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Review

Research Progress on Nitrogen and Phosphorus Loss in Small Watersheds: A Regional Review

by
Chunbo Wang
1,
Chengtao Huang
1,*,
Shuai Zhang
1,
Lei Zhang
1,
Tingzhen Li
1,
Jiyou Peng
2 and
Liuyi Zhang
1,*
1
Chongqing Key Laboratory of Water Environment Evolution and Pollution Prevention and Control in Three Gorges Reservoir Area, Chongqing Three Gorges University, Chongqing 404000, China
2
Xiangxi Satellite Application Technology Center, Jishou 416800, China
*
Authors to whom correspondence should be addressed.
Water 2023, 15(16), 2894; https://doi.org/10.3390/w15162894
Submission received: 3 July 2023 / Revised: 2 August 2023 / Accepted: 8 August 2023 / Published: 10 August 2023
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Abstract

:
As an ecological subsystem, a small watershed is mainly located upstream from lakes, rivers, or other water bodies. The characteristics of non-point source (NPS) pollution in a small watershed are random and complex. Rainfall is the direct driving force of NPS pollution, and different land-use types are the main factors affecting NPS output in small watersheds. At present, the NPS pollution of small watersheds is serious, and the problem of eutrophication of watershed water is prominent. Nitrogen (N) and phosphorus (P) are essential nutrients for aquatic organisms, but excessive amounts can lead to water pollution and ecological imbalances. The study of N and P loss in small watersheds can provide a decision-making basis for NPS pollution control in small watersheds. This paper introduces the research progress on small watersheds in detail, focusing on the main influencing factors of N and P output in small watersheds, including rainfall, different land-use types, N and P loss prevention, and control measures; it also provides a prospective view of the current problems, hoping to provide references for the study of NPS pollution in small watersheds.

Graphical Abstract

1. Introduction

To better comprehend ecological dynamics and soil-erosion processes in small watersheds and to devise efficient and targeted management strategies for mitigating soil erosion, conserving natural resources, and encouraging sustainable land-use practices in these crucial ecosystems, current research primarily emphasizes studying the ecological environment and integrating soil-erosion management in small watersheds. As an ecological subsystem, a small watershed is not only the smallest unit of rainfall runoff convergence but also the smallest unit of the occurrence and development process of soil and water loss. Its area is usually between 3~50 km2, generally located upstream from lakes, rivers, and other water bodies. The NPS pollution of small watersheds is characterized by randomness and complexity, and the pollution range is wide and difficult to determine [1]. The occurrence of NPS pollution in small watersheds is influenced not only by factors such as rainfall, geological conditions, and climate change, but also by human activities, including excessive fertilization and sewage discharge [2]. Rainfall serves as the primary driving force behind NPS pollution and is the main factor contributing to N and P loss in small watersheds. The erosive impact of rainfall leads to the influx of excessive N and P pollutants into small watersheds, with particularly severe losses occurring during heavy rainfall. Such N and P losses intensify water eutrophication [3]. Therefore, it is becoming increasingly urgent to study the characteristics of N and P loss caused by rainfall in small watersheds.
Research studies have determined that rainfall and runoff are the primary driving forces behind nutrient loss, and the NPS pollution load is strongly influenced by rainfall conditions. There is a significant correlation between the NPS pollution load and factors such as rainfall amount, rainfall intensity, and rainfall duration [4]. Furthermore, different land-use types are significant factors influencing N and P loss in small watersheds. Current studies investigating N and P loss in small watersheds with varying land-use types primarily concentrate on examining the impact of rainfall characteristics, the physical and chemical properties of the land, landform, and geomorphology, as well as changes in vegetation cover. Different land-use types have a direct impact on soil nutrient characteristics, as well as the storage and transformation of nutrients. Land-use types also play a significant role in N and P fixation in the soil and vegetation, as well as the N and P output through runoff in small watersheds. Thus, an in-depth study of its mechanism can better analyze the process of N and P loss in small watersheds under different land-use types [5,6].
Global research on small watersheds began in the 1930s. In 1933, the Soviet Union first established an experimental station for the study of small watersheds. Coweeta Hydrologic Laboratory of USA was set up in 1934 for forest hydrology and ecological research in two main basins with 32 sub-watersheds under various treatments [7]. The Harz Mountains experiment of Germany with a pair of catchments, Wintertal and Large Bramke, and focusing on the hydrological effects of land use, began in 1948 [8]. The significant development of the study of small watersheds can be attributed to the International Hydrological Ten-year Scientific Plan (1965–1974), which proposed the establishment of a working group focused on small watersheds. Simultaneously, considerable attention was paid to small experimental watersheds, leading to a substantial advancement of hydrological experiments in small watersheds across various countries [9]. Hydrological watershed experimentation in China originated in the 1950s. With the passage of time, China’s hydrological research on small watersheds has seen continuous development. Currently, the country has established a vast network of hydrological observation stations and experimental basins, conducting systematic research on various hydrological aspects. This research encompasses basin hydrological processes, runoff-formation mechanisms, water-cycle dynamics, flood warnings, and more. Such efforts hold immense significance for water-resource management, flood-risk assessment, and ecological protection [10,11].
At present, scholars have begun to use some models to further study the process of N and P loss in small watersheds and have achieved phased results. For example, the SWAT (Soil and Water Assessment Tool) model (CMADSV1.0), which can better simulate the long-term runoff process [12], is used to simulate rainfall runoff in watersheds. The SWMM (Storm Water Management Model) model (5.0) is used to simulate the rainfall runoff from urban roads. The use of rainfall-runoff modeling offers a powerful scientific tool for comprehending and managing hydrological processes in watersheds, developing rational water-resource management strategies and predicting natural disasters. Its application significantly enhances our decision-making ability and promotes the sustainable utilization of water resources and environmental protection. However, it is crucial to recognize that small watersheds serve as the starting point for surface-source pollution. Over the decades, research on nutrient loss, specifically N and P, in small watersheds has transitioned from traditional experimental observations to modern technical approaches. While many results have been achieved, these findings remain fragmented and lack comprehensive summarization. Therefore, this paper will summarize and look forward to the research on small watersheds, hoping to provide some reference for colleagues in this field.

