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Article

Global Meta-Analysis of Nitrate Leaching Vulnerability in Synthetic and Organic Fertilizers over the Past Four Decades

by
Naila Sumreen Hina
ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland
Water 2024, 16(3), 457; https://doi.org/10.3390/w16030457
Submission received: 8 September 2023 / Revised: 20 October 2023 / Accepted: 30 October 2023 / Published: 31 January 2024
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Abstract

:
The significance of nitrogen in agricultural ecosystems cannot be overstated; however, it can pose a threat to the environment when it leaches into groundwater. This comprehensive meta-analysis sheds light on the complex relationship between organic and inorganic fertilizers and nitrogen leaching, encompassing 39 years of data. The results indicate that the impact of fertilizers is dependent on crop type, soil properties, and fertilization practices. Vegetables treated with synthetic fertilizers were found to have the highest levels of nitrogen leaching, while grasses exhibited the lowest. Soil texture was also determined to be a significant factor, with coarser soils leading to higher levels of leaching than medium or fine soils. The meta-analysis showed that organic sources resulted in an average of 16% higher losses of nitrate-N, but there was no significant difference between organic and synthetic N fertilizers in terms of leaching overall. These findings provide valuable insights for the responsible management of nitrogen and to further our understanding of the impact of fertilizers on nitrate pollution.

