Measures for Reducing Nitrate Leaching in Orchards: A Review

Measures for Reducing Nitrate Leaching in Orchards

Introduction

Nitrogen (N) is crucial for plant growth.
Improper N fertilization leads to nitrate leaching, especially in fruit production.
High nitrate levels in drinking water are harmful.
Excessive nitrate in rivers causes eutrophication.
The study reviews methods to reduce nitrate leaching in orchards.
Approaches like grass cover, controlled-release N fertilizer, and nitrification inhibitors are evaluated.
Integrated measures are essential for agricultural sustainability.
N fertilizer increases agricultural yield and quality.
Excessive N fertilization causes high groundwater nitrate levels.
Groundwater nitrate pollution disrupts ecological balance and harms human health.
In the 1960s, nitrate contamination was linked to chemical N fertilizer in the US and Europe.
Groundwater has limited self-purification capacity (renewal period of 1400 years).
Effective measures are needed to prevent nitrate pollution.
Many countries have set nitrate concentration standards for groundwater.
- WHO: Nitrate content should not exceed 50 mg L⁻¹ (11.3 mg L⁻¹ for $NO_3^- - N$ ).
- EU: Highest concentration should not exceed 50 mg L⁻¹, recommended allowable $NO_3^- - N$ concentration is 5.6 mg L⁻¹.
- US EPA, Japanese and Canadian environmental organizations: Maximum limit of $NO_3^- - N$ in groundwater is 10 mg L⁻¹.
Reducing nitrate levels can produce by-products, increasing processing costs.
Prevention from the source is the most effective solution.
Studies focus on reducing fertilizer and changing irrigation methods in corn, wheat, and other field crops.
Orchards differ due to the varied root distribution of fruit trees.
Herringbone and dichotomy root structures capture nitrate most effectively.
These structures produce high-density root systems early, speeding up nitrate consumption and reducing penetration into deeper soil layers.
Fruit trees store nutrients, complicating nitrate leaching.
Appropriate management strategies are crucial to reduce nitrate leaching in orchards.

Methodology

Literature was collected from Web of Science using keywords like "nitrate leaching", "orchards", and "fruit trees".
The number of articles on nitrate leaching increased from 2000 to 2019.
More scholars are studying nitrate pollution, indicating a threat to human health.
Most articles on reducing nitrate leaching focus on pasture and field crops.
Few reviews specifically address nitrate leaching in orchards.
Sanz-Cobena et al. (2017) discussed nitrate leaching as part of greenhouse gas emissions mitigation.
Fan et al. (2010) studied nitrate-N accumulation in the soil profile under dryland agriculture in northern China.
Orchard production contributes significantly to agricultural production, and nitrate leaching poses a serious ecological risk.
This review aims to aid further research on nitrate leaching in orchards.

Methods for Assessing Nitrate Leaching in Orchards

Isotopes are chemical elements with the same atomic number but different masses.
Stable isotopes are non-radioactive and used for tracking pollution sources.
$^{15}N$ is used to mark different forms of N fertilizers to understand N absorption and transportation in plants.
d15N and d18O isotopes of nitrate are used to track nitrate pollutants in groundwater.
Different nitrate sources have different N and O isotope ratios, which aid in identifying the source of pollution.
In small catchments, surface water and groundwater interaction affects the d18O value of nitrate.
Combining d15N and d18O isotopes of nitrate with dD and d18O isotopes from water improves understanding of nitrate sources.
This method is rarely used for large rivers due to multiple water sources.
Other indicators like hydrological conditions, water quality, and land use types need to be considered for large rivers.
Land use types, hydrochemistry, multi-isotopes and dual isotopes of nitrate are combined to reduce the effect of isotopic fractionation caused by space-time factors.
Limitations of isotopic techniques in tracking nitrate pollution sources:
- Large spatial difference in nitrate.
- Lack of database reflecting global and local changes of d15N and d18O of nitrate.
- Expensive measurement method.
- Complicated operation.
- Unclear analysis of the variation degree of original d15N and d18O of nitrate and the influence of such variation on the experimental results.
To overcome these limitations:
- Establish global and regional isotope databases.
- Further study biochemical reactions in N migration and factors affecting these reactions.
- Explore new isotope detection methods.
- Improve quantification of N and O isotope fractionation.