2. Effect of Rainfall on the Output of N and P in Small Watersheds

2.1. Effect of Rainfall Amount

The loss of N and P through surface runoff represents a significant form of agricultural NPS pollution, and the magnitude and variation of runoff are determined by the characteristics of the rainfall and the mechanisms of runoff generation within the watershed. Whether it is natural rainfall or simulated, rainfall runoff serves as the driving factor behind NPS pollution [13]. Through the study of major small watersheds such as the Three Gorges Reservoir, Chaohu Lake, Taihu Lake, and Dianchi Lake, it is found that summer rainfall is frequent and heavy, and this season is the main period of N and P output. Rainfall is the main influencing factor of N and P loss in the watersheds [14]. According to relevant studies, the correlation between rainfall and N and P output is reflected in exponential-function correlation, power-function correlation, logarithmic-function correlation, primary-function correlation, and quadratic-function correlation under the influence of topography and geomorphology, the physical and chemical properties of soil, etc. Table 1 presents the relationship characteristics observed in different research areas.
The relationship between rainfall and N and P in small watersheds has become one of the hotspots of current research. Millar et al. [15] simulated water quality through the SWMM model, and the results showed that the rainfall and output of pollutants were manifested as an index-function relationship. According to the dynamic changes of pollutants with the dynamic changes in the rainfall-runoff process of pollutants, Li et al. [17] established the mathematical–statistical model between rainfall-runoff and runoff-pollutant output, which is reflected as a power-function relationship. Li et al. [18] studied the sediment behavior in a small agricultural watershed during natural rainfall and found that the relationship between rainfall and pollutant output was a logarithmic function. Li et al. [19] studied the characteristics of soil N and P loss under natural rainfall conditions and found that the relationship between rainfall and total N loss was not obvious, but the relationship between rainfall and total P loss was a linear function. Geng et al. [20] established the relationship between the annual average rainfall of the watershed and the annual inflow of NPS pollutants through regression analysis based on the NPS pollution-load data of Miyun Reservoir and found that the relationship between the annual N and P load and rainfall conforms to a quadratic function. The diverse functional correlations between rainfall and N and P output result from the complex interaction of several factors. Soil permeability and water-retention capacity play a crucial role in determining the retention and infiltration of rainfall. The type and density of vegetation cover affect water and nutrient retention, transformation, and evapotranspiration. Rainfall characteristics, such as intensity, frequency, duration, and spatial distribution, influence the distribution and transport of N and P output. Topographical undulation and gradient impact the speed and direction of water flow, affecting the transport of rainfall to N and P outputs. Human activities, such as agriculture, industry, and urbanization, significantly influence the input and output of N and P to water bodies, and the type and intensity of human interventions may lead to distinct functional relationships. Understanding the impact of these various functional relationships on water-resource management and conservation strategies is vital. This understanding will facilitate the development of precise and feasible management measures aimed at minimizing N and P pollution impact on the water environment. Ultimately, such efforts will contribute to the sustainable use of water resources and ecological health.

2.2. Effect of Rainfall Intensity

Rainfall intensity is defined as the volume of rainfall per unit time or during a specific period. It is categorized into different levels, including light rain, moderate rain, heavy rain, and rainstorms. Due to the difference of runoff yield and flow models, the characteristics of N and P loss caused by rainfall with different intensities are different, and under the same rainfall conditions, rainfall intensity is the main factor affecting N and P loss [21]. At present, the effect of rainfall intensity on N and P loss in small watersheds is usually studied through artificial rainfall simulation or natural rainfall. In the study conducted by Yang et al. [21], three different rainfall intensities were simulated: low intensity at 0.83 mm min−1, medium intensity at 1.17 mm min−1, and high intensity at 1.67 mm min−1. The researchers observed that the start of flow production took longer under low intensities compared to high intensities, indicating a significant influence of rainfall intensity on flow production. Furthermore, the average sand content in runoff was 2.85 g L−1 under high rainfall intensity, while the sand content ranged from 1.30 to 1.79 g L−1 for other rainfall intensities. The data clearly show that as rainfall intensity increased from 0.83 to 1.67 mm min−1, the sediment loss doubled. However, the sediment loss escalated significantly, reaching 7.51 times the initial amount, when the rainfall intensity increased from 1.67 to 2.50 mm min−1. These results indicate that runoff generated under high rainfall intensities is considerably more vulnerable to sediment loss compared to runoff generated under medium and low rainfall intensities. Zhang et al. [22] studied rainfall intensities (30, 50, 65, and 100 mm h−1) and land slope gradients through simulated rainfall experiments (0°, 5°, 10°) on P loss in runoff and found that P loss increased with increasing rainfall intensity at the same slope. Wang et al. [23] also reported that sediment and effective phosphorus loss increased with increasing rainfall intensity and slope, while total nutrient loss was dominated by sediment-related fractions. Lu et al. [14] conducted an analysis of the variation rules of N output in the Chenjiagou Watershed of the Three Gorges Reservoir area under three different rainfall intensities. Their study revealed that the concentration of various forms of N changed significantly over time during different rainfall processes. Under moderate rainfall (rainfall intensity of 0.43 mm h−1) conditions, the average concentrations of DTN (dissolved total N), NH4+-N (ammonia N), NO3−-N (nitrate N), and DON (organic N) were 2.81, 0.71, 1.43 and 0.68 mg·L−1, respectively. Under heavy rainfall (rainfall intensity of 1.98 mm h−1) conditions, the average concentrations of DTN, NH4+-N, NO3−-N and DON are 3.39, 0.63, 2.39 and 0.37 mg·L−1, respectively. Under rainstorm (rainfall intensity of 2.89 mm h−1) conditions, the average concentrations of DTN, NH4+-N, NO3−-N and DON are 4.64, 1.32, 2.40 and 0.93 mg·L−1, respectively. In the three rainfall processes observed, all forms of N initially increased and then decreased with the rise of runoff. During the early stage of peak runoff, the N concentration showed a slow increase, but once peak runoff was reached, N concentrations rapidly surged to their highest levels and then gradually declined. The trends of NO3−-N and NH4+-N followed a pattern similar to that of DTN, while the variation in DON was relatively small. These fluctuations can be attributed to the dry soil conditions and strong water-storage capacity during the initial stage of rainfall. As the amount of rainfall reached a certain threshold, the soil gradually became saturated, leading to reduced river dilution and subsequently decreasing all forms of N in the runoff [24].

