that sense, after evaluating the effects of conventional and organic fertilization on pecan, Noperi-Mosqueda et al. (63) concluded that the nutritional properties of the pecan nut were improved under a balanced combination of both fertilization practices, with phenolic compounds N, P, and K being in the highest values. In addition, falling pecan husks have been proven to absorb heavy metals like chromium (64); thus, food produced under this type of agroecosystem can be considered environmentally safer than other AFSs.

In future studies, effort should be put on understanding the response of the biophysics (soil temperature, soil moisture, etc.) to management in the pecan orchard when integrated to agroforestry, in order to develop robust models to predict C cycling in these systems.

Unlike North America, South American countries have their pecan orchards mostly in the initial stage of implantation and production; thus, most of their trees have not yet attained the potential nut yield. Although most actions promoting the increase of planting area and nut production are being carried out in Brazil and Argentina, pecan is growing in importance in other South American countries like Uruguay and Peru.

As repeatedly mentioned earlier, pecan trees represent a singular sink for atmospheric C; however, the literature seems to have overlooked the global dimension of this ecosystemic service for this edible fruit tree. Given this knowledge gap and through a rather simplistic method, we made the first approach by gathering the data of planted area during the period 2012–2022 for all the Americas’ countries only excluding Peru due to the scarcity of temporal data. The planted area was multiplied by an estimated mean tree density of 70 trees ha^-1, and then, the mature pecan tree population by country was weighed by a biomass of 1.2 t tree^-1 year^-1 and a gain of 0.2 t year^-1 tree^-1 for mature trees (104). Immature trees were considered to have 50% of mature tree biomass and 50% of the yearly gain in biomass. Carbon stored in biomass was considered to be half the biomass, and CO₂ equivalents removed from the atmosphere were calculated by multiplying C stored in biomass by 44 and dividing it by 12.

The results shown in Table 3 reflect a statistical indicator that has been neglected in the bibliography so far: the existence of ca. 23 million pecan trees in the Americas (excluding Peru), storing 14 Mt C in their biomass or, what is the same, removing 51.3 Mt CO₂eq from the atmosphere. However, this is only part of the story since, if we consider a mean annual C sequestration rate of 9.11 t C ha^-1 yr^-1 (18) for mature trees only, including the vegetation surrounding pecan, and integrate the planted area from 2012 to 2022, the cumulative stored C in the system (pecan biomass + soil) during that period results in 21.92 Mt C, having removed 80.36 Mt CO₂eq, with mature orchards occupying 72.3%–77.3% of the planted area in the Americas for the considered period. This denotes not only the environmental potential of pecan orchards as C sinks alone but also a hypothetical increase in C sequestration when integrated into other activities, for example, silvopastoral systems or alley crops. Although we found some studies on pecan pioneering with calculations of this sort (16, 94), future studies should provide friendly tools for pecan farmers to be able to calculate environmental gains in their orchards.

The global apprehension is that carbon management should be in forests to mitigate the increased concentration of greenhouse gases in the atmosphere. However, the worldwide forest cover is diminishing fast in view of great biotic stress, industrialization, urbanization, land use changes for developmental activities, and the conversion of forests to agricultural land. The importance of terrestrial vegetation as significant sinks of atmospheric carbon dioxide and its other derivatives is highlighted under the Kyoto Protocol (119).

Pecan trees have recently drawn attention as climate-smart production systems for temperate regions as they can provide high net carbon (C) gains per area and generally occupy a relatively small fraction of the agricultural landscape (44). Pecan trees form an important component in the different production forest systems, which provide extensive ecosystem services. In this context, biomass and carbon inventory in these production systems are essential (119).

Pecans are grown in many climates under different environmental conditions, and this diversity can generate variations in the carbon sequestration in aboveground biomass between regions. In the wetter areas of the USA, pecans can grow much larger and at rates different than in the arid Southwest, simply as a result of soil moisture. Usually, the age structure of the pecan population in humid regions is older than in the West. Although this may not be good for the constant production of pecan, the longer a tree lives, the more carbon is stored in aboveground biomass (107).

Globally, the estimated range between 40 and 150 t C ha^–1 is the average quantity of carbon stored in the aboveground components of agroforestry systems (19). One of the most important benefits of pecan trees is their high photosynthetic rate due to their long phenology, which increases its potential annual atmospheric CO₂ capture (18). The pecan can store significant amounts of carbon in aboveground biomass for a long time. Based on the calculations of Kraimer et al. (104), a mature pecan (approximately 15 years old) has a biomass of 1.2 t per tree and accumulates approximately 0.2 t of biomass per year, depending on climatic and edaphic conditions and nutritional and cultural management (107).

