A Comprehensive Review on Ecological Buffer Zone for Pollutants Removal

Abstract

The issue of agricultural non-point source pollution has attracted global attention. A buffer zone is an effective, eco-friendly, and economically feasible remediation ecosystem to reduce the impact of agricultural non-point source pollution on water bodies. They can effectively remove pollutants in agricultural drainage through physical processes (infiltration, filtration, deposition, etc.), plant absorption and assimilation, and microbial processes, improving the water quality of water bodies. This article provides a comprehensive review of the current studies on using buffer zones to remediate agricultural non-point source pollution, with a focus on the key affecting factors for pollutant removal efficiencies. The main factors included buffer zone width, vegetation type, slope, seasonal variation, soil variation, and vegetation density. The influencing mechanisms of these factors on the pollutant removal efficiencies of buffer zones were also discussed. This review can serve as a reference for a deep understanding of buffer zones and help optimize their design and management in real ecological remediation projects.

1. Introduction

With the rapid development of the social economy, an increasing level of attention is being paid to the issues of water environments. At present, eutrophication has become one of the global water pollution issues [1,2]. According to relevant research [3,4], the usage of chemical pesticides increased from 0.73 million tons in 1990 to 1.66 million tons in 2017 in China. The usage of mineral fertilizers increased from 8.84 million tons in 1978 to 58.59 million tons in 2017. Obviously, agricultural growth relies on intensive inputs of production factors, leading to serious non-point source pollution in water environments [5]. In the United States, agricultural activities are also a major source of surface water pollution, including excessive nutrients from fertilizers and pesticides, as well as an increase in water turbidity caused by soil erosion [6], accounting for approximately 55% of surface water pollution from non-point sources. Additionally, global agriculture production releases around 31 million tons of nitrogen and 2.9 million tons of phosphorus into freshwater bodies per year [7].

Agricultural non-point source pollution is characterized by its extensive dispersion, complex migration routes, hidden nature, and cumulative effects, leading to challenges in effectively controlling such pollution [8]. Although significant efforts have been conducted to reduce fertilizer application and adopt optimal land management practices [9], nutrient pollution in water bodies persists. This is partly due to the continuous loss of function of natural riparian wetlands [10]. Extensive studies have been focused on the prevention and controlling of agricultural non-point source pollution to reduce the impact on aquatic ecosystems. At present, the commonly used technologies include developing precision agriculture, ecological ditches, buffer zones, compost technology, and soil microbial fertilizers [11,12,13]. Among them, buffer zone technology has become a widely accepted and effective technology due to its pollutant removal capabilities. It is widely used to control non-point source pollution and support the development of more sustainable agriculture [14,15]. The detailed arrangement and functionality of buffer zones are shown in Figure 1 [16].

Buffer zones serve as critical interfaces between surface water and groundwater systems [17], aiming to improve water quality by capturing pollutants from surface water and shallow groundwater and absorbing excess pollutants [18]. In addition, buffer zones exhibit complex bio-geochemistry processes that play an important role in maintaining the river balance of nature, promoting biodiversity conservation and providing a variety of ecological services [19]. As shown in Figure 2, after pesticides and nutrients are discharged from agricultural fields, they enter buffer zones and can be effectively removed through processes such as soil filtration, plant absorption, and microbial degradation, significantly reducing their impact on water bodies and ecosystems [20]. Liu et al. [21] studied the impact of buffer zones on controlling non-point source pollution in Chaohu lake. They found that the pollutant removal efficiency of the buffer zone was significantly better than that of the constructed wetland and the permeable pedestrian pathways. The reduction rates of non-point source pollution load for total nitrogen and total phosphorus were 15.29% and 15.03%, respectively. Further studies demonstrated that buffer zones could reduce nitrogen fluxes by up to 90% through a series of complex processes, including plant absorption, denitrification, and storage [22]. In summary, buffer zones have been demonstrated as an effective, eco-friendly, and economically viable method for trapping runoff and sediment. Therefore, it is becoming a popular non-point measure of soil and water conservation [23,24].

Previous studies mainly focused on the removal effect of buffer zones on agricultural pollutants, the restrictive factors of buffer zones, and the development of models or technical methods for evaluating buffer zones in the field [4,25,26,27,28,29,30]. During the processes of pollutant removal via buffer zones, it is necessary to clarify the mechanism of nitrogen and phosphorus removal. The pollutant removal processes are influenced by many factors, such as the buffer zone width, vegetation type, slope, seasonal variation, soil composition, and vegetation density. Therefore, the mechanisms of pollutant removal via buffer zones are quite complex and require further research. However, to the best of our knowledge, few comprehensive reviews were focused on pollutant removal via buffer zones, which prompted us to write this critical and comprehensive review. The specific objectives are the following: (1) provide insights on recent study trends and the developing progress of buffer zones, aiming to demonstrate the importance of buffer zones in reducing the introduction of pollutants from agricultural activities into water bodies; (2) clarify the main retention process and mechanisms of pollutant removal in buffer zones; and (3) explore the influencing factors for the performance of buffer zones in pollutant removal. This review can provide important references for the design and construction of buffer zones, improving the development and application of buffer zones in water pollution control and ecosystem protection.

2. Methods

2.1. Literature Acquisition Sources

To search for the relevant literature, the Web of Science (WoS) and ScienceDirect (SD) databases were used, which cover the main academic journals and published papers with a high degree of authority and credibility in almost all major subject areas. The search process is summarized in

Table 1. Literature search strings.

