Long-Term Increases in Continuous Cotton Yield and Soil Fertility following the Application of Cotton Straw and Organic Manure

Long-term continuous cotton cropping results in a significant decrease in soil quality and soil organic carbon, threatening cotton yield. The application of organic amendments is considered an effective management practice for the sustainability of soil productivity and often increases yield. However, the sustainable improvement in the cotton yield, stability, and soil fertility over time resulting from organic amendments with cotton straw and organic manure still need to be confirmed with research, especially under a continuous cotton cropping system. This study evaluated the effect of 12 years of consecutive application of cotton straw and organic manure on continuous cotton yield, soil quality, and soil organic carbon. Four treatments, i.e., chemical N and P fertilizers (NP, control), NP plus cotton straw (NPS), NP plus manure (NPM), and NP plus cotton straw and manure (NPSM), were carried out. The results indicated that the addition of cotton straw and organic manure improved the temporal stability and sustainability of cotton yield. The combination of cotton straw and organic manure resulted in the greatest improvement, increasing the average annual cotton yield by 32.28% compared with the control (NP). A correlation analysis revealed that cotton yield was closely related to soil quality and soil organic carbon. The application of cotton straw and organic manure increased cotton yield by enhancing soil fertility, especially the quantity and quality of soil organic carbon, which improved the supply and cycling of soil nutrients and benefited the stability and sustainability of the cotton yield. Reusing cotton straw and organic manure could improve the sustainable productivity of cotton soil and provide additional environmental value as well as having great potential for cleaner and sustainable cotton production.


Introduction
China is a major producer and consumer of cotton. In 2022, the total production and demand for cotton in China were 5.98 million tons and 7.60 million tons, respectively, with an annual shortfall of approximately 1.62 million tons [1]. It is relatively difficult to increase the sown area of cotton under the conditions of shrinking arable land area and limited cotton cultivation benefits. Long-term continuous cotton cropping results in a significant decrease in soil quality and organic carbon exhaustion, affecting both yield and the environment. Protecting and improving the quality of cotton fields are key to ensuring sustