Introduction

Ensuring global access to sufficient, safe, and nutritious food to achieve the United Nations (UN) Sustainable Development Goals (SDGs) is one of the key challenges facing humanity. The UN Food and Agriculture Organization (FAO) estimates that nearly one billion people worldwide are currently chronically undernourished (McGuire 2015). The challenge of preventing or reducing undernourishment will become even greater in future as the world population is projected to rise to 9.7 billion people in 2050 (UN 2022), representing almost 50% increase over the 2005 population of 6.5 billion people (UN 2017). Agricultural expansion and intensification have been responsible for substantial productivity gains that have contributed to the reduction of undernourished people and improvement in human health and well-being (Corvalan et al. 2005). Since the efforts to boost agricultural production are motivated not only by food security enhancement, but also by income generation and economic growth, enormous pressure on agricultural systems will continue over the next decades. Despite the obvious advantages of these strategies, especially in safeguarding global food security, concerns exist both, in terms of their sustainability and their impacts on the environment and ecosystem services. For example, previous studies have linked expansion and intensification of agriculture to soil degradation (Tully et al. 2015), biodiversity loss (Kehoe et al. 2017), nutrient depletion and imbalances (Vitousek et al. 2009), soil acidification (Li et al. 2020b) and greenhouse gas emissions (van Loon et al. 2019). In this context, soil degradation is a serious and growing global problem of the 21st century. It compromises soil health and its ability to ensure sustainable delivery of ecosystem services, including provisioning (e.g., food and water), regulating (e.g., carbon sequestration and storage), supporting (e.g., nutrient cycling and habitat for biodiversity) and cultural (e.g., education and recreation) services.

Consequently, there is an increasing interest in agroecological approaches that have potential for intensifying food production in the long run, while simultaneously improving or maintaining other aspects of the environment, particularly soil quality, fertility, and health. Agroforestry systems, including those that integrate animals and trees (i.e., silvopastoral), crops, animals and trees (i.e., agrosilvopastoral), or crops and trees (i.e., agrosilvicultural) on the same piece of land, have increasingly been recognized as promising agroecological approaches for sustainable agricultural intensification (Ravi et al. 2015; Rosati et al. 2021; Smith et al. 2012). Agroforestry, located at the intersection between agriculture and forestry, can provide multiple benefits at the same time; including the enhancement of food security and income generation (Fahmi et al. 2018), conservation of biodiversity (Torralba et al. 2016), as well as improvement of ecosystem services, e.g., by soil erosion and soil fertility control (Fahad et al. 2022), as well as carbon sequestration (Guo et al. 2020). Indeed, studies in different climatic regions, including tropical, temperate, and Mediterranean climate, have improved our understanding of the impacts of agroforestry on soil quality indicators, such as physical, chemical, and biological properties (Amare et al. 2022; Beule et al. 2022; Cherubin et al. 2019; Guillot et al. 2021; Lagerlöf et al. 2014; Liu et al. 2019; Mesfin and Haileselassie 2022; Zhu et al. 2019), and their influencing factors e.g., climatic conditions, plant species selection, and soil and water management (Bracken et al. 2023; Souza et al. 2012; Zuazo et al. 2014). Nevertheless, there remains a notable gap in research when it comes to a comprehensive meta-analysis encompassing various climatic zones. However, such a detailed study would be highly valuable as it would yield essential insights and a deeper understanding of the existing patterns and linkages between agroforestry and soil properties across climatic zones. Additionally, it would enable the identification of regulatory factors that pose threats to agroforestry success, as well as factors that promote positive outcomes.

Here, we collected data from studies that reported effects of agroforestry on physical, chemical, and biological indicators of soil quality and their controlling factors across tropical, temperate, and Mediterranean climates. We focused on tropical, temperate, and Mediterranean climates because they are the climates where agroforestry is most commonly practiced and are highly conducive to its implementation. Other climates, such as deserts and polar regions, either lack substantial agroforestry observations or are underrepresented in the available literature (Elrys et al. 2022; Marsden et al. 2019; Spiegelaar et al. 2013). This is probably because these challenging zones are characterized by extreme temperatures, water scarcity, and soil erosion, which greatly impede plant growth and restrict agroforestry research in those areas. Therefore, the collected data were quantitatively assessed to address the following research questions: (i) Does agroforestry improve all soil quality indicators? (ii) Do agroforestry-induced improvements in soil health and fertility show consistent patterns across the three climate zones? (iii) What are the key regulators of agroforestry practices and their subsequent effects on soil quality in tropical, temperate, and Mediterranean environments? The following hypotheses were addressed: (a) Agroforestry improves soil quality across the climate zones studied; (b) improvement of soil properties by agroforestry still varies across climate zones owing to their differences in regulating factors, including temperature, precipitation and plant characteristics, and (c) climatic factors (i.e., temperature and precipitation) are the main determinants of net effects of agroforestry on soil quality of all three aforementioned climate regions.

