Effects of Soil Temperature on Cotton Growth Under Phosphate Fertilizer Drip Application Conditions

WANG Yiqi¹,², MAI Wenxuan², ZHANG Wentai¹, WANG Yanyan², TIAN Changyan²
¹College of Resources and Environment, Xinjiang Agricultural University, Urumqi, Xinjiang 830052, China
²Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China

Abstract

This study investigates cotton growth responses to soil temperature under phosphate fertilizer drip application conditions, aiming to explore the role and mechanism of soil temperature regulation of cotton root growth in improving phosphate fertilizer utilization efficiency. A pot experiment was conducted with three soil temperature gradients: low temperature (11–18 °C), medium temperature (22–26 °C), and high temperature (30–34 °C) as a single-factor design with water bath temperature control. The effects of different soil temperatures on cotton growth traits, biomass, root distribution, soil available phosphorus distribution, and phosphorus utilization efficiency were analyzed. Results showed that with increasing soil temperature, cotton plant height, stem diameter, leaf number, and biomass all exhibited parabolic trends, peaking under medium temperature conditions (22–26 °C). Root length in the 0–5 cm soil layer increased with soil temperature, with the high temperature treatment showing 5.2%–126.9% and 4.9%–62.3% greater root length compared to low and medium temperature treatments, respectively. Below the 5 cm soil layer, root length decreased with increasing temperature, with the medium temperature treatment showing the longest root length—81.68%–98.43% longer than low and high temperature treatments, respectively. Soil available phosphorus content decreased with higher temperatures, with medium and high temperature treatments showing 13.7% and 20.5% lower available phosphorus than the low temperature treatment. Phosphorus absorption and utilization efficiency were highest under medium temperature, followed by low temperature, and lowest under high temperature. Specifically, phosphorus absorption under medium temperature was 49.69% and 89.36% higher than under low and high temperatures, respectively, while phosphorus utilization efficiency was twice that of the high temperature treatment and 50% higher than the low temperature treatment. Considering the comprehensive effects of soil temperature on cotton growth, root length, soil available phosphorus distribution, phosphorus absorption, and utilization efficiency, the optimal soil temperature range for cotton growth and improved phosphorus utilization efficiency is 22–26 °C.

Keywords: soil temperature; cotton; drip phosphate fertilizer; phosphate fertilizer utilization

Introduction

Xinjiang is China's largest cotton production base, with total cotton output exceeding 5.1×10⁶ t, accounting for over 90% of national production. Currently, more than 90% of Xinjiang's cotton fields use subsurface drip irrigation with integrated water and fertilizer management. This technology significantly improves water use efficiency (from 0.4 to 0.7 kg·m⁻³) and nitrogen efficiency by supplying water and nutrients according to cotton demand. However, compared with traditional basal phosphorus application, drip phosphorus application only improves phosphorus fertilizer utilization efficiency by 2%–5%, failing to address the persistently low phosphorus use efficiency. This suggests that even in integrated production systems where water and fertilizer supply can be easily matched to crop demand, improving seasonal phosphorus use efficiency remains a complex and challenging scientific and technical problem.

Phosphorus fertilizer has very limited vertical mobility in soil after drip application. A series of indoor simulation experiments using ³²P isotope tracer technology have found that phosphorus fertilizer generally moves less than 13 cm vertically in soil. More than 90% of phosphorus accumulates within the 0–2 cm surface soil layer. On the other hand, the plastic film mulching used in Xinjiang's drip irrigation technology increases soil temperature, particularly after full canopy closure. Field observations show that under mulching conditions from seedling to squaring stage (before early July), the 0–5 cm soil layer temperature can reach 13–34 °C in the afternoon, with daily temperature variations of 13–34 °C. The optimal soil temperature for plant root growth is approximately 25 °C, with growth inhibition occurring above this threshold. Thus, the 0–5 cm soil layer temperature often exceeds the critical value for root growth, coinciding with the critical period for cotton root system establishment (rapid root length increase). The spatial mismatch between cotton root distribution and phosphorus fertilizer distribution under drip application conditions is therefore the fundamental reason for low phosphorus utilization efficiency, with soil temperature variation playing a crucial role. Based on this, this study uses cotton under drip phosphorus application as the research object, employing pot experiments with different soil temperatures to investigate the effects of soil temperature on cotton growth and phosphorus fertilizer utilization efficiency, and to determine the optimal soil temperature range for cotton growth and improved phosphorus utilization efficiency in Xinjiang.

