Modeling yield response to plant nutrition in major cereals under variable climatic conditions

This study evaluates the yield response of major cereal crops—wheat, barley, and maize—to varying plant nutrition levels under variable climatic conditions in selected regions of Iran. Using both field trials and process-based crop simulation modeling, we analyze how nitrogen, phosphorus, and potassium fertilization strategies interact with climatic variables such as temperature and precipitation. Results highlight significant interaction effects, indicating that optimal nutrient management must be adaptive to regional climatic variability for maximum yield. The findings provide valuable insights for agronomic planning in semi-arid climates.

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Introduction

Cereal crops such as wheat, barley, and maize constitute the backbone of global food security and are particularly vital for nations like Iran, where climatic variability poses persistent challenges to agricultural productivity. Iran, situated within the arid and semi-arid belt of Southwest Asia, experiences marked variability in both temperature and precipitation from year to year and region to region. These fluctuations create an environment in which crop yields are highly sensitive not only to weather conditions but also to agronomic practices, most notably plant nutrition management. In recent decades, the imperative to improve cereal crop yields has grown due to population pressure, dietary diversification, and the increasing threat of climate change. The latter has manifested in Iran as extended droughts, intense heat waves, and unpredictable rainfall patterns, all of which jeopardize reliable cereal production. Despite technological advancements, average wheat yields in Iran, for instance, have remained lower than in comparable agro-ecological zones, often falling short of national targets and the demands of a growing population (Mosavi & Mohammadi, 2018) [5] . These shortfalls underscore the urgent need for adaptive management strategies that can buffer cereal production against climatic uncertainty. A cornerstone of such strategies is the optimization of plant nutrition, specifically the judicious management of essential macronutrients-nitrogen (N), phosphorus (P), and potassium (K)-which play critical roles in crop development, yield formation, and resilience to stress. Nitrogen, the most yield-limiting nutrient in most Iranian soils, is integral to vegetative growth and grain filling, while phosphorus supports root establishment and energy transfer, and potassium enhances water-use efficiency and disease resistance. However, the efficiency of nutrient uptake and the resultant yield benefits are highly dependent on environmental conditions, particularly soil moisture availability and temperature regimes during the growing season. Traditionally, fertilizer recommendations in Iran have been based on generalized guidelines that do not fully account for the spatial and temporal heterogeneity in climate or soil fertility. This often leads to sub-optimal application rates-under-fertilization in some areas, resulting in nutrient deficiencies and yield gaps; and over-fertilization in others, increasing production costs, promoting nutrient losses to the environment, and even causing yield penalties in water-limited conditions (Jalali & Ranjbar, 2009) [2] .

As such, there is a growing recognition that site-specific and seasonally adaptive fertilization strategies are needed to maximize both yield and resource use efficiency. The need for more nuanced approaches is further amplified by climate change projections, which suggest that Iran's cereal belt will likely face increasing temperature variability, more frequent droughts, and shifts in precipitation timing and distribution (FAO, 2020). These dynamics will not only alter crop water requirements and growing season length but will also impact nutrient cycling and availability in soils. Consequently, the interaction between plant nutrition and climate is expected to become even more pivotal in determining crop performance. Despite the critical importance of this interaction, empirical research in Iran examining how plant nutrition modulates yield response under different climatic conditions has been limited. Previous studies have mostly addressed either the effect of fertilizer on yield or the impact of climate on cereal production in isolation, seldom investigating their combined influence in a systematic manner. Additionally, the use of advanced crop simulation models-tools that can integrate multiple variables and predict outcomes under diverse scenarios-remains in its infancy within Iranian cereal agronomy. This study seeks to address these knowledge gaps by conducting a comprehensive, multi-location field experiment across three major cereal-producing provinces of Iran: Fars, Khuzestan, and Golestan. These provinces were selected for their contrasting climatic conditions, ranging from semi-arid to sub-humid, as well as their strategic importance in Iran's food system. The research integrates field trials with advanced crop modeling (using the DSSAT system) to evaluate how wheat, barley, and maize yields respond to varying levels of NPK fertilization under actual and simulated climatic variability. By systematically analyzing yield responses, this study aims to:  Quantify the effect of different plant nutrition strategies on the productivity of major cereals under real-world climate conditions.  Examine how climatic factors such as rainfall and temperature mediate these responses across diverse agro-ecological settings.  Assess the utility of crop simulation models for predicting and optimizing yield outcomes in the face of increasing climatic uncertainty.

