Agroforestry boosts soil health in the humid and sub-humid tropics: A meta-analysis

Agroforestry and Soil Health in the Tropics

Introduction

Agricultural intensification has improved human well-being and economic development but at the cost of natural resource degradation.
There's a growing demand for agroecological approaches for long-term sustainable intensification to feed a growing global population (9.7 billion by 2050, 11.2 billion by 2100).
Ecological intensification approaches increase soil organic matter (SOM) and soil-based ecosystem services.
Agroforestry, integrating trees and crops, is a promising sustainable intensification pathway.
Agroforestry is agroecology in practice, adapting ecological concepts to agroecosystem design and management.
Agroecology contributes to Sustainable Development Goal 2 (SDG 2): ending hunger, achieving food security, improved nutrition, and promoting sustainable agriculture.
Agroforestry contributes to household income, food security, biodiversity conservation, and ecosystem services.
It serves as a climate change mitigation and adaptation intervention.
Optimizing canopy cover is important to avoid compromising ecosystem services.
Agroforestry is defined as:
- A dynamic, ecologically based, natural resources management system.
- Integrates trees on farms and in the agricultural landscape.
- Diversifies and sustains production.
- Increases social, economic, and environmental benefits.
Two types of agroforestry practices:
- Simultaneous: trees and crops on the same land during the same cropping season (e.g., alley cropping, intercropping).
- Sequential: trees and crops on the same land in a temporal sequence (e.g., improved fallows).
These practices impact soil health through direct and indirect effects of trees.
Soil quality and soil health: reflect a shift from focusing on soil physical and chemical properties to recognizing soil as a living entity.
Soil health definition:
- An integrative property of soil.
- Reflects the capacity of soil to respond to agricultural intervention.
- Supports agricultural production and provides other ecosystem services.
Soil health is one of the three components of environmental quality (water, air, and soil).
Soils provide provisioning, regulating, and cultural ecosystem services:
- Provisioning: food, feed, fiber, energy, genetic materials.
- Regulating: Nutrient storage and supply, greenhouse gas sequestration, flood mitigation, pest/disease control, detoxification.
- Cultural: non-material benefits affecting physical and mental state.
The Koronivia Joint Work on Agriculture recognizes the importance of soil carbon, soil health, and soil fertility in responding to climate change.
Meta-analysis focus: humid and sub-humid tropics, due to:
- Greater potential for productivity increase.
- Significant soil degradation impacting the rural population.
- High risks to biodiversity losses driven by agricultural expansion and shifting cultivation.
- Dominated by low-activity clay soils prone to acidity, toxicities, low nutrient reserves, nutrient deficiencies, and erosion.
Agroforestry can alleviate constraints, increase food production, improve nutrition/health, and conserve natural resources.
Reviews and meta-analyses have improved understanding of agroforestry's impact on provisioning (crop yields) and regulating (pest/disease control, carbon sequestration) services.
This synthesis addresses research questions and hypotheses not addressed by earlier syntheses, focusing on mechanisms by which agroforestry impacts soil health and soil-mediated ecosystem services.
The objective of this meta-analysis:
- Quantify the contribution of agroforestry practices to soil-mediated ecosystem services.
- Specifically, regulation of soil erosion, storage of soil C and N, availability of soil N and P, and alleviation of soil acidity.
The overall aim is to create awareness among researchers, development practitioners, and policymakers about:
- The roles that agroforestry can play in climate change adaptation and mitigation.
- Management of land degradation.
- Useful for countries engaging in the Koronivia Joint Work on Agriculture and preparing nationally determined contributions (NDCs) to the UNFCCC.

Methods

Selection of Indicators

Indicators are used to describe, represent, monitor, assess, or model complex processes for decision-making.
No consensus exists on practical indicators for soil ecosystem services.
Several chemical, physical, and biological variables can be used as indicators.
Indicators were chosen based on:
- High frequency of reporting in published studies for meta-analysis.
- Ability to represent major soil health constraints.
Limited studies on agroforestry's impact on soil biological parameters limits their use in meta-analysis.
This quantitative synthesis focussed on key physical and chemical indicators.
Specific indicators:
- Soil erosion rate: eroded soil, infiltration rate, runoff, macroaggregates, and mean weight diameter (MWD).
  - Aggregate stability: measure of how well soil aggregates resist disintegration; a key indicator of resistance to erosion.
- Soil C storage: soil organic carbon (SOC) and macroaggregate-associated C.
  - SOC: key indicator of soil health, a universal proxy of multiple ecosystem services, and an important driver of agricultural sustainability.
- Soil N storage: total N and macroaggregate-associated N.
  - Total N: allows assessment of the contribution of agroforestry to soil N stocks.
- Nutrient availability: indicators of regulating and supporting ecosystem services.
  - Soil N availability: ammonium-N, nitrate-N, and inorganic N (ammonium-N + nitrate-N).
  - Soil P availability: inorganic P.
- Amelioration of soil acidity: soil pH.
  - Soil pH: has a direct influence on physical, chemical (e.g. nutrient availability, toxicity) and biological (e.g. microbial activity) characteristics that influence crop growth.