Methods and Assessment Tools for Tracking Nitrate Sources

15N isotope labeling is used to track N transformation in plants and soil.
Stable isotopes of nitrates are used to identify nitrate contamination sources in groundwater.
Stable N and O isotopes of nitrate molecules are used to assess the source and process of nitrate in groundwater.
Isotopic fractionation affects the accuracy of nitrate source determination using stable N and O isotopes alone.
Combining environmental tracers and nitrate stable isotopes enhances the accuracy of nitrate source and conversion processes.
Koh et al. (2010) demonstrated that chemical fertilizers are the main nitrate source in groundwater using stable isotopes.
Pasten-Zapata et al. (2014) used a multi-tracer method to prove that animal feces cause groundwater nitrate pollution in orchards and vegetable agriculture.
Multiple isotopic techniques determine the source of nitrate pollution and provide an effective tool for controlling it from the source.
A nitrate pollution map reflects nitrate contamination in an orchard.
Assessing spatial scale data and map information minimizes the risk of groundwater nitrate contamination.
Grey Water Footprint (GWF) measures and generates a map of nitrate contamination in groundwater.
Serio et al. (2018) used GWF to measure groundwater nitrate pollution, obtaining a nitrate pollution map to determine environmental factors in areas with serious nitrate pollution.
Portable, inexpensive instruments for rapid nitrate monitoring play an important role in studies on nitrate contamination.
Mitschele et al. (2012) usedCardy meter to measure soil nitrate N solutions quickly and accurately.
Identifying the source of nitrate pollution is of great importance to solve nitric acid pollution from the source and to ensure the safety of drinking water.
Nitrate pollution in groundwater has various sources, with different d15N and d18O values, so the source of nitrate pollution can be identified using N and O isotope tracking technology.
There are overlaps between d15N and d18O of some nitrate pollution sources, which may lead to inaccurate identification for the source of nitrate pollution.
The combined use of d15N, d11B, d18O, d17O d2H, d34S and other isotope tracer techniques improves the accuracy of nitrate pollution identification, but the accuracy was not satisfactory due to the existence of isotope fractionation.
Future studies must improve the identification and quantification of isotope fractionation to increase the accuracy of the isotope determination method.

Prediction of Nitrate Leaching by Modelling

Understanding water and N balance provides a basis for optimizing fertilization strategies in orchards.
Numerical models balance water and N, predicting N fertilizer demand and nitrate leaching.
Different models evaluate combinations of irrigation and fertilizer scheduling to optimize water and N uptake and reduce leaching.
The primary factor of excessive nitrate content in groundwater is excessive N input.
The HYDRUS-1D model predicted future nitrate change in coastal areas of Israel.
- Reducing the fertilization rate by 25% (original value is 200 kg N ha⁻¹) did not change nitrate content in groundwater over 40 years.
- Decreasing fertilization by 50% reduced nitrate concentration according to drinking water standards.
Hydrus-2D/3D model predicts nitrate leaching from fertilizer treatments.
Changing the fertilization method from sprinkler to drip irrigation reduces nitrate leaching by nearly 37%.
The root zone water quality model (RZWQM2) accurately predicts the impact of irrigation schemes on nitrate leaching.
Reasonable irrigation and reduced fertilization effectively reduce nitrate leaching.
The leaching estimation and chemistry model (LEACHM) simulates water and solute transport in unsaturated soils.
It is an effective tool for understanding N conversion and transportation in soils.
The LEACHN model predicts nitrate leaching far better than the compartment model.
Models with one and three layers predict nitrate concentration in the soil.
Epistics combines biophysical and decision-making models, simulating N dynamics in orchards.
Agronomic rules and crop models should be linked to more rational N management.
Models simulate the relationship between N fertilizer input and nitrate leaching.
The ANIMO model shows that reducing N application rate from 600 to 400 kg N ha⁻¹ yr⁻¹ reduces nitrate leaching by 28%-47%.
Optimal N input for wheat may range from 100 to 150 kg N ha⁻¹.
Optimal N input for orange trees may range from 150 to 350 kg N ha⁻¹, depending on tree age, soils and climate.
The STICS model demonstrates that nitrate leaching is significantly reduced when N input in apple orchards is less than 100 kg N ha⁻¹.
The numerical model effectively understands water and nutrient dynamics, assessing deficiencies in current production practices.
Commonly used models: STICS, HYDRUS, Hydrological model SWAP, Nutrition model ANIMO, Root zone water quality model RZWQM2 model, LEACHAM model, etc.
These models have disadvantages such as inability to simulate water and nitrogen competition, failure of tree structure to explain nitrogen storage, high parameterization, and complex calibration.
Multiple potential scenarios may lead to uncertainty in the assessment of agricultural practices.