Database	Retrieval String	Number	Search Data
Web of Science	First search string: buffer zone, second search string: water pollution, third search string: agriculture	364	30 June 2024
	First search string: buffer zone, second search string: water pollution, third search string: mechanism	138	30 June 2024
ScienceDirect	Keywords in the title or abstract: buffer zone, pollution	28	30 June 2024
	Keywords in the title or abstract: buffer zone, agriculture	35	30 June 2024
Total	—	565	30 June 2024

. The initial search was conducted based on specific keywords, and 565 papers published from 2010 up to 30 June 2024 were obtained, including 502 papers in WoS and 63 papers in SD.

2.2. Literature Selection Criteria and Classification

According to the topic of this review, the relevant literature was further screened out by titles, abstracts, keywords, and full-text articles in turn, and the duplicate and irrelevant articles were eliminated manually, followed by intensive reading to determine the eligible articles. The detailed screening process is shown in Figure 3. Eventually, 318 relevant articles were identified.

On the basis of the above searching results, further classification was carried out according to the corresponding retrieval string, as shown in

Table 2. Literature classification.

Retrieval String	Number of Articles in the Initial Searching	Number of Relevant Articles Based on the Exacting Screening
Mechanism	52	28
Buffer zone width	83	56
Vegetation type	67	45
Slope	58	37
Seasonal variation	48	38
Soil composition	201	28
Vegetation density	22	15
Runoff intensity	20	12
Others	—	—

. Among them, when the articles involved multiple influencing factors, they were separately counted in each influencing factor.

3. Mechanism of Buffer Zones in Removing Pollutants

Buffer zones can reduce the concentration of pollutants in the water through a series of complex physical, chemical, and biological processes [31]. The nitrogen and phosphorus removal processes are closely related to some factors, such as buffer zone width, vegetation type, slope, seasonal variation, soil composition, and vegetation density. A combination of these internal and external factors is essential for complete understanding and effective management of the pollutant removal capacity of buffer zones. Nitrogen and phosphorus removal via buffer zones mainly involve physical adsorption, vegetation absorption and assimilation, soil adsorption, and microbial absorption and transformation, as well as denitrification processes [32]. Although many researchers have studied the migration and transformation processes of nitrogen and phosphorus in different ways using many experimental methods, there is still much controversy regarding the relative importance of each process [33].

3.1. Physical Processes

Buffer zones protect aquatic ecosystems by effectively reducing nitrogen and phosphorus in water through physical processes, such as precipitation, filtration, infiltration, absorption, and degradation [34]. Firstly, vegetation within buffer zones plays a crucial role. Vegetation covering the soil surface can effectively increase runoff resistance and slow down surface runoff velocity, allowing more surface runoff to infiltrate through soil pores and become subsurface flow [35]. As a result, most solid particles carrying pollutants gradually settle, and particulate pollutants or suspended solids in runoff are effectively filtered and intercepted [36]. At the same time, soluble pollutants in the subsurface infiltrate into deeper soil layers through the relatively loose soil in the buffer zone. The transport capacity of surface runoff for soluble pollutants is decreased [20], thus reducing the loss of total nitrogen and total phosphorus [37]. The root system of vegetation can penetrate the deep layer of soil and increase the structural stability, water permeability, and aeration of the soil. It also plays a role in filtering and adsorbing nutrients such as nitrogen and phosphorus, thus purifying groundwater and surface water. Goloran et al. [38] found that plant roots could absorb and intercept nutrients from underground runoff, reducing the loss of total nitrogen and total phosphorus from underground runoff. The more developed root system and the higher biomass of the buffer zone can promote root absorption and microbial degradation of plant roots, thus increasing the efficiency of pollutant interception in runoff [39].

Wu et al. [40] found that the nitrogen and phosphorus removal efficiency of a buffer zone exceeded 60% at 2–5% slopes, which was significantly higher than that of runoff. Alemu et al. [41] observed that 99% of total phosphorus and 85% of nitrate nitrogen could be reduced through approximately 10 m of herbaceous buffer zone. This result suggests that the establishment of herbaceous buffer zones on both sides of riverbanks can reduce the entry of nitrogen and phosphorus into the water body. It was also found that the populus buffer zone in Taihu significantly reduced sediment and nitrogen loss from surface runoff and the loss flux decreased with the increasing plant density in the buffer zone [42].

Furthermore, the soil layer of the buffer zone also possesses the ability to adsorb, filter, and immobilize nutrients. When the soil contains large clay, small silt, and sand particles, the runoff velocity of water slows down [43]. Meanwhile, soil pores serve as channels for water transport, retaining pollutants in the soil through water absorption. Moreover, the infiltration capacity of soil is increased and the erosion of surface runoff on the soil surface is reduced under the action of gravity. In addition, soil texture, soil–water interaction, organic matter content, and soil nutrient coverage were found to affect phosphorus release [44]. The retention and transformation of phosphorus in buffer zones are primarily driven by physical processes [45], including the adsorption effect by particles in surface runoff and the infiltration effect of the soil, thus the retained phosphorus can be absorbed and utilized by plant roots. These physical processes lead to the reduction in phosphorus in the surface runoff.

In addition, the types and forms of pollutants in surface runoff also have a significant influence on the interception effect of buffer zones. When surface runoff containing particulate nitrogen and phosphorus and dissolved nitrogen flows through a buffer zone, the vegetation in the buffer zone effectively suppresses soil erosion due to the runoff, increases the roughness of the surface, and effectively decreases the runoff velocity. It also improves the hydraulic permeability of the soil and assists in the effective removal of nitrogen and phosphorus from runoff [16,46]. Besides, the width, slope, and other factors of buffer zones also play an important role in their removal of pollutants, improving the exchange and transformation of substances among the vegetation, soil, and water.