2.3. Effect of Rainfall Duration

The duration of rainfall plays a crucial role in the output of N and P in small watersheds. It determines the duration during which N and P, along with other nutrient elements, are washed off the surface and transported during rainfall events. In general, when the rainfall intensity remains constant, longer rainfall durations and larger rainfall amounts result in greater runoff and higher N and P loss loads. These findings emphasize the importance of considering rainfall duration when assessing the impacts of rainfall on nutrient loss in small watersheds [25]. However, with the increase of rainfall duration, the variation trend of N and P concentrations in small watersheds is more complex. Shen et al. [26] conducted a study to examine the impact of rainfall duration on nutrient loss from a black soil slope. They observed that the concentrations of NO3−-N and P in runoff rapidly decreased at the beginning of the runoff-production period, stabilizing after about 20 min. When the duration of rainfall varied from 30 to 90 min, the loss of nitrogen and P in runoff decreased with increasing rainfall duration. Specifically, the N loss increased from 4.8 mg to 11.6 mg, corresponding to a decrease in concentration from 4 mg·L−1 to 2 mg·L−1. This trend is attributed to the prolonged rainfall duration leading to the gradual saturation of the soil infiltration rate. NO3−-N, being a non-adsorption chemical element with weak interaction between soil particles, adheres only to the surface layer of the soil and migrates rapidly, getting lost with surface runoff. Similarly, P loss increased from 2.6 mg to 8.8 mg, while its concentration decreased from 1 mg·L−1 to 0.5 mg·L−1. This is explained by the shorter duration of rainfall causing rainwater to dissolve and leach P to the runoff, which then migrates into the runoff, leading to nutrient depletion in the surface soil. However, when the rainfall duration extended to 120 min, the N concentration tended to stabilize without significant changes, whereas the P concentration increased significantly from 0.5 mg·L−1 to 0.8 mg·L−1. This was due to the increased rainfall and severe soil loss conditions, leading to the destruction of the topsoil and leaching of nutrients from the lower soil layer into the runoff. Moreover, P also attached to the topsoil layer, resulting in elevated levels of P in the later stages of rainfall [27].
Table 2 selects some watersheds and explores the effect of rainfall duration on N and P loss in these watersheds during the rainfall process, in which the study of N loss in the Baimachahe small watershed shows that the greatest amount of N is lost during heavy rainfall, in which the loss of total N (TN), NO3−-N, and NH4+-N from the watershed outlet accounts for 84.7%, 64.4%, and 91.7% of the total loss, respectively. This is not only because of the high rainfall intensity, but also because of the longer rainfall duration [28]. Two rainfall events in the Chaohe watershed were selected for analysis. Event 1 had a rainfall of 75.6 mm and a rainfall duration of 37 h, while event 2 had a rainfall of 71.3 mm and a rainfall duration of 32 h. With similar rainfall, the rainfall duration of event 1 was 5 h longer than that of event 2. The results of the study were that the TN output of event 1 was 5 times higher than that of event 2, and the total P (TP) output of event 1 was 1.4 times higher than that of event 2 [29]. Similar findings apply in regions such as the Qingshuihe River watershed [30], the Wuchuan watershed [31], and the Shibetsu watershed [32]. The main factors influencing N and P loss in small watersheds are the amount and duration of rainfall. However, it is evident that small watersheds experience lower output loads of N and P during rainfall compared to medium and large watersheds, and this difference is closely related to the size of the watershed. As the watershed area increases, the output load of N and P during rainfall also rises accordingly. Larger watersheds have a greater capacity to accumulate and transport runoff, resulting in higher nutrient loss during rainfall events. Conversely, smaller watersheds possess limited water-holding capacity and are more efficient in retaining nutrients, leading to lower N and P output loads during rainfall.