1. Introduction

Nitrogen (N) is the most important fertilizer for crops, and its environmental impact cannot be ignored. Six billion people consume approximately 25 million tons of protein N each year; by 2050, this figure is expected to rise to 40–45 million tons [1]. N plays a crucial role in plant growth and development because it is a major component of chlorophyll, amino acids, and proteins, all of which are essential for various physiological processes in plants. When crops receive an adequate supply of N fertilizers, they tend to increase leaf and stem development, improve vegetative growth, and enhance photosynthesis. This ultimately leads to higher crop yields and improved agricultural productivity [2,3]. Therefore, nitrogen is considered as one of the most important fertilizers for crops, and its availability in the soil can significantly influence the success of agricultural systems worldwide.
However, inefficient use of chemical fertilizers in the agricultural ecosystem is a major contributor of diffuse N load in ground water [4]. Excessive concentrations of N in water sources can cause severe long-term environmental concerns and threaten both the economy and human health. In general, applying N fertilizers to crops is very cost-effective; the additional crop value obtained greatly exceeds the cost of the fertilizer. This has pushed farmers to use excessive N to assure a higher crop yield [5]. There is currently a high spatial and temporal variation in the global fertilizer application mean, which is approximately 133 kg N ha−1year−1 [6]. It should be noted that excess supplies of N can pollute the biosphere including soil, air, and water ecosystems. N contamination as nitrate (NO3−) can result in significant environmental and health issues. It leads to water pollution, disrupts aquatic ecosystems, and contributes to eutrophication. One of the most prevalent and detrimental effects of agriculture, connected to N fertilizers, is the deterioration of groundwater quality and contamination of drinking water supplies [7]. Furthermore, NO3− can cause methemoglobinemia in infants, linked to certain cancers (gastric and colorectal), and potential reproductive and developmental effects. Nitrate can accumulate in food crops, posing risks when consumed, and may exacerbate asthma and respiratory issues [8]. Agricultural soils contribute far more nitrate to surface water than semi-natural grasslands and forests due to enormous amounts of supplementary N applied as inorganic fertilizers and manures [9]. As a result, the nitrogen input-output balance is one of the main indicators for sustainable agricultural development, and it is also used to assess NO3−-N leaching in groundwater [10]. Effective management of nitrate sources, improved agricultural practices, and safe drinking water treatment are essential to mitigate these environmental and health concerns.
Over time, numerous spatially variable and interacting factors, including land-use, vegetation type, climate, soil properties, and total nutrient inputs, define the nitrate stocks, fluxes, and leaching at a farm or catchment level [11,12]. Most importantly, fertilizer N is considered to be strongly connected with NO3−-N leaching in groundwater and surface waters [13,14,15]. However, in addition to the well-documented leaching losses from N fertilizers, several studies have shown that organic N mineralization can also result in abundant N leaching [13,16].
Mineralization of organic nitrogen can cause loss of available nitrogen from the soil via leaching, most significantly during the fallow period after the crop harvest [17]. When comparing organic and inorganic fertilizers on a large scale, organic fertilizers usually have 30–40% lower leaching losses [18,19]. In contrast, few studies have reported 20% more leaching in organic fertilizers than in synthetic fertilizers [20,21]. However, the differences in leaching rates from organic and synthetic fertilizers are either non-significant [22,23] or in favor of synthetic fertilizers [24,25].
A meta-analysis is a technique for synthesizing information that employs a specific methodological process for gathering and evaluating data from several independent scientific investigations [26]. Due to the importance of the N source on N transfer from agricultural soils to water resources, the published meta-analysis are summarized in Table 1 demonstrated great strides in the subject matter. Among the published meta-analyses, Boy-Roura et al. [27] and Wang et al. [28] analyzed the maximum number of observations to see the effect of animal urine and fertilizer N rate on N leaching. The former did not report direct losses but calculated a N leaching and emission factor, and the latter one used the data collected only from New Zealand. In comparison, a recent published meta-analysis by Li et al. [29] used maximum factors to evaluate their effect on NO3−-N leaching and reported a significant decrease in NO3−-N leaching with crop residue incorporation, which was the focus of this meta-analysis. However, none of these have compared the leaching losses from organic and synthetic N inputs globally despite a significantly ambiguity and the issue is still debatable and important due to environmental concerns. Despite the widespread use of both organic and synthetic fertilizers, no meta-analysis has been conducted to assess the relative impact of each on NO3−-N leaching in agricultural ecosystems. Therefore, this meta-analysis aims to bridge the existing research gap by conducting a comprehensive global comparison of NO3−-N leaching losses from agricultural lands treated with organic and synthetic fertilizers. It seeks to answer critical questions about the relative impact of these fertilizers, including whether there are significant differences in leaching between them, and how crop types, fertilization sources, application methods, and soil properties interact with these differences. By addressing these questions, this study contributes valuable insights into the environmental implications of fertilizer choices in agriculture, which is vital for informed decision-making and sustainable farming practices worldwide. Furthermore, a meta-analysis is chosen as the methodology for this research because it offers advantages such as synthesizing a large volume of diverse studies, quantifying effect sizes, enhancing statistical power, and promoting objectivity, making it the most suitable approach to comprehensively address the research questions on NO3−-N leaching from agricultural lands treated with organic and synthetic fertilizers.

