Estimating nitrate leaching rate using SSM model in wheat agroecosystems

Document Type : Research Paper

Authors

1 1. PhD of Agronomy, Department of Agronomy, Faculty of Plant Production, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.

2 Department of Horticulture Science, Faculty of Plant Production, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.

3 Department of Agronomy, Faculty of Plant Production, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.

4 Department of Agrotechnology, ّFaculty of Agriculture, Ferdowsi University of Mashhad, Iran.

10.22059/ijswr.2025.402220.670005

Abstract

Simulation models are effective tools for estimating nitrogen leaching and reducing its loss from agricultural fields. In this study, the SSM-Wheat model was applied to estimate nitrate leaching in wheat fields of Bandar-e-Turkmen County during 2019–2020. A total of 59 fields were randomly selected across the region using a uniform sampling approach. Meteorological data from the Bandar-e-Turkman synoptic station (minimum and maximum temperatures and daily sunshine hours), field management information (nitrogen fertilizer timing and amount, irrigation frequency, plant density, and planting date), and soil characteristics (texture, depth, nitrate and ammonium levels, salinity, and pH) were collected through farmer questionnaires and laboratory analyses at Gorgan University of Agricultural Sciences and Natural Resources. These data were then used as inputs for the SSM-Wheat model. Results indicated that three irrigated fields in the central part of the county experienced nitrate leaching exceeding 30 kg/ha. Additionally, 10.18% of the fields (six fields) in the southern region exhibited leaching levels of 20–30 kg/ha. High nitrogen application before planting, large fertilizer doses during tillering, and heavy rainfall following fertilizer application contributed to elevated leaching levels. The model also showed that 32.2% of the fields had no nitrate leaching, and 10.17% demonstrated minimal leaching (1–5 kg/ha), categorizing them as environmentally healthy regarding groundwater contamination. Lower fertilizer use, split nitrogen applications, and appropriate timing were key factors in reducing nitrate leaching. Overall, the findings offer practical guidance for optimizing nitrogen fertilizer management, mitigating groundwater nitrate pollution, and improving the sustainability of wheat production systems in northern Iran.

Keywords

Main Subjects

Introduction

 Nitrogen (N) plays an important role in crop plants. It is involved in various critical processes, such as growth, leaf area expansion, and biomass-yield production (Anas et al., 2020). Excessive use of nitrogen fertilizers disrupts the balance of the global nitrogen cycle and has led to major environmental, health and economic problems. On a global scale, nearly 50 percent of agricultural nitrogen fertilizers cannot be effectively absorbed and used by plants and are lost to the environment as nitrate (NO3-), ammonia (NH3), and nitrous oxide (N2O). This increases the costs of agricultural production and leads to water pollution and climate change (Ding et al., 2020). Finding a balance between agricultural production and environmental protection is a prerequisite for the sustainable development of ground and surface waters and soil quality (Spijker et al., 2021). Therefore, considering the necessity of providing the nitrogen required by the plant to maintain yield and also to reduce nitrate pollution in groundwater due to inappropriate use of nitrogen fertilizers, the design and parameters of fertilization management should be optimized for different soil types and crops (Azad et al., 2020). Simulation models constitute efficient tools not only for predicting and managing nitrate-N pollution of surface and ground waters but also for understanding the physical, chemical, and biological processes defining nitrate-N transport from the soil–plant system to water bodies (Singh & Craswell, 2021).

In this study, the nitrate leaching index, as a factor threatening the health of agroecosystems, was investigated using the SSM-Wheat model to calculate and estimate the environmental impact of nitrogen fertilizers used in wheat fields in Bandar-e-Turkmen County.