Materials and methods

Data collection

A total of 125 peer-reviewed articles were selected from 932 studies conducted between 1990 and 2022 (Fig. S1). These articles were found using Google Scholar (https://scholar.google.com) and Web of Knowledge (http://apps.webofknowledge.com). The search keywords were “agroforestry,“ “agroforestry system,“ and “soil quality,“ “soil health,“ “soil fertility,“ “tropical,“ “temperate,“ “Mediterranean,“ “physical,“ “biological,“ “chemical,“ or “microbiological properties” (Fig. 1). To avoid publication biases, the following inclusion criteria were applied during the selection of appropriate studies: (a) experiments included both a control treatment (monoculture) and a treatment plot (agroforestry system); (b) observations were conducted in tropical, temperate, and/or Mediterranean environments; (c) additional information, such as replication, soil layer and agroforestry system type, was provided; and (d) the studies directly reported the means, sample sizes and standard deviations (SD) or standard errors (SE) of the target variables. When the results of a study were reported without SD or SE, we calculated the SD using OriginPro version 2021 software (Origin Lab Corporation, Northampton, MA, USA). The data were extracted from figures using Graph Grabber software (https://www.quintessa.org/software/downloads-and-demos/graph-grabber-2.0.2).

Fig. 1
figure 1

 A linked concept between agroforestry and soil quality, health, and fertility

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Agroforestry systems and soil quality indicators

The following agricultural management practices were considered as agroforestry system: (1) Silvopastoral Agroforestry (combining trees, forage, and livestock in a single system); (2) Alley Cropping (planting trees in rows and then planting crops between the rows); (3) Windbreaks (planting trees in a line to reduce wind speed and protect crops from wind damage); (4) Forest Farming (growing crops and trees together in a managed forest setting); (5) Riparian Buffers (planting trees along rivers and streams to reduce erosion and improve water quality); and (6) Homegardens (growing a variety of crops and trees in a small area around a home).

To evaluate the impact of agroforestry system on soil quality, we used monoculture systems as a control. To compile a database of physical properties, we included MWD: mean weight diameter; WSA: water stable aggregate; water content; water infiltration, water holding capacity; soil moisture; bulk density; clay content; silt content; sand content; porosity and soil erosion. For Chemical properties, we included soil pH; EC: electrical conductivity; CEC: cation exchange capacity; OM: organic matter; soil organic carbon (SOC) content and concentration; TC: total carbon; SN: soil nitrogen; DON: dissolved organic nitrogen; TN: total nitrogen; C/N ratio; C seq: carbon sequestration; Nmineral: nitrogen mineralization; Avail. P: available phosphorus; TP: total phosphorus; OlsenP; K+; Al3+; Ca2+; Mg2+; NO3−; Zn2+; Mn2+; and Fe2+ content. For Microbiological properties, we included microbial community composition; MBC: microbial biomass carbon; MBN: microbial biomass nitrogen; MBP: microbial biomass phosphorus; bacterial and fungal abundance; basal respiration; Rs: soil respiration; urease activity; protease activity and β-Glucosidase activity.

Assessment of factors regulating agroforestry effects on soil quality

To explore the regulating factors of agroforestry effects on soil quality, we collected data on the following 10 variables: (i) Agroforestry management and type: Tree planting and spacing (design and layout), agroforestry time, pruning, and intercropping, as well as agroforestry system types (i.e., agrisilvicultural, silvopastoral, or agrosilvopastoral systems) were taken into account; (ii) Biodiversity: It is among the key components of agroforestry systems as it promotes ecosystem resilience, stability and productivity through the provision of multiple ecosystem services, including soil fertility, nutrient cycling, and climate regulation; (iii) Climate (i.e., temperature, and precipitation): It plays a crucial role in determining the success of agroforestry systems through its direct effects on crop growth, tree health, and overall ecosystem functioning; (iv) Crop species selection: The selection of appropriate crops that are well-suited for agroforestry systems is important. This selection generally considers factors such as potential for intercropping, compatibility with tree species, and market demand; (v) Farmer collaboration and training: Proper training and collaboration among farmers and other relevant stakeholders are crucial for the successful implementation of agroforestry systems. It should be noted that sharing experiences, knowledge, and best practices during these trainings and collaborations can enhance the understanding of agroforestry techniques, improve decision-making, and foster innovation in the field; (vi) Policy support and markets: Implementing favorable policies, incentives, and certification schemes, as well as creating market opportunities and ensuring fair prices for agroforestry products can motivate and encourage farmers to adopt agroforestry practices; (vii) Socio-economic factors, including local community involvement, cultural practices, and economic incentives, play an important role in the adoption and successful implementation of agroforestry systems; (viii) Soil management, including techniques such as mulching, cover cropping, and organic matter addition; (ix) Tree species selection: This takes into consideration factors such as tree growth rate, adaptability to local environmental conditions, economic value, and tree ecological functions; and (x) Water management: It includes techniques such as the use of improved irrigation practices, drought-resistant crop plants, and cover crops.

