Perspective Chapter: Agroforestry Strategies for Integrated Soil and Water Conservation

Abstract

Land degradation has a substantial influence on food security, health, and socioeconomic development, highlighting the critical role that land plays as a resource necessary for maintaining life. As a result, agroforestry interventions become essential tactics for resource preservation and improving sustainable production. Many agroforestry techniques, including agri-horticulture, silvipasture, and agri-silviculture systems, have been developed over the course of substantial study in a variety of agroclimatic zones with the goal of reducing land degradation. The United Nations Convention to Combat Desertification has acknowledged these strategies as essential to reaching land degradation neutrality. The benefits of agroforestry techniques for reducing soil erosion and runoff, increasing soil fertility, and enhancing carbon sequestration are explained in this chapter. It is crucial to promote these affordable and sustainable technologies to guarantee their widespread adoption. As a result, putting in place agroforestry systems is essential for healing impacted regions and addressing issues with livelihoods, environmental sustainability, and food security. In order to make sure that stakeholders receive the proper incentives, national policy programs should incorporate the valuation of the advantages of soil protection. Furthermore, future research endeavors should prioritize the development of economically viable agroforestry systems designed to restore degraded lands, enhance water efficiency, and minimize competition between trees and crops.

1. Introduction

Meeting the increasing global demands for agricultural products, along with the need to enhance ecosystem services, is a significant challenge driven by rising populations and incomes [1, 2]. Amid these concerns, safeguarding the world’s soils and preventing soil erosion takes center stage [3, 4]. Global soil and water erosion is on the rise, affecting soil quality and carbon storage and leading to biodiversity loss, reduced crop yields, and even poverty [5, 6, 7, 8]. It also hampers the achievement of various sustainable development goals [9, 10, 11].

Soil erosion is a historical and persistent challenge that has influenced human civilization and the pursuit of a better quality of life. It arises from natural processes and can be exacerbated by socioeconomic developments over time. The materials eroded due to this phenomenon have detrimental consequences, affecting both the immediate environment and areas farther away, impacting plant and animal life. These repercussions can be magnified by the intricate interactions within the ecosystem, involving both interconnections among ecosystem components and internal dynamics [12]. In regions characterized by an increasing population and expanding agricultural production, construction, urban development, and diverse human activities, soil erosion presents a prominent challenge. This issue has been noted in various studies [13, 14]. Among the contributing factors to soil erosion is inadequate land management, which leads to soil degradation and the subsequent surface runoff of water, rather than facilitating proper soil infiltration [15, 16, 17].

Soil erosion poses significant threats to plant growth, agricultural productivity, water quality, and recreational spaces, affecting diverse land types as evidenced by various studies [15]. Its primary drivers are water and wind, leading to substantial annual soil loss. Erosion can manifest gradually and unnoticed or as a rapid process, resulting in the loss of valuable topsoil. In agricultural contexts, it translates into reduced crop yield potential, compromised surface water quality, and disrupted drainage systems [16, 18]. On a global scale, soil erosion stands as a significant environmental concern, depleting soil nutrients and causing land degradation while also giving rise to off-site problems such as flooding, water siltation, and pollution [19, 20, 21]. Effectively addressing this challenge necessitates comprehensive monitoring and assessment to ensure the sustainable management of natural resources and the environment.

Soil erosion as a substantial contributor to soil degradation; it is well-established that agroforestry practices hold the potential to enhance rainwater infiltration and reduce runoff, effectively mitigating soil erosion. As raindrops make initial contact with the soil surface, the collective action of trees, annual crops, and grasses through their canopy, surface litter, and intricate root systems comes into play. These components act as a physical barrier, effectively decelerating the movement of surface water and diminishing erosive forces, leading to a reduction in soil erosion [22]. Moreover, the application of various agroforestry techniques, including agri-silviculture, agri-horticulture, silvopasture, and alley cropping, plays a pivotal role in addressing soil erosion. These techniques not only help in erosion control but also contribute to enhanced soil fertility, improved water quality, increased biodiversity, and carbon sequestration. Recent literature underscores the importance of agroforestry in developing integrated, diverse, and sustainable land use systems, bridging the gap between agriculture and conservation [23, 24, 25]. By integrating trees, shrubs, crops, and livestock, agroforestry provides a cost-effective and efficient alternative to traditional soil conservation methods, ensuring sustainable land productivity [22, 26]. This chapter delves into the significance of agroforestry strategies in comprehensive soil and water conservation, particularly in regions where perennial ground cover is essential, such as in ravines and poor lands.