Materials and Methods

1.1 Materials

This study employed pot experiments using gray desert soil with basic physicochemical properties: pH 7.76, organic matter 11.5 g·kg⁻¹, available nitrogen 83.7 mg·kg⁻¹, available phosphorus 378 mg·kg⁻¹, and available potassium 115 mg·kg⁻¹. The soil was air-dried and passed through a 2 mm sieve. The cotton variety used was "Jiumian 18". Temperature monitoring was conducted using right-angle soil thermometers. Experimental pots were soft plastic seedling pots without bottom holes. The drip irrigation system used 250 mL medical infusion bottles with controllable flow rates.

1.2 Experimental Design

The experiment was conducted in June 2023 at the Halophyte Botanical Garden Network Room in Karamay, Xinjiang (45°26′26″N, 85°00′39″E). The experiment featured a single factor—soil temperature—with three gradients: low temperature (LT: 11–18 °C), medium temperature (MT: 22–26 °C), and high temperature (HT: 30–34 °C). Each treatment had 9 replicates. Soil temperature was regulated using a water bath with heating and ice addition, monitored with soil thermometers. Each pot contained 1.2 kg of air-dried soil. Nitrogen fertilizer (urea, CH₄N₂O) was applied as basal fertilizer at 0.6 g per pot and mixed thoroughly with the air-dried soil. Initial watering was 600 mL. Cotton was direct-seeded at the pot center, and when plants reached 3 true leaves, each pot was thinned to one plant. The pots were then moved to water baths set to the target temperatures to begin formal treatment. The drip irrigation system was installed with the emitter at the pot center. Phosphorus fertilizer (NH₄H₂PO₄) was applied through drip irrigation at 0.6 g per pot. Irrigation water volume was controlled gravimetrically, with 50–100 mL of water added every 2 days to maintain soil moisture at 60–70% of field capacity. The experiment lasted 45 days.

1.3 Sample Collection and Analysis

Soil samples were collected using the stratified cutting method at 5 cm intervals (0–5, 5–10, 10–15, and 15–20 cm). Cotton roots in each layer were extracted with tweezers, washed with distilled water, and scanned using a Phantom 9980XL scanner to measure root length. Soil samples from each layer were collected, air-dried, ground, and passed through a 0.25 mm sieve. Soil available phosphorus content was determined using the NaHCO₃-Mo-Sb anti-colorimetric method. After measuring plant height, stem diameter, and leaf count, aboveground cotton biomass was divided into stem and leaf portions. Fresh weight was measured before oven-drying at 105 °C for 30 minutes, then at 75 °C to constant weight. Dry samples were ground and digested with H₂SO₄-H₂O₂ for 30 minutes, and phosphorus content was determined using the vanadium molybdate yellow colorimetric method.

1.4 Data Processing

Data were analyzed using one-way ANOVA and least significant difference (LSD) multiple comparison methods with SPSS 29.0 (IBM Corp., Armonk, NY, USA). Spatial distribution maps of soil phosphorus were created using Surfer 21 software, and figures were prepared using Microsoft Excel.

Results

2.1 Effects of Soil Temperature on Cotton Growth Traits

Medium temperature treatment resulted in the best cotton growth performance. Compared with high and low temperature treatments, plant height was 17.06% and 73.85% higher, respectively; stem diameter was 28.62% and 29.86% higher; and leaf number was 3.61% and 72.22% higher under medium temperature conditions. All growth traits showed a parabolic trend with increasing soil temperature, peaking at medium temperature.

2.2 Effects of Soil Temperature on Cotton Biomass and Allocation

Medium temperature treatment produced the maximum cotton biomass, followed by low temperature, with high temperature showing the minimum. Root biomass allocation increased continuously with surface temperature. Stem biomass allocation showed minimal differences among treatments, with high temperature treatment being the lowest. Leaf biomass allocation was highest under low and medium temperature treatments, with no significant difference between them, while high temperature treatment showed the lowest allocation.

