Critical Nitrogen Dilution Curve for Diagnosing Nitrogen Status of Cotton and Its Implications for Nitrogen Management in Cotton–Rape Rotation System

Abstract

Based on a 2-year in situ nitrogen fertilization experiment, this study aims to establish a critical nitrogen concentration (CNC) dilution curve model for cotton under straw incorporation, analyze the effects of the nitrogen application rate on the cotton yield and nitrogen use efficiency (NUE), and determine the optimal nitrogen application rate by integrating the nitrogen nutrition index (NNI). The experiment setup was a randomized block design with five nitrogen application levels under a straw incorporation: 0, 60, 120, 180, and 240 kg N ha⁻¹ (denoted as N0, N60, N120, N180, and N240, respectively). The cotton dry matter accumulation and nitrogen concentration were measured at the flowering and boll stage, peak boll stage, and boll opening stage. The CNC dilution curve was developed using the data from 2021 and validated with those of 2022. Results showed that the cotton biomass and seed cotton yield at the boll opening stage increased with nitrogen application rates up to 180 kg N ha⁻¹. However, no further increase was found in the yield with an N rate higher than 180 kg N ha⁻¹. The CNC dilution curve was formulated as y = 3.4921x^−0.416 (R² = 0.8741). The validation using 2022 data yielded a root mean square error (RMSE) of 0.21% and a normalized RMSE (nRMSE) of 13.40%, confirming the model’s robustness. The NNI, calculated based on the CNC, indicated that an application rate of 120 kg N ha⁻¹ maintained NNI values close to one across all growth stages, reflecting an optimal nitrogen status. Significant positive correlations were observed between the NNI and both the seed cotton yield and harvest index (p < 0.05). Nitrogen use efficiency parameters, including the agronomic NUE (NUEa), nitrogen partial factor productivity (NPFP), and internal NUE (NUEi), exhibited quadratic declines with the increasing nitrogen input. Within the range of 120–240 kg N ha⁻¹, the highest NPFP was achieved at 120 kg N ha⁻¹. In conclusion, the critical nitrogen dilution curve model combined with the NNI effectively diagnoses the nitrogen status in cotton under straw incorporations. Considering the NNI, yield, and nitrogen utilization efficiency, the recommended nitrogen application rate for cotton in a cotton–rape rotation system with a straw incorporation is 120 kg N ha⁻¹.

1. Introduction

The cotton–rape rotation has emerged as a pivotal dryland cultivation system in the Yangtze River Basin, effectively optimizing land utilization and light–thermal resources while elevating economic returns from cotton and rapeseed production. This sustainable practice currently maintains an annual cultivation area exceeding 7 × 10⁵ ha, demonstrating a significant expansion momentum in recent years [1,2]. In the wheat cotton rotation pattern in East China and the corn cotton rotation pattern in West Africa, where cotton participates, wheat, maize, and cotton straw—an organic-rich byproduct containing substantial nitrogen (N), phosphorus (P), and potassium (K) reserves—play a crucial role in soil fertility enhancements through microbial decomposition, and under the condition of returning straw to the field, applying 150 kg ha⁻¹ of nitrogen could store nitrogen fertilizer in the form of organic nitrogen (40.9%) and microbial biomass nitrogen (11.8%), effectively improving soil fertility [3,4]. Empirical evidence confirms that a strategic straw incorporation not only reduces synthetic N fertilizer requirements but also improves soil nutrient availability and yield potential, thereby advancing dual objectives of nitrogen reduction and carbon sequestration [5,6,7].

As a key determinant of cotton productivity, nitrogen management presents both opportunities and challenges. While an optimized N application drives yield improvements and economic returns through established fertilization protocols [8,9], an excessive reliance on chemical fertilizers creates economic burdens and environmental concerns. China’s current nitrogen use efficiency (NUE) lags behind developed nations despite increasing fertilizer costs, underscoring the urgent need for precision nitrogen management strategies [10,11]. This imperative is particularly pronounced in straw-incorporated cotton–rape rotation systems, where an accurate nitrogen status diagnosis and application optimization constitute critical pathways toward achieving synergistic goals of a high yield, superior quality, and sustainable production [12].