In summary, buffer zones effectively intercept and transform nitrogen and phosphorus in surface runoff through physical processes involving vegetation, soil, and topography. The amount of nitrogen and phosphorus entering water bodies can be reduced, thus protecting the health of aquatic ecosystems.

3.2. Absorption and Assimilation Process by Plants

Absorption and assimilation via plants play an important role in the removal of nitrogen, phosphorus, and other pollutants in buffer zones. Firstly, root systems absorb pollutants like nitrogen and phosphorus from the soil and convert them into nutrients for plant growth [37]. Pollutants from water and soil are effectively transferred into plants through this absorption process, reducing their pollution levels in water bodies and the soil. When dissolved nitrogen (usually in the form of nitrate or ammonia) infiltrates into the root zone, it is absorbed by plant roots [25]. Then, it is converted into organic nitrogen through a series of biochemical reactions, mainly existing in the forms of amino acids and proteins stored in plants [47]. This process involves the participation of various enzymes, such as nitrate reductase and glutamine synthetase, which catalyze the transformation and synthesis of nitrogen. Inorganic phosphorus in the soil is absorbed by plant roots and converted into organic phosphorus, such as phospholipids and nucleic acids. This process involves biochemical reactions such as phosphorylation and esterification. Organic phosphorus plays important biological functions in plants, such as energy conversion and cell metabolism.

Secondly, plants absorb CO₂ and release O₂ during their growth through photosynthesis. Plant roots also release O₂, which can boost O₂ content in water bodies. The release of O₂ is essential for the survival of aquatic organisms in water bodies. It maintains the balance of aquatic ecosystems and promotes the decomposition and degradation of organic matter, further reducing the concentration of pollutants in water bodies. At the same time, it can promote the growth and activity of soil microorganisms and root-associated microorganisms, accelerating the degradation and removal of pollutants. As displayed in Figure 4 [48], oxygen and root exudates are transferred from the upper biomass to the microbial communities on the root surface. With the action of rhizosphere microorganisms and enzymes, the ability of plants to remove pollutants is significantly enhanced.

Moreover, it was found that the majority of nitrogen and phosphorus in plants was returned to the soil through plant aging or leaf litter [49]. Peterjohn et al. also found that nitrogen in leaf litter returned to the soil accounted for up to 80% of nitrogen absorption in a deciduous forest-dominated buffer zone [50]. This phenomenon is similar to Yang et al.’s study [51]. Nevertheless, plant assimilation remains an important mechanism for nitrogen and phosphorus removal via buffer zones [47]. It can alter the forms of nitrogen and phosphorus in soil. The mineral decomposition of plant residue can produce a lot of inorganic salts and available carbon sources, thus creating favorable conditions for microbial activity.

In summary, absorption and assimilation via plants play an important role in removing nitrogen and phosphorus in buffer zones, involving the absorption of pollutants, release of oxygen, and promotion of microbial activity. This contributes to the improvement of environmental quality in both the water and soil.

3.3. Microbiological Effects

Microorganisms are an important driving factor for nitrogen and phosphorus removal in buffer zones. They promote the transformation and absorption of nitrogen and phosphorus through biodegradation, phosphorus dissolution, and precipitation, as well as symbiotic interactions with plants, improving the removal efficiency of buffer zones and maintaining the health of aquatic ecosystems [52].

In buffer zones, microorganisms biodegrade organic matter to release organic nitrogen and phosphorus, converting complex organic molecules into different forms of inorganic compounds such as ammonia nitrogen, nitrates, and phosphates, which can be more easily absorbed by soil or water. These microbial metabolites and activities influence the cycling processes of nitrogen and phosphorus, promoting their removal efficiency in buffer zones and maintaining healthy aquatic ecosystems. The intensity of microbial processes varies in different types of vegetative buffer zones [53,54].

Ammonification, nitrification, and denitrification processes by microorganisms play important roles in nitrogen transport and transformation [25,26]. Organic nitrogen is converted to ammonia nitrogen through ammonification. Ammonia nitrogen can be absorbed and assimilated by microorganisms, as well as converted to nitrate nitrogen by nitrifying bacteria and nitroso bacteria. Studies have shown that the main mechanisms of nitrogen removal in buffer zones are plant absorption and microbially mediated denitrification in soil [25,26]. As shown in Figure 4 and Figure 5, nitrate nitrogen can be more easily removed via the denitrification effect of denitrifying bacteria, reducing it to N₂ and releasing it into the atmosphere, which can be completely removed from the buffer zone [48,55]. Additionally, a portion of nitrate nitrogen is reduced to ammonia nitrogen via nitrate reductase and is further synthesized into amino acids and proteins. Denitrification and microbial assimilation are important processes for nitrogen removal in buffer zones [53,54]. Gu et al. [55] argued that microbial nitrogen removal processes play much larger roles than plant uptake, and microbial fixation plays a minor role in the nitrogen removal processes [56]. Different types of buffer zones also exhibit varying degrees of denitrification intensity. Studies have demonstrated that denitrification can remove 20–1600 kg of nitrogen per hectare of buffer zone annually [57].

Microorganisms also play an important role in the migration and transformation of phosphorus in buffer zones. Soluble phosphorus can be assimilated and absorbed by plant roots and microorganisms and subsequently converted into organic phosphorus in plants, effectively reducing phosphorus in water [34]. In addition, microorganisms are involved in phosphorus cycling processes such as desorption, adsorption, and mineralization, which affect the availability of phosphorus in soil [58].