2. Unearthing the challenge: soil erosion in India and global

Land resources are essential for human well-being and societal progress [27]. However, land degradation has escalated since the twentieth century due to ecological decline, increased food demand, rapid urbanization, industrialization, and unsustainable land use [28]. Approximately 60% of the world’s land is now degraded, posing a significant challenge for sustainable land use [29]. Water and wind erosion contributes to this problem, leading to an annual loss of 24 billion tons of fertile soil and a financial loss of $400 billion [30]. Sustainable soil management is gaining global attention, spearheaded by the Global Soil Partnership (GSP) [4]. The FAO reports an alarming annual global soil loss of 75 billion tons, with India experiencing an average loss of 16 tons per hectare [31]. Rapid land degradation has led to a significant economic loss of 3.17 lakh crore ($46.90 billion) [32]. Soil erosion assessment traces back to Myres [33], and the United Nations Environment Programme (UNEP) has monitored global soil degradation trends from the 1980s to the 1990s [34]. UNEP’s Global Assessment of Soil Degradation focuses on static land use and land cover data, while India’s land use is becoming more susceptible to surface erosion due to population pressure [4]. Currently, 16.7% of land is allocated for crop cultivation, showing a significant increase since 1985 [35]. The Global Assessment of Soil Degradation Survey highlights that soil erosion, a significant contributor to land degradation, is responsible for the degradation of 84% of affected land [36]. This results in an annual global socioeconomic loss of approximately $40 billion [37]. The United Nations’ Sustainable Development Goals (SDGs) have set a target to eradicate land degradation by 2030 [38]. Monitoring reports within the European Union indicate that over 201,000 square kilometers of land are at risk of severe erosion (>10 Mg ha⁻¹ year⁻¹). Soil erosion plays a critical role in monitoring progress toward SDGs related to climate action, zero hunger, and life on land. This pressing issue is recognized and acknowledged by Lal [37].

Despite the widespread occurrence of water erosion, there is a notable scarcity of comprehensive global data regarding the extent and severity of this issue. The Global Assessment of Human-Induced Soil Degradation (GLASOD) study, as outlined in Table 1, estimated that approximately 15% of the world’s ice-free land area is affected by various forms of land degradation. Within this assessment, accelerated soil erosion, primarily caused by wind and water, accounts for roughly 28 and 56%, respectively. This indicates that the regions impacted by water erosion span approximately 11 million square kilometers, while wind-induced erosion affects approximately 5.5 million square kilometers. Notably, tillage erosion remains undocumented in some affected areas. As soil is a finite resource formed over time, the global significance of the erosion problem has been gaining recognition only recently.

Degradation	World	Asia	West Asia	Africa	Latin America and Caribbean	North America	Australia and Pacific	Europe
Water erosion	1094	440	84	227	169	60	83	115
Wind erosion	548	222	145	187	47	35	16	42
Nutrient depletion	135	15	6	45	72	—	+	3
Salinity	76	53	47	15	4	—	1	4
Contamination	22	2	+	+	+	—	—	19
Physical	79	12	4	18	13	1	2	36
Others	10	3	1	2	1	—	1	2
Sum	1964	747	287	494	306	96	103	218

Table 1.

Estimation of human-induced soil degradation by GLASOD.

Source: Bai et al. [39].

In India, 120.7 million hectares (M ha) of land are affected by degradation, with soil erosion being the main contributor, affecting 71% of degraded land (85.7 M ha) (Table 2). Water erosion alone affects 60.7% (73.3 M ha), and wind erosion affects 10.3% (12.4 million hectares). Furthermore, deforestation (2.07 M ha, 2001–2021), intense rainfall (>7.5 mm ha⁻¹), uncontrolled grazing (5.65 M ha), excessive fertilizer usage (32 mt year⁻¹), and shifting cultivation (7.6 M ha) compound the issue of land degradation. Presently, 97.8 M ha (29.7% of the total area) face degradation, with states such as Rajasthan, Maharashtra, Gujarat, Karnataka, Ladakh Union Territory, Jharkhand, Odisha, Madhya Pradesh, and Telangana contributing about two-thirds of the nation’s degraded land (23.7% of TGA) [40]. India, a country characterized by its riverine systems and monsoon patterns, faces an escalating threat in the form of soil erosion, a pressing issue in several regions [41]. This erosion, attributed to natural and human-induced factors [42], is exacerbated by climate variations and unscientific agricultural practices, such as deforestation and construction [43, 44, 45]. The loss of the topsoil layer poses a significant threat to soil fertility and food security [46]. India has managed to meet food production targets despite monsoon challenges, largely due to advanced mechanization, the use of chemical fertilizers, and extensive tillage [33]. However, this heightened mechanization adversely impacts soil structure and diminishes water-holding capacity [31].

Degradation	Arable land (M ha)	Open forest (<40% canopy) (M ha))	Source
Water erosion (>10 Mg ha^−1 year^−1)	73.27	9.30	Soil Loss Map of India-IISWC, Dehradun
Eolian erosion	12.40	—	Wind Erosion Map of India, CAZRI, Jodhpur
Sub-total	85.67	9.30
Chemical degradation
Exclusively salt-affected soils	5.44	—	Salt-affected Soils of India, CSSRI, Karnal; NBSS&LUP, Nagpur; NRSA, Hyderabad, and others
Salt-affected and water-eroded soils	1.20	0.10
Exclusively acidic soil (pH < 5.5)	5.09	—	Acid Soils of India, CSSRI, Karnal; NBSS&LUP, Nagpur
Acidic (pH < 5.5) and water-eroded soil	5.72	7.13
Sub-total	17.45	7.23
Physical degradation
Mining and industrial waste	0.19		Wasteland Map of India-NRSA, Hyderabad
Waterlogging (permanent surface inundation)	0.88
Sub-total	1.07
Total	104.19	16.53
Grand total (arable land open forest)	120.72

Table 2.