2.3 Effects of Soil Temperature on Cotton Root Length Distribution

Analysis of the vertical distribution of cotton root length under different soil temperatures revealed distinct patterns. In the 0–5 cm layer, root length increased with temperature, with high temperature treatment showing the maximum root length of 530.5 cm—5.2%–126.9% and 4.9%–62.3% greater than low and medium temperature treatments, respectively. Below the 5 cm layer, root length decreased with increasing temperature, with medium temperature treatment showing the longest root length—81.68%–98.43% longer than low and high temperature treatments, respectively. The spatial distribution of cotton roots showed that high temperature treatment had more roots in the shallow soil layer, while medium and low temperature treatments showed increasing root length with soil depth. Specifically, in the 0–5 cm layer, root length increased with temperature, while below this layer, the medium temperature treatment showed the most pronounced increase in root length with depth.

2.4 Effects of Soil Temperature on Soil Available Phosphorus Distribution

Overall, soil available phosphorus content decreased with soil depth. All treatments showed higher available phosphorus content in the surface layer near the drip emitter. The available phosphorus content exhibited a trend of decreasing with higher soil temperatures. In the 0–5 cm layer, medium and high temperature treatments had 13.7% and 20.5% lower available phosphorus content, respectively, compared to the low temperature treatment.

2.5 Correlation Between Soil Available Phosphorus Content and Cotton Root Length Under Different Temperatures

Correlation analysis between soil available phosphorus content and cotton root length under different temperatures revealed a significant negative correlation in all three temperature treatments. The negative correlation was stronger under low and medium temperature treatments and weaker under high temperature treatment. This indicates that higher soil available phosphorus content is not conducive to cotton root length increase.

2.6 Effects of Soil Temperature on Cotton Phosphorus Use Efficiency

Analysis of phosphorus absorption and utilization efficiency under different soil temperatures showed that both parameters were highest under medium temperature, followed by low temperature, and lowest under high temperature. Phosphorus absorption under medium temperature was 49.69% and 89.36% higher than under low and high temperatures, respectively. Phosphorus utilization efficiency differed significantly among treatments, with medium temperature treatment showing the highest efficiency—twice that of high temperature treatment and 50% higher than low temperature treatment. All plant organs showed similar patterns.

Discussion

3.1 Effects of Soil Temperature on Cotton Growth

Plants obtain water, nutrients, air, and heat required for growth and development from soil. Cotton plant height, stem diameter, leaf number, and biomass are important indicators reflecting cotton growth and development. Biomass allocation represents a plant's resource distribution strategy, resulting from photosynthesis and respiration allocation among different organs. This allocation is regulated by both environmental changes and plant growth characteristics. Previous studies have found that cotton plant height, stem diameter, leaf number, and biomass all increase to varying degrees with soil temperature improvement. Low soil temperature leads to thin stems, short plants, reduced leaf number and biomass, and decreased yield. Within a certain range, increasing soil temperature can promote cotton growth and development. However, excessively high temperatures or prolonged high temperature exposure affect cotton seed survival and germination rates, seedling height, stem diameter, leaf number, root length, flowering and boll setting during the flowering and boll stage, lint and seed cotton yield, and fiber quality, potentially causing yield reduction or even plant death. High temperatures disrupt physiological activities such as photosynthesis in most crops and can directly affect yield. This study found that all cotton growth traits showed a trend of increasing then decreasing with soil temperature, with medium temperature treatment producing the best growth performance, consistent with findings from related scholars.