Meta-analysis

Using the METAWIN software version 2.1 (Sinauer Associates, Inc. Sunderland, MA, USA), we examined variables that could explain the responses of soil physical, chemical, and biological properties to agroforestry. The response ratio (RR) was calculated by comparing the variable-level outcome with the control group (CK). A logarithm of RR (lnRR) was calculated to determine the effect size of each observation (Hedges et al. 1999) as follows:

$${\text{lnRR=ln}}\left( {\frac{{{{\overline {{\text{X}}} }_{\text{t}}}}}{{{{\overline {{\text{X}}} }_{\text{c}}}}}} \right)={\text{ln}}{\overline {{\text{X}}} _{\text{t}}} - {\text{ln}}{\overline {{\text{X}}} _{\text{c}}}$$
(1)

where \(\overline{\text{X}_{\text{t}}}\) and \(\overline{\text{X}_{\text{c}}}\) represent the values of physical, chemical, and biological properties in the treatment and control groups, respectively. The variance (ν) of lnRR was computed as: (Eq. 2)

$${\text{v =}}\frac{{{\text{S}}_{{\text{t}}}^{{\text{2}}}}}{{{{\text{n}}_{\text{t}}}\overline {{{\text{X}}{}_{{\text{t}}}^{{\text{2}}}}} }}{\text{+}}\frac{{{\text{S}}_{{\text{c}}}^{{\text{2}}}}}{{{{\text{n}}_{\text{c}}}\overline {{{\text{X}}{}_{{\text{c}}}^{{\text{2}}}}} }}$$
(2)

where nt and nc are the sample sizes for the treatment and control groups, respectively, and St and Sc are the standard deviations for the treatment and control groups, respectively.

The weighting factor (w) was computed as the inverse variance for each observation to provide a final weighting factor (w′), which was then used to compute the mean effect size (RR++). The following equations were used:

$$w{\text{=1/v}}$$
(3)
$$w^\prime {\text{=w/n}}$$
(4)
$$RR_{++}=\frac{\sum\nolimits_i\ln\;RR^\prime}{\sum\nolimits_i w_1^\prime}$$
(5)

where lnRR′ = w′lnRR is the weighted effect size, n is the total number of observations per study, and i is the ith observation.

The standard deviation (SD) of all variables was computed as:

$${\text{SD=SE}} \times \sqrt {\text{N}}$$
(6)

where N is the number of replications.

From the RR of pairwise comparisons between treatment and control, we calculated the weighted effect size (RR). To explain the response of the estimated values of physical, chemical, and biological properties, the response ratio to agroforestry was converted back to the percent change as follows:

$$({e^{R{R_{++}}}} - 1){\text{ }} \times {\text{ }}100\%$$
(7)

Bootstrapping (9,999 iterations) was used to generate confidence intervals (CIs) for the weighted effect size. To determine statistical significance, we computed 95% confidence intervals for lnRR++. If the 95% CIs for control (CK) and treatment did not overlap by 10% (vertical lines in the graphs), then the comparison was considered significant.

Results

Effects of agroforestry on soil physical properties

Agroforestry improved soil aggregate stability. Specifically, the mean weight diameter (MWD) increased in agroforestry systems by 74%, 65% and 59% in temperate, tropical, and Mediterranean climate, respectively, compared to monoculture systems (Fig. 2a). The water stable aggregate (WSA) was increased after adopting agroforestry by 102%, 100%, and 69% in temperate, Mediterranean, and tropical climate, respectively (Fig. 2a). Moreover, agroforestry promoted soil water regulation. Water content, infiltration and water holding capacity were increased on average by 40% (21–84%), 50% (52–96%) and 81% (61–101%), respectively, across all climates (Fig. 2a). However, our study showed that agroforestry decreased soil moisture, especially in Mediterranean (-88%) and tropical climate (-12%). Additionally, agroforestry improved soil texture and conservation, but its effect on soil porosity was not consistent (Fig. 3).

Fig. 2
figure 2

Effects of agroforestry on (a) physical, (b) chemical, (c) microbiological and (d) C, N and P cycling in temperate, tropical, and Mediterranean climates. MWD: mean weight diameter; WSA: water stable aggregate; MBC: Microbial biomass carbon; MBN: Microbial biomass nitrogen; MBP: Microbial biomass phosphorus; Rs: soil respiration; EC: electrical conductivity; CEC: cation exchange capacity; OM: organic matter; TC: total carbon; SOC: soil organic carbon; SN: soil organic nitrogen; C seq: Carbon sequestration; DON: dissolved organic nitrogen; TN: total nitrogen; Nmineral: nitrogen mineralization; Avail. P: available Phosphorus; TP: total Phosphorus. Black bars represent 95% confidence intervals. The different letters in parentheses represent the number of observations. *The effect is significant when it does not cross over the dashed vertical line at 10%

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Fig. 3
figure 3

Schematic diagram illustrating the effects of agroforestry management practices on different soil properties. Lines with arrows in the flow diagram show causal relations. OM: organic matter; EC: electrical conductivity; CEC: cation exchange capacity. (-) not improved in all climate zones; (+) improved in specific climate zone; (++) improved in all climate zones

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Effects of agroforestry on soil chemical properties

Agroforestry systems increased soil pH by 128% in tropical soils and by 96% in Mediterranean soils, but decreased this parameter by 104% in temperate soils. The soil Ca2+, Mg2+ and K+ contents were increased by agroforestry practices in tropical and Mediterranean climate, but not in temperate climate, where Ca2+ was decreased by 68% (Fig. 2b). Temperate agroforestry systems were characterized by increased Fe2+ (129%) and Al3+ contents (235%) compared to monocultures (Fig. 2b). Agroforestry increased soil NO3− content in temperate and tropical, but not in Mediterranean climate (Fig. 2b). In addition, the CEC was decreased due to agroforestry practices, except for Mediterranean agroforestry systems (Fig. 3).