Recent methodological advances employ critical nitrogen concentration (CNC) dilution curves as diagnostic tools for crop nitrogen status quantification. Although successfully implemented across multiple crops through nitrogen nutrition index (NNI) models [13,14], existing CNC frameworks face limitations in cotton systems due to insufficient early-growth data and a low temporal resolution [15,16,17]. Notably, the model reliability improves with biomass accumulation [18], as demonstrated by the superior predictive accuracy of wheat CNC models developed during reproductive stages [19]. Field observations in Yangtze River Basin cotton reveal a significant divergence in dry matter accumulation and nitrogen concentration patterns during post-flowering developmental phases under differential N inputs [20,21], highlighting the necessity for a dedicated CNC model development targeting flowering-to-boll opening stages.

Regional heterogeneity in nitrogen response mechanisms further complicates management strategies. Contrasting patterns emerge between northwest China (yield response decoupled from N uptake) and central China (enhanced uptake with diminishing yield returns) [22,23], emphasizing the need for region-specific solutions. The study on the nitrogen application gradient from 0 to 240 kg ha⁻¹ has been confirmed to be scientifically reasonable [24], so we hypothesize that under a straw incorporation, cotton exhibits a dilution trend in its nitrogen concentration with biomass accumulation, and within the nitrogen application gradient range of 0–240 kg ha⁻¹, an optimal NNI range exists for diagnosing sufficiency. Therefore, we aimed to establish and validate a CNC model and identify nitrogen rates optimizing the yield and NUE.

To address critical knowledge gaps in Jiangxi Province’s cotton–rape rotation systems, we conducted a biennial nitrogen gradient field experiment with three primary objectives: (1) establish and validate a straw-incorporated cotton CNC dilution model; (2) quantify nitrogen rate effects on the dry matter dynamics, yield formation, and NUE; and (3) develop scientific guidelines for precision nitrogen management in sustainable rotation systems. This research provides novel insights into nitrogen diagnostics under straw amendments while advancing practical strategies for ecological intensification in cotton production.

2. Materials and Methods

2.1. Site Description

The experiment was conducted at the Jiangxi Economic Crops Research Institute (30°05′ N, 116°54′ E). The region has a subtropical monsoon climate with an annual average temperature of 16.8 °C and precipitation of 1450 mm. The soil pH value in the 0–20 cm soil layer before broadcasting in 2021 was 7.45, with an organic matter content of 9.96 g kg⁻¹, total nitrogen content of 1.0 g kg⁻¹, available phosphorus content of 32.91 g kg⁻¹, and available potassium content of 196.79 g kg⁻¹. To complement the chemical profile, future studies should include microbial biomass N, mineralization potential, and soil enzyme activities, as these are closely linked to the N release rate from decomposed straw.

2.2. Experimental Design

The study was a random block design with five nitrogen application rates (0, 60, 120, 180, and 240 kg N ha⁻¹ as urea, hereafter referred to as N0, N60, N120, N180, and N240, respectively) with three replications. Each plot was 6.84 m long and 6.5 m wide. The cotton variety was Ganzi Cotton 0906, which was sown in mid-May every year with a density of 97,500 plants ha⁻¹. After harvesting in mid-October, the cotton stalks were not removed and rapeseed was directly sown. After the rapeseed harvest in May of the following year, the cotton and rapeseed straw in each plot were crushed and returned to the field. The amount of cotton and rapeseed straw returned to the field was controlled at 7.5 t ha⁻¹ and 9.0 t ha⁻¹, respectively. The nitrogen uptake of cotton and rapeseed straw was 135 kg ha⁻¹ and 40 kg ha⁻¹, respectively. The dosage of elemental P and K for each treatment was 75 kg ha⁻¹ and 180 kg ha⁻¹, respectively, and they were buried in a ditch at once during the seedling stage. The ratio of nitrogen fertilizer base to topdressing was 3:2. The base fertilizer was buried in the ditch during the seedling stage, and the topdressing was applied during the initial flowering stage. Other field management measures should refer to the local high-yield management level. The same processing in the same plot was set up for two consecutive years, with a buffer cell setup between each processing area, whose area was consistent with the area of the repeated cell setup.