Research Questions and Hypotheses

Five Research questions:
- Does agroforestry reduce soil erosion?
- Does agroforestry build soil organic C and N stocks?
- Does agroforestry increase soil N availability?
- Does agroforestry increase soil P availability?
- Does agroforestry alleviate soil acidity?

Literature Search

Meta-analyses compared soil properties of sequential or simultaneous agroforestry practices with crop monocultures.
The literature search focussed on randomized studies that compared plots.
Publications were identified using the ISI Web of Science, focusing on literature published up to July 2017.
Two searches were conducted using 20 keywords on different agroforestry practices in combination with either 19 or 25 key words representing response variables associated with soil structure maintenance/soil C storage and nutrient cycling, respectively.
Followed by an intensive review of abstracts and papers to be included in the meta-analysis, resulting in a total of 119 articles.
Reference lists of papers including previous syntheses on related topics were examined.
Following factors were included in the database: location (country, latitude, longitude and altitude), mean annual rainfall, soil type (WRB classification), soil texture, agroforestry practice (i.e. simultaneous or sequential), tree species, crop species, study type (experimental or observational), soil response variable (e.g. soil available P), soil depth, data collected in both control and intervention treatments.

Criteria for Inclusion in Meta-Analysis

Criteria:
1. Study originated in the humid or sub-humid tropics (annual rainfall > $600$ mm, within $30$ °North/South of Equator).
2. Compared plots representing:
  - Simultaneous or sequential agroforestry practices with plots of crop monocultures ("control").
  - Agroforestry and control plots located on the same farms with the only difference being the presence or absence of trees.
  - Agroforestry practices were classified into simultaneous and sequential practices.
  - Studies involving organic inputs from outside or tree effects confounded with other inputs (e.g., manure) were excluded.
  - Rotational woodlots (trees grown > $3$ years) and home-gardens were excluded due to lack of proper control plots.
3. The same crop species grown in the agroforestry plot and the corresponding control plot.
4. Quantified one or several indicators of aggregate ecosystem function and soil health.
5. Studies conducted on research stations and at the farm scale were included, excluding landscape-scale and laboratory studies.

Data Extraction

Extracted data on:
- Soil erosion, infiltration, runoff, % macroaggregates, MWD
- Soil organic carbon (SOC), total N, macroaggregate C, macroaggregate N
- Soil inorganic N, nitrate-N, ammonium-N, soil inorganic P
- Soil pH
Soil ammonium-N and/or nitrate-N were discriminated from soil inorganic N.
Only soil inorganic P data extracted by the Olsen, Bray or Mehlich methods were included.
Other ancillary data: geographic coordinates, altitude, mean annual precipitation, soil type and texture were extracted.
Soil texture categories: sandy soils (< $20$ % clay), loam soils ( $20–32$ % clay), clay soils (> $32$ % clay).
Data were extracted from results section, tables, appendices, graphs and figures from each of the papers using IMAGE J software.
Multiple agroforestry treatments with different tree species were considered as separate data point.
Treatments based on different tree species compared with the same control were considered as unique observations.
If a paper reported results from more than one soil depth, only the upper soil layer (till layer) was considered.
In cases where tests were repeated over the growth period, soil measurements made before the last growing season of the experiment were selected.

Effect Size

Response ratio (RR) was used as the effect size.
$RR = T/C$ (T: treatment value, C: control value).
Logarithm of RR (logRR) was used for the meta-analysis as recommended by Hedges et al. (1999).