Soil carbon, nitrogen, and phosphorus contents were increased on average by 18% (8–61%), 41% (28–76%), and 51% (39–68%) in response to agroforestry practices (Fig. 2c; p < 0.05). The soil organic carbon (SOC) contents were higher in temperate and Mediterranean agroforestry soils than in tropical agroforestry soil (Fig. 2c). Carbon sequestration was enhanced in agroforestry systems across all climate types, especially in the tropics (Fig. 2c). Furthermore, our meta-analysis showed increased N mineralization, soil nitrogen (SN), and dissolved organic nitrogen content (DON), particularly in temperate and tropical climate (Fig. 2c; p < 0.05). Regarding soil P content, agroforestry positively affected all investigated soil P forms, except total phosphorus (TP) that was decreased, but only in tropical climate (Fig. 2c; p < 0.05, for all).

Effects of agroforestry on soil microbiological properties

Soil microbial communities, biomass and enzyme activities were generally increased by agroforestry compared to monoculture systems across all investigated climate types (Fig. 2d). The positive effects of agroforestry on soil microbial communities were highest in tropical climate (103%) and lowest in Mediterranean climate (71%). Regarding microbial biomass, agroforestry promoted microbial biomass phosphorus (MBP) more than microbial biomass nitrogen (MBN) and microbial biomass carbon (MBC). Similar to microbial communities, MBC, MBN and MBP were enhanced in tropical climate more than in the other climate types analyzed (Fig. 2d). Agroforestry increased soil respiration in temperate (119%) and tropical climate (105%), but it decreased this parameter by 29% in Mediterranean climate (Fig. 2d). Temperate and tropical agroforestry systems were generally characterized by greater activities of soil enzymes (urease, protease and β-glucosidase) than Mediterranean agroforestry systems (Fig. 3).

Factors regulating agroforestry effects on soil quality

In the order of temperate > tropical > Mediterranean environments, the regulating effects of agroforestry management and tree species selection on agroforestry practices and their subsequent impacts on soil quality decreased significantly. Similarly, the effects of climatic conditions decreased substantially in the order of Mediterranean > temperate > tropical environments (Fig. 4a). Soil and water management factors had a much more pronounced effect on Mediterranean than on tropical and temperate agroforestry systems (Fig. 4a). While the regulating effects of biodiversity and crop species selection were higher in tropical environments, those of farmer collaboration, training, policy support and markets were greater in temperate environments, and those of socio-economic factors were higher in the Mediterranean, with differences among the three climatic regions being significant only for crop species selection (Fig. 4a). Overall, the net effects of agroforestry practices on soil quality were predominantly regulated by climatic conditions, followed by agroforestry management and tree species selection (Fig. 4b).

Fig. 4
figure 4

Relative importance of regulating factors of agroforestry practices and their subsequent effects on soil quality across different climatic zones: (a) temperate, tropical, and Mediterranean environments; and (b) three climatic regions combined. The different letters in parentheses represent the number of observations

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Discussion

Soil physical properties responses to agroforestry systems

Soil physical quality is determined mainly by the soil structure, which indicates the aggregation of soil mineral particles to form different size classes of soil aggregates. Agricultural intensification, especially through strong physical disturbances of soil and excessive inputs of chemical fertilizers, has been observed to decrease soil aggregate stability (Aziz et al. 2013; Karlen et al. 2006; Xue et al. 2022). We found that soil aggregate stability, expressed by the mean weight diameter (MWD) and water stable aggregate (WSA) indices, was improved by agroforestry systems compared to crop monocultures (Fig. 2a). Previous studies showed that MWD is positively correlated with plant species richness, root length and mass densities (Gould et al. 2016), CEC (Nsabimana et al. 2021), total soil porosity (Li et al. 2012), soil SOC and N contents (Le Bissonnais et al. 2018), soil enzyme activities as well as carbon and N mineralization rates (Green et al. 2007). The quantity and quality of soil organic matter (SOM) were found to be positively related to WSA (Pardon et al. 2017; Pikul et al. Jr 2009). These factors that determine soil particle aggregation are generally enhanced in agroforestry systems compared to monocropping systems, as evidenced in Fig. 2. Therefore, agroforestry has the potential to improve soil aggregation and aggregate stability, which can ultimately regulate a wide range of soil physical and biogeochemical processes, properties or functions. These processes and functions include soil compaction, root density and penetration, nutrient cycling and nutrient leaching losses, water storage capacity, water infiltration rates and surface runoff volumes, biological activities, soil erosion, and crop production (Fattet et al. 2011; Li et al. 2020a; Nunan et al. 2003; Pervaiz et al. 2020; Rabot et al. 2018; Sithole et al. 2019; Zhu et al. 2022).