3. Effect of Land-Use Type

3.1. Agricultural Watershed

Agricultural surface pollution is strongly associated with agricultural production, and its contribution to water pollution exceeds 50% in most countries worldwide [34,35,36]. Therefore, studying the characteristics of nitrogen and phosphorus loss from small agricultural watersheds is of great practical significance for controlling and preventing agricultural surface pollution [37]. Small agricultural watersheds have the characteristics of randomness, complexity, and difficulty in determining the scope of pollution. Their main sources of pollution are the excessive use of fertilizers and pesticides, as well as the discharge of rural domestic sewage and livestock and poultry wastewater (Table 3). The absorption of fertilizer by crops increases with the increase of fertilizer application, but the utilization rate of fertilizer is only between 25% and 45%, which will cause 55~75% waste of fertilizer and damage the ecology of the small watershed [38]. Sui et al. [39] used double isotopes (15N and 18O) to analyze the sources of nitrate in the surface runoff of small agricultural watersheds in Northeast China and found that the sources mainly came from fertilizer and precipitation. Long-term fertilization can lead to the accumulation of N in the soil, and excess N will be lost with surface runoff. Zhang et al. [40] used the distributed hydrological model SWAT to simulate changes in watershed runoff and N and P loss processes under different climate change scenarios and cropping structures, and the results showed that the losses of N and P were positively correlated with fertilizer application. With each 10% increase in fertilizer use, the losses of N and P increased by 1% and 4%, respectively. Therefore, controlling the amount of fertilization and improving fertilizer-utilization efficiency is one of the ways to prevent NPS pollution [41].
Affected by soil type, slope, planting type, and other factors, agricultural small watersheds resulted in different degrees of N and P loss. Guo et al. [42] simulated the effect of rainfall on the spatial distribution of N and P in the three types of soil and found that, under simulated rainfall conditions, N loss in cinnamon soil was higher than that in brown and red soil, which was caused by higher original N content and easy leaching, and P loss in red soil was higher than that in brown and cinnamon soil, which was caused by higher clay content in red soil than in brown and cinnamon soil. Wang et al. [3] studied the loss characteristics of soil TN, NH4+-N and NO3−-N in exposed karst-slope farmland (slopes 5° and 10°) under extreme rainfall conditions, and found that under the same rainfall intensity, the nutrient loss was more serious for farmland with a slope of 10° than that with a slope of 5°. This indicates that slope has a certain influence on nutrient loss in small agricultural watersheds. Yang et al. [43] studied the rule of P loss in farmland and found that different crop types caused different P loss. The concentration of P loss was the highest in vegetable land (1.1 mg·L−1) and the lowest in green land (0.42 mg·L−1). Corn fields (0.86 mg·L−1) were slightly higher than peanut fields (0.77 mg·L−1), These differences can be attributed to factors such as long-term P fertilizer application in vegetable fields, higher vegetation coverage, lower coverage in corn and peanut fields, and comparatively lower fertilization in green fields. Wang et al. [44] studied the rule of N and P loss in dryland farmland, finding that the main form of N loss was NO3−-N, and the main form of P loss was particulate P (PP). According to the characteristics of N and P loss in small agricultural watersheds, effective measures can be developed to mitigate the impact of agricultural activities on water quality and ensure sustainable agricultural practices.
Table 3. Loss of N and P in small watersheds with different land use types.
Table 3. Loss of N and P in small watersheds with different land use types.
Watershed TypeWatershed CharacteristicsMain Pollution SourceN, P Loss CharacteristicsReferences
Small agricultural watershedRandomness, complexity; the scope of pollution is not easy to determineFertilizer, pesticide, rural domestic sewage, livestock, and poultry breeding wastewaterN is mainly lost in the form of soluble TN and nitrate N. P is mainly lost in the form of a PP. [40,43]
Small wetland watershedEcological vulnerability, biological diversity, and strong purification abilityIndustrial wastewater, breeding sewage, domestic sewageN is mainly lost in the form of dissolved state N, and P is mainly lost in PP.[45]
Small grassland watershedHigh vegetation coverage area and single structureAnimal husbandry, domestic sewageN is mainly lost in the form of TN, and P is mainly lost in PP[46,47]
Small forest watershedHigh spatial densityAtmospheric depositionSurface runoff has great influence on N and P loss in small forest watersheds. N is lost in the form of NH4+-N, NO3−-N, and P is mainly lost in the form of PP[48]
Small urban watershedNature is regional and hierarchical in structureDomestic sewageUnderlying surface has a large impact on N and P loss in small urban watersheds. N is mainly lost in the form of dissolved N and P is mainly lost in the form of PP[49]

3.2. Wetland Watershed

As a unique ecosystem formed by the interaction of land and water on earth, the wetland is the most important ecological environment for human beings and one of the most biodiverse ecosystems in nature. A small wetland watershed is characterized by ecological fragility and biodiversity. At present, small wetland watersheds are facing the impact of natural environment deterioration and human activities, and their pollution is mainly from industrial wastewater, aquaculture sewage, and domestic sewage (Table 3) [50]. The research on the characteristics of N and P loss in small wetland watersheds is mainly conducted from the aspects of pollution sources and the spatial and temporal distributions of nutrients. Wang et al. [45] studied the distribution characteristics of N and P in the sediment of the estuarine wetland in the Erhai watershed and found that the spatial distribution of the estuarine wetland of Luoshijiang River was affected by exogenous pollutants, aquaculture activities, and wetland aquatic plants. The content of N and P in sediments of aqueducts with aquaculture activities was significantly higher than in small wetlands without aquacultural activities.
The spatial and temporal distributions of a small wetland watershed can affect N and P output. Pan et al. [51] studied the temporal and spatial distribution characteristics of soil nutrients in a small watershed of Huixian wetland, and they found that TN content in river-sediment soil, water-fluctuation-zone soil, and cultivated soil had significant seasonal variability; N content in the wetland was mainly affected by the N fixation capacity of organisms and plants and the covered area of vegetation. Studies on the characteristics of N and P loss in wetlands show that N mainly exists in water in the form of dissolved N, but part of N is volatilized to the atmosphere in the form of ammonia, which is absorbed and deposited by plants, while the other part is easily lost due to leaching and scouring [52]. P mainly exists in water in the form of PP. Under the effect of rainfall erosion, the loss of N and P will increase with the increase of wetland runoff [53]. At the same time, the wetland watershed also has the characteristics of pollutant interception and purification, but more research is needed on artificial wetlands to mitigate the loss of N and P in agricultural runoff. Planting emergent plants in a wetland watershed can intercept, remove, and reduce the loss of runoff nutrients [54]. As shown in Figure 1, the optimal substrate and plant performance of the artificial wetland has a good retention of N and P from agricultural runoff pollution [55].