Data Analysis

A linear mixed modelling procedure was applied for all analyses because:
- Many of the studies did not report either the SD or SE.
- Data gathered across studies were unbalanced with respect to predictor variables and sample sizes.
The mixed model process:
- Categorical variables (e.g. agroforestry type, ability to fix N and soil texture) were entered as fixed effects.
- The source of data (i.e. study) was entered as the random effect.
- Model parameters and their $95$ % confidence intervals ( $95$ % CI) were estimated using restricted maximum likelihood (REML) estimation.
Where moderator variables were not applicable, for example the overall effect of agroforestry on a given variable, the $95$ % CIs were estimated by bootstrapping (resampling with replacement) with $9999$ random replicates.
Population marginal means and $95$ % CI of the back- transformed RR are presented.
Means were considered to be significantly different from one another only if their $95$ % CI were non-overlapping.
Where sample sizes were small (< $30$ ), the results were interpreted cautiously.
If there is no significant difference between agroforestry and the control for a given variable, the $95$ % CI of RR will encompass $1$ .
If the $95$ % CL of RR is greater than $1$ it means significant increases under the given agroforestry practice over the control.
The agroforestry effect was interpreted as significantly negative (leading to reduction) when the $95$ % CL < $1.0$ .
Data on macroaggregate-associated C and macroaggregate- associated N, infiltration rates, runoff and porosity were scarce.

Results

Regulation of Soil Erosion

Erosion rate were reported in a total of $17$ studies and a sample size of $69$ was available for analysis.
In all studies, RR was less than $1$ indicating that soil erosion rates were significantly lower under agroforestry compared to the corresponding crop monocultures.
Overall, agroforestry trees reduced soil erosion by $50$ %.
Infiltration rates were $75$ % higher under agroforestry than crop monocultures.
Runoff was $57$ % lower under agroforestry than crop monoculture.
Soil macroaggregates (> $0.25$ mm) and mean weight diameter (MWD) were significantly higher under agroforestry than in the crop monocultures; the increases being $22$ and $30$ % for macroaggregates and MWD, respectively.

Storage of Soil Carbon (SOC)

SOC was reported in $71$ studies and a total of $225$ pairs of observations were available for analysis.
With overall effect size of $1.21$ (CL: $1.15–1.27$ ), agroforestry significantly increased SOC storage compared to crop monocultures.
SOC storage under agroforestry was significantly greater in sandy soils compared to loamy soils.
Aggregate-associated C was significantly higher under agroforestry than in the crop monocultures.
Closer examination using soil physical fractionation techniques shows that $13–29$ % more soil C is stored in macro- aggregates under agroforestry practices.

Storage of Soil Nitrogen (N)

Total N was found in $48$ studies with a total sample size of $167$ RR values.
The overall mean effect size (RR = $1.13$ ; CL: $1.08–1.19$ ) was significantly greater than $1$ indicating that soil N stocks under agroforestry were higher than in crop monocultures.
The effect of agroforestry on soil total N levels was significantly influenced by soil texture.
Total N was significantly higher in sandy soils than loamy soils.
Aggregate- associated N was significantly higher under agroforestry than in the crop monocultures.
Closer examination using soil physical fractionation techniques shows that $22–43$ % more soil N is stored in macroaggregates under agroforestry practices.

Availability of Soil Nitrogen

Data on pre-planting soil inorganic N were found in $34$ studies with a total of $117$ RR values.
The overall mean RR ( $1.46$ ; CL: $1.32–1.59$ ) was significantly greater than $1$ indicating that soil inorganic N under agroforestry was $46$ % higher than in crop monocultures.
Agroforestry increased soil inorganic N by up to $52$ % on clay soils as compared to the $25$ % increase on loamy soils.

Availability of Soil Phosphorus

Soil inorganic P was found in $49$ studies with a total sample size of $165$ RR values.
The overall mean RR was $1.11$ (CL: $1.05–1.68$ ) was significantly greater than $1$ .
P availability was significantly higher on loamy soils than sandy soils.

Soil Acidity Alleviation

Soil pH was found in $46$ studies with a total sample size of $138$ RR values.
Overall, agroforestry practices significantly increased soil pH (RR = $1.02$ ; CL: $1.01–1.03$ ) over the crop monoculture.
RR values greater than $1$ were found in pH below $6$ , while above pH $7$ the RR values remained close to $1$ .
The effect of agroforestry on soil pH also marginally differed with soil type; the most significant increase in pH being on Nitisols, Ferralsols and Acrisols, which are naturally prone to acidification.