2. Materials and Methods

2.1. Search Criteria through Online Research Databases and Study Selection

The metadata was collected in compliance with the PRISMA (Preferred reporting items for systematic reviews and meta-analyses) guidelines [35,36]. The PRISMA guidelines are a set of standardized guidelines designed to improve the reporting of systematic reviews and meta-analyses in research publications. The purpose of these guidelines are to provide a structured framework for authors to report the methods, results, and findings of their systematic review or meta-analysis studies in a way to promote clarity and reliability in research reporting [37]. A thorough literature search was conducted for peer-reviewed articles published before 6 July 2022 that mentioned fertilizer-induced NO3−-N leaching from soils. The SCOPUS® “http://www.scopus.com (accessed on 7 July 2022)” databases were searched for relevant articles using the search terms, (“nitrate leaching” OR “NO3 leaching”) AND “nitrogen fertilizer” AND “soil” AND (“chemical” OR “organic”). The search criteria were refined to peer-reviewed articles published only in English language. The steps involved from identification to selection for meta-analysis are reported in the PRISMA diagram (Figure 1).
The database search yielded 1005 studies, and 307 duplicates were removed. The following selection criteria were used for the final selection of articles:
  • Chemical (synthetic) or organic fertilizers must have been used in each study.
  • At least one fertilizer application rate must have been mentioned.
  • NO3−-N leaching must have been monitored.
The inclusion criteria for selecting articles for meta-analysis mentioned above were followed to meet the aim to quantify NO3−-N leaching losses from agricultural lands treated with organic and synthetic fertilizers. Abstract and full article reading according to the selection criteria, 82 articles (both organic and synthetic fertilizers) comprising 211 individual studies, including 187 synthetic fertilizer and 24 organic fertilizer, from 22 countries (Figure 2) were selected for meta-analysis.
The selected papers spanned almost four decades (39 years) from 1983 to 2022 (Figure 3). The data extracted from each study includes the mean, number of observations and standard deviation (SD). In the studies where standard error (SE) was reported instead of SD, it was converted to SD using the equation ( S D = S E × N ). Data was extracted from graphs using the Web Plot Digitizer (version 2.26: “http://getdata-graphdigitizer.com/download.php (accessed on 29 October 2023)”. The data related to parameters influencing NO3−-N leaching irrespective of source were extracted from studies, including crop type (cereals, legumes, vegetables, grasses, and all other), soil texture (coarse, medium and fine), soil pH (acidic, neutral and alkaline), source of N (organic and synthetic), fertilization method, and leaching measuring method (porous cups and lysimeter).

2.2. Meta-Analysis

The use of log response ratios as the matrices of effect sizes is a commonly employed method in multiple studies when comparing treatments to a control. However, this study took a different approach by comparing synthetic and organic fertilizers directly, using single untransformed (raw) means with the metamean function of ‘dmetar’ package in the R environment (https://r-project.org/). This allows for a clearer and direct interpretation of the differences between the two fertilizers, without the potential biases of a control group as mentioned in Doing Meta-Analysis in R: A Hands-on Guide by Harrer et al. [37]. The generic inverse variance method was used to assign the weights using ‘meta’ [38] and ‘metafor’ [39] packages in R for NO3−-N leaching means of both organic and synthetic fertilizers’ observations. A heterogeneity test was performed before designing the meta-analysis model to identify whether a fixed or random/mixed effect model should be employed. The τ2 was estimated using restricted maximum likelihood method which represents the heterogeneity between studies. The value of τ2 (3938.39) was highly significant demonstrating the certainty of a random/mixed-effects approach. The Q profile method was also employed on a full dataset, including 623 observations, which was also significant (Cochran’s Q = 534916.83, df = 622, p < 0.001) and also confirmed the use of the random/mixed effect model [40]. In addition, Hartung–Knapp adjustment was used in this meta-analysis in order remove the biasness due to small number of studies and high values of heterogeneity [41]. The Hartung–Knapp adjustment provide a more accurate estimate when there was significant variability between studies. Traditional methods might underestimate this variability, giving false confidence in the pooled results. The Hartung–Knapp method corrects for this by often resulting in wider confidence intervals, thus reflecting a more realistic level of uncertainty. Its use is crucial when studies in the meta-analysis differ notably from one another, ensuring that the overall conclusions are more robust and reliable.
The estimated pooled means of NO3−-N leaching, together with their 95% confidence intervals (CI), were shown in forest plots created using the ‘ggplot2’ package [42] in R. The impact of organic or synthetic fertilizers was deemed significant if the 95% CI did not overlap the zero line. Overlaps on the zero line show that synthetic/organic fertilizer had no meaningful effect and are denoted as ‘ns’ [43]. A positive number implies an increase in the NO3−-N leaching values, whereas a negative number suggests a reduction, as denoted by percent change (±%).