3.2 Effects of Soil Temperature on Cotton Root Distribution

Temperature is a crucial factor affecting root growth. Roots require an appropriate temperature range to maintain normal growth rates and function, with their optimal temperature often lower than that of the shoot. As the medium for plant-soil "communication," root quantity and distribution in soil directly determine crop productivity. Different light and temperature environments affect cotton root diameter, surface area, biomass, and root length density distribution in soil. Excessively high soil temperature reduces carbohydrate translocation from shoots to roots, limiting root development and decreasing the root-to-shoot ratio, while also altering root architecture. Root architecture determines the soil volume accessible to roots and is a primary factor controlling plant nutrient absorption efficiency. Studies on wheat, sweet potato, sorghum, and maize have shown that under high temperature stress, primary roots become shorter, lateral root growth and number decrease, root growth angles reduce, and the number of larger-diameter secondary and tertiary roots increases. This is explained by high temperature reducing root cell division rates. Soil temperature changes significantly affect root growth and development, with high surface soil temperature limiting crop root establishment and survival. This study found that in the 0–5 cm layer, high temperature treatment produced the maximum root length of 530.5 cm, 5.2%–126.9% and 4.9%–62.3% greater than low and medium temperature treatments, respectively. Below the 5 cm layer, medium temperature treatment showed the longest root length, 81.68%–98.43% longer than low and high temperature treatments. High temperature roots were more distributed in the surface soil layer, likely because high temperature reduces carbohydrate translocation from shoots to roots, limiting root development and decreasing root-to-shoot ratio, resulting in reduced belowground dry weight. These results indicate that cotton roots demonstrate stronger tolerance to high soil temperature compared to other crops. Additionally, moderately increasing low temperature benefits cotton root establishment in the surface soil layer, which is advantageous for improving drip phosphorus fertilizer utilization efficiency. However, the soil temperature gradient range in this study could be expanded, and further research is needed.

3.3 Effects of Soil Temperature on Soil Available Phosphorus Distribution and Cotton Phosphorus Fertilizer Utilization

Soil available phosphorus content is an important indicator of soil phosphorus supply capacity. Due to poor phosphorus mobility, drip application through drip tapes leads to high phosphorus accumulation in surface soil. Roots are the plant organs in direct contact with soil, and phosphorus absorption is closely related to root spatial distribution. Studies have shown that greater root surface area (lateral roots, root hairs) in surface soil is an ideal root characteristic for efficient phosphorus absorption. A series of experiments have demonstrated that relatively shallow root systems facilitate efficient phosphorus acquisition. This study found that in the 0–5 cm soil layer, root length increased with temperature while available phosphorus content decreased, showing obvious depletion. Therefore, increased spatial matching between roots and phosphorus contributes to phosphorus absorption, corresponding with previous research findings. Studies have shown that both phosphorus absorption amount and phosphorus fertilizer utilization efficiency can serve as indicators for phosphorus fertilizer yield and efficiency improvement. Appropriate soil temperature enhances crop respiration and phosphorus absorption capacity while promoting various enzymatic reactions in crops, increasing respiration, and thereby improving root phosphorus absorption. This study found that medium temperature treatment produced the highest cotton phosphorus absorption and utilization efficiency, consistent with previous research conclusions.

Due to pot size limitations, cotton plant growth, root size, and spatial distribution differed from field conditions, but the growth responses to soil temperature changes were adequately reflected. Overall, maintaining soil temperature at 22–26 °C is most beneficial for cotton phosphorus absorption and utilization efficiency. In practical production, soil temperature can be increased through double-layer mulching, organic fertilizer application, and windbreak establishment, or decreased through irrigation water temperature control and straw mulching measures to regulate cotton root proliferation in specific zones and improve drip phosphorus fertilizer utilization efficiency. Therefore, these results provide valuable reference for achieving efficient drip phosphorus fertilizer utilization through soil temperature regulation. Future research should conduct full growth period verification experiments under field conditions and develop operable soil temperature control technologies based on Xinjiang's specific cotton production patterns.

Conclusion

With increasing soil temperature, cotton plant height, stem diameter, leaf number, and biomass all showed initial increases followed by decreases. Medium temperature treatment (22–26 °C) was most favorable for cotton growth and development, producing the highest phosphorus absorption and utilization efficiency (0.12 kg·kg⁻¹). The underlying mechanism involves appropriate soil temperature promoting root proliferation in the 0–5 cm soil layer, with corresponding depletion of available phosphorus in this layer providing supporting evidence. In summary, maintaining soil temperature at 22–26 °C benefits both cotton growth and drip phosphorus fertilizer utilization efficiency.

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