Soil water availability that depends greatly on water infiltration and retention can constrain plant productivity. Our meta-analysis showed greater improvement in soil water storage in agroforestry than in monoculture systems (Fig. 2a). This improvement can be attributed to multiple factors, including better soil water infiltration, root decay, enhanced macropore development and continuity, and soil aggregate formation and stability (Anderson et al. 2009; Zhu et al. 2019). We found that the effects of agroforestry practices were stronger on soil water infiltration than on soil moisture content (Fig. 2a). These findings are consistent with a previous meta-analysis in sub-Saharan Africa (Kuyah et al. 2019), where improved infiltration was suggested as the primary mechanism by which trees enhance water regulation. This study also suggested that the effects of trees on soil moisture content in agroforestry systems are generally determined by tree soil water uptake and transpiration. Other studies reported that, compared to monocultures, agroforestry improves soil moisture through multiple mechanisms, including minimizing water loss via soil evaporation and crop transpiration (Lin 2010; Siriri et al. 2013), improving water infiltration rates (Muchane et al. 2020; Sun et al. 2018) and increasing water retention capacity (Udawatta et al. 2017; Wang et al. 2017). Thus, the potential mechanisms underlying the relationship between agroforestry practices and soil moisture content are complex and need further investigation.

Soil erosion is a serious environmental and agricultural problem in many parts of the world. Previous studies revealed that the productivity of soils can decline by up to 50% as a result of erosion (Eswaran et al. 2019; Liu et al. 2010), associated with decline in soil quality (An et al. 2008; Pimentel 2006). We found that agroforestry leads to significant reductions in soil erosion compared to monocultures (Fig. 3a). Following the land use conversion from natural vegetation to agriculture, soil erosion is often enhanced, primarily due to tillage practices and removal of litter, which acts as both, a protective cover and a rainfall re-distributor (Borrelli et al. 2017; Mohammad and Adam 2010). In agroforestry, trees provide organic input through litter fall or pruning, thereby covering the soil surface. Thus, trees can act as physical barriers to soil erosion. In addition, the belowground input of organic matter to the soil through root turnover in agroforestry and the increased activities of soil fauna can promote soil structural stability and in turn, contribute to soil erosion reduction. The improved MWD by agroforestry (Fig. 2a) might also be partly responsible for the reduction of erosion because the large relative values of this soil aggregate stability index indicate the dominance of aggregate forming over aggregate destroying processes. Taken together, these findings strongly support the roles of agroforestry systems in improving soil aggregate stability and soil water content, as well as in reducing soil erosion.

Soil chemical properties responses to agroforestry systems

Soil acidification is one of the major problems of land degradation facing agricultural regions globally, as acidic soils are widespread. They cover approximately 40% of the total global arable land area (Yadav et al. 2020). Soil acidity can result in nutrient deficiencies due to leaching-induced depletion of basic cations (i.e., Ca2+, Mg2+, Na+ and K+) and nutrient toxicities owing to enhanced solubility of toxic metals (i.e., Al3+ and Mn2+) in soils. These conditions may in turn adversely affect soil flora and fauna (Luizão et al. 2007; Yadav et al. 2020). Our results show that agroforestry can contribute to soil acidity alleviation compared to monocropping (Fig. 2b). This is likely due to trees in leguminous agroforestry systems that can fix atmospheric N2, thereby increasing soil pH and, thus, ameliorate acidic soils (Muchane et al. 2020; Sileshi et al. 2014). Moreover, the presence of trees in agroforestry systems creates a more diverse and healthy soil microbial community, which is beneficial for soil health and acidity alleviation. In addition, agroforestry systems are known to add soil organic matter (via the trees) that is beneficial for nutrient retention, water infiltration, and soil structure (Moraes et al. 2017). The organic matter can also help to increase soil pH buffering capacity that allows for better resistance to acidification and helps to maintain a more neutral pH (Sahrawat 2005), which is beneficial for crop growth. Furthermore, in agroforestry trees may reduce soil acidity through decreasing water drainage as well as deep capture and recycling of nutrients.

Soil organic matter is an important soil quality indicator that influences soil physical, chemical, and biological properties. Generally, our study showed greater accumulation of SOM and nutrient stocks in agroforestry relative to monocultures (Fig. 3). In general, agroforestry systems are characterized by increased species diversity that can improve soil structure, water infiltration and nutrient cycling. They are also subject to regular addition of above- and below-ground residue input (i.e., crop residues, tree prunings, leaf and root litter) to the soils. These properties could ultimately lead to the greater accumulation of SOM and nutrient stocks. Significant positive correlations were found between SOM content and both, tree density and species richness in the agroforestry homegardens (Islam et al. 2015). Pardon et al. (2017) assessed the nutritional potential of hybrid poplar (Populus × canadensis Moench.) in agroforestry systems and found that poplar leaf litter can make a large contribution to the yearly nutrient input, e.g., 12.6 kg K ha−1 and 5.8 kg Mg ha−1. In addition, substantial amount of nutrients, especially K+ and Na+, can be supplemented via nutrient-enriched throughfall water. Trees in agroforestry systems can also provide shelter from rainfall, thereby reducing leaching of nutrients to deeper soil layers. Our findings generally indicate that agroforestry systems have the capacity to store significantly larger amounts of soil organic matter and nutrients than monocultures. This is important for maintaining soil fertility and providing numerous ecosystem services.