Moreover, the microbial communities around plant roots symbiotically enhance the absorption capacity of plants for nitrogen and phosphorus, thereby promoting the effectiveness of buffer zones in removing nitrogen and phosphorus. In summary, microorganisms provide strong support for nitrogen and phosphorus removal in buffer zones through their diverse metabolic pathways and interactions with plants, contributing to the maintenance of the health and stability of aquatic ecosystems.

4. Affecting Factors for Pollutants Removal

As shown in Figure 6a, buffer zones have been increasingly applied in agricultural pollution control and have attracted more and more attention from researchers from 2010 to 2023. The migration and transformation of nitrogen and phosphorus in buffer zones is highly complex, involving many physical, chemical, and biological processes [25,26,27]. These processes are influenced by many factors, such as the width of the buffer zone, vegetation type, seasonal variation, soil composition, vegetation density, slope, and runoff intensity, which in turn affect the effectiveness of buffer zones in removing pollutants such as nitrogen and phosphorus [28,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78]. The number of published articles on different affecting factors are summarized from 2010 to June 2024 as shown in Figure 6b.

Table 3. The key factors for buffer zones.

Buffer Zone Width	Vegetation Type	Slope	Soil Composition	Average Annual Precipitation	Average Temperature	Reference
1, 3, 7 m	Grass	<3%	—	665 mm	8.2 °C	[59]
5, 9, 13 m	Weeds, sweet clover, and sweet clover/Chinese wingnut	10–20%	Sand, clay, and silt	665 mm	Minus 13.7 °C to 23.7 °C	[60]
10, 30 m	White clover, meadow fescue, and timothy	1–14%	Fine sand	—	—	[61]
10, 15, 30 m	Grass, deciduous trees, and trees	—	Hagerstown and Opequon	1050 mm	—	[62]
12, 24, 36, 48, 60 m	Woody vegetation, shrubs, or grass	8–9%	Clay	1650 mm	18 °C	[63]
0, 10, 20 m	Willows and poplars	—	Loess soil	350–600 mm	8 °C	[64]
12, 36, 60 m	Grass vegetation, shrubs, and woody vegetation	8–9%	—	1650 mm	18 °C	[65]
2, 4, 8 m	Native tallgrass prairie grasses and forbs	5–10%	Sand or clay	1035.8 mm	8 °C	[66]
25, 45 m	Tall forbs or swamp non-forest communities	0.5–3.0%	Soil, fine sands, grits, and coarse sand	600 mm	8.6 °C	[67]
3.05, 6.1, 9.14 m	Ordeum vulgare, medicago sativa,	0.5–2.0%	Loamy sand, sandy loam, loam, and silty loam	150 mm	0–23.1 °C	[68]
	bromus marginatus, and pascopyrum smithii
0–200, 200–500,	Agriculture, forest, grassland, and urban	0–5%	—	1900 mm	17 °C	[69]
500–1000 m
1, 3, 4, 6 m	Forest and tillage crops	8%	Loamy soils	450–700 mm	6.25 °C	[70]
100–700 m	Forest, paddy field, and tea field	0–80.30°	Ultisols, anthrosols, and inceptisols	1340 mm	17.5 °C	[71]
500, 1000 m	Site buffer, riparian buffer, and catchment buffer	—	—	—	—	[72]
500, 800, 1000	Forest land, water area, agricultural land,	—	—	1680 mm	17.5 °C	[73]
1200, 1500, 1800 m	bare land, construction land
—	Silvopastoral systems, silvoarable agroforestry, and	—	—	—	—	[74]
	linear tree plantings	—
—	Rice plant and grass samples	—	Ponds, rice fields, and natural wetlands	1200 mm	20 °C	[75]
—	Arboraceous, herbaceous, and aerenchymous	—	Organic and mineral	—	—	[14]
4.5 m	Trees and grasses	1–5%, 5–9%	Putnam silt loam soil and armstrong loam soil	978 mm	—	[76]
—	Phragmites australis	—	Gravel, gravel + biochar, ceramsite + biochar,	1191.5 mm	16.1 °C	[77]
			and modified ceramsite + biochar
30 m	—	≤10%, >10%	—	—	—	[78]

displays the key factors for buffer zones. These factors should be carefully considered when a buffer zone is applied for protecting agricultural ecosystems. The following sections assess the important affecting factors for pollutant removal in buffer zones.

4.1. Buffer Zone Width

The width of buffer zones plays an important role in the removal of pollutants such as nitrogen and phosphorus [28]. Scholars have explored the impact of buffer zone width on pollutant removal efficiency through field experiments, numerical simulations, and modeling studies. The optimal width of buffer zones was also determined to optimize the design of buffer zones and enhance their pollutant removal capacity.

The width of a buffer zone determines whether it can completely exhibit its ecological service function [4]. Chen et al. [79] found that the removal capacity of a buffer zone for pollutants such as nitrogen, phosphorus, and others depended highly on its width. Jiang et al. [80] discovered that the total nitrogen in water was reduced by 23.21, 50.39, and 56.20% for buffer zones 20, 40, and 60 m wide, with the removal efficiency of total phosphorus of 18.16, 45.93, and 52.14%, respectively. Clearly, wider buffer zones result in better efficiency in trapping and transforming pollutants. Wang et al. [81] also found that the optimal widths of buffer zones in Dianchi Lake, Erhai Lake, and Fuxian Lake were 450, 100, and 150 m, respectively. The findings of these studies indicate that the width of buffer zones is positively correlated with their efficiency in improving water quality and protecting aquatic environments, and a wider buffer zone plays an important role. It could be attributed to the increased vegetation area provided by the wider buffer zone, which in turn increases the contact area between pollutants, soil, and vegetation and promotes the removal and degradation of pollutants [82]. Aguiar et al. [63] studied the effects of buffer zone width (12, 24, 36, 48, and 60 m) on nutrient removal. They found that the vegetation strip with a 60 m width exhibited the optimum removal efficiency, especially for nitrogen. The finding differs from Johnson et al.’s study, which suggests that additional buffer zone width does not necessarily produce proportional groundwater water quality benefits [83].