Detailed information on India’s wastelands and degraded land (in M hectares).

Source: Jinger et al. [40].

Water-induced soil erosion is a leading cause of land degradation in India. The severity of this erosion varies, spanning from moderate (over 10 Mg ha⁻¹ year⁻¹) to exceedingly severe (exceeding 80 Mg ha⁻¹ year⁻¹). Globally, water erosion is a significant contributor to land degradation, affecting approximately 2 billion hectares, with a significant presence in tropical regions. This pervasive issue has substantial implications for essential natural resources, particularly soil and water [41]. The worldwide loss of water and sediment due to soil erosion presents a notable environmental concern [42]. Soil erosion is exacerbated by several factors, including high-intensity rainfall [43], steep terrain [44], and the vulnerability of topsoil [45]. In many regions of India’s tropical areas, the annual rainfall is concentrated within a short 4–5-month period (June–September). During the subsequent 7–8-month dry season, water scarcity leads to a severe shortage of fodder on farmlands, resulting in heightened grazing pressure on forest and community lands. It is worth noting that nearly one-third of India’s fodder requirements are met through the utilization of forest resources for grazing and harvested fodder [46]. Land degradation in croplands and grasslands is predominantly accelerated by inappropriate land use [31, 33] and mismanagement [32]. The runoff water generated by water erosion poses a substantial and far-reaching threat to soil quality, resulting in the depletion of organic carbon, disturbances in nitrogen balance, soil compaction, reduced soil biodiversity, and contamination from pesticides and heavy metals.

3. Impacts of erosion: assessing the consequences on water and soil erosion

3.1 Impact on nutrient mobilization

The examination of erosion processes provides valuable insights into the intricate environmental dynamics of our planet. Notably, water erosion emerges as a dominant force, contributing to an annual sediment flux of roughly 28 Pg. In addition, tillage and wind erosion introduce approximately 5 and 2 Pg, respectively, leading to a total sediment flux of roughly 35 ± 10 Pg per year. This erosional activity also results in an agricultural carbon erosion flux of around 0.5 ± 0.15 Pg C annually, with an estimated 0.08 ± 0.02 Pg C transported to river systems through water erosion. When it comes to nitrogen dynamics, soil erosion is a significant factor, mobilizing roughly 23–42 Tg of nitrogen every year, a magnitude similar to the nitrogen applied to agricultural land via chemical fertilizers, the nitrogen removed by harvested crops, and the estimated riverine fluxes of particulate nitrogen. In terms of phosphorus, we estimate that soil erosion leads to the movement of 2.1–3.9 Tg of organic phosphorus and 12.5–22.5 Tg of inorganic phosphorus on an annual basis. While global mean phosphorus fluxes are notably lower than the 40 Pg stored in soils worldwide, they are similar in magnitude to crop uptake and fertilizer phosphorus additions to agricultural land. However, in specific regions, erosion-induced phosphorus fluxes surpass phosphorus additions, posing challenges for soil fertility and food production. This comprehensive evaluation highlights the intricate global interplay of erosion processes and their far-reaching implications for soil and nutrient dynamics [45, 47].

3.2 Impact on carbon cycle

Soil erosion encompasses the processes of soil mobilization, transport, and deposition. A holistic understanding of its effects on the carbon cycle requires considering all three phases. When soil material is mobilized, it disrupts soil structure, potentially leading to a significant increase in soil organic carbon (SOC) mineralization, resulting in the loss of over 20% of the total SOC as carbon dioxide. Regarding the influence of transport on SOC mineralization, it is essential to distinguish between SOC deposited locally after relatively short-distance transport (<500 m) via water or tillage within a brief timeframe (<1 day) and the fate of SOC transported to rivers. Field observations and erosion-deposition simulations based on 137Cs inventories indicate that the additional SOC mineralization during soil transport over land is of limited significance [48].

Recent field observations have also showed that SOC losses from soil redeposited after a short transport phase are relatively minor, constituting less than 2.5% of eroded SOC and, therefore, having a negligible impact on the global carbon budget. Understanding erosion’s implications for the carbon cycle necessitates a comprehensive examination of both short-term and long-term impacts [49]. Contemporary research suggests that erosion plays a dual role in influencing carbon dynamics. When soil structure is disrupted during erosion, it can trigger the immediate release of carbon dioxide. Over extended timeframes, the increased carbon emissions are associated with eroded soils losing their capacity to support plant growth, leading to reduced carbon inputs from plant and root matter. Surprisingly, erosion can also contribute to carbon sequestration. As soil erosion integrates carbon-poor subsoil into the plow layer, newly exposed mineral surfaces may bind organic matter, offering potential for expanding soil carbon inventories [50].

The process of erosion, characterized by the removal of soil, also has the effect of bringing subsoil and parent material closer to the surface. Both empirical and theoretical evidence are increasingly pointing to a connection between erosion and heightened rates of chemical weathering in areas with silicate-rich parent material under steady-state conditions. This is significant because the weathering of silicate minerals consumes carbon dioxide, suggesting a potential link between erosion-induced weathering and the consumption of carbon dioxide [48]. However, it is expected that this carbon dioxide flux is relatively small. On the contrary, in regions with calcareous parent materials, accelerated weathering could lead to the release of carbon dioxide into the atmosphere. For instance, in the Canadian prairies, it has been estimated that approximately 10% of the carbonates acidified during erosion might be emitted as carbon dioxide, resulting in an estimated loss of carbon ranging from 0.12 to 1.2 Mg ha⁻¹ year⁻¹ [51].