3. Results

3.1. Effect of Fertilizer Type on NO3−-N Leaching as a Function of Crop Type

In this meta-analysis, NO3−-N leaching from organic and synthetic fertilizers varied with crop type. The pooled means of all crops in Figure 4 shows that NO3−-N leaching from cereals were significantly less (p = 0.046, 40%, k = 358) with the application of synthetic fertilizers (40.9 kg N ha−1) as compared to the application of organic fertilizers (68.1 kg N ha−1). In addition, NO3−-N leaching from vegetables was significantly more (p = 0.043, 88%, k = 53) with synthetic fertilizers than with organic fertilizers. Among all crop types, the lowest NO3−-N leaching was recorded from grasses [17.8 kg N ha−1 (organic) and 32.5 kg N ha−1 (synthetic)] having significant difference (p = 0.047). On the other hand, NO3−-N leaching from legumes was 76.7 kg N ha−1 in synthetic fertilizers and 58.2 kg N ha−1 in organic fertilizers and was not significantly different. No significant differences were observed in overall NO3−-N leaching among type of fertilizers (synthetic vs. organic) applied, as shown in Figure 4.

3.2. Effect of Nitrogen Source on NO3−-N Leaching

All the organic and synthetic N materials used in the studies included in this meta-analysis are shown in Figure 5. In comparison to urea and all other ammonium-based synthetic fertilizers, calcium ammonium nitrate (CAN) and complex fertilizers resulted in almost equal and maximum N losses. Among all organic N fertilizers, pig slurry was more vulnerable to N leaching (184.4 kg N ha−1) but a smaller number of observations (k = 7) were available for the analysis. The same was true for animal urine, where only two observations were available.

3.3. Effect of Fertilizer Type on NO3−-N Leaching as a Function of Soil Properties

In order to make the results more understandable, the soil pH was classified into three main classes as acidic (pH ≤ 6.5), neutral (pH 6.6–7.3), and alkaline (pH ≥ 7.4) as shown in Figure 6. Mean N leaching was significantly different in alkaline (p = 0.033) and neutral (p < 0.001) soils, with 99% more and 47% less leaching from synthetic fertilizers in neutral and alkaline soils, respectively, compared to organic fertilizers. NO3−-N leaching was higher from synthetic material only when the soil was comparatively neutral in terms of pH.
Irrespective of fertilizer types, maximum leaching losses were estimated from coarse-textured soils than fine soils (Figure 7), but no study of organic material on fine soil was found to be included in this meta-analysis. Aside from that, mean nitrogen leaching from organic fertilizers was always slightly higher in coarse (59.4 kg N ha−1) and medium-textured soils (49.7 kg N ha−1) compared to synthetic fertilizers, but not significantly different.

3.4. Effect of Fertilizer Type on NO3−-N Leaching as a Function of Measuring Method

The lysimetric method had a significant effect (p = 0.037) on NO3−-N leaching losses measured from synthetic and organic materials with 47% less from the former with 44.3 kg N ha−1 leaching losses compared to 82.2 kg N ha−1 from organic fertilizers (Figure 8). The N leaching measurement using the porous cup method had no significant effect on mean N leaching. However, the leaching losses from synthetic fertilizers were estimated 10% less (47.6 kg N ha−1) compared to organic fertilizers (52.7 kg N ha−1). All other methods (excluding the two mentioned earlier) also had a significant effect and showed 269% more leaching from inorganic material.