Mayer et al. [84] conducted a meta-analysis on 89 buffer zones with different widths. They found that the reduction rate of nitrate nitrogen in water was significantly enhanced as the width increased from 0 to 25 m. However, increasing the width from 25 to 50 m did not significantly enhance the removal rate of nitrate nitrogen. Lv and Wu observed a similar phenomenon [85]. The highest nitrogen removal was achieved at a width of 15 m. When the width exceeded 15 m, the increasing trend in nitrogen removal rate noticeably slowed down and even decreased. However, Valkama et al. [86] used a meta-analysis and found no effect of width on nitrogen removal efficiency, which is contrary to the model predictions of Zhang et al. [78]. Wanyama et al. [87] found that there was no linear relationship between buffer zone width and phosphorus removal efficiency in water. As the width increased (7.5 and 15 m), the pollutant removal efficiency by unit width of buffer zone continuously decreased [88]. Based on previous studies, it can be concluded that buffer zones cannot be expanded indefinitely, which will not only increase the cost and complexity, but also may lead to resource wasting and performance reduction. Therefore, it is necessary to discuss the optimal width of buffer zones.

The optimal width of buffer zones has been extensively studied by many researchers through field investigations or mathematical models, which suggested suitable widths [64]. Fischer et al. [89] suggested that different widths of buffer zones can meet the requirements of various types of ecological and environmental protection. In general, a 3–10 m buffer zone can be used for removing organic matter, a 10–20 m buffer zone is suitable for stabilizing streams, a 5–30 m buffer zone is suggested for water quality protection, a 20–150 m buffer zone can be applied for flood controlling, and a 30–500 m buffer zone can provide riparian habitats. Additionally, some studies suggested that buffer zone width should exceed 500 m, aiming to observe the ecological status of the forest riparian zone. The buffer width was recommended to be between 0.9 and 30.5 m and the planting gap should not exceed 7.6 cm, which is more conducive to the removal of pollutants [36]. It was also found that the optimal width of buffer zones ranged from 5 to 12 m in the small-scale research, whereas it was wider than 15 m in field-scale research [90]. The buffer zone width used in the United States is usually 30 m, but the removal of pollutants is still significant when the buffer zone width exceeds 30 m [91]. As shown in Table 3, it is clear that although various studies were focused on the optimal width of buffer zones, especially on small slopes, there is no consensus on the optimal width of buffer zones. This may be due to differences in natural conditions such as geographical location, types of pollutants, composition and structure of soils, plant communities, and climate change. These factors increase the difficulty in establishing a unified optimal width for buffer zones. Moreover, the fixed-width buffer zone may fail to achieve the desired objectives in certain areas. As shown in Figure 7, it is recommended that the buffer zone width should be extended by 5–15 m compared to the practical width for enhancing their function [92]. Thus, more in-depth studies are needed. As shown in Figure 8 and Figure 9, the optimum buffer zone width should be determined according to the specific environmental conditions and pollutant types, aiming to effectively improve the performance of buffer zones and the water quality and protect the aquatic environment [81,93].

4.2. Vegetation Type

Vegetation types play an important role in intercepting and removing pollutants in buffer zones, including wetland, aquatic, riparian, and grassland vegetation, as well as wetland trees. The influencing factors for the purification effectiveness of vegetation buffer zones mainly include vegetation type and the structure and characteristics of pollutants.

Different types of vegetation have different root structures, growth characteristics, and absorption capacities, leading to differences in their effectiveness in removing pollutants. Compared with non-vegetated buffer zones, vegetated buffer zones have better runoff stagnation capacity, effectively enhancing soil hydraulic permeability and improving removal efficiencies of nitrogen, phosphorus, and other pollutants [94]. At the same time, plant roots can effectively increase soil porosity, and aboveground portions of plants can enhance water storage and conductivity of soil through transpiration and root absorption, facilitating the transformation of dissolved nitrogen and plant absorption [95]. Trees have certain advantages in protecting groundwater, stabilizing riverbanks, and resisting floods [96]. A grass-based buffer zone is suitable for absorbing pollutants, improving plant and animal habitats, and increasing agricultural biodiversity [97]. Forest-based buffer zones can more effectively intercept rainfall [98]. Arundinaria gigantea is excellent at increasing water penetration rates, controlling surface runoff, and reducing total suspended sediment and total phosphorus concentrations [99]. Selection of suitable vegetation types is essential to minimize nutrient loss and maximize nutrient removal efficiency.