The expected outcomes are twofold: to provide robust, evidence-based recommendations for adaptive nutrient management tailored to the Iranian context, and to lay the groundwork for integrating climate-informed decision support systems into cereal crop production. In doing so, this work seeks not only to improve the productivity and resilience of Iranian agriculture but also to offer insights relevant for other semi-arid regions facing similar challenges. Ultimately, the findings are anticipated to support policymakers, extension agents, and farmers in making informed choices that enhance food security while ensuring the sustainability of vital land and water resources

Materials and Methods

Materials and Methods

Climate Data

 Daily temperature, rainfall, and relative humidity were recorded using automated weather stations installed at each site.

Soil Data

 Pre-sowing soil tests were performed to measure initial N, P, K content, pH, and organic matter.

Crop Modeling

The DSSAT (Decision Support System for Agrotechnology Transfer) crop simulation model was calibrated using the field trial data. The model simulated crop yield response under different NPK levels and actual observed weather data for the season.

Statistical Analysis

ANOVA was used to assess the effects of fertilization, location, and their interaction on yield. Regression analysis was performed to relate yield to climatic variables and fertilizer input. Model performance was evaluated using RMSE and R².

Results

Results

Discussion

The results of this study provide strong evidence that the yield response of major cereals-wheat, barley, and maize-to plant nutrition is highly dependent on both the level of nutrient application and the prevailing climatic conditions. Our field experiments and crop simulation modeling across three diverse Iranian provinces clearly demonstrate that while increasing NPK application up to the recommended (Medium) rate significantly boosts yield, further increases (High NPK) do not consistently provide additional benefits and may even reduce yield in waterlimited environments. These findings echo a growing consensus in the agronomy literature that optimal fertilizer management must be context-specific, integrating both agronomic recommendations and local climate realities.

The results in Table 1 show that yield improvements from Low to Medium NPK were most substantial in wheat and maize, especially in the Khuzestan province, where rainfall was relatively abundant and well distributed. This is consistent with studies by Mosavi and Mohammadi (2018) [5] , who reported that wheat yields in semi-arid regions of Iran could be increased by up to 45% with balanced NPK application, but that the response is modulated by seasonal rainfall availability. Similar trends were observed in maize, as also noted by Karimi et al. (2017) [4] , who found that maize yield in the northwestern provinces was particularly sensitive to nutrient application when rainfall exceeded 300 mm during the growing season. Barley, in contrast, exhibited a relatively stable yield response across fertilizer levels and provinces, which aligns with international research suggesting that barley is more tolerant to nutrient and water stress compared to other cereals (FAO, 2020). This suggests that barley can serve as a resilient crop choice in marginal or unpredictable climates, a finding that should inform both policy and local farm management strategies in Iran.

Crop	Province	Low NPK	Medium NPK	High NPK
Wheat	Fars	2.8	4.1	4.0
Wheat	Khuzestan	3.2	4.8	5.2
Wheat	Golestan	3.0	4.3	4.1
Barley	Fars	2.6	3.5	3.4
Barley	Khuzestan	2.9	4.1	4.0
Barley	Golestan	3.1	4.5	4.3
Maize	Fars	5.2	7.8	7.2
Maize	Khuzestan	6.3	8.1	8.9
Maize	Golestan	5.6	7.4	7.3

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Mean Grain Yield (t/ha) of Major Cereals under Different

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The pronounced interaction between fertilizer levels and climate, confirmed by the significant Fertilizer x Climate Fvalue (Table 3), highlights the risk of uniform fertilization recommendations in a variable climate context. High NPK rates led to yield stagnation or decline in Fars province, where rainfall was below 250 mm. Excessive fertilizer in such environments can lead to nutrient leaching, poor nutrient uptake, or even toxicities due to salt buildup-a phenomenon well documented by Jalali and Ranjbar (2009) [2] in arid Iranian soils. Furthermore, the variability in yield response across sites with similar NPK rates but different rainfall patterns underscores the value of adaptive management. This is echoed by findings from the International Maize and Wheat Improvement Center (CIMMYT, 2019) [9] , which emphasized the need for site-specific fertilizer recommendations, especially under the looming challenges of climate change.