3.3. Grassland Watershed

Grassland watersheds are the catchment areas of grassland and its related ecological and economic system, which is a natural, social, and economic complex with certain hydrological characteristics. The grassland watersheds are a natural geographical unit with systematic functions and properties and serve as the basic unit of sediment production, abortion, and soil erosion control in China. The grassland watersheds have the characteristics of a high vegetation coverage area and a single structure, and its pollution comes mainly from animal husbandry and domestic sewage discharge (Table 3). In recent years, due to the special geographical location of small grassland watersheds, a growing number of studies has been conducted on the characteristics of N and P loss in small grassland watersheds, especially in terms of vegetation cover and nutrient output [56]. Jin et al. [57] used the improved SWAT model to simulate the ecohydrological processes in grassland watersheds with different vegetation cover in the upper reaches of the Bayin River basin, and the results showed that the improved SWAT model can better estimate runoff and sand production and can simulate the hydrological processes in arid alpine grasslands with different vegetation cover. Han et al. [46] selected lightly grazed, heavily grazed, and non-grazed grasslands in the Kelulun River watershed of Hulunbuir Grassland and conducted rainfall simulation experiments to study the influence of surface physical properties and vegetation types of grasslands with different grazing intensities on factors such as rainfall runoff production and nutrient element loss. It was found that due to the sparse vegetation in heavily grazed grasslands during rainfall, serious erosion occurred on the surface soil, and the runoff carried more sediment, which had the most serious impact on the loss of TP and relatively little impact on TN. In lightly grazed and non-grazed pastures, the surface vegetation had a certain buffering effect on rainfall, and the slightly grazed pastures had a small effect on TP and a large effect on TN. Tiessen et al. [47] believed that runoff during spring snowmelt and summer rainfall was a common source of sediment and nutrient load in small catchments of the Canadian prairies, and PP loss accounted for a relatively high proportion. The influence of N and P loss was caused by precipitation, discharge changes, vegetation, and the duration of runoff in vegetation, etc.
However, excessive vegetation in small watersheds may have a negative correlation. Relevant studies have shown that leaf length, plant height, leaf area and leaf dry matter of plants are significantly correlated with soil water. Excessive vegetation will consume more soil water, destroy the water balance of plants in the growing period, and aggravate soil drought, while at the same time, soil water plays an important role in regulating the transport and loss of nitrogen and phosphorus. Therefore, maintaining a certain amount of vegetation is more beneficial to the ecology of grassland small watersheds [57].

3.4. Forest Watershed

As an important renewable resource in nature, forests play a crucial role in regulating the global water balance and water cycle, while also effectively improving the ecological environment. Forest watersheds are characterized by high spatial density and complex community structures, with atmospheric deposition being the main source of pollution. At present, small forest watersheds are affected by various factors such as reduction of forest vegetation, long-term high N deposition, and high intensity of rainfall, among which rainfall has the greater impact (Table 3) [25]. A series of studies have been carried out on the characteristics of N and P loss in small forest watersheds. Xiang et al. [48] conducted a study on the Caijiatang forest watershed and found that the synergistic effect of high N deposition and the forest canopy could enhance the soil N pool and promote nitrification. However, excessive N deposition led to the leaching of excess N in the form of NO3−, and prolonged exposure to high N deposition intensified soil acidification and the risk of forest degradation. Chu et al. [58] studied the characteristics of nutrient loss in the surface runoff of three kinds of forestlands and found that NH4+-N was the main form of N loss, and PP was the main form of P loss; the proportion of PP to TP in these three types of woodlands was 71.04%, 73.55% and 78.18%, respectively.
An increase in the type and amount of vegetation in small forest watersheds helps to slow nutrient loss. Chu et al. [59] conducted a study investigating the impacts of plantation conversion on soil erosion and nutrient loss. Their findings revealed that transitioning from a monoculture plantation to a mixed forest had significant effects on rainfall infiltration, surface runoff, soil erosion, and nutrient loss. The study also demonstrated that compared to a single plantation cultivation, planting a mixed forest significantly reduced surface runoff and soil erosion. It was observed that the annual loss of N and P in surface water decreased by 42% to 60% and 44% to 64%, respectively. These findings indicate that planting mixed forests can enhance the role of forests in soil and water conservation and contribute to nutrient retention. Rajaei et al. [60] demonstrated that changes to small forest watersheds can have a significant impact on the quality of water within those watersheds. During the rainy season, P attached to soil particles tends to be transported into rivers as sediment through runoff. However, a greater extent of forest cover and well-developed plant roots can facilitate infiltration, enhance soil moisture content, and promote groundwater recharge. Consequently, these factors contribute to mitigating the loss of N and P.