Aguiar et al. [63] found that, under the same buffer width, the interception capacity of woody vegetation for nitrogen, phosphorus, and nitrate was 100%, while removal rates by shrub vegetation were 83%, 66%, and 80%, and the rates for grassland vegetation were 61%, 53%, and 52%, respectively. Lv and Wu discovered that the order of removal efficiency of total nitrogen was as follows: Taxodium hybrid ‘Zhongshanshan’ + poplar (Nanlin-95) (65.57%) > poplar (Nanlin-95) (62.67%) > Taxodium hybrid ‘Zhongshanshan’ (60.63%). However, the order of removal efficiency for nitrate and ammonium nitrogen was as follows: poplar (Nanlin-95) > Taxodium hybrid ‘Zhongshanshan’ > Taxodium hybrid ‘Zhongshanshan’ + poplar (Nanlin-95) [85]. Dunn et al. [100] studied the effects of different vegetation on runoff and sediment loss. They found that the order of runoff reduction was as follows: willow (49%) > deciduous woodland (46%) > grass (33%). The decreasing order of suspended substance loss was as follows: willow (44%) > deciduous woodland (30%) > grass (29%). Apparently, the willow-based buffer zone showed a strong capacity for removing pollutants. Stutter et al. [13] reported that there was an increasing interest in willow (Salix spp.) due to its potential as a biomass energy source and its effectiveness as a barrier to prevent the flowing of soil and nutrients from agricultural land to rivers. Additionally, it possesses the ability for rapid regeneration after coverage and a high adaptability for various growing conditions. The differences among different types of riparian buffer zones are mainly attributed to the quality and quantity of organic carbon, aerobic and anaerobic conditions, and the composition of microbial communities [101]. However, some studies suggest that there is no difference in the maximum pollutant removal capacity among different vegetation types, which is possibly due to vegetation coverage exceeding 80%, resulting in an insignificant effect of vegetation composition on the buffer zone [102]. Therefore, in the designing process of buffer zones, suitable vegetation types for the local environment and water features should be selected and combined with the function of vegetation and ecological benefits to effectively improve water quality and protect the ecological environment.

Besides considering buffer zone width, it is difficult to summarize the effects of different vegetation coverage. Under the same herbaceous vegetation and external environment, a higher biomass of vegetation usually exhibits a stronger ability for reducing pollutants [103]. Hou et al. also considered that there was a positive correlation between vegetation coverage and the purification capacity index, and the optimal vegetation coverage should be higher than 84% [8]. However, some studies indicate that the impact of different degrees of vegetation coverage on pollution control is up to 20% with the same buffer width [41]. It is also suggested that the impact of vegetation coverage may be very limited or nonexistent [104].

Due to the importance of plant communities in the buffer zone, particular attention has been paid to their effects on pollutants entering buffer zones [93]. Xu et al. [34] found that Platycladus orientalis was more effective in removing nitrogen, phosphorus, and other pollutants from rivers, while Pinus tabuliformis could effectively intercept pollutants. Different vegetation types usually lead to differences in plant composition, root system type, and microbial community composition, thus affecting the absorption and transformation of nitrogen, phosphorus, and other pollutants via different types of buffer zones. Therefore, further studies are needed to deeply explore the effects of different vegetation types on pollutants. Quantitative analysis of the impact of vegetation types, biomass, and morphology on pollutant removal and their potential mechanisms should be emphasized.

4.3. Slope and Runoff Intensity

The slope and runoff intensity of buffer zones are important influencing factors for the removal of agricultural non-point source pollutants [105]. They significantly affect the transport rate and extent of soluble pollutants in runoff and sediments [106]. The slow-moving surface runoff in a gently sloping riparian zone can provide a longer contact and buffer time, and the sediment deposition can be utilized to increase the interception and degradation efficiency of pollutants. In contrast, the steep slope accelerates the speed of runoff, following pre-existing channels and bypassing vegetation and greatly diminishing the effectiveness of the buffer zone. Rainfall intensity has a significant effect on runoff, which increases with rainfall intensity [107]. Therefore, previous studies have focused on slope and runoff intensity as key factors for studying the effectiveness of rainwater runoff pollutant retention. It is worth noting that the slope of a buffer zone may not have a significant impact on the removal of pollutants without exceeding a certain threshold of rainfall duration and intensity.

Wu et al. [40] conducted a quantitative study on the removal loads of nitrogen and phosphorus using a constructed buffer zone and runoff hydrological measuring devices in fields with different slopes (2, 3, 4, and 5%). The initial runoff outflow time for a buffer zone with a 2% slope was 16.4 min, whereas it was only 9.1 min with a 5% slope. This indicates that the capacity for hindering runoff decreases with the increase in slope, likely since buffer zones with gentler slopes can enhance soil hydraulic permeability, significantly slowing down runoff. It was also found that the infiltration removal rates with the slopes of 2, 3, 4, and 5% reached 71.66, 68.14, 64.39, and 61.93%, respectively. The result indicates that lower slopes result in higher infiltration rates and infiltration ability for removing pollutants. This finding is consistent with Hille et al.’s study [46]. They attributed the higher pollutant removal rates to soil retention, filtration, microbial degradation, and root absorption [46]. However, Zhang et al. [78] found that a 10% slope was a crucial turning point for the water quality protection function of ecological buffer zones. The different results may be due to the sensitivity of slope parameters. The slight increase in slope will greatly increase the runoff velocity, which will then affect the retention and absorption of pollutants. These results provide a scientific basis for the designing and construction of buffer zones to effectively control non-point source pollution.

Additionally, some studies indicate that the total amount and intensity of rainfall should be considered along with buffer zone slope when determining the recommended width of a buffer zone [20]. In summary, slope plays a crucial role in determining buffer zone width. Furthermore, it is unsuitable to construct a wide enough buffer zone due to the limitations of construction costs and land resource. Therefore, it is necessary to conduct comparative analyses of the differences in pollutant removal effectiveness between runoff and infiltration and comprehensively consider buffer zone slopes and widths to improve the removal capacity for agricultural non-point source pollution via buffer zones.