2.3. Measurements

2.3.1. Biomass and Nitrogen Uptake

During the cotton flower and boll stage (the cotton flower and boll stage is a period during which cotton is in a state of flowering and ringing), the peak boll stage (on the fifteenth day after the sampling date during the cotton flower and boll stage), and the boll opening stage, we took one intact and disease-free sample (including roots) from each plot. These samples were deactivated in a 105 °C oven for 30 min, then dried at 80 °C to constant weight before weighing. The crushed samples were analyzed for total nitrogen content using the Kjeldahl method [24]. For boll opening stage samples, vegetative organs and reproductive organs were separately weighed and measured for nitrogen content. Population biomass and nitrogen uptake across treatments were calculated based on sample dry weight and total nitrogen content.

2.3.2. Yield Components

We harvested seed cotton from each plot on 15 October 2021 and 15 October 2022. Fifty cotton bolls were collected from each plot at harvest in both years. After air-drying, they were weighed to calculate single boll weight. Ten consecutive intact cotton plants with uniform growth and no disease were selected to determine the number of mature bolls per plant. All plots underwent actual seed cotton yield measurement through complete harvest, followed by ginning and weighing to calculate lint percentage using formula [25]:

Lint percentage = (Lint yield/Seed cotton yield) × 100%.

2.4. Calculations

2.4.1. Critical Nitrogen Dilution Curve

The methodology for constructing the critical nitrogen dilution curve model in this study was as follows: (1) Comparative analysis of dry matter under different nitrogen levels. Through ANOVA, nitrogen treatments were classified into N-limited and non-N-limited groups. (2) For the N-limited group, a linear regression was established between dry matter and nitrogen concentration. (3) For the non-N-limited group, the critical nitrogen concentration was determined as the intersection point between the average dry matter accumulation and the regression line. The data in the figure were based on the average nitrogen content of five different treatments of plants during three sampling periods and are presented after dividing and calculating the nitrogen restriction group and non-restriction group. Using the average nitrogen content measurements from each period and treatment, we calculated one data point, resulting in a total of 15 points (3 × 5). The critical nitrogen dilution curve model for cotton was established as follows:

N_c = a_c·DW^−b

(1)

where N_c (%) was the critical nitrogen concentration of cotton, a_c (%) was the nitrogen concentration when cotton dry matter reached 1 t·ha⁻¹, DW (t·ha⁻¹) was the dry matter accumulation of cotton, and b was the slope of the critical nitrogen dilution curve [26]. Establish critical nitrogen concentration dilution curves using 15 data points from the flower and boll stage, peak boll stage, and boll opening stage in 2021, and validate the established cotton nitrogen dilution model using data from repeated experiments in 2022. That is to say, the data points from 2022 would be used as measured values and input into the critical nitrogen concentration dilution curve model established above to obtain one-to-one corresponding simulated values. Then, the model would be tested using root mean square error (RMSE) [27] and standardized root mean square error (nRMSE) [28], and a scatter plot based on a 1:1 straight line between the simulated and measured values would be established to demonstrate the model’s fit and reliability. The calculation formulas for R_RMSE and R_nRMSE were as follows:

$$RRMSE=\sqrt{{\displaystyle \frac{{\displaystyle \sum _{\mathrm{i}=1}^N{(s_i-m_i)}^2}}{N}}}$$

(2)

$$RnRMSE={\displaystyle \frac{RRMSE}{\overline{X}}}\times 100\%$$

(3)

where sᵢ was the simulated value, mᵢ was the measured value, N was the number of datapoints, and $\overline{X}$ was the mean of measured data.

RMSE (root mean square error) quantifies the average deviation between simulated and measured values, retaining the original data units. A smaller RMSE indicates higher consistency and lower bias between simulations and measurements. nRMSE (normalized foot mean square error) is a dimensionless metric for comparing model performance across datasets with different units. nRMSE ≤ 10% represents an excellent performance, 10% < nRMSE ≤ 20% represents a good performance, and nRMSE > 30% represents a poor performance.

2.4.2. Nitrogen Nutrition Index

The nitrogen nutrition index (NNI) for each treatment was calculated based on simulated critical nitrogen concentration values and measured nitrogen concentration values:

NNI = N_t/N_c

(4)

where NNI was the nitrogen nutrition index, N_t was the measured nitrogen concentration (%), and N_c was the predicted nitrogen concentration derived from the critical nitrogen dilution curve model (%). Interpretation: the NNI provides a direct indicator of crop nitrogen status; NNI = 1 represents optimal nitrogen nutrition, NNI > 1 represents nitrogen excess, and NNI < 1 represents nitrogen deficiency [29].