3.5. Urban Watershed

A small urban watershed refers to a water-collection area defined by the boundaries of the watershed and the outlet section of the urban river channel, distinguished by the presence of urban streams. In recent years, with the expansion of urban scales and the growth of urban populations, more attention has been paid to the problem of NPS pollution in small urban watersheds. Numerous experimental research findings have provided a scientific basis for mitigating runoff pollution in small urban watersheds, remediating black and odorous water bodies, and enhancing urban water-treatment processes to improve water quality [49]. Small watersheds in towns have distinctive regional characteristics and hierarchical structures, with the main source of pollution being domestic sewage (Table 3). Studies on N and P loss characteristics in small urban watersheds mainly focus on rainfall and runoff pollution. Nutrient elements such as N and P are major pollutants in rainfall runoff in small urban watersheds. These nutrients are leached and scoured by rainwater during the natural rainfall process, and ultimately enter surface water bodies as runoff, which can have an impact on the water environment of small urban watersheds [61].
For different ground types, there are differences in pollution generated during rainfall. The average concentration of N and P in general rainfall runoff shows that a road surface is higher than a roof, and a roof is higher than green space [62]. Li et al. [63] analyzed the rainfall water quality in Xuzhou city and found that the TN mass concentration of road-runoff rainwater was mostly higher than that of roof-runoff rainwater in the three rainfall events, which was mainly related to the underlying surface characteristics. Different land use types and landforms also resulted in different pollutant content in runoff. Studies have shown that roofing materials have a great impact on the water quality of runoff and rainwater, especially the release of roofing material pollutants, which will seriously affect the water quality of the initial runoff. Wang et al. [64] studied the emission characteristics of nutrient pollutants and heavy metals in the surface runoff of small urban watersheds, and they found that most pollutants in small urban watersheds had an initial scour effect, and N in each underlying surface mainly existed in a dissolved state, while P was in granular state. Investigating roof runoff found that the TN concentration from cement tile roofs was (4.36 ± 2.66) mg·L−1, for asbestos tile roofs, (3.36 ± 2.74) mg·L−1, and for cement flat roofs, (3.01 ± 1.54) mg·L−1. These concentrations surpass the Category V indicators outlined in the Surface Water Environmental Quality Standard in China. This also suggests that atmospheric wet deposition makes a significant contribution to the TN content in surface runoff [65]. Due to the continuous usage of motor vehicles, road runoff and rainwater often carry various pollutants, including metals, rubber particles, and fuel oil. The sources of road pollution are complex, as they are influenced by factors such as high vehicle density and the emission of diverse pollutants from vehicle exhaust. Consequently, road runoff and rainwater are susceptible to multiple unstable factors, such as tire debris, brake pad wear, and corrosion products from the road surface [66].

4. Prevention and Control Measures of N and P Loss

Numerous research findings demonstrate that N and P pollution are primarily influenced by driving factors such as climate and land use. Furthermore, climate change is anticipated to induce seasonal variations in rainfall, consequently impacting regional hydrological processes. As a result, it becomes imperative to implement preventive measures and control measures to mitigate the loss of N and P during rainfall events and land use activities [6,67].

4.1. Prevention and Control Measures in Rainfall

Currently, pertinent studies have demonstrated that rainfall plays a pivotal role in driving the dynamic changes of nutrients in rivers. During the rainfall process, pollutants leach from terrestrial soils in upstream areas and subsequently migrate downstream through water runoff [68]. At present, there are many control technologies for N and P loss caused by rainfall and runoff, such as source-reduction-control technology, process-blocking technology, nutrient-reuse technology, etc. Source reduction is mainly controlled through fertilization management and adjustment of planting structure [69]. Common measures for process blocking include building terraces, setting vegetation buffer zones, and conservation tillage in marginal zones [22]. Nutrient reuse is mainly for constructed wetlands, and nutrient resources such as N and P lost by runoff are used to re-enter the wetland to control the loss of N and P [51].

4.2. Prevention and Control Measures for Different Land-Use Types

Different land-use types differ greatly in the degree of interception, elimination, and absorption of NPS pollution. Some types are the “sources” of NPS pollution, such as farmland, etc., while others are the “sinks” of NPS pollution, such as forest and grassland, etc. [70]. Land-use types exert significant influences on soil and vegetation N and P fixation, as well as runoff N and P outputs. Rainfall serves as a driving force, leading to substantial variations among different land-use types. Land-use types with high N and P losses are primarily agricultural and small urban watersheds. In agricultural areas, practices such as fertilizer application, irrigation, and pesticide use contribute to increased N and P runoff. Additionally, urbanization and industrialization lead to higher sewage and wastewater discharges, further contributing to significant N and P losses in these regions. On the other hand, land-use types with lower N and P losses are typically forest, grassland, and small wetland watersheds. These areas exhibit strong self-regulation mechanisms and possess considerable N and P storage capacity. They can effectively absorb and retain large quantities of N and P, reducing their losses through runoff. This ability to naturally regulate and store nutrients plays a crucial role in minimizing the environmental impact of N and P in these land-use types.
According to relevant studies, farmland exhibits higher concentrations of N and P losses than other land-use types under rainfall conditions. To mitigate N and P loss, employing fertilizer-application methods such as hole application and deep application can be effective. The use of mixed fertilizers can aid in reducing NO3−-N leaching, while the appropriate combination of N and P fertilizer serves as an effective control measure to enhance crop yield and minimize leaching incidents. In the study conducted by Cadahia et al. [71], it was observed that applying N fertilizer alone led to an increase in the soil’s NO3−-N content. A significant portion of this NO3−-N was found to accumulate below the active layer of roots, making it less accessible for crop absorption. Consequently, there was an elevated risk of NO3−-N leaching, which can be environmentally detrimental. However, when N-P or N-P organic fertilizers were mixed, the migration of NO3−-N back to the root zone or above the root zone was observed, reducing the risk of leaching. This suggests that the combined application of N and P can enhance nutrient uptake by crops and minimize the loss of NO3−-N through leaching. Nevertheless, it is essential to be cautious about the excessive application of P fertilizer or organic fertilizers containing P. Excessive P application can result in the infiltration of NO3−-N content below the root zone, leading to a substantial increase in NO3−-N concentration, and in some cases, surpassing the effects of using N fertilizer alone [71]. This emphasizes the importance of balanced nutrient management to avoid unintended consequences on soil and water quality.
The loss of N and P can also be effectively controlled through the implementation of artificial wetland technology and adjusting the planting structure. R. Haberl et al. [72] in their collaboration with developing countries on wastewater treatment discovered that artificial wetland technology offers several advantages. Notably, this technology is characterized by low operating costs, a high pollutant purification effect, and significant ecological and environmental benefits. As a result, artificial wetlands are considered highly suitable for implementation in countries like China and other developing nations, especially considering the prevailing basic conditions of these countries. By utilizing artificial wetland technology, these countries can achieve effective wastewater treatment while also minimizing financial burdens. Additionally, the purification efficiency of artificial wetlands ensures the removal of pollutants from wastewater, contributing to improved water quality and environmental preservation. Furthermore, the ecological benefits of such systems, like habitat creation for wildlife and promotion of biodiversity, align with the goals of sustainable development [73]. By making strategic changes to the cropping structure and minimizing the use and loss of chemical fertilizers, the risk of diffuse pollution from agricultural practices can be significantly reduced. This proactive approach helps safeguard the health of water environments and ecosystems. Implementing these measures leads to multiple benefits. Firstly, it enhances the efficiency of nutrient utilization on farmlands, ensuring that crops receive the necessary nutrients while minimizing wastage. Secondly, it curtails the negative impact of agricultural activities on the environment, particularly in terms of nutrient runoff and leaching, which contribute to water pollution. By reducing these pollution sources, water bodies can remain healthier and support a more diverse range of aquatic life. [40].
Forests and grasslands have the capacity to intercept surface runoff to a certain extent, effectively reducing both surface runoff and runoff volume. Numerous studies have indicated that the well-developed root systems in forests and grasslands contribute to increased surface roughness, which significantly aids in the interception of N and P in runoff [74]. Yang et al. [12] used the SWAT model and analyzed land use change and found that the reduction degree of N and P was different. The greatest reduction in TN was observed when cropland and grassland were converted to forest land. In the case of TP, the largest reduction occurred when cropland and construction land were converted to forest land.