3.3 Impact on nutrient cycles

Current research on erosion’s impact on nitrogen and phosphorus cycling has primarily concentrated on assessing how these nutrients are mobilized and delivered to aquatic ecosystems. However, there is limited understanding of how erosion influences nitrogen and phosphorus cycling in terrestrial environments. Soil organic matter serves as a significant reservoir for nitrogen and phosphorus. Therefore, increased soil carbon mineralization due to erosion-induced soil mobilization leads to a proportional rise in dissolved nitrogen and phosphorus, making them more readily available to organisms than particulate or organic forms. Conversely, the burial and preservation of deposited carbon contribute to the stability of organic nitrogen. Nitrogen’s stability in depositional settings may be high, primarily influenced by the rate of carbon mineralization. This phenomenon may account for the consistent C:N ratios observed in surface soils within specific ecological contexts, as well as the similarities between contemporary and ancient soil C:N values. In eroding environments, the dynamic replacement of carbon also promotes nitrogen stabilization. Nevertheless, nitrogen can also influence carbon cycling, particularly in environments where nitrogen availability directly limits biomass production and, consequently, dynamic carbon replacement [48].

Erosion significantly reduces soil phosphorus levels over time, not just through physical removal but also due to the exposure of low-phosphorus subsoil. This decline is influenced by erosion rates and chemical weathering. The soil’s phosphorus composition changes, shifting from various forms to predominantly organic and occluded form [52]. Depositional areas receive a significant phosphorus influx, as seen in Hawaii, where dust deposition alleviates phosphorus limitations in aging soils [53]. In regions dominated by erosion with limited nitrogen and phosphorus inputs, primary production exponentially declines as erosion intensifies [54]. This is driven by nutrient loss, soil structure degradation, and reduced water availability due to thinner soils. Water erosion enriches sediment with carbon, nitrogen, and phosphorus relative to the parent soil during mobilization and deposition [54]. This leads to changes in the relative abundance of these elements in soils, with water and wind erosion causing greater enrichment than tillage erosion. Loss of carbon, nitrogen, and phosphorus from mobilization sites may initiate a feedback loop, reducing plant productivity and increasing erosion vulnerability. Soil resistance to erosion is closely tied to organic matter and vegetation cover [54]. Conversely, deposition sites experience higher nutrient and carbon contents, boosting primary productivity and enhancing soil fertility, plant growth, and erosion resistance [16].

4. Agroforestry’s real-world effects on biodiversity

Agroforestry offers many advantages associated with the conservation of biodiversity (BD) and is found to have a major role in BD conservation and protection, according to recent reviews. Studies show that a variety of aboveground vegetation types and their interactions have a significant impact on belowground biodiversity; belowground communities are enhanced by systems that combine above- and belowground features. Comparing practices such as alley cropping and silvi-pasture to monocropping, the latter shows lower leaching losses and higher plant growth, along with superior soil attributes such as improved soil porosity, water dynamics, and nutrient cycling efficiency. The temperature of the surface soil rises when shade trees are removed, which has an impact on the soil communities, decomposition rates, humidity, and nutritional status. In Missouri, Agroforestry buffers outperform monocrop regions in terms of soil thermal characteristics and resistance to high temperatures. Additionally, there were differences in the microclimate characteristics of crop and buffer areas. The buffers had higher soil temperature, humidity, and wind speeds than the crop areas. Numerous studies conducted globally have demonstrated advantageous soil characteristics, microclimates, and nutritional condition, hence endorsing a wider range of agroforestry management approaches. These results suggest that specific species combinations at appropriate spacing and configurations can contribute to the improvement and conservation of BD by fostering a favorable microclimate and favorable soil properties [55].

5. Agroforestry approaches to mitigate soil erosion: measures and techniques

Soil is the most crucial resource in agriculture for meeting the varied food needs of the world’s rapidly expanding population. It is an essential component of ecosystems for providing the main ecosystem services. But according to recent reports, increased soil erosion poses a serious risk to soil, and approximately every year, 24 billion tons (BT) of rich soil are lost only as a result of erosion caused by water [55]. Erosion strongly affects soil quality by decreasing the available soil nutrients, which subsequently affects the plant growth, yield, and carbon stock potential [56]. The idea of soil conservation has evolved during the last 20 years in a number of ways. Soil conservation was once associated with soil erosion, and efforts to limit it were conducted independently of other land management activities. By the 1970s, the phrase had come to refer to not just keeping the soil in its proper location but also preserving or even improving its production. Different agroforestry system in different parts of the country have been mentioned (Figures 1–6). Here are some key strategies and practices within agroforestry systems that help prevent soil erosion:

Figure 1.

Sapota-based agri-horticulture (sapota + castor + cowpea) system on bench terrace in Mahi ravine land of Central Gujarat, India.

Figure 2.