2.4.3. Nitrogen Use Efficiency (NUE)

Based on the two-year average cotton biomass, seed cotton yield, lint percentage, nitrogen uptake, and other parameters, the nitrogen fertilizer utilization efficiency is calculated as follows:

Nitrogen partial factor productivity NPFP = Y_n/F_n

Nitrogen internal use efficiency NUEi = Y_n × LP/NU

Nitrogen agronomic efficiency NUEa = (Y_n − Y₀)/F_n

Nitrogen physiological use efficiency NUEp = (Y_n − Y₀)/(NU_n − NU₀)

The nitrogen uptake NU = biomass × nitrogen content.

Harvest index HI = Y_n/biomass × 100%

Nitrogen fertilizer recovery rate AREN = (NU_n − NU₀)/F_n

where Y_n is the yield of seed cotton in the nitrogen application zone, Y₀ is the seed cotton yield of unfertilized plots, F_n is the nitrogen application rate, LP is the lint percentage, NU is the nitrogen absorption rate, NU_n is the nitrogen absorption rate in fertilized plots, and NU₀ is the nitrogen absorption rate in the unfertilized plots [30,31].

2.5. Data Analysis

Excel 2010 was used for data organization and SPSS 19.0 for analysis of variance and correlation.

3. Results

3.1. The Effects of the Nitrogen Application Rate on Cotton Biomass and Nitrogen Uptake

The results in Figure 1 showed that within the range of nitrogen applications from N0 to N180, the biomass of the cotton reproductive organs, vegetative organs, and population showed a continuous increasing trend with the increase in the nitrogen application. Compared with N60, N180 and N240 treatments significantly increased the biomass of vegetative organs and reproductive organs during the boll opening stage in 2021 by 12.52% and 10.55% and 17.01% and 11.30%, respectively. In 2022, the biomass of vegetative organs during the boll opening stage significantly increased by 24.52% and 16.69%, respectively. Compared with N120, N180 and N240 treatments showed a significant increase of 29.90% in reproductive organ biomass during the opening period, except for the N180 treatment in 2021. Two years of N180 and N240 treatments did not show significant differences in the biomass of reproductive and vegetative organs at different stages. This indicated that increasing the nitrogen fertilizer application under straw returning conditions was beneficial for the accumulation of biomass in various parts of cotton plants. However, increasing the nitrogen application rate from 120 to 180 or from 180 to 240 kg ha⁻¹ no longer significantly increased the accumulation of vegetative organ biomass. If the nitrogen application rate exceeded 180 kg ha⁻¹, it was not conducive to the accumulation of dry matter in reproductive organs.

In terms of nitrogen absorption, the results of Figure 2 showed that, compared with the N0 treatment, the N60~N240 treatments significantly increased nitrogen absorption during the flower and boll stage by 38.48%, 40.90%, 89.64%, and 118.89% in 2021 and by 21.44%, 42.40%, 69.52%, and 87.06% in 2022, respectively. The N180 and N240 treatments showed significant improvements in nitrogen absorption during the flower and boll stage compared to the N0~N120 treatments. Compared with the N0 treatment, only the N180 treatment showed a significant increase of 42.93% in the nitrogen uptake by reproductive organs during the boll opening stage in 2021, while the other treatments did not show a significant increase in nitrogen uptake. There was no significant difference in the nitrogen uptake by reproductive organs between the N60 and N240 treatments in the two years.

3.2. The Development and Validation of a Critical Nitrogen Concentration Dilution Curve Model for Cotton

The nitrogen concentration in cotton plants showed a decreasing trend with an increasing dry matter accumulation (Figure 3a). Based on Formula (1), the critical nitrogen concentration dilution curve model for cotton in 2021 was established as y = 3.5774x^−0.42 (R² = 0.864, p < 0.05). The model was validated using 2022 experimental data, yielding a correlation coefficient (R²) of 0.6091, a root mean square error (RMSE) of 0.22%, and a normalized RMSE (nRMSE) of 14.279% (Figure 3b). The model parameters indicated a good to excellent simulation performance, demonstrating that the critical nitrogen concentration dilution curve model developed in this study could be effectively applied to estimate cotton nitrogen concentrations under the cotton–rapeseed rotation with a straw incorporation system.