4.3. Limitations of N and P Loss Prevention and Control Measures

In developing and implementing strategies for N and P loss prevention and control in small watersheds, it is vital to take into account the unique characteristics and challenges of these ecosystems. Adopting targeted measures and conducting continuous monitoring and assessment are essential for achieving sustainable water-resource management and protection in small watersheds. However, there are several limitations that may be encountered during the implementation of N and P loss prevention and control measures in small watersheds:
(1)
Lack of comprehensive management: Small watersheds involve multiple stakeholders and decentralized management, which can hinder effective coordination and implementation of prevention and control measures. Developing comprehensive management mechanisms and coordinating bodies is necessary to ensure the success of these strategies.
(2)
Land use variability: Small watersheds often consist of diverse land-use types, including agriculture, forests, and urban areas. Each land-use type contributes to N and P loss differently, making it challenging to design universal measures that address the unique characteristics of each land use.
(3)
Balancing agricultural development and environmental protection: Agriculture is crucial for food security in small watersheds, but it is also a significant source of N and P loss. Striking a balance between agricultural development and environmental protection requires the implementation of scientifically sound agricultural management practices to sustain production while reducing N and P loss risks.
(4)
Financial and technical constraints: Effectively preventing and controlling N and P loss requires substantial financial and technical resources. However, many small watersheds face limitations in terms of economic conditions and technical capacity, leading to difficulties in implementing prevention and control measures.
(5)
Uncertainty of influencing factors: The occurrence and extent of N and P loss are influenced by various factors, such as rainfall, soil type, and vegetation cover. However, there is uncertainty in understanding the variations and mechanisms of these factors, making it challenging to accurately predict and address N and P loss effectively.
Addressing these limitations necessitates collaborative efforts among stakeholders, including local communities, government agencies, and researchers. Implementing adaptive management practices and investing in research to better understand the influencing factors will contribute to the successful prevention and control of N and P loss in small watersheds, ensuring the sustainable use and protection of water resources in these crucial ecosystems.

5. Summary and Prospect

This paper reviews the investigation of the characteristics of N and P loss from small watersheds in terms of rainfall and different land-use types. Rainfall amount and rainfall intensity are the characteristic factors of rainfall, which are the main factors of nutrient loss from the surface, and rainfall ephemeris determines the time of nutrient transport to the surface during rainfall. Source-reduction-control technology, process-blocking technology, and nutrient-reuse technology are effective measures for preventing and controlling the loss of N and P pollutants caused by rainfall runoff. These measures have proven to significantly reduce N and P losses. The impact of different land-use types on nutrient output in small watersheds is primarily studied by examining rainfall characteristics, the physical and chemical properties of the land, topography, and geomorphology, as well as changes in vegetation cover. It is found that fertilizer application, pesticides, domestic sewage discharge, and livestock and poultry-breeding wastewater discharge are the main sources of surface source pollution in small watersheds. The application of efficient fertilizers, adjustment of planting structure, use of artificial wetland technology, and construction of forest and grass-slope protection are ways to slow down the loss of N and P pollutants from small watersheds.
In general, the current research on small watersheds has achieved staged progress, but two aspects of rainfall and different land-use types need to be strengthened in exploring the loss N and P from small watersheds. Further research can address the following aspects in the future:
(1)
Conducting extensive research on rainfall processes in small watersheds, as well as the migration and transformation patterns of N and P under various land-use types. This includes studying the temporal and spatial patterns of rainfall changes, rainfall-formation mechanisms, the interrelationship between rainfall and climate change, and the overall impact of rainfall on water resources and ecosystems.
(2)
Establishing long-term monitoring of N and P concentrations in runoff. To gain a comprehensive understanding of nutrient status, N and P sources, and the processes of migration and transformation, it is crucial to monitor the concentrations of various forms of TN, NH4+-N, NO3−-N, TP, and dissolved P in water bodies. Additionally, monitoring eutrophication indicators, such as the N-P ratio, can help assess the nutrient status and overall water quality of the watershed.
(3)
Investigating the transport and fate of N and P in small watersheds. This can be achieved by utilizing isotope-tracer techniques and chemical analysis to trace the sources and transport pathways of N and P. Such research will provide a deeper understanding of N and P transfers, transformations, and destinations within small watersheds.
By addressing these research aspects in the future, we can further advance our knowledge of N and P dynamics in small watersheds. This knowledge will be instrumental in developing effective strategies for preventing and controlling nutrient loss, contributing to the sustainable use and protection of water resources and ecosystems in small watersheds.