Melia dubia-based silvo-aromatic (Melia dubia + lemon grass) system in gully bed of Mahi ravine land of Central Gujarat, India.

Figure 3.

Dragon fruit-based horti-silviculture (Dragon fruit + Melia dubia) system in Mahi ravine land of Central Gujarat.

Figure 4.

Silvo-aromatic system (Melia dubia + lemon grass) in degraded lands of Shivalik hills of Chandigarh.

Figure 5.

Silvo-aromatic system (Melia dubia + lemon grass) in degraded lands of Dehradun under Western Himalaya.

Figure 6.

Silvi-agriculture system (Melia dubia + green gram) in red soil of Southern Karnataka, India.

Establish windbreaks and shelterbelts: planting rows of trees and shrubs along field boundaries or across the prevailing wind direction helps reduce wind erosion. These vegetative barriers slow down the wind and minimize the impact of wind-driven soil erosion.

Create riparian buffers: in areas adjacent to water bodies like streams, rivers, or ponds, plant trees and shrubs as riparian buffers. These buffer zones filter runoff water, reduce soil erosion, and improve water quality.

Integrate contour farming: implement contour farming, where crops are planted along contour lines of the land, thereby reducing the speed of surface runoff and minimizing soil erosion.

Terracing: construct terraces on sloping lands to create flat surfaces that trap water and soil, reducing soil erosion. Plant trees or shrubs on terraces to further stabilize the soil.

Cover cropping: intercrop or rotate main crops with cover crops like legumes or grasses. Cover crops protect the soil from erosion, enhance soil structure, and add organic matter.

Agroforestry alley cropping: in alley cropping systems, rows of trees or shrubs are planted with crop alleys. The tree canopy and the root systems help protect the soil from erosion and provide shade to the crops.

Silvopasture: integrate trees with livestock grazing areas. Trees provide shade, reduce trampling of the soil by livestock, and contribute to soil stabilization.

Minimize soil disturbance: implement no-till or reduced-till farming practices to reduce soil disturbance during planting and harvesting. This preserves the soil structure and reduces the risk of erosion.

Maintain ground cover: keep the soil covered with crop residues or mulch, which reduces the impact of raindrops and minimizes water erosion. Mulching can be a crucial practice in agroforestry systems.

Proper agroforestry design: plan and design agroforestry systems to ensure that tree and shrub species are compatible with crops and provide adequate soil protection. The arrangement and spacing of trees should be carefully considered.

Soil fertility management: maintain proper soil fertility through nutrient management and organic matter addition. Healthy soils with adequate nutrients are less susceptible to erosion.

Manage livestock access: if livestock are part of the agroforestry system, manage their access to sensitive areas and prevent overgrazing, which can lead to soil compaction and erosion.

Regular monitoring and maintenance: continuously monitor the agroforestry system and address any issues promptly, such as damaged windbreaks or deteriorating terraces. Agroforestry practices can vary depending on the specific ecosystem and agricultural goals. Combining these techniques can provide a holistic approach to preventing soil erosion while enhancing biodiversity, improving soil health, and increasing overall agricultural sustainability.

6. Prevention of soil erosion

Agroforestry is a valuable approach to prevent soil erosion while also promoting sustainable land use and agricultural practices (Figure 7). Here are some studies indicate the strategies within agroforestry systems that helps to prevent soil erosion: A study reported that a single bamboo plant can bind up to 6 m of soil [57]. However, in Jiulongjang and Dayingjang rivers in China, it was noted that each clump can protect 12 m of river embankment, also yielding shoots and bamboo timber [58]. Wiersum [59] found that different agroforestry systems cause the lowest soil erosion. Compared to pure cropping systems, agroforestry produced the following results: 50% less erosion, 75% higher infiltration, 57% lower runoff, 22% greater macro-aggregation, and 30% higher mean weight diameter [60]. In Brazil, Beliveau et al. [61] compared erosion in short cycle cropping systems, 2-year-old agroforestry systems, and forest systems. They discovered that while soil particle loss, total Hg, and cation mobilized in agroforestry systems were similar to those in forests, they were significantly lower in short cycle cropping systems. The ability of agroforestry systems to mitigate soil erosion varies depending on their kind and management style. Only a small percentage of the current agroforestry systems are able to prevent farmland from eroding, according to an Indonesian study. This suggests that more agroforestry is required to reduce soil erosion and preserve watershed quality while promoting sustainable agriculture [62]. Purwaningsih et al. [63] recommended use agroforestry with proper species and arrangement as technique to control landslide reactivation on volcanic foot slopes in Java. Agroforestry functions in two ways to control erosion of soil: supplementary and directly; by supplementary function, plants stabilize soil structure and improve texture and enhance soil productivity, while vegetation directly by itself reduces erosion by reducing impact of rainfall.

Figure 7.

Ecosystem services received from agroforestry system (Source: Rathore et al. [2]).