3.3. The Diagnosis of the Nitrogen Nutrition Status in Cotton Based on the Critical Nitrogen Concentration Dilution Curve

The nitrogen nutrition index (NNI) of cotton during the flower and boll stage and the peak boll stage and the boll opening stage is presented in

Table 1. Effect of nitrogen application rate on nitrogen nutrient index (NNI) of cotton under straw returning to field.

Treatment	2021			2022
	Flower and Boll Stage	Peak Boll Stage	Boll Opening Stage	Flower and Boll Stage	Peak Boll Stage	Boll Opening Stage
N0	0.88 ± 0.06 d	1.00 ± 0.03 b	0.84 ± 0.11 b	0.86 ± 0.02 c	0.80 ± 0.02 d	0.79 ± 0.02 b
N60	1.08 ± 0.02 c	1.09 ± 0.01 ab	1.00 ± 0.09 ab	0.92 ± 0.01 c	0.88 ± 0.02 c	0.85 ± 0.04 b
N120	1.04 ± 0.04 c	1.02 ± 0.01 b	1.09 ± 0.09 b	1.00 ± 0.01 b	0.99 ± 0.02 a	0.98 ± 0.10 ab
N180	1.25 ± 0.02 b	1.12 ± 0.06 ab	1.13 ± 0.02 ab	1.04 ± 0.03 b	0.96 ± 0.02 b	1.10 ± 0.05 a
N240	1.38 ± 0.02 a	1.18 ± 0.04 a	1.10 ± 0.00 a	1.11 ± 0.00 a	1.02 ± 0.02 a	1.15 ± 0.07 a

Note: Data show the means ± standard error. Different lowercase letters after the same column of data indicate a significant difference between treatments at p < 0.05. N0, N60, N120, N180, and N240, representing nitrogen application rates of 0, 60, 120, 180, and 240 kg N ha⁻¹, respectively. The same applies below.

. With increasing nitrogen (N) application rates, the NNI during these stages showed an overall upward trend. In both 2021 and 2022, the NNI values at the flower and boll stage and the peak boll stage and the boll opening stage under the zero-N treatment were consistently below one, indicating a nitrogen deficiency in the cotton plants. For treatments with 60 kg·ha⁻¹ and 120 kg·ha⁻¹ N applications, the NNI ranged between 0.85 and 1.09 and 0.98 and 1.09, respectively, from the flower and boll stage and the peak boll stage to the boll opening stage. Notably, in 2022, the NNI at the boll opening stage did not exceed one under these N rates, suggesting that cotton plants did not consistently achieve an optimal nitrogen status at these application levels. We have considered the reasons, which might be the following: the experiment started in 2021, and the nutrient content of the basic soil was relatively high, with a total nitrogen content of 1.0 g kg⁻¹. Therefore, in 2021, the NNI of the N60 treatment had just reached the nitrogen suitable state, while in 2022, the NNI of the N60 treatment was in a deficit state at all measurement periods. Only the N180 and N240 treatments had an NNI exceeding one during the two years of the boll opening stage, indicating that the cotton plant had reached a stable nitrogen suitable state and there was already a luxury absorption. The simulation results in Figure 4 indicated that the relationship between the seed cotton yield and the NNI at the boll opening stage was well fitted by a univariate quadratic equation, with R² = 0.7592 and R² = 0.7549 for the two years, respectively. This result indicated that under the conditions of this study, the NNI derived based on the critical nitrogen concentration could be used as an evaluation and diagnostic indicator of the cotton nitrogen nutrition status under different nitrogen application rates, and there was a strong correlation between the NNI during the boll opening stage and the seed cotton yield. Regulating the NNI of cotton during the boll opening stage through nitrogen applications could have a certain impact on the seed cotton yield.

3.4. The Effects of the Nitrogen Application Rate on the Cotton Yield and Its Components

The results in

Table 2. Effect of nitrogen application rate under straw returning on cotton yield and yield components.