SOC is one of the most important soil quality indicators and its storage in the soil represents a critical mechanism for climate change mitigation. We found higher SOC stocks in agroforestry soils than in monocultural soils (Fig. 2c). This indicates that agroforestry systems enhance SOC build-up and storage rates relative to their monoculture counterparts. In agroforestry trees fix atmospheric CO2 and transform it into organic carbon, which is then transferred to the different soil compartments via litter and root decay. The enhancement of soil aggregate stability in agroforestry systems (Fig. 3) might have contributed to SOC storage and protection because soil aggregates are considered as the store house of SOC (Wankhede et al. 2020). Increasing the storage of SOC, which is the main constituent of SOM, can influence multiple physical properties of soils (e.g., soil aggregate stability, infiltration rate and bulk density) and, thus, has substantial implications for provisioning and regulating ecosystem services (e.g., increased productivity of crop plants, carbon sequestration, soil erosion prevention and water conservation). Regulating ecosystem services were remarkably improved by agroforestry compared to monocropping (Fig. 2a, c).

Plant productivity in various terrestrial ecosystems, including agricultural ecosystems, is constrained by limited nitrogen availability. The incorporation of legume trees into agricultural lands has been considered as an ecological approach to increase N availability without increasing fertilizer addition (Rosenstock et al. 2014). Our meta-analysis showed that agroforestry systems had greater soil N availability compared to monoculture systems (Fig. 2c). This is because leguminous trees, in association with bacterial root symbionts, can fix atmospheric N2, which becomes available to co-cultivated crop plants after the decomposition and mineralization of leaf and root litter. For example, litter input from the mature nitrogen-fixing Faidherbia albida trees was estimated to supply over 18 kg N ha−1 yr−1 (Yengwe et al. 2018), and thereby contributed to soil fertility restoration in degraded agricultural land. According to Wartenberg et al. (2020), some non-leguminous agroforestry trees (e.g., Nephelium lappaceum and Durio zibethinus) can also positively affect soil C and N contents. In addition, the physical protection of SOM fractions within soil aggregates represents a crucial N storage mechanism in soil, since the majority of soil N is present in organic form as part of SOM (Chen and Xu 2008; Korhonen et al. 2013).

Multiple lines of evidence indicate widespread P-limitation to primary productivity in global terrestrial ecosystems (Du et al. 2020; Hou et al. 2020; Vitousek et al. 2010). Our results indicate that agroforestry practices can increase soil P availability as compared to monocropping practices (Fig. 2c). Unlike C and N, no process comparable to atmospheric CO2 and N2 fixation exists for biological P inputs into agroforestry systems. Although tree residues are known to supply less P to the soil due to their low P concentrations, they can still influence P availability in soils for crop uptake (Dagar and Gupta 2020). There are various mechanisms by which soil P availability can be enhanced. These mechanisms include (i) transformation of less available organic P pools into more readily available inorganic P pools, (ii) mineralization of the organic P, (iii) blocking of the P-sorption by organic C radicals (Sanchez et al. 1997), and (iv) enhanced P mobilization by the symbiotic association of tree roots with mycorrhizal fungi (Liu et al. 2020). Even though agroforestry trees can improve soil P availability to crops, the strategic mineral P fertilizer inputs are still necessary for sustained increase in agricultural production in P-limited soils as long as breeding of crops with low P requirement has not been successfully invented (Netzer et al. 2019).

Soil microbiological properties responses to agroforestry systems

Soil microorganisms, which play key roles in decomposition of organic matter and nutrient cycling, thereby influencing the physical and chemical properties of soils, are known to be highly sensitive to environmental perturbations. Agroforestry systems can improve soil health, as a result of increased soil organic matter contents and nutrient availability, decreased soil compaction and erosion, and improved water availability, which in turn create diverse habitats for the long-term survival and growth of the soil microorganisms (Dollinger and Jose 2018; Fahad et al. 2022). This view is supported by the present study, as higher abundance of soil microbial communities, including microbial, bacterial and fungal biomass, were found in agroforestry than in monoculture soils (Fig. 2d). SOM contents and nutrient availability have been observed to correlate positively with soil microbial biomass (Lepcha and Devi 2020; Yengwe et al. 2018) as well as bacterial and fungal abundance and biomass (Banerjee et al. 2016). However, soil microbial parameters were negatively related to soil pH (Ge et al. 2013) and bulk density (Lepcha and Devi 2020). This suggests that SOM and nutrient availability can be used as essential indicators of not only soil quality and soil fertility, but also soil microbial, bacterial and fungal abundance and biomass, and that changes in soil pH and bulk density can significantly affect soil microbes and their activities.

Soil respiration – defined as the flux of CO2 from soils to the atmosphere – is considered a good indicator of the overall soil biological activity and soil quality (Dutta et al. 2010; Gajda et al. 2013; Mondini et al. 2010). In the present meta-analysis, the generally higher soil respiration in agroforestry compared to monoculture cropping systems indicates that agroforestry can increase total soil respiration as a result of its positive influence on both, autotrophic respiration (i.e., by increasing root biomass) and heterotrophic respiration (i.e., by increasing soil organic matter and microbial biomass and activity). This view is supported by previous studies, reporting that soil respiration is positively correlated with root biomass, organic matter, microbial biomass (Bae et al. 2013; Lee and Jose 2003) and enzyme activities (Borase et al. 2020; Wu et al. 2021).