4. Discussion

The data identification and selection for this meta-analysis revealed the importance of NO3−-N leaching from agricultural soils. After 1995, more research on NO3−-N leaching was published, as shown in Figure 3, highlighting the significance of this environmental issue that started in the second half of the 20th century. Galloway et al. [44] reported that the second half of 20th century was the time when agriculture underwent significant changes and artificial nitrogen fertilization became a pillar in modern farming.
Although overall, leaching losses were 16% greater with the use of organic fertilizers compared to synthetic fertilizers, there was no significant difference between the two. However, Dahan et al. [45] have shown that nitrogen leaching is significantly higher from organic fertilizers, while others have found little to no differences between organic and conventional fertilizers [46,47]. The small differences in NO3−-N leaching between organic and conventional fertilizers, as estimated in this meta-analysis, could be due to the reason that organic fertilizers were applied to legume-based cover crops in most of the studies, which can increase both nitrogen input and crop dry matter production, leading to higher NO3−-N leaching due to a lack of synchronicity between mineralization and crop N demand [47,48]. The asynchrony between nutrient release from organic fertilizers and crop nitrogen demand can result in higher leaching rates. Slow-release organic fertilizers may supply nutrients when crops do not require them, increasing the likelihood of excess nitrogen in the soil. This excess nitrogen can then be susceptible to leaching into groundwater or surface water, contributing to higher leaching rates compared to chemical fertilizers, which release nutrients more synchronously with crop demand. In contrary, a number of studies have also found lower nitrogen leaching from organic fertilizers [49,50,51], possibly due to the slow release of nutrients from organic sources, which limits the available nitrogen in soil solution and reduces the risk of contamination of water sources [52]. Overall, it is important to carefully consider the potential impacts of different fertilizers on nitrogen leaching to protect the water quality.
The type of crops significantly impacted nitrogen leaching from both organic and synthetic fertilizers, except for legumes. The highest nitrogen leaching was observed from vegetables and cereals when using synthetic and organic fertilizers, respectively. A number of research has shown that the risk of nitrogen leaching increased from vegetables is due to the high fertilization rates (200 to >300 kg N ha−1) often used in their production, irrespective of the N source, e.g., in the studies reported by Zhang et al. [53], Jiang et al. [54], Wilson et al. [55] and a global meta-analysis by Qasim, Xia, Lin, Wan, Zhao and Butterbach–Bahl [33]. In fact, vegetables have been found to have the highest nitrogen leaching potential among all land uses, followed by arable cropping, pasture ploughing, grazed pasture, cut grassland, and finally, forests, which have the lowest risk of nitrogen leaching. To reduce nitrogen leaching losses in vegetable production, it is important to use the optimal amount of nitrogen, as applying excess nitrogen can lead to leaching without improving crop yields [53]. Apart from this, grasses had the lowest risk of nitrogen leaching among all land uses in the meta-analysis. This is likely due to the low fertilization rates used in grassland management, which reduces the amount of nitrogen available in the soil solution and lowers the risk of leaching. In addition, grasslands have a large pool of nitrogen in soil organic matter, which is slowly released over time due to the low net nitrogen mineralization rate (due to less disturbance of soil) and long residence time of nitrogen in soil organic matter contributing to the reduced risk of NO3−-N leaching in grasslands [56]. In conclusion, to minimize nitrogen leaching in crop production, farmers and agricultural planners should consider the specific crop type and its associated leaching potential. For high-leaching potential crops like vegetables, it is crucial to apply the optimal amount of nitrogen and avoid over-fertilization, as excess nitrogen can lead to leaching without enhancing crop yields. Furthermore, transitioning some areas to grasslands, which inherently have lower leaching risks due to lower fertilization rates and high soil organic matter, can be a strategic decision. In short, adopting crop-specific fertilization rates, enhancing soil organic matter, and integrating grasslands into the cropping system can be key strategies to protect water quality and ensure sustainable agriculture.