Year	Treatment	Boll Density (Bolls m−2)	Boll Weight (g)	Lint Percentage (%)	Seed Cotton Yield (kg·ha−1)
2021	N0	48.10 ± 4.23 b	5.64 ± 0.20 a	45.03 ± 0.55 a	2612.46 ± 196.01 c
	N60	63.38 ± 3.13 a	5.91 ± 0.12 a	44.75 ± 0.71 a	3094.31 ± 13.88 b
	N120	64.68 ± 2.28 a	5.99 ± 0.05 a	44.84 ± 0.45 a	3491.10 ± 69.52 a
	N180	59.15 ± 0.33 a	5.86 ± 0.27 a	45.43 ± 0.25 a	3564.99 ± 138.75 a
	N240	60.78 ± 2.83 a	5.70 ± 0.41 a	44.90 ± 0.21 a	3443.33 ± 54.42 a
2022	N0	49.73 ± 3.94 b	4.01 ± 0.16 b	43.87 ± 0.48 a	1442.50 ± 86.82 c
	N60	67.93 ± 3.66 ab	4.56 ± 0.09 a	43.26 ± 0.54 ab	1515.50 ± 64.49 bc
	N120	79.63 ± 9.36 a	4.57 ± 0.06 a	43.11 ± 0.52 ab	1755.50 ± 84.45 ab
	N180	82.88 ± 9.96 a	4.56 ± 0.16 a	43.35 ± 0.35 ab	1877.50 ± 91.69 a
	N240	82.88 ± 10.43 a	4.66 ± 0.09 a	42.45 ± 0.43 b	1821.00 ± 33.08 a

Note: Data show the means ± standard error. Different lowercase letters in the same column of data indicate a significant difference between treatments at p < 0.05.

showed that within the N0 to N120 range, the seed cotton yield and boll density gradually increased with higher nitrogen (N) application rates. Compared to N0, the N120, N180, and N240 treatments in 2021 significantly increased the seed cotton yield by 33.63%, 36.46%, and 31.80% and the bolls density by 34.55%, 23.06%, and 26.44%, respectively. In 2022, these treatments increased the seed cotton yield by 21.70%, 30.16%, and 26.24% and the bolls per plant by 60.13%, 66.67%, and 66.67%, respectively. Among all N treatments, compared to N60, the N180 and N240 treatments significantly increased the yield by 11.28–15.21% and 20.16–23.89% over the two years, respectively. However, no significant differences in the seed cotton yield were observed between the N120, N180, and N240 treatments in either year. Regarding yield components, compared to N60, the N120, N180, and N240 treatments showed no significant improvement in the boll weight or ginning outturn (lint percentage) (p < 0.05). Additionally, there were no significant differences in the boll density, boll weight, or ginning outturn among the N120, N180, and N240 treatments (p < 0.05). These results indicated that an increasing nitrogen application enhances the boll density, thereby boosting the seed cotton yield. However, under the straw incorporation, exceeding the N120 application rate (120 kg ha⁻¹) provided no further yield benefits.

3.5. Effects of Nitrogen Application Rate on Nitrogen Use Efficiency in Cotton

The results in Figure 5 revealed quadratic relationships between nitrogen use efficiency (NUE) metrics and nitrogen application rates. Under the conditions of this experiment, within the range of a nitrogen application rate of 60–240 kg ha⁻¹, the nitrogen agronomic efficiency (NUEa), the nitrogen physiological use efficiency (NUEp), the nitrogen partial factor productivity (NPFP), and the nitrogen internal use efficiency (NUEi) gradually decreased with the increasing nitrogen application rate, while a nitrogen application rate of 60 kg ha⁻¹ maintained a high nitrogen fertilizer utilization efficiency.

The results of Figure 6 indicated that, compared to the 60 kg ha⁻¹ treatment, the NPFP in 2021 and 2022 under the 120, 180, and 240 kg ha⁻¹ applications decreased significantly by 43.59%, 61.60%, and 72.18% and 42.08%, 58.70%, and 69.96%, respectively. Notably, no significant differences in any NUE parameters were observed between the N180 and N240 treatments across both years. This result indicated that under the condition of returning straw to the field, an excessive nitrogen application would reduce the nitrogen fertilizer utilization efficiency in cotton fields. Combined with the yield performance mentioned above, reducing the nitrogen application from 240 kg ha⁻¹ to 120 kg ha⁻¹ could significantly improve the nitrogen fertilizer utilization efficiency while maintaining stable yield levels.