7. Bioengineering measures

For soil and water conservation, structural and mechanical methods are frequently employed; nevertheless, they have drawbacks, including a limited lifespan, soil disturbance, high cost, labor intensity, and technical requirements. On the other hand, using plants, bioengineering techniques provide an affordable way to increase output while preserving natural resources [64]. These techniques, which have been successfully shown in Dehradun, use plant materials to alleviate erosion problems. At Nalotanala, for example, a 4 ha area was completely restored and revegetated in less than 10 years, greatly lowering the sediment load from 320 to 5.5 Mg ha⁻¹ year⁻¹ over a 30-year period. In addition, when therapy ended, the flow of dry weather rose from 100 to 250 days. The impact of bioengineering techniques on the Doon Valley’s Shahastradhara mining watershed was evaluated by Juyal et al. [65]. Over a 20-year period, they discovered that afforestation raised vegetation cover from 10 to 90%. After treatment, bulk density dropped from 1.63 to 1.45 Mg m⁻³, pH dropped from 8.1 to 7.4, and slope was cut in half. Soil organic carbon increased from 0.13 to 0.45%. Main drainage channels also became perennial, and in November and February, respectively, water flow reached 265 and 100 cubic meters per day.

8. Soil health improvement through agroforestry practices

Under both sequential and simultaneous agroforestry, the incorporation of trees into croplands can aid in maintaining the physico-biochemical qualities. In addition to supplying soil organic matter, tree litter and prunings increase soil fertility by releasing nutrients into the soil through mineralization. The amount and quality of tree litter or prunings, the local climate, the kind of soil, and the growing activity of microorganisms in the root zone all play a role. The roots of trees typically extend considerably beyond the crop zone, allowing them to absorb nutrients from the lower soil layers and boost nutrient and water consumption efficiency while posing minimal threat to crops. Improved environmental conditions provided by shade trees for crops and silvo-pastoral systems stimulated the activity of earthworms, termites, and other soil engineers, leading to an increase in the development of both tiny and large faunal aggregates [66]. The soil fertility enhancement varies with the species and the quantity of litter added to the soil Table 3. Various workers have reported that trees directly or indirectly influence the soil physic-chemical-biochemical properties. Rivest et al. [70] found that windbreaks increased the soil organic C, total N, and C:N ratio while reducing the soil pH, relative to adjacent crop fields. Meta-analysis by Muchane et al. [60] revealed that agroforestry can reduce soil erosion rates by 50%, enhances soil organic carbon by 21%, raises nitrogen by 13%, enhances available phosphorous by 11%, and alleviates pH of soil by 2% than pure cropping. This might be due to reduced runoff, high infiltration, increased macro-aggregation, and more stable soil structure in agroforest. The physical and chemical characteristics of the soil under coconut monocropping and 5-year-old and 20-year-old Gliricidia-coconut-based mixed systems were studied for 3 years by Raveendra et al. [71]. They discovered that the 20-year-old Gliricidia-coconut-based mixed systems had 22% higher organic matter content, 20% higher available P content, and 69% higher total exchangeable K content than the coconut monocrop. This study demonstrated the potential of Gliricidia-coconut-based mixed system for rehabilitation of degraded coconut-growing soils in Sri Lanka.

Species	Soil fertility enhancement	References
Populus deltoides	SOC: 2.9–4.8 Mg ha^−1	[67]
Acacia nilotica	SOM: 1.4 g kg^−1 soil	[68]
S. cannabina	209 kg N ha^−1	[69]

Table 3.

Soil fertility enhancement through trees.

Biological nitrogen fixation (BNF) constitutes a key nutrient input to agro ecosystems. The contribution of leguminous trees to building up N in degraded soils through BNF is well recognized as an important component of the ecosystem service of nutrient cycling [72]. There are significant differences in estimates of BNF in trees, ranging from high rates up to 472 kg N ha⁻¹ year ⁻¹ in L. leucocephala, Gliricidia sepium, C. calothyrsus to low rates <50 kg N ha⁻¹ year⁻¹ in Acacia melanoxylon and A. holoserica[73], whereas roots of Casuarina species fix nitrogen up to 350 kg N ha⁻¹ year⁻¹. Improvement of soil organic carbon by 36.5% and total nitrogen stocks by 33.2% at 0–10 cm soil layer was observed in alley cropping of rubber trees with Coffea liberica as compared to rubber monoculture [74]. In agroforestry system, BNF trees contribute toward complementary interaction, which makes agroforestry system sustainable for nutrient requirements.

Leguminous trees contribute to soil nitrogen enrichment through the process of N-fixation, which occurs when nitrogen-rich biomass is accumulated in the soil through root decomposition, litter fall, and prunings. In addition to being leguminous, some nonleguminous shade plants, such as rambutan and durian, improved the soil’s carbon and nitrogen content when added to cocoa agroforestry [75]. Humid and subhumid tropical regions are at risk of losing biodiversity due to soil degradation, and the majority of these soils have inadequate nutrient reserves, are prone to erosion, are acidic, and have other related toxicities. Most of these obstacles may be greatly alleviated by agroforestry, which would also guarantee increased food production, better health and nutrition, and the preservation of natural resources [76]. Stocker et al. [76] also estimated the agroforestry improves several soil physical attributes over a short time, by diversification of tree root systems and accumulation of plant residues. Piza et al. [77] compared decomposition and release of nutrients in different land use systems in Columbia and found that litter decomposition and nutrient release in coffee agroforests were similar to a nearby forest.