The use of lysimeters and ceramic suction cups (porous cups) for measuring NO3−-N concentration in leachate has been widely used in the literature, with lysimeters showing the highest levels of leaching in this meta-analysis, as shown in Figure 8. Both lysimeters and ceramic suction cups offer unique insights into NO3−-N concentration in leachate, yet they come with distinct limitations that are crucial when interpreting their results in the context of real-world conditions. Lysimeters provide a comprehensive understanding of both leachate volume and NO3−-N concentration, making it a reliable method for determining nitrate loss. However, they might not capture the heterogeneity of a broader field due to their smaller variants and potentially skewing results when scaled up [57]. Their installation often involves soil disturbance, which can inadvertently alter natural soil structure, thereby affecting water flow and nutrient leaching patterns [58]. On the other hand, suction cups, due to their specific point measurements, can potentially miss areas of high or low leaching, introducing biases. Furthermore, suction cups only measure the NO3−-N concentration in soil solution, requiring the calculation of drainage volume to determine total nitrate leaching loss, which may under or overestimate the cumulative NO3−-N leaching losses [59]. Despite the uncertainty associated with the use of porous ceramic cups to estimate NO3−-N leaching from soil solution to groundwater, a higher number of studies (k = 239) used this method for NO3−- N leaching measurement compared to the studies (k = 182) that used lysimeters. This might be because the installation and usage of porous ceramic cups is a simple and cost-effective way of extracting soil solution, making it a more convenient choice for on-site leaching measurements. While both methods have their strengths and limitations, and provide invaluable data, their inherent biases underscore the importance of careful interpretation. For practical applications, it is essential to weigh these limitations and, where possible, employ a combination of techniques to ensure a comprehensive and accurate understanding of nitrogen leaching potential.
Soil texture was found to have a significant impact on NO3−-N leaching, regardless of fertilizer type. In line with studies, such as those by Khodabin et al. [60] and Pandey, Li, Askegaard, Rasmussen and Olesen [47]), this meta-analysis has found that coarse-textured soils, like sand, are more prone to NO3−-N leaching due to their better aeration and lower water holding capacity. These properties of coarse soil leads to enhanced release of nitrates from added fertilizers and mineralized organic N, as well as increased loss of soluble nitrate with excess water movement below the root zone [61,62]. In contrast, fine-textured soils, like clay, have higher water retention and chemical reactions that inhibit NO3−-N leaching [47]. Therefore, the coarser the soil texture, the greater the risk of leaching from agricultural ecosystem.
NO3−-N leaching is much higher in alkaline soils with organic fertilizers compared to synthetic fertilizers and all other soil types (neutral and acidic). This difference can be ascribed to a variety of factors including the negative influence of high soil pH on root development, which limits plants’ ability to absorb excess nitrates, resulting in higher NO3−-N leaching [63]. Furthermore, the lower microbial activity in alkaline soils [64] reduces the conversion of nitrates to other forms of nitrogen that are less susceptible to leaching. As a result, the leaching of nitrates from alkaline soils due to the combined effects of high soil pH, slow N release for plant uptake, reduced microbial activity, and poor root development. These findings on the role of soil texture and pH in NO3−-N leaching underscore critical implications for soil management and agricultural practices. In agricultural settings dominated by coarse-textured soils like sand, it becomes crucial to recalibrate fertilizer application rates and timings to minimize nitrate losses. Strategies such as split nitrogen applications or the use of slow-release fertilizers can be employed to synchronize nutrient release with plant uptake, thus conserving nitrogen and safeguarding groundwater quality [9,65]. In addition, the estimated leaching in alkaline soils with organic fertilizers necessitates the careful selection of soil amendments. By optimizing soil pH through techniques like liming or the addition of sulfur, we can strike a balance between nutrient availability and reduced leaching [66]. Ultimately, these insights provide a roadmap for a more comprehensive approach to improve agriculture productivity and environmental conservation.