3.6. Interrelationships Among Cotton Nitrogen Nutrition Index (NNI), Nitrogen Use Efficiency (NUE), and Yield

The correlation heatmap in Figure 7 revealed the following relationships: the NNI at the boll opening stage showed a strong positive correlation with the seed cotton yield (correlation coefficient range: 0.96–1.00) and harvest index (HI) (correlation coefficient range: 0.89–0.99). Conversely, the NNI exhibited significant negative correlations with the NUEa (correlation coefficient range: −0.69 to −0.88) and NPFP (correlation coefficient range: −0.94 to −0.99). These results indicated that the nitrogen nutritional status of cotton plants during the boll opening stage significantly influenced the yield and HI. Specifically, the seed cotton yield increases with a higher NNI, while NUEa and NPFP declines as the NNI rises. The main reason for the above correlation might be that increasing the nitrogen application could enhance plants’ nitrogen uptake, thereby increasing NNI values and improving the yield performance. However, as the nitrogen application rate increased, the nitrogen fertilizer utilization efficiency gradually decreased, resulting in different correlations between the yield, nitrogen fertilizer utilization efficiency, and NNI. The correlation coefficients between the NNI, NUEp, and AREN varied between −0.99 and 0.65 and −0.75 and 0.60, indicating that the NUEp, AREN, and nitrogen nutrition index during the boll opening stage did not show a strong stable correlation.

4. Discussion

4.1. A Feasibility Analysis of the Critical Nitrogen Concentration Dilution Curve Model for Cotton

In recent years, the critical nitrogen dilution curve has been applied and validated as a diagnostic indicator of the nitrogen nutritional status across various ecological regions and crops [32]. However, in straw incorporation systems, the decomposition process of the incorporated straw in the cultivated soil layer increases the soil organic matter content, thereby reducing the crop demand for exogenous nitrogen during growth, while simultaneously consuming part of the soil nitrogen [33]. This necessitates a more precise and rapid nitrogen diagnosis in straw-incorporated systems. The relatively lower a and b coefficients suggest a more gradual dilution rate, which may reflect the sustained N mineralization from the incorporated straw and the improved N retention in the rhizosphere, delaying depletion. In this study, by analyzing the cotton aboveground biomass, nitrogen accumulation, and nitrogen concentration under different nitrogen application levels, we constructed a critical nitrogen concentration dilution curve (Nc = 3.5774x^−0.42, R² = 0.864) that reveals the dynamic coupling between the nitrogen uptake and dry matter accumulation in cotton. Compared to parameters in critical nitrogen dilution curves for different cotton varieties established by Yin et al. [34] and Feng et al. [35], both the a and b values in our model were lower. This suggests a flatter slope in the critical nitrogen curve under the cotton–rapeseed rotation with a straw incorporation, likely due to the enhanced soil nitrogen supply capacity and overall fertility from the increased organic matter [36]. These factors delay the nitrogen dilution during late growth stages, improving the plant nitrogen storage capacity and elevating critical nitrogen concentrations—a finding consistent with Lv et al. [37] in wheat–cotton rotation systems. Studies indicate significant differences in parameters a and b between critical nitrogen dilution curves for winter rapeseed and wheat in China and those developed in Europe and North America [38,39,40], highlighting the necessity of considering regional variations in model applicability. The methodology proposed in this study simplifies nitrogen status assessments: by the measuring cotton dry matter and nitrogen content post-flowering and boll-forming stages, values can be directly input into the curve for a rapid diagnosis. Compared to traditional methods, such as soil testing and fertilizer recommendation, petiole nitrate test strips, chlorophyll meters, and UAV-based spectral remote sensing, this approach offers three advantages: reduced environmental interference; applicability across the entire growth period with holistic nitrogen status reflections; and lower costs and technical complexity. Integrating the critical nitrogen dilution curve with the nitrogen use efficiency analysis advances the development of precision nitrogen management technologies for cotton fields under straw incorporations, promoting sustainable agricultural practices.