9. Effect of agroforestry on erosion factors

Agroforestry is the intentional cultivation of trees, crops, and/or animals in interdependent combinations for a range of purposes. Although agroforestry has been used for a long time all over the world, it was not until the late 1970s that the phrase and concept gained widespread recognition as a viable choice for sustainable land management. These days, a wide range of land-use systems in various contexts are included by the phrase [78]. Agroforestry, which bridges the gap between agriculture and forestry, is currently viewed as a potential method of land use, particularly in emerging tropical and subtropical nations [79].

The idea that agroforestry systems might lessen rainfall erosivity is one that is commonly accepted. In some agroforestry scenarios, it might not hold true, though. When there is a tall, broad-leaved canopy, the kinetic energy of raindrops falling can be increased. Raindrops aggregate to form larger droplets that, when descending from a height of roughly 30 meters above the ground, can reach a high speed and significantly erode splash zones due to their contact. Although field measurements from such agroforestry systems have been sparse, it is probable that a low and dense canopy will lower erosivity.

When wind is a significant contributor to soil erosion, agroforestry techniques such as windbreaks and shelterbelts are crucial for managing the erosion. Protecting fields, houses, canals, and other places from wind and blowing sand is done by planting windbreaks, which are small strips of trees, shrubs, and/or grasses. Protecting agricultural fields from tidal wave flooding is the purpose of shelterbelts, a type of windbreak that are long, multiple rows of trees and bushes, commonly along sea shores. In the semiarid tropics [80] as well as in semiarid temperate parts of North America, Europe, and Asia, windbreaks have long been used to protect crops and soil from wind and wind erosion. When properly designed and maintained, a windbreak reduces the velocity of the wind and thus its ability to carry and deposit soil and sand (Table 4). It can improve the microclimate in a given protected area by decreasing evaporation from the soil and plants. In many cases, windbreaks have been shown to increase the productivity of the crops they protect. Additionally, windbreaks can provide a wide range of useful products, from poles and fuel wood, to fruit, fodder, fiber, and mulch [78].

Degraded land	Location	Agroforestry system	Impact	Reference
Degraded sloping land	Northeast India	Hedgerow cropping	There was a 94 and 78% decrease in soil loss and runoff, respectively. Michelia oblonga enhances soil physical behavior and SOC significantly due to its continuous leaf litter and root exudates.	[81]
	Dehradun	Silvi-pasture system	Zero soil loss, with eucalyptus providing additional revenue of about Rs. 4000 per hectare per year just from commercial grass.	[82]
	Shivalik hills	Horti-pasture system	4.9–30.7 cm of water and 862–2818 kg ha^−1 of soil were preserved in the Emblica officinalis + Chrysopogon fulvus horti-pasture system.	[83]
	Karnataka	Ley farming (Vegetative barriers)	Plants such as Cenchrus ciliaris and Cymbopogon martini reduced soil loss by 16% and discharge by 38%.	[84]
	Kashmir	Silvi-agriculture	Lower temperatures and decreased soil erosion combined with higher agricultural yields.	[85]
Gullied and ravine lands	Gujarat	Agri-horticultural	They showed that sapota with bench terraces and trenches decreased runoff by 16–34% and soil loss by 15–25%.	[86]
	Gujarat	Cowpea, Castor, and Sapota in an Agroforestry System	Compared to a single tree plantation, there was a reduction in total soil loss and runoff of 37.7 and 19.1%, and an increase in system productivity of 81.9%.	[22]
	Uttar Pradesh	Agri-horticulture	Increased yields of fuel wood, wheat, pearl millet, and ber were recorded.	[87]
	Rajasthan	Alley cropping	In alley cropping systems based on Leucaena, higher yields, land equivalent ratios, and soil organic carbon levels were observed.	[88]
Shifting cultivation land	Northeast India	Nitrogen-fixing, fast-growing multipurpose tree plantations	SOC increased by 96.2%, aggregate stability increased by 24.0%, porosity increased by 10.9%, accessible soil moisture increased by 33.2%, bulk density decreased by 15.9%, and erosion ratio decreased by 39.5%.	[81]
	Odisha	Alley cropping	There was a 23–32% decrease in runoff and a 49–52% decrease in soil loss.	[89]
	East India	The grass filter strip and hedgerow of Gliricidia sepium	There was a 32% decrease in runoff and a 35% decrease in soil loss.	[90]

Table 4.

Agroforestry practices with potential for control of soil erosion.