Author Contributions

Conceptualization, C.W.; methodology, L.Z. (Liuyi Zhang); validation, L.Z. (Lei Zhang), and C.H.; formal analysis, C.W. and L.Z. (Liuyi Zhang); investigation, C.H.; resources, S.Z.; data curation, J.P.; writing—original draft preparation, C.W.; writing—review and editing, L.Z. (Liuyi Zhang) and C.H.; visualization, C.W.; supervision, S.Z.; project administration, T.L. and C.H.; funding acquisition, L.Z. (Liuyi Zhang) and T.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Commission of Chongqing, project (No. CSTB2022NSCQ-MSX0818).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Thanks to Jia Wang and Lilin Xia for their work in structuring and revising the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 02894 g001
Figure 1. Wetland mitigating N and P from agricultural runoff [55].
Figure 1. Wetland mitigating N and P from agricultural runoff [55].
Water 15 02894 g001
Table 1. Relationships between rainfall and the output of N and P in small watersheds.
Table 1. Relationships between rainfall and the output of N and P in small watersheds.
Research AreaAreaAverage SlopeAnnual PrecipitationResearch MethodRelationship between Rainfall and N P OutputFormulaReferences
A catchment area in Canada0.026 km2__SWMM model for rainfall runoff managementExponential-function correlationL = Cp[15]
The Yangtze River Delta__1050 mmData analysisPower-function correlationL = kPb[16]
Yangzikeng small watershed,
Guangdong Province		90 km27°1875.3 mmData analysisPower-function correlationL = kPb[17]
Xiangshui Village, Sichuan Province0.1389 km23.16°961.3 mmData analysisLogarithmic- function correlationL = k1ln(P) + M[18]
Jiangjin District of Chongqing Municipality3219 km220°1030.7 mmStudy-area monitoring and data analysisFirst-function correlationL = k2P + N[19]
Miyun reservoir15,788 km20~79°300~700
mm		Water-quality monitoring data and data analysis Quadratic-function correlationL = k3P2 + mP + Q[20]
Notes: L is the amount of NPS pollution (t); P is rainfall (mm); C, k, k1, k2, k3, b, m, Q, M, N are constants.
Table 2. Effect of rainfall on N and P output in small watersheds.
Table 2. Effect of rainfall on N and P output in small watersheds.
Watershed NameArea
(km2)		GeologyRainfall Amount
(mm)		Rainfall Duration
(h)		TN
(kg)		NO3−-N
(kg)		NH4+-N
(kg)		TP
(kg)		PP
(kg)		TDP
(kg)		References
Baimachahe small watershed (China)4.08hilly123.445.3577.683.9265.4———[28]
66.33176.134.312.2———
19.733.528.211.911.8———
Qingshuihe River watershed
(China)		2380mountainous13.861249.921074.1841.17181.70162.6619.04[30]
20.752094.121545.84141.15262.99237.5925.41
8.031765.371195.44182.82185.85163.6822.18
14.591170.831046.5449.9674.6756.5618.11
Chenjiagou small watershed (China)8.94mountainous5.1128.464.182.05———[14]
23.81220.8014.063.97———
34.71291.2646.6824.24———
Wuchuan watershed (China)1.88hilly38.917684121———[31]
15.152357———
58.2191035602252———
101.292117657337———
Chaohe watershed (China)5340mountainous127.624274.73211.719.7190.2188.911.29[29]
75.637186.72146.079.252.972.310.66
71.33237.5233.790.582.100.471.63
29.6717.9316.580.430.720.200.52
Jiulong River
watershed (China)		9570hilly2329127,46517,5137943———[33]
1187840191775478———
Shibetsu watershed
(Japan)		679—801618.36.3 ———[32]
9123168.424.9————
551556.923.6————
501958.021.7————
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Wang, C.; Huang, C.; Zhang, S.; Zhang, L.; Li, T.; Peng, J.; Zhang, L. Research Progress on Nitrogen and Phosphorus Loss in Small Watersheds: A Regional Review. Water 2023, 15, 2894. https://doi.org/10.3390/w15162894

AMA Style

Wang C, Huang C, Zhang S, Zhang L, Li T, Peng J, Zhang L. Research Progress on Nitrogen and Phosphorus Loss in Small Watersheds: A Regional Review. Water. 2023; 15(16):2894. https://doi.org/10.3390/w15162894

Chicago/Turabian Style

Wang, Chunbo, Chengtao Huang, Shuai Zhang, Lei Zhang, Tingzhen Li, Jiyou Peng, and Liuyi Zhang. 2023. "Research Progress on Nitrogen and Phosphorus Loss in Small Watersheds: A Regional Review" Water 15, no. 16: 2894. https://doi.org/10.3390/w15162894

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