Comparisons and implications of soil properties responses to agroforestry systems across tropical, temperate, and Mediterranean environments

Agroforestry systems were found to reduce soil erosion much more in the temperate and Mediterranean environments than in the tropics (Fig. 2a). The high worldwide variability in soil erosion, which generally peaks in tropical environments (owing to the large amount and high intensity of precipitation) and decreases towards higher latitudes (Borrelli et al. 2021), may have contributed to the soil erosion reduction patterns observed in the current meta-analysis. The agroforestry-induced accumulations of OM and SOC were much higher in both, temperate and Mediterranean soils than in the tropical soils (Fig. 2b, c). This could be due to the higher temperature and precipitation at lower than at higher latitudes that enhances organic matter decomposition and leaching, thereby lowering rates of OM and SOC accumulation (Tian et al. 2018; Wieder et al. 2014). Agroforestry systems resulted in higher rates of soil respiration in temperate and tropical soils relative to Mediterranean soils (Fig. 2d). The low soil moisture contents in Mediterranean agroforestry systems (Fig. 2a) could help to explain this observation. Apparently, the soil water status can exert substantial influence on soil respiration in Mediterranean ecosystems (González-Ubierna and Lai 2019; Morillas et al. 2017). Soil pH in agroforestry systems was increased in tropical and Mediterranean soils, but decreased in temperate soils. This pattern is similar to soil Ca2+ contents (Fig. 2b). This finding implies strong weathering and leaching processes in temperate agroforestry systems, indicated by low Ca2+ and high Fe2+ and Al3+ contents in temperate agroforestry soils (Fig. 1b). Therefore, we suggest to particularly use trees in temperate agroforestry systems, which positively influence Ca2+ cycling and availability. For instance, Senna siamea trees were shown to recycle Ca2+ from the calcium-rich agroforestry subsoils, thereby increasing the pH in agroforestry topsoils (Vanlauwe et al. 2005). Tree species that produce calcium-rich litter (e.g., Acer platanoides and Acer saccharum) were shown to be associated with increases in soil exchangeable Ca2+, base saturation and soil pH (Dijkstra 2003; Reich et al. 2005). Although agroforestry-induced soil carbon sequestration was relatively more enhanced in tropical than in Mediterranean and temperate agroforestry systems, perhaps due to greater plant species density, diversity and richness in the tropics, our meta-analysis proved that agroforestry practices can significantly increase carbon capture and storage compared with monoculture practices (Fig. 2c), thereby contributing greatly to climate change adaptation and mitigation in these climate zones.

Key regulators of agroforestry effects on soil properties

Our meta-analysis showed that climatic conditions, followed by agroforestry management and tree species selection, predominantly regulated the effects of agroforestry practices on soil quality (Fig. 4b), with significant variations among the three climate zones studied (Fig. 4a). Climatic factors, such as temperature and precipitation, directly influence growth and development of trees and crops in temperate (Ehret et al. 2015; Maracchi et al. 2005; Schroeder and Naeem 2017), tropical (Ong et al. 2007; Rao et al. 2007; Swamy and Tewari 2017) and Mediterranean environments (Damianidis et al. 2021; Querné et al. 2017; Temani et al. 2021). Temperature influences several key processes, including photosynthesis, respiration, and nutrient uptake, as well as plant phenological stages, such as flowering, fruiting, and leaf senescence (Thorlakson and Neufeldt 2012). While optimal temperatures may promote plant growth and productivity by enhancing nutrient mineralization and photosynthetic activity (Beule et al. 2022; Udawatta et al. 2017), extreme temperatures (i.e., heatwaves or frost) can induce stress in plants and retard their development (Ehret et al. 2015; Schroeder and Naeem 2017). Also, the precipitation variations, in terms of intensity, frequency, duration, and seasonal distribution, significantly affect plant growth, development, and productivity in agroforestry systems by altering soil moisture availability. For instance, insufficient rainfall can lead to water stress that may affect the establishment, development and overall productivity of trees and crops (Droppelmann et al. 2000; Lin and Hülsbergen 2017). Conversely, excessive precipitation can cause soil anaerobiosis, erosion and nutrient leaching, thereby negatively affecting soil fertility and plant growth (Kreuzwieser and Rennenberg 2014; Querné et al. 2017).

Our meta-analysis shows that climatic conditions significantly regulate the effects of agroforestry on soil quality, especially in Mediterranean and temperate environments (Fig. 4). In Mediterranean environments, there is a pronounced hot-dry summer season, which is characterized by low or erratic precipitation patterns and high temperatures (Maracchi et al. 2005). During this period, agroforestry trees and crops can experience retarded growth and productivity, primarily due to water scarcity and associated stress. In contrast, temperate environments experience more moderate and evenly distributed precipitation throughout the year. They are also generally characterized by milder summers and winters with less extreme temperature fluctuations. The relatively favorable temperate climatic conditions can greatly enhance the overall plant diversity and productivity in agroforestry systems and associated ecosystems services including the improvement of soil quality (Borden et al. 2020; Quinkenstein et al. 2009). In summary, the most striking differences in soil quality responses to agroforestry systems across tropical, temperate, and Mediterranean environments were associated with climatic factors.

