Soil available phosphorus (AP) deficiency and shortage of phosphate rocks limit cotton production in China. Therefore, pool-culture experiments were conducted in 2019 and 2020 using two cotton cultivars (CCRI-79, low-P tolerant; SCRC-28, low-P sensitive) under three soil AP levels (P₀: 3 ± 0.5, P₁: 6 ± 0.5, and P₂ (control): 15 ± 0.5 mg kg⁻¹) to ascertain the effect of soil AP on boll biomass and lint yield. P₀ and P₁ decreased the P concentration and net photosynthetic rate (Pn) of subtending leaves, thus, reducing boll biomass and lint yield. Additionally, soil AP deficiency decreased boll wall:boll, lint:boll, and lint:seed, and increased seed:boll ratio. Upper fruiting branch positions (FB_9–12) had higher lint:seed ratio and proportion of the total lint yield under low soil AP. Moreover, soil AP deficiency also reduced the sucrose transformation rate (Tr) and activities of sucrose-metabolizing enzymes, such as ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), sucrose phosphate synthase (SPS), and sucrose synthase (SuSy), while increased carbohydrate levels (soluble sugar, sucrose, and starch) and the activity of cytosolic fructose-1,6-bisphosphatase (cy-FBPase) in the subtending leaves. The sucrose and starch contents, cy-FBPase, and SPS activities of SCRC-28 were more sensitive to low soil AP than CCRI-79. Higher Tr and activities of initial Rubisco and SuSy in the subtending leaves improved boll biomass and lint yield.

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.

Long-termrotary tillage has led to a deterioration in cotton production. It remains unclear whether soil inversion with subsoiling could halt the deterioration. Here, a field experiment was set from 2015 to 2017 with three treatments: CK, 15 cm rotary tillage; T1, replace the top 20 cm soil layer with the 20–40 cm soil layer and loosen the 40–55 cm soil layer; T2, replace the top 20 cm soil layer with the 20–40 cm soil layer and loosen the 40–70 cm soil layer. The results showed that the total nitrogen(N) content, alkaline N content, total phosphorous(P) content, available P content, and available potassium(K) content of T1 and T2 in the 20–40 cm soil layer were significantly higher than those in the top 20 cm soil layer in 2017, and the soil nutrient contents in the top 20 cm soil layer increased with each planting season. Compared with CK, the root dry matter of T1 and T2 increased by 13.1% and 15.2%, respectively, and the boll number and boll weight were also significantly higher, and the seed cotton yield under T1 for the three years increased by 7.7%, 7.6%, and 6.1%, respectively, and the seed cotton yield under T2 for the three years increased by 6.1%, 8.6%, and 8.2%, respectively. The results suggest that soil inversion with subsoiling is a continuously effective tillage practice for increasing the output of cotton in the North China Plain.

There is a lack of systematic research on the comprehensive regulatory effects of urea and organic fertilizer application on soil quality and cotton yield in summer direct-seeded cotton fields in the Yangtze River Basin. Additionally, there is a redundancy of indicators in the cotton field soil quality evaluation system and a lack of reports on constructing a minimum dataset to evaluate the soil quality status of cotton fields. We aim to accurately and efficiently evaluate soil quality in cotton fields and screen nitrogen application measures that synergistically improve soil quality, cotton yield, and nitrogen fertilizer utilization efficiency. Taking the summer live broadcast cotton field in Jiangxi Province as the research object, four treatments, including CK without nitrogen application, CF with conventional nitrogen application, N1 with nitrogen reduction, and N2 with nitrogen reduction and organic fertilizer application, were set up for three consecutive years from 2022 to 2024. A total of 15 physical, chemical, and biological indicators of the 0–20 cm plow layer soil were measured in each treatment. A minimum dataset model was constructed to evaluate and verify the soil quality status of different nitrogen application treatments and to explore the physiological mechanisms of nitrogen application on yield performance and stability from the perspectives of cotton source–sink relationship, nitrogen use efficiency, and soil quality. The minimum dataset for soil quality evaluation in cotton fields consisted of five indicators: soil bulk density, moisture content, total nitrogen, organic carbon, and carbon-to-nitrogen ratio, with a simplification rate of 66.67% for the evaluation indicators. The soil quality index calculated based on the minimum dataset (MDS) was significantly positively correlated with the soil quality index of the total dataset (TDS) (R² = 0.904, p < 0.05). The model validation parameters RMSE was 0.0733, nRMSE was 13.8561%, and the d value was 0.9529, all indicating that the model simulation effect had reached a good level or above. The order of soil quality index based on MDS and TDS for CK, CF, N1, and N2 treatments was CK < N1 < CF < N2. The soil quality index of N2 treatment under MDS significantly increased by 16.70% and 26.16% compared to CF and N1 treatments, respectively. Compared with CF treatment, N2 treatment significantly increased nitrogen fertilizer partial productivity by 27.97%, 31.06%, and 21.77%, respectively, over a three-year period while maintaining the same biomass, yield level, yield stability, and yield sustainability. Meanwhile, N1 treatment had the risk of significantly reducing both boll density and seed cotton yield. Compared with N1 treatment, N2 treatment could significantly increase the biomass of reproductive organs during the flower and boll stage by 23.62~24.75% and the boll opening stage by 12.39~15.44%, respectively, laying a material foundation for the improvement in yield and yield stability. Under CF treatment, the cotton field soil showed a high degree of soil physical property barriers, while the N2 treatment reduced soil barriers in indicators such as bulk density, soil organic carbon content, and soil carbon-to-nitrogen ratio by 0.04, 0.04, 0.08, and 0.02, respectively, compared to CF treatment. In summary, the minimum dataset (MDS) retained only 33.3% of the original indicators while maintaining high accuracy, demonstrating the model’s efficiency. After reducing nitrogen by 20%, applying 10% total nitrogen organic fertilizer could substantially improve cotton biomass, cotton yield performance, yield stability, and nitrogen partial productivity while maintaining soil quality levels. This study also assessed yield stability and sustainability, not just productivity alone. The comprehensive nitrogen fertilizer management (reducing N + organic fertilizer) under the experimental conditions has high practical applicability in the intensive agricultural system in southern China.