Effects of Fertilizer Management on Nitrate Leaching

Optimal Use of Chemical N Fertilizer

Rational use of chemical fertilizers promotes healthy crop growth and improves yields.
Unreasonable use has negative impacts on crops and the agro-ecological environment.
Exploring the optimal application rate of chemical fertilizers is significant for agricultural production.
The optimal N fertilizer input varies from region to region.
For citrus, the optimal amount of N fertilizer input is usually between 150 and 250 kg N ha⁻¹.
Studies show that the excessive input of fertilizer N is the pri- mary factor leading to high risk of nitrate leaching and ground- water pollution.
Excessive application of N fertilizer can lead to high concentration of nitrate in the soil.
An optimal N input should maximize fruit yield and minimize N losses.
The N balance method can help understand the difference between N input and N output in an orchard.
Calculating the annual cumulative water flux density and annual cumulative $NO_3^- - N$ flux density showed that under the standard fertilizer application rate, the annual average concentration exceeds 10 mg L⁻¹.
When fertilizing fruit trees, we also need to consider the use of fruits and avoid wasting fertilizer.
In order to reduce nitrate leaching caused by excessive fertil-ization, on the one hand, we need simple tools to quickly measure the residual N content in the soil, to determine the N content that the crop needs to supplement, such as N balance method.
On the other hand, there is an urgent need to formulate fertilization standards for different cash crops, to formulate fertilization technical manuals to guide farmers to correctly fertilize, and to increase economic subsidies for farmers who comply with fertil-ization laws.

Optimal Use of Manure and Organic Fertilizer

The application of manure or organic fertilizers also has an impact on soil nitrate concentration.
As the amount of organic fertilizers increases, the nitrate content in the soil also increases.
Therefore, controlling the input of organic fertilizer is the key to reducing N loss.
The application of mineral fertilizer and compost increased the total N in the soil regardless of the rate.
Long-term use of compost in orchards can reduce the use of mineral N, reduce nitrate leaching from the soil, and increase N pools.
Fruit trees need different nutrients to grow, and they can get all or part of their nutrients from compost, reducing the need for other fertilizers.
The application of organic fertilizer stimulated the growth of peach tree roots, increased the ability of roots to retain nitrate and reduced the risk of nitrate leaching in the soil.
The application of organic fertilizers increase soil microbial carbon, organic matter, total N, P, K, improve the soil environment, and increase soil fertility.
High soil total N promotes the increase of available N in the soil and guarantees the growth requirements of the fruit tree, but if the rate of mineralization increases above the demand of the fruit tree, this causes excessive N to accumulate in the soil, which may become a potential leaching risk.
Therefore, when applying compost, the amount of N and the growth needs of the tree should be carefully evaluated to avoid over-fertilization.
In addition, the litter will contain a large amount of N returned to the soil every year, and may be absorbed by plants. This part of N must also be considered when studying the amount of N applied.
Regular application of compost during the growth period of the orchard can reduce the use of chemical N fertilizer, and reduce environmental pollution problems caused by long-term application of chemical N fertilizers.
Therefore, further studies is needed to determine the optimal ratio between the use of compost and chemical fertilizer in orchards to ensure that N release is synchronized with crop demand.