The responses of soil quality indicators to agroforestry greatly relied on agroforestry management (Fig. 4b), especially in temperate and tropical environments (Fig. 4a). It should be noted that alley cropping systems (that involve growing trees or shrubs and agricultural crops in alternate rows) and silvopastoral systems (that integrate animals and trees) are widely practiced in temperate environments. These systems can improve soil structure, nutrient cycling, and water management (Lovell et al. 2018), provide shade and windbreaks to reduce damage to co-cultivated crops (Lelle and Gold 1994). Temperate agroforestry systems often prioritize the integration of economically valuable tree species, including oak, walnut, and cherry, which provide multiple benefits e.g., timber production, biodiversity conservation, and carbon sequestration (Pardon et al. 2017; Partey et al. 2011). In tropical environments, agroforestry systems are commonly practiced in the form of multistrata systems, in which different tree species are grown in different layers along with agricultural crops (Liu et al. 2019; Schroth et al. 2001). Tropical agroforestry systems are also characterized by diverse combinations of trees, crops, and livestock. This diversification not only enhances soil fertility, overall system productivity and sustainable livelihoods, but also provides wildlife habitats and conserves biodiversity (Oelbermann et al. 2004; Partey et al. 2011), but may also lead to higher productivity compared to monoculture systems (Castle et al. 2021.

Moreover, proper agroforestry design and layout may contribute to increased light interception, nutrient cycling, and microclimate regulation, which are essential for crop growth and development. For example, tree arrangement and spacing in tropical agroforestry systems were found to significantly influence the amount of light reaching the crops, thereby maximizing crop yield (Burgess et al. 2022). When trees are strategically placed, they can reduce heat stress on crops by providing shade during intense sunlight periods (Leakey et al. 2006). Additionally, tree spacing and arrangement may significantly determine light penetration, wind effects on crops, and tree-crop resource competition. Furthermore, designing diverse and multi-layered agroforestry systems, with different tree species and functional group combinations, can provide multiple ecosystem services, including enhanced carbon sequestration and soil moisture retention, and improved microclimate regulation (Paul et al. 2017, thereby promoting ecological stability and reducing vulnerability to climate extremes (Gwali 2014; Mbow et al. 2014).

The choice of tree species to be incorporated into the agroforestry systems is important as it determines the ecological functions and services provided, including nutrient cycling, soil improvement, and biodiversity enhancement (German et al. 2006; Lemma et al. 2006). Our study revealed that tree species selection is the third key factor that regulates the effects of agroforestry practices on soil quality in tropical, temperate, and Mediterranean (Fig. 4b). However, this selection can also be influenced by several other factors, firstly, the adaptability of tree species to the specific agroecological conditions e.g., soil type, climate, and water availability (Bayala and Prieto 2020; Germon et al. 2020). This is based on the fact that different tree species have distinct tolerance levels to the agroecological conditions (Mwase et al. 2015). In temperate environments, it is recommended to match tree species to their specific environmental conditions, as the success of temperate agroforestry systems heavily relies on the ability of trees to adapt and thrive in their surroundings (Pardon et al. 2017). In tropical environments, where the climate is typically warm and humid throughout the year, tree species that are adapted to high rainfall and heat are often prioritized (Montagnini and Nair 2004; Oelbermann et al. 2004). In Mediterranean environments, Temani et al. (2021) emphasized the necessity of planting tree species with higher drought tolerance and greater ability to withstand high temperatures. Therefore, it is favorable to use native tree species as they are well adapted to the local climatic conditions (e.g., high temperatures and intense rainfall), soil types (e.g., nutrient-poor soils), and pests, to increase chances of survival and growth (Montagnini and Nair 2004).

Secondly, the selection of tree species is influenced by their functional characteristics, including root system architecture, tree nutrient cycling abilities, and tree-microorganism-soil interactions. For instance, deep-rooted species can access deeper soil layers, thus enhancing their water and nutrient uptake efficiency (Borden et al. 2020). Nitrogen-fixing tree species (e.g., leguminous trees) can improve soil fertility, and ultimately promote crop growth by nursing effects (Schroth 1995). Thirdly, the selection of tree species should take into account the specific objectives of the particular agroforestry system (Mwase et al. 2015). For instance, in temperate environments, where agroforestry systems are commonly used for timber production, the selection of fast-growing tree species with high timber value is prioritized (Montagnini and Nair 2004). In tropical environments, where agroforestry systems are often used for multiple purposes (e.g., food production, shade, and soil conservation), the selection of tree species is based on providing multiple benefit (Paul et al. 2017). In addition, it is important to consider the compatibility of tree species with crops or livestock as this can directly affect the productivity and sustainability of the agroforestry systems (Querné et al. 2017; Ranjitkar et al. 2016). Generally, if tree species selection in agroforestry systems is managed holistically and appropriately, it will not only maximize economic benefits, but also provide multiple ecosystem services, including enhanced soil fertility, water conservation, and biodiversity conservation.

In summary, our meta-analysis underscores the significance of tree species selection as a determinant of agroforestry success. Nonetheless, when considering the broader context of agroforestry systems across tropical, temperate, and Mediterranean climates, the impact of climatic conditions and agroforestry management outweighs the influence of tree species selection in shaping overall outcomes. Furthermore, in order to deepen our comprehension of the effects of agroforestry systems, it is imperative that future studies delve into the distinctive attributes of tropical soils and the specific implications of this biome, particularly within the context of Brazil. By doing so, we can acquire a more comprehensive and accurate understanding of how agroforestry practices impact different regions and countries,