4.4. Seasonal Variation

Seasonal variation has a significant influence on the pollutant removal ability of buffer zones. During the growing season, plant absorption is one of the main pathways for nitrogen removal. However, plant absorption may significantly decline or even cease in winter. Salazar et al. [108] pointed out that nitrogen fixation mainly occurs in the autumn and winter, whereas nitrogen mineralization primarily leads to approximately 70% absorption of nitrogen during the spring and summer.

Seasonal variation primarily affects nutrient absorption by plants through changes in temperature, light, precipitation, and plant communities. As shown in Figure 10, buffer zones exhibit good performance in reducing nutrient concentrations in warm or temperate climates [28]. However, several issues occur in colder climates, such as vegetation flattening or dying due to ice and snow and reduced infiltration due to frozen soil, resulting in poorer nutrient removal via buffer zones [37]. Additionally, Kumwimba et al. also found that many biological activities in buffer zones decreased during the spring snowmelt period, reducing plant uptake and assimilation of nutrients [18,28]. At the same time, some plants may be flattened or submerged by snow/ice during melting, these plants may decay and decompose to release nitrogen and phosphorus into surface runoff, consequently leading to an increase in pollutant concentrations in water. Duan et al. [97] also found that the removal rate of ammonia nitrogen via buffer zones was higher than that of total nitrogen in all experimental groups. This is mainly due to the absorption function of plant roots and the denitrification process in soil caused by water saturation and anoxic conditions during the warm growing season [109].

In addition, vegetation can be periodically harvested during the winter to reduce the loss of nutrients through runoff or leaching [110]. Vegetation in buffer zones absorbs the phosphorus in soil throughout its growing season. But vegetation begins to transport phosphorus from the branches to the roots in late autumn. Harvesting vegetation in autumn is beneficial for removing phosphorus and reducing the phosphorus releasing capacity of vegetation. However, more phosphorus is released from vegetation in completely unmanaged buffer zones, which may lead to a higher phosphorus content in soil and surface runoff. Zhang et al.’s study also suggests that harvesting vegetation before the end of October can avoid backflow and reduce agricultural non-point source pollution [49]. Therefore, it is necessary to understand the growth rate, life history, and community structure of vegetation in different seasons to fully utilize the function of buffer zone vegetation [111], providing a scientific basis for buffer zone management.

4.5. Soil Composition

Physical and chemical properties of soil, such as particle size, organic carbon content, texture, structure, and moisture status, have a certain impact on the pollutant removal efficiency of buffer zones. Soil serves as a sink for pollutants like nitrogen and phosphorus, providing important functions such as interception, adsorption, and degradation. However, as phosphorus accumulates in soil, its fixing capacity for phosphorus gradually decreases, eventually leading to soil phosphorus entering water bodies through surface runoff, becoming a significant source of phosphorus in rivers. As shown in Figure 11, there are differences in phosphorus pools and their distribution between farmland and buffer zones, indicating that components of soil phosphorus and phosphorus stocks can be used to assess changes in the behavioral characteristics of soil phosphorus [112]. Compared with the paddy field without a soil plant buffer zone, the effluent concentration of each indicator in the paddy field with the operation mode of a soil plant buffer system is significantly reduced, and the interception rates of total dissolved nitrogen and phosphorus are 64.28% and 83.73%, respectively [113]. Thus, soil plant buffer zones can effectively reduce non-point source pollution in paddy fields and enhance yield and fertilizer utilization.

Walton et al. [14] studied the impact of organic and mineral soil on buffer zones, as depicted in Figure 12. They found that the average removal efficiency of nitrate nitrogen in organic soil and mineral soil was 52% and 51%, while that of total nitrogen was 45% and 36%, respectively. Clearly, wetland buffer zones with organic soil showed better pollutant removal effectiveness under higher loading rates, which may be related to their higher content of organic matter and stronger capacity for water infiltration. The combination of industrial by-products and buffer zone was also studied, which provides a potential strategy for improving the removal of soluble phosphorus from agricultural runoff. Some studies used industrial by-products containing large amounts of activated aluminum, iron, and calcium to reduce the release of soluble reactive phosphorus through adsorption or precipitation reactions [114,115]. Additionally, the adsorption capacity could be maintained under a wide pH range. Moreover, it has been found that phosphorus that adsorbed onto these materials was not easily desorbed. This study suggests that the combination of appropriate industrial by-products with buffer zones is an economical, efficient, and environmentally friendly method that can minimize the loss of soluble phosphorus.

4.6. Vegetation Density

Vegetation density is one of the important influencing factors for the pollutant retention efficiency of buffer zones. Currently, there are not many studies on the pollutant retention efficiency of buffer zones with appropriate vegetation density. Tang et al. [116] suggested that increasing vegetation coverage was an important approach to reduce soil erosion in aquatic ecosystems. Yang et al. [117] also found that the recharge depth of soil water increased by 0.1 m after planting grass, which significantly improved rainwater infiltration and reduced the spatiotemporal changes in soil water porosity compared with non-vegetated sloping land. Liang et al. [107] found that a negative correlation between runoff and vegetation coverage (10, 30, and 50%) existed under the same rainfall intensity. This is due to the increase in vegetation coverage enhancing the roughness, with leaves and roots blocking more runoff. When the vegetation coverage increased from 0 to 50%, the surface runoff decreased by 4.10, 12.32, and 19.10%, respectively, indicating that increasing vegetation density can effectively reduce surface runoff and improve water resource management and soil conservation. Liang et al. [107] suggested that vegetation coverage should be higher than 50% to produce significant benefits in soil and water conservation.