Optimal In-Season Nutrient Management

Fertilization times and methods affect nitrate leaching.
Split application of urea (60% and 40% after autumn) improves N use efficiency and reduces residual N in the soil.
Applying N at the beginning of the season has the highest NUE, while applying N after harvest has low NUE and high nitrate leaching.
Early N-fertilized trees have more flowers, shorter leaves, and buds.
Spring is the best time to apply N fertilizer.
The limited N supply before harvest does not increase fruit yield but increases the risk of nitrate leaching.
Applying fertilizer in April increases total yield without reducing fruit size and increases NUE.
Under all N systems, plants preferentially absorbed $NH_4^+ - N$ .
However, $NH_4^+ - N$ has a strong impact on pecan growth, which is characterized by reduced total biomass and root growth.
With the same amount of N fertilizer, the form of N fertilizer can lead to different surplus of nitrate in the soil.
For growers, optimizing N management requires comprehensive measures. Fertilization should be managed according to the 4R principle. That is “right fertilizer type”, “right application rate”, “right application time” and “right application method”. For example, the growers could use controlled release N fertilizer (relatively expensive though), apply optimal amount of fertilizer with proper split, and apply appropriate ratio of organic and inor- ganic fertilizers.

Effects of Irrigation on Nitrate Leaching in Orchards

Irrigation affects nitrate leaching; quantity and mode of water supply influence nitrate migration.
Drought stress and excessive irrigation are not conducive to crop N accumulation.
Reasonable irrigation is significant for improving water use efficiency (WUE) and reducing N loss.
High-frequency irrigation minimizes nitrate leaching and ensures a stable nutrient supply in the root zone, improving N absorption efficiency.
Reducing irrigation and N by 20% reduces drainage by 15.8% and nitrate leaching by 46.4%.
High frequency and low N concentration (HFLC) fertigation maintains water and nitrate in the effective zone of the root.
High-frequency underground irrigation further reduces irrigation and N loss.
Micro-irrigation helps the root zone absorb nutrients and reduces the risk of leaching.
Over-optimized water input leads to large leaching of water and N, which reduces water and NUE. However, slight suboptimal irrigation and fertilization can increase water and NUE.
Reducing water input to around 75% of ETC and to N requirements (200 kg N ha⁻¹) can significantly reduce nitrate leaching without affecting yield.
Irrigation below ground can bring more advantages, such as reducing soil evaporation and preventing weed growth.
Calculating crop ET accurately and optimizing irrigation management schemes to synchronize the application of water and N with the demand of crops will remain a major challenge in the future.

Effect of the Soil C/N Ratio on Nitrate Leaching

Effect of Biochar on Nitrate Leaching

Biochar helps retain nitrate ions in pores, affecting nitrification and denitrification and increasing N fertilizer use.
The amount of biochar that should be added is an important research topic.
Different biochar application methods have different effects on nitrate leaching.
The interaction potential of biochar with $NO_3^- - N$ is determined by soil properties and ion concentration rather than soil properties and aging.
The study shows that the effect of biochar addition on nitrate leaching in soil is related to the biochar heating tempera- ture, biochar pH, fertilizer amount and C/N ratio.
Fewer biochar types significantly reduce nitrate leaching in soils at C/N ratios between 100 and 200.
A pH between 7.8 and 8.9 increases the nitrate concentration in the soil, but acidic to neutral and strongly alkaline biochar reduces nitrate leaching.
This new modified biochar can be used as an effective tool for removing nitrate from groundwater or rivers with its clean production effect, and more new modified biochar will be paid more and more attention.
Future research needs to better understand the function of biochar, seek for biochar that can be modified or engineered to improve the effectiveness of biochar adsorption on soil nitrate.
The application of biochar and fertilizer should also be explored to maximize the role of biochar in improving soil.

Effects of Grass or Organic Covering on Nitrate Leaching

Grass or mulch fixes and absorbs excess N, reducing nitrate leaching.
Natural vegetation cover reduces nitrate concentration in soil.
Even the use of cover crops requires the addition of organic fertilizers to maintain good yields and suf- ficient tree vigor.
Weeds as an overlay reduce nitrate leaching in the soil.
Some studies have shown that the use of weeds as an overlay in orchards reduces the risk of nitrate leaching in the soil.
Winter is the season when nitrates are most likely to leach in the orchard. If perennial weeds start to grow active in the winter and dormant in the summer, there will be no nutrient competition with fruit trees and better nitrate absorption from the soil.
Compost and wood chip coverings increase the concentration of organic matter and nitrate in the soil, making trees use N more efficiently.
By using a bean cover, the N fixation of legumes can effectively reduce nitrate leaching and improve fertility.