5. Conclusions

NO3−-N leaching from agricultural systems, particularly from fertilizer sources, is a challenging issue worldwide. This meta-analysis reveals that synthetic fertilizers, especially when applied to vegetables and grasses in neutral pH soils, tend to have higher NO3−-N leaching. In contrast, cereal crops grown in alkaline soils saw increased leaching with organic fertilizers. Harnessing this knowledge allows us to refine nitrogen management in agriculture, offering promising avenues for water quality preservation and broader environmental conservation. It is imperative for policymakers and farming communities to internalize these findings, adapting fertilizer use according to specific crop-soil configurations. A concerning revelation is the limited research on organic fertilizers’ impact, especially given the global shift towards organic farming practices. This research gap necessitates more in-depth studies, providing crucial data to ensure sustainable farming in the future. For researchers seeking to expand upon this work, exploring other factors like precision fertilizer application methods and the long-term results on soil health might be insightful. In conclusion, while our findings elaborated the crucial aspects, they also pave the way for future inquiries, ensuring agriculture remains productive without compromising on sustainability.

Funding

The APC was funded by ETH Zurich, Switzerland.

Data Availability Statement

The data will be provided on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 16 00457 g001
Figure 1. PRISMA flow diagram for this meta-analysis.
Figure 1. PRISMA flow diagram for this meta-analysis.
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Figure 2. Locations of studies (Synthetic fertilizer and Organic fertilizer) included in the meta-analysis, the size of shape representing number of studies from the relevant country.
Figure 2. Locations of studies (Synthetic fertilizer and Organic fertilizer) included in the meta-analysis, the size of shape representing number of studies from the relevant country.
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Figure 3. Number of research articles published yearly from 1983 to 2022 selected for meta-analysis.
Figure 3. Number of research articles published yearly from 1983 to 2022 selected for meta-analysis.
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Figure 4. Effect of crop type on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
Figure 4. Effect of crop type on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
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Figure 5. Effect of fertilizer type on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
Figure 5. Effect of fertilizer type on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
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Figure 6. Effect of soil pH on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
Figure 6. Effect of soil pH on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
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Figure 7. Effect of soil texture on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
Figure 7. Effect of soil texture on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
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Figure 8. Effect of leaching measuring method on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
Figure 8. Effect of leaching measuring method on the pooled NO3−-N leaching where organic (green color) and synthetic (blue color) fertilizers were applied as nitrogen source. The size of scatter point represents the number of studies. p value indicates the significant difference between organic vs. synthetic fertilizer (NS represents non significance) and the percentage represent the percent change in the significant difference. Variables were significant for nitrate leaching if the error bars did not overlap with zero line.
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Table 1. Summary of meta-analysis published on NO3− leaching.
Table 1. Summary of meta-analysis published on NO3− leaching.
Data SpanTreatmentsSub-GroupsNumber of Studies, ObservationsOutput ParametersObservations for N LeachingReference
38 years (1969–2007)Fate of N from grain cropsSoil order, soil texture, field plot size, soil organic C, and latitude and longitude of the experimental site217 studiesN pools and fluxes6 studies[30]
23 years (1990–2013)Urine application rate-12 studies, 82 observationsNO3−-N leaching-[27]
Published before March 2016Effect of N management on grain yield and N lossesFertilisers N management, and N rate376 studies, 1166 observationsGrain yield, NUE, NH3 and N2O emission and N leaching and runoff4 observations[31]
Published before August 2016Livestock manureManure type, crop type141Crop productivity,
NH3 emission, N leaching and N run off		61 observations[32]
Published before October 2018N fertilizer rateCrop type, fertilizer type, soil pH, total N, measuring method86 studies, 324 observationsSoil NO3−-N leaching emission factors-[28]
Published before 10 September 2020Effect of fertilizer types and application rate on vegetables Fertilizer types and application rates477 observationsN2O emission and N leaching220 observations[33]
Published before 11 January 2020Effect of crop residuesClimatic conditions, land use type, soil pH, soil texture, synthetic fertilizer application, crop residue type, tillage, and duration of experiment90 studies, 345 observationsNO3− leaching and N2O emission90 observations[29]
Published between 1990 and 2021Effect of animal manure on crop productivity and reactive N lossesReactive N, crop productivity, soil chemical properties, dissolve organic carbon334 studiesCrop productivity,
NH3 and N2O emission, N leaching		-[34]
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Hina, N.S. Global Meta-Analysis of Nitrate Leaching Vulnerability in Synthetic and Organic Fertilizers over the Past Four Decades. Water 2024, 16, 457. https://doi.org/10.3390/w16030457

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Hina NS. Global Meta-Analysis of Nitrate Leaching Vulnerability in Synthetic and Organic Fertilizers over the Past Four Decades. Water. 2024; 16(3):457. https://doi.org/10.3390/w16030457

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Hina, Naila Sumreen. 2024. "Global Meta-Analysis of Nitrate Leaching Vulnerability in Synthetic and Organic Fertilizers over the Past Four Decades" Water 16, no. 3: 457. https://doi.org/10.3390/w16030457

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