10. Agroforestry approaches to mitigate water erosion: measures and techniques

Water erosion poses a significant threat to soil health and agricultural productivity, leading to the degradation of arable land and the loss of essential nutrients. In recent years, agroforestry has emerged as a sustainable and effective approach to mitigate water erosion, offering a holistic solution that integrates the cultivation of trees and crops. This portion explores various measures and techniques within agroforestry systems that prove instrumental in combating the detrimental effects of water erosion. Integrating cover crops and living mulches into agroforestry systems is another effective strategy. The roots of cover crops bind soil particles together, preventing them from being washed away by water runoff. Leguminous cover crops, such as clover and vetch, offer the added benefit of fixing nitrogen in the soil, enhancing overall soil fertility. These living mulches also provide a protective cover, reducing the impact of raindrops on the soil surface and minimizing soil compaction. Uttar Pradesh implemented an agri-horticulture system, resulting in higher yields of Ber, pearl millet, wheat, and fuel wood, as reported by Prakash et al. [87]. Another instance in Northeast India involved multipurpose tree plantation, specifically fast-growing nitrogen-fixing trees, in fallow areas. This approach led to a 15.9% reduction in bulk density, a 39.5% decrease in erosion ratio, and significant increases in soil organic carbon (96.2%), aggregate stability (24.0%), porosity (10.9%), and available soil moisture (33.2%), as observed by Saha et al. [81]. Contour planting is a key technique in agroforestry that helps mitigate water erosion. By planting along the contours of the land, the flow of water is slowed down, giving it more time to infiltrate the soil. This reduces the risk of surface runoff and soil erosion. Agroforestry systems that incorporate contour planting often include a mix of deep-rooted trees and shrubs alongside crops. The roots of these trees play a crucial role in stabilizing the soil structure. Buffer strips comprising trees, shrubs, and other perennial vegetation can be strategically established between agricultural fields and water bodies. These buffer strips act as a buffer zone, filtering and slowing down surface runoff before it reaches water bodies. This not only prevents soil erosion but also helps in retaining nutrients and reducing the influx of sediment into water sources. Native species are often preferred for buffer strips to enhance biodiversity and ecosystem services. In Dehradun, a silvipasture system was adopted, demonstrating no soil loss and yielding an annual return of about Rs. 4000 ha⁻¹ year⁻¹ from commercial grass alone, in addition to returns from Eucalyptus, according to Sharda and Venkateswarlu [82]. The horti-pasture system in the Shivalik hills, incorporating Emblica officinalis and Chrysopogon fulvus, proved effective in water and soil conservation, saving 4.9–30.7 cm of water and 862–2818 kg ha⁻¹ of soil, as documented by Prasad et al. [87]. In Karnataka, Ley farming with vegetative barriers using Cenchrus ciliaris and Cymbopogon martini resulted in a 38% reduction in runoff and a 16% decrease in soil loss, according to Ramajayam et al. [84]. In Kashmir, the adoption of silvi-agriculture not only reduced scorching heat but also mitigated soil erosion and increased crop production, as highlighted by Mughal and Makaya [85]. Gujarat saw success with an agri-horticulture system incorporating soil moisture conservation practices, revealing that sapota with trenches and bench terraces reduced runoff by 16–34% and soil loss by 15–25%, as indicated by Kumar et al. [88]. Additionally, an agroforestry system involving cowpea, castor, and sapota in Gujarat, as studied by Jinger et al. [22], exhibited a 37.7% reduction in total soil loss, a 19.1% decrease in runoff, and an 81.9% increase in system productivity compared to sole tree plantation. Alley cropping, where rows of trees are planted between rows of crops, is an agroforestry technique that promotes water conservation and reduces erosion. The tree rows serve as windbreaks and contribute organic matter to the soil through leaf litter, enhancing soil structure. Additionally, the shading effect of the trees helps in moderating soil temperature and moisture, creating a more favorable environment for crops. In Northeast India, hedgerow cropping was implemented, resulting in a substantial reduction of soil loss and runoff by 94 and 78%, respectively, as reported by Saha et al. [81]. Rajasthan adopted alley cropping, leading to higher yields, a favorable land equivalent ratio, and increased soil organic carbon in Leucaena-based systems, according to Dhyani et al. [88]. In Odisha, the implementation of alley cropping resulted in a noteworthy reduction of 23–32% in runoff and 49–52% in soil loss, as reported by Adhikary et al. [89]. Finally, in East India, the use of Gliricidia sepium hedgerow and grass filter strip effectively reduced runoff by 32% and soil loss by 35%, according to Lenka et al. [90]. Agroforestry presents a multifaceted approach to address water erosion challenges in agriculture. The combination of windbreaks, cover crops, contour planting, buffer strips, and alley cropping creates a resilient and sustainable system that not only mitigates erosion but also enhances overall soil health and productivity. As we move forward in our quest for sustainable agricultural practices, agroforestry stands out as a promising solution to combat water erosion and promote the long-term sustainability of our agricultural landscapes (Figure 8).

Figure 8.

Manifold benefits of agroforestry system.

11. Conclusion

Many agroforestry techniques, including agri-silviculture, silvi-pastroral, and agri-horticulture, have demonstrated a high potential for generating both financial and environmental gains from these lands. India has a large number of species that could be used for agroforestry; all that is needed is to fully realize their potential. Farmers can significantly boost their revenue by widely cultivating and adopting high-value trees. On an average, soil loss and runoff could be reduced by 45–55 and 55–65%, respectively, by different agroforestry system. Moreover, SOC and system productivity could be enhanced by 100–150 and 30–40%, respectively, under different agroforestry system. More studies are required to develop agroforestry practices that can be widely accepted by farmers and stakeholders. Understanding fundamental processes and implementing technology well would be crucial for successfully managing and repairing soil eroded land through agroforestry. Therefore, to reduce soil erosion, it is necessary to develop appropriate agroforestry practices, policies, and action plans for the greater promotion and adoption of agroforestry in India.