Carrying out studies related to carbon sequestration in aerial biomass in pecan forests, associated with different production systems in India, the tree density of an 11-year-old C. illinoinensis orchard was 238 trees ha^-1, and the total tree basal area was 6.11 m² ha^–1. The pecan orchard had a mean diameter at chest height (dbh) of 18.04 cm and a wood volume of 0.266 m³. The maximum values of biomass (56.5 t ha^–1) and carbon (25.3 t ha^–1) were determined (119). With annual herbaceous productions cultivated between pecan lines, these mixed productions raise the possibility of maximizing global production per surface unit, evaluating positive interactions between pecan and herbaceous crops, and controlling weed development.

Tests carried out with organic amendments recorded a significant increase in both height and diameter ( Figure 8 ).

Timber production in pecan silvopastures is often neglected despite pecan’s wood use for cabinetmaking, paneling, furniture, and flooring (121). Most pecan timber comes from tree thinning aimed at reducing overcrowding to sustain nut production. A fundamental step towards the improvement in the management of the timber resource in pecan silvopastures is the determination of standing stocks and annual increments, thus enabling the derivation of potential extraction rates (122).

In summary, pecan orchards represent an important reservoir of C in the aerial biomass, representing a stable compartment of the CO₂ fixed from the atmosphere, given the longevity of the tree and its potential timber use. In addition to the commercialization of nuts, the implementation of walnut forests is considered of great environmental and commercial value.

Pecan trees require approximately 12–15 years of growth after establishment to deliver economic yield with nuts; thus, the use of an agroforestry system where the alley between tree rows can be used with perennial crops can be used to sustain livelihood security and maintain soil cover and quality (112). In addition to the diversification of products that the agroforestry system with pecan delivers (food, fodder, fruit, pruned material such as firewood, nuts, etc.), it provides carbon sequestration, capturing CO₂ from the atmosphere and storing it in the soil as organic matter–stabilized forms, while increasing productivity (116).

Agroforestry system with pecan has the potential to increase soil fertility and microbial biomass carbon due to the constant input of the deciduous leaves and root turnover to the soil in a long term (47 years). The presence of older trees also leads to a shading effect, with a reduction in soil temperature and an increase in soil moisture, fine root biomass, and soil carbon content when compared to young trees (17). Another agroforestry system with pecan in the Ozark Highlands region of northwest Arkansas, United States, that received poultry litter as organic fertilizer for 17 years increased superficial (0–15 cm) nutrient concentrations and reduced bulk density when compared to inorganic fertilization (123). That management practice was reflected in higher soil organic matter concentrations and soil carbon stock, which was also influenced by the differential decomposition of pecan leaf litter (123). In that same agroforestry system study, pecan soil that received poultry litter application got higher soil quality index (SQI) than agroforestry with red oak (124). The SQI index used soil organic carbon, pH, P, K, and bulk density as part of the methodology Soil Management Assessment Framework (SMAF), which is a quantitative soil quality evaluation tool that is used to check soil responses to management systems (125).

The pecan orchards with organic management have increased concentrations of nitrogen, microbial biomass carbon, and soil organic carbon when compared to conventional practices (113). Furthermore, the soil organic matter in the organic pecan management is more stabilized than more recalcitrant forms, reducing the CO₂ released to the atmosphere, which promotes more efficiency in carbon sequestration when compared to conventional management (113). In an agroforestry system with 19-year-old pecan trees (C. illinoinensis) and bluegrass (Poa trivialis), the total soil organic carbon and total nitrogen were higher in the tree rows than in the middle of the alley, but it found no difference in the particulate organic matter and microbial biomass carbon between those sites (126). Litter quality also influences soil carbon dynamic, as found in a 21-year-old pecan and bluegrass (P. trivialis) intercrop study in which the bluegrass litter mineralized significantly higher amounts of C and decomposed 2.3 times faster than pecan litter, which has higher C:N and lignin compounds (127).

Additionally, aged, mature pecan orchards present better soil aggregate stability, mostly because of the higher levels of soil organic matter and microbial activity, which result in better soil quality when compared to row crop fields (61). Pecan roots are capable of increasing nutrient cycling and cation exchange capacity (CEC) as they can remove nitrogen from the deeper soil profiles and bring it to the upper layers and can also reduce nitrate leaching below the root zone (61). The use of a perennial cover cropping with Kura clover (a nitrogen-fixing legume) between pecan tree rows in an agroforestry system promoted a restoration of soil physical properties by rhizodeposition, which encouraged soil-stable aggregation, reducing erodibility and enhancing soil organic matter and providing economic benefits for pecan while achieving maturation (112). Deficiency in mineral nitrogen in pecans may lead to nutrient imbalance and toxicity; thus, that nutrient is very important for the growth and development of that species, which is more inclined to use NH 4 + –N than NO 3 − -N, with a ratio of 75:25 considered the most favorable for pecan seedlings (128).

As observed in this part of the review, there are some studies that assessed soil and nitrogen soil content, but few studies evaluate soil carbon stock in pecan orchards and their potential to increase it when compared to a monoculture or native vegetation. That marks a research gap in the study of the environmental aspect of pecan for climate resilience production.