4.2. Biomass and Nitrogen Uptake

Optimizing field management practices and the nitrogen fertilizer input is a critical pathway to improve nitrogen use efficiency. The NNI model, based on the critical nitrogen concentration dilution curve, quantifies nitrogen luxury uptake or deficiency states, where an NNI around 1.0 indicates an optimal nitrogen status [41]. An insufficient N application induces nitrogen deficiency, suppressing crop growth and yield formation, while an excessive N application hinders improvements in the NUE. Diagnostic results based on the NNI revealed that cotton in rotation systems often experiences nitrogen deficiency (NNI < 0.9) after the flowering and boll-forming stage, which is strongly linked to the competitive nitrate absorption by preceding crops in the rotation system [42]. In this study, the NNI calculated via the critical nitrogen dilution curve showed that N application rates of 180–240 kg·ha⁻¹ during the flower and boll stage to the boll opening stage frequently led to nitrogen excess (NNI: 1.10–1.18 in 2021 and 0.96–1.15 in 2022). In contrast, 120 kg·ha⁻¹ maintained the NNI within the 0.98–1.09 range, achieving an optimal nitrogen status. This aligns with findings by Saudy et al. [43], who reported a suppressed nitrogen agronomic efficiency under high N inputs. The analysis of NNI dynamics and NUE characteristics across treatments revealed that the NUEa and AREN initially increased and then declined with higher N rates, while the NUEp and NPFP consistently decreased. Although the NUE under the optimal nitrogen status (120 kg N ha⁻¹) was lower than under nitrogen deficiency (60 kg N ha⁻¹), no significant reductions in the NUEa, NUEp, or NUEi were observed. Importantly, maximizing NUE alone was insufficient; maintaining a high NUE while achieving target yields holds practical significance [44,45]. The increased risk of nitrogen leaching due to high nitrogen application rates will lead to increased groundwater pollution and eutrophication risks in aquatic ecosystems, posing a threat to farmland soil and biodiversity. Therefore, a nitrogen fertilizer application plan that balances a stable yield and high nitrogen fertilizer utilization efficiency not only has economic value but also ecological significance [46]. The nitrogen fertilizer application experiment on maize in Nepal showed that reducing the nitrogen application rate from the traditional local 210 kg ha⁻¹ to 180 kg ha⁻¹ could maintain the same level of maize yield while significantly improving the NPFP and NUEa [47]. This study demonstrated that 60 kg·ha⁻¹ achieved the highest NUEa, NUEp, NPFP, and NUEi, with no significant differences in the nitrogen uptake by reproductive organs across the N60–N240 treatments. However, compared to N60, N180 and N240 increased the yield by 11.28–15.21% and 20.16–23.89%, respectively, over two years. This indicates that while higher N rates reduce NUE and minimally affect plant nitrogen uptake, they significantly enhance the yield—a key criterion for assessing the optimal nitrogen status, which is consistent with findings by Khatun et al. [48] and Zhang et al. [49]. An excessive N application promotes vigorous vegetative growth but may reduce boll numbers, lower boll weight, and increase boll rot, potentially leading to yield stagnation or decline [50]. Although the NUEa and NPFP decline beyond 60 kg N ha⁻¹, the yield continues to rise until 180 kg N ha⁻¹, likely due to sufficient N enabling a sustained boll retention and sink filling. However, diminishing returns and the onset of luxury uptake (NNI > 1.1) suggest inefficiency and ecological loss beyond 120 kg N ha⁻¹. In this study, increasing N from 120 kg N ha⁻¹ to 180 kg N ha⁻¹ did not significantly improve the yield, and 240 kg N ha⁻¹ failed to further boost the yield despite the superior nitrogen status. By integrating critical nitrogen dilution curve-based diagnostics with yield and NUE optimization, this study provides actionable insights for precision nitrogen management in cotton systems of the Yangtze River Basin, balancing productivity and sustainability.

5. Conclusions

The critical nitrogen dilution curve (Nc = 3.58·DW^−0.42, R² = 0.864, p < 0.05) accurately estimates nitrogen requirements for cotton under straw returning conditions. The NNI-based diagnosis and yield–NUE optimization recommend 120 kg N ha⁻¹ for cotton–rape rotation systems in the Yangtze River Basin.