Jin and Römkens found that when the vegetation density of buffer zones increased from 2500 to 10,000 clumps/m², the runoff decreased significantly, and the removal rate of suspended solids increased by 45% [118]. Lv and Wu studied the effect of different vegetation densities (400, 1000, and 1600 plants/hm²) in buffer zones on nitrogen removal. They found that when the buffer zone width was 30–40 m, the average removal rates for total nitrogen, nitrate nitrogen, and ammonia nitrogen gradually increased with the increase in vegetation density [85]. However, the optimum nitrogen removal efficiency of the buffer zone with a plant density of 1000 plants/hm² was achieved with a width of 5 m. This may be due to a higher amount of litter in the buffer zone with a greater vegetation density, resulting in more nitrogen released from litter to the soil through litter decomposition [115], as shown in Figure 5. Approximately 46% of the nutrient returns to the soil through decomposition of plant litter in the whole Dinghushan forest area [119]. Similarly, suitable vegetation density facilitates the removal of phosphorus from water bodies via buffer zones [42]. Hénault-Ethier et al. [120] also found an insignificant difference in buffering capacity between a 3 m wide buffer zone planted with willow and a buffer zone with natural regeneration herb coverage, regardless of density. Therefore, the different buffer zone configuration has a significant impact on its pollution interception effect in the process of buffer zone construction, which is also the main reason for the difference in the pollutant removal effect of buffer zones.

4.7. Other Factors

Besides the above six factors, the types and contents of pollutants, microbial activity, and other factors can also have a certain influence on the ability of buffer zones to remove pollutants.

4.7.1. Types and Contents of Pollutants

One of the main factors that determine the change in nitrogen removal efficiency is the pollution source [86]. Agricultural non-point source pollution originates from various sources, primarily including wastewater discharged from livestock farming, runoff of pesticides and fertilizers, and soil erosion and nutrient loss during heavy rainfall [103]. As shown in Figure 13, different types of pollution correspond to different types of buffer zones. Within the 0–50 m range, the riparian buffer zone is predominant, where natural vegetation can be effectively utilized to intercept and absorb nutrients during runoff. The 300–1000 m range typically comprises agricultural zones, where implementing a mulberry and rapeseed intercropping system can effectively control nitrogen and phosphorus loss. In forested or grassland areas, or orchards beyond 1000 m, reducing nitrogen and phosphorus into rivers can be achieved through soil and water conservation projects (such as increasing understory vegetation cover) and afforestation efforts [121]. The efficiency of buffer zones is different based on the source, type, and form of pollutants. Among them, nitrogen usually exists in the form of soluble nitrogen such as nitrate nitrogen and ammonia nitrogen. Phosphorus mainly consists of soluble, particulate, and organic phosphorus [28]. When the pollutants in the runoff pass through the buffer zone, the removal rate of the particulate adsorbed pollutants is the highest, while that of the dissolved pollutants is the lowest. However, buffer zones can improve groundwater quality to the same extent regardless of the source of contamination. In addition, it was found that nitrogen retention in surface runoff and groundwater was linearly correlated with the initial nitrogen concentration entering buffer zones by performing a robust weighted meta-analysis. The higher initial nitrogen concentration led to the greater amount of nitrogen retained in the buffer zone [86].

4.7.2. Microbial Activity

The presence of vegetation root systems in buffer zones promotes soil biodiversity and provides a habitat for the growth and reproduction of microorganisms [122]. Microorganisms form a symbiotic relationship with vegetation, mutually enhancing their growth and development. Microorganisms are abundant and diverse components of buffer zone systems, which influence the ecological function and water quality of buffer zones via biodegradation, nitrogen cycling, and degradation of toxic substances [28]. These processes not only degrade pollutants such as nitrogen and phosphorus, but also improve soil texture, increase microbial diversity, and reduce ecological damage [123]. At the same time, the presence and management of buffer zones can also affect microbial diversity, with specific impacts on particular environments. Microorganisms are influenced by the physical characteristics of the buffer zone (such as topography and width), vegetation structure (such as type and density), and physical attributes of the river (such as width and hydrology) [124,125]. Therefore, more attention should be paid to maintaining the diversity and activity of microorganisms in buffer zones, which is of great significance for the protection of the ecological environment and the improvement of water quality.

5. Conclusions and Prospects

A buffer zone is one of the most commonly implemented management methods for controlling agricultural non-point source pollution. This review summarized the main affecting factors for buffer zones, including width, vegetation type, slope, seasonal variation, soil composition, vegetation density, types and forms of pollutants, and microbial activity. All these factors have an appropriate range to achieve maximum pollutant removal efficiency. Their range is closely related to the types of pollutants and their external environment. The inappropriate ranges for these factors may lead to the weakening or invalidation of buffer zones, consequently affecting the removal effect for agricultural non-point source pollution.

So far, great achievements have been focused on the pollutant removal efficiency of buffer zones, but few reviews were conducted on the remediation of agricultural non-point source pollution via buffer zones. Many issues require further research. Due to insufficient dynamic studies, the mechanisms through which single factors affect the pollutant removal efficiency of buffer zones is unclear. More attention should be paid to the combination of multiple factors. At the same time, studies on the mechanisms of pollutant removal are not deep enough, which may lead to insufficient understanding of the specific processes of buffer zones for removing pollutants. More research on this topic should be carried out to provide effective and scientific guidance for the practical application of buffer zones.