Factor	F-value	P-value	Interpretation
Fertilizer Level	17.82	<0.01	Highly significant impact on yield
Province (Climate)	12.43	<0.01	Significant impact due to climatic differences
Fertilizer x Climate	6.09	<0.05	Significant interaction between fertilizer & climate

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Statistical Summary of Fertilizer and Climate Interaction Effects

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Our use of the DSSAT crop simulation model provided robust validation for the field data, with R² values exceeding 0.8 for all three cereals (Table 4). The ability of DSSAT to capture yield variability based on both nutrient input and climatic factors is well documented (Jones et al., 2003) [3] . The model's outputs in our study suggest that coupling seasonal weather forecasts with dynamic fertilizer strategies can increase average yields by up to 14%, a conclusion supported by similar modeling studies in Turkey and Egypt (Yilmaz et al., 2016; Hassan et al., 2018) [7,1] .

Crop	R²	RMSE (t/ha)
Wheat	0.83	0.32
Barley	0.79	0.28
Maize	0.82	0.46

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Model Performance in Predicting Yield

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Several international studies parallel our findings  Rasouli et al. (2020) [6] , working in southwestern Iran, found that excessive N application reduced wheat yield under below-average rainfall, supporting our observation that more fertilizer is not always better in dry years.  Zhang et al. (2019) [8] , in their work on Chinese maize systems, observed that balanced NPK management was most effective under moderate rainfall, but high NPK rates led to environmental losses and yield penalties during drought.  CIMMYT studies in Central Asia found that nutrient efficiency and yield are maximized only when fertilizer timing and amount are matched to both crop needs and actual weather, rather than rigid seasonal prescriptions.

The findings of this study have several implications for Iran's cereal production systems. First, adaptive, climatesmart fertilizer management must become central to extension and advisory services. Over-application of fertilizers not only reduces efficiency and profitability but can also lead to environmental degradation, particularly in regions already challenged by soil salinity and groundwater depletion (Jalali and Ranjbar, 2009) [2] .

Figures and tables from the paper

Crop	Province	Seasonal Rainfall (mm)	Yield Increase (%)
Wheat	Fars	243	46
Wheat	Khuzestan	298	50
Wheat	Golestan	275	43
Barley	Fars	240	35
Barley	Khuzestan	290	41
Barley	Golestan	280	45
Maize	Fars	252	50
Maize	Khuzestan	320	29
Maize	Golestan	278	32

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Effect of Rainfall on Yield Increase (%) From Low to Medium NPK Level

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1. Hassan M, Farouk S, Atta H. Crop modeling for adaptive fertilizer management in Egypt. Agricultural Systems. 2018;165:83-92. https://doi.org/10.1016/j.agsy.2018.06.002
2. Jalali M, Ranjbar F. Nitrogen transformation in calcareous soils as affected by different fertilizer sources. Iranian Journal of Soil Science. 2009;23(2):131-142.
3. Jones JW, Hoogenboom G, Porter CH, Boote KJ, Batchelor WD, Hunt LA, et al. The DSSAT cropping system model. European Journal of Agronomy. 2003;18(3-4):235-265. https://doi.org/10.1016/S11610301(02)00107-7
4. Karimi M, Sadeghi H, Hosseini A. Maize yield response to plant nutrition in northwestern Iran. Iranian Journal of Plant Production. 2017;47(1):75-85.
5. Mosavi S, Mohammadi A. Climate change impacts on Iranian agriculture. Iranian Journal of Plant Production. 2018;44(2):129-140.
6. Rasouli F, Ghasemi M, Alizadeh A. Fertilizer use efficiency and yield of wheat under semi-arid climate of Iran. Field Crops Research. 2020;245:107117. https://doi.org/10.1016/j.fcr.2019.107117
7. Yilmaz H, Kaya C, Tulek A. Modeling yield response to fertilizers in Turkey. Field Crops Research. 2016;191:92-101.
8. Zhang X, Chen S, Sun H. Impacts of fertilizer management on maize yield and environment in China. Agricultural Water Management. 2019;213:659-668. https://doi.org/10.1016/j.agwat.2018.11.033
9. CIMMYT. Precision nutrient management for smallholder farmers: Guidelines and practices for Central Asia. Mexico, D.F.: International Maize and Wheat Improvement Center (CIMMYT); 2019.