Materials and methods

This study was conducted in Bandar-e-Turkmen county, one of the northern cities of Golestan province, Iran. This county has an area of 854.18 km2 and is located in the geographical coordinates between 53˚ 54ʹ and 54˚ 28ʹ E longitudes and 36˚ 49ʹ and 37˚ 24ʹ N latitudes. This county is 20 meters below the level of the Caspian Sea, and the slope decreases towards the sea. Bandar-e-Turkmen has a semi-humid climate, but it is located in the semi-desert climate zone from the northeast. The annual rainfall of this county is less than 420 mm. The total rainfall of the region in this crop year is about 325.2 mm and the average temperature of the coldest and hottest months of the year is 4˚C in February 2019 and 30.50˚C in June 2020, respectively. The crops of this county include wheat, barley, canola, and cotton, which are cultivated in both irrigated and rainfed system.

This study was conducted to evaluate the health of wheat agroecosystems in 59 wheat fields in the agricultural lands of Bandar-e-Turkmen County during 2019-2020. The soil samples were taken based on the W-shaped pattern and from a depth of 60 cm using an auger. samples are placed in the shade so that the moisture of the samples is completely lost and dried. They were pounded and passed through a 2 mm sieve kept at room temperature and air-dried until the tests were performed.

In this study, the SSM-Wheat model was used to simulate nitrate leaching in wheat fields. To implement this model, field management information (crop density, variety, number of seed used, planting depth, amount and frequency of fertilization and irrigation), soil characteristics (soil texture, soil bulk density, usable water, soil depth, amount of nitrate, ammonia, and total nitrogen) and weather conditions (daily data of minimum and maximum temperatures, solar radiation and rainfall) are required. These data were collected through questionnaires, laboratory experiments, and the meteorological station of the county.

Results

The results of the SSM-Wheat model showed that three fields in the central part of the county had more than 30 kg/ha of nitrate leaching, which are dedicated to irrigated farming. Also, 10.18 percent (6 fields) in the southern part of the region showed leaching of 20 to 30 kg/ha. The use of total amount of fertilizer used in the pre-planting stage, the use of high amounts of nitrogen in the tilling stage, and the occurrence of heavy rains immediately after the use of fertilizer can be the reasons for the higher leaching of nitrates in these fields. Also, the model under study identified 32.2% of the fields as having no nitrate leaching and 10.17% as having the lowest nitrate leaching rate (1-5 kg/ha), which were recognized as healthy fields in terms of groundwater environmental pollution. Low consumption of nitrogen fertilizer, distribution of fertilizer in several growth stages of wheat, and proper timing of fertilizer consumption can be the reasons for reducing nitrate leaching in these fields. Kostensalo et al (2024), in a site-specific prediction model for nitrogen leaching in conventional and organic farming, utilized up to 16 years of field measurements from two leaching fields in Finland. They developed prediction equations for nitrogen leaching for two soil types: sandy and clay soil classes. Results showed that organic farming, with a crop rotation resembling that of conventional farming, resulted on average in 20 percent less nitrogen leached per hectare as compared to conventional farming. Developed models are suitable for integration into a life cycle assessment framework, and especially the models utilizing nitrate nitrogen were shown to be applicable to a wide range of different crop types, making the model well-suited for plots with diverse crop rotations.

Conclusions

Due to the low livelihood situation of dryland farmers in the region and the high purchase price of chemical fertilizers, nitrogen fertilizer is used one or two times, and the amounts are 50-100 kg/ha, at the right time. This increases efficiency and reduces waste and leaching of nitrate from fields.

Author Contributions

The author was responsible for developing the main ideas of the paper and preparing both the initial manuscript and the revised versions.

Data Availability Statement

The datasets used in this study can be obtained from the author upon reasonable request.

Acknowledgements

We express our sincere appreciation for the financial support provided by the Gorgan University of Agricultural Sciences and Natural Resources.

Ethical considerations

The author adhered to ethical research standards and refrained from any form of data manipulation, plagiarism, or academic misconduct.

Conflict of Interest

The author certifies that there are no conflicts of interest associated with this work.

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Iranian Journal of Soil and Water Research
Volume 56, Issue 11
January 2026
Pages 2989-3007

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