Other Ways to Reduce Nitrate Leaching

Foliar Fertilization

Foliar fertilization reduces the amount of soil fertilization needed while fully exerting a fertilizer effect and reducing the risk of nitrate leaching in the orchards.
The results showed that there were no significant differ-ences between foliar and soil fertilization, and that foliar fertiliza-tion prevents surplus of nitrate in the soil.
Therefore, fertilizing the soil and leaves or fertilizing the leaves only also reduces the loss of N while maintaining the productivity and fruit quality.
However, the efficiency of foliar nutrient absorption is low, foliar fertilization is easy to drip from the foliar surface, spraying fertilizer is easy to be lost by rain, and the limited nutri-ents provided by foliar fertilization cannot meet all the crops’ nutrient requirements.
Therefore, exploration of foliar nutrient absorption mecha-nism and spraying theory is still the basis for future research. Ac-cording to the nutritional characteristics of crops, selecting suitable nutrient forms and ratios, developing high-efficiency composite additives, and applying multifunctional organic active substances to improve application effects are the directions of future research.

Controlled-Release N Fertilizer

Controlled-release fertilizer releases fertilizer slowly with plant growth, improving the fertilizer use rate and reducing fertilizer loss.
Sulfur-coated urea provides a good N balance for citrus orchards, reducing nitrate leaching.
The use of controlled-release N fertilizer is clearly a feasible N source for controlling nitrate leaching.
However, controlled-release N fertilizers are not suitable for use on arid lands, and nutrients cannot be effectively released in the absence of water, which may cause crops to reduce yields because they cannot absorb effective nutri-ents.
The membrane material and production process of the controlled-release fertilizer may produce pollutants, and the encapsulating agent remaining in the soil may cause secondary pollution.
In order to reduce the price of controlled-release N fertilizer, efficient and cheap controlled-release N fertilizer is the key to the current research.

Nitrification Inhibitors and Urease Inhibitors

The use of nitrification inhibitors in the orchard can increase the use of N fertilizer, improve the quality of the fruit, and reduce the potential pollution of groundwater by nitrate leaching.
Nitrification inhibitors can inhibit the conversion of ammonium N into nitrate N by microorganisms in the soil, thereby reducing the formation and accumulation of nitrate N in the soil.
The 3,4-dimethylpyrazole phosphate (DMPP) nitrification inhibitor is also a very good choice, and it can improve the fruit quality and reduce the risk of nitrate flowing into the groundwater.
Both nitrification urease inhibitor (NUI) and the fungicide chlorothalonil can reduce nitrate leaching in the orchard.
*Some compounds secreted by the rhizosphere of plants also have an inhibitory effect on soil nitrification. These compounds are called biological nitrification inhibitors (BIN).
Chemically synthesized nitrification inhibitors such as DCD and DMPP have been widely used. However, chemical nitrification inhibitors are costly and easily cause environmental pollution, especially some water-soluble nitrification inhibitors can cause groundwater pollution

Soil Texture

The mobility of nitrate in soil is very strong, and the influence of different soil texture on the flow of nitrate is quite different.
The loss of nitrate was the lowest in silty loam, but the highest in silty clay loam and sandy loam.
Rough soils usually have large pores and high permeability through which nitrate and water can easily penetrate.
Improvement of the soil are required for rough soils to increase soil water retention capacity.
Reasonable improvement of soil texture can allow soil to better retain fertilizer and water, which reduces costs and environmental pollution for agricultural production.

Conclusion and Future Perspectives

The misuse of chemical N fertilizers and manure is a direct cause of nitrate pollution.
Understanding the sources and causes of nitrate in soil and groundwater is of great importance to formulate effective and efficient N management strategies and measures.
The application of some green and less polluting methods can also reduce nitrate leaching. For example, adding biochar or planting natural vegetation can reduce the nitrate leaching without affecting the yield and quality of the fruit.
Therefore, effectively reducing nitrate leaching from orchards and other agricultural systems requires comprehensive measures, which include scientific and quantitative tools as well as policy and subsidy supports.