Discussion

Agroforestry Reduces Erosion Rates

Soil erosion is a pervasive feature of land degradation globally, especially in mountainous agricultural landscapes in humid tropical and sub-tropical regions.
Soil erosion has on-site and off-site impacts:
- On-site: decline in soil quality, loss of key soil constituents, reduction in water holding capacity and nutrient reserves, loss of topsoil, inefficient use of nutrients.
Agroforestry practices significantly reduce soil erosion rates compared to crop monocultures in humid and sub-humid tropics.
Supported by reduction in erosion rates, higher infiltration rates and macroaggregation, and lower runoff recorded under agroforestry.
Trees provide physical barriers to soil erosion.
The belowground organic inputs through root turnover and the increased biological activity of soil ecosystem engineers promote soil structural stability and contribute to the reduction in soil erosion rates.
The abundance of large macroaggregates and MWD under agroforestry could also partly explain the reduction in erosion rates.
Aggregate stability is an important indicator of the structural stability of soil and its resistance to erosion.
These results together provide evidence that agroforestry can play a vital role in erosion control, which is one of the key regulation services in agroecosystems.

Agroforestry Increases Storage of SOC

Agroforestry contributes to greater SOC build-up than crop monocultures.
Soil C is protected inside soil aggregates, leading to as much as $30$ % greater soil C stored in soil macroaggregates under agroforestry practices.
Increase in SOC affects nutrient availability and growth of crop plants, but also soil biodiversity and bottom-up effects on crop pests and their natural enemies.
High SOM content in soil can support a greater diversity of soil organisms, which provide alternative food sources for natural enemies that help to suppress crop pests.
SOC also affects multiple soil physical properties including aggregate stability, bulk density and water infiltration rates.
A recent meta-analysis highlights that contribution to the overall increase in available water capacity seems to be lower than commonly thought as $1$ % mass increase in SOC on average increased available water capacity by about $1.2$ %.
Even small changes in SOC stock can have considerable impacts on the atmospheric $CO_2$ concentrations and the global climate.

Agroforestry Increases Storage and Availability of Soil N

Agroforestry significantly contributes to greater soil total N levels than crop monocultures.
Protection inside aggregates is an important mechanism for N storage in soil.
Leguminous agroforestry trees have been shown to significantly influence the soil N economy thanks to their capacity to fix atmospheric N in their biomass.
The contribution of N-rich tree biomass to the soil organic N pool occurs through aboveground inputs (e.g. litterfall, prunings) or belowground inputs (e.g. roots) which, following decomposition and mineralization processes, adds soil inorganic N (i.e. ammonium-N and nitrate-N) to the soil solution.
Agroforestry trees with higher organic tissue quality and faster decomposition rates have been shown to make greater short-term contributions to soil inorganic N than trees with lower tissue quality.

Agroforestry Increases Soil Available P

Agroforestry significantly increases availability of soil inorganic P compared with crop monocultures.
The possible mechanisms for improved P availability in agroforestry include:
- the mineralization of organically bound P in the organic inputs
- the transformation of less available pools of inorganic P into more readily available organic P that is mineralized
- organic C radicals blocking P-sorption sites
Trees used in agroforestry systems are highly dependent on arbuscular mycorrhizal fungi (AMF) and cluster roots to adapt to P-deficient soils.
Overall, the results provide evidence that agroforestry can lead to increases in P availability although the increases are marginal.

Agroforestry Alleviates Soil Acidity

Agroforestry contributes to alleviating soil acidity compared to crop monocultures.
N-fixing trees can contribute to reduce soil acidification.
Trees could minimize soil acidification both by decreasing drainage and through deep-capture and recycling leached nutrients.
The soil acidity alleviating effect of plant materials depends on their chemical composition, especially their ash alkalinity.
Trees producing litter rich in Ca are often associated with soils with higher exchangeable Ca, per cent base saturation and pH.
Increases in soil pH have often been associated with greater abundance and activity of soil organisms that can influ- ence C and nutrient cycling.

Conclusions

While the effect of agroforestry may vary with soil, climate, crop type and tree management, its overall effect is towards improving soil health and soil-mediated ecosystem services in the humid and sub-humid tropics.
Agroforestry practices significantly reduce soil erosion rates, increase SOC and N storage, increase the availability of inorganic N and marginally increase the availability of inorganic P and pH in the soil compared to crop monocultures.
Agroforestry can be an option for increasing soil nutrient availability to crops when access or use of mineral fertilizers is limited.
Attention is needed to improve management of canopy-cover and optimize trade- offs among production, climate and sustainability goals.
Agroforestry can significantly contribute to eco- logical intensification trajectories that support agroecological transi- tions towards sustainable agriculture and food systems in the humid and sub-humid tropics.
It can also provide significant climate change adaptation and mitigation benefits.
Agroforestry be considered in the nationally determined contributions of parties to the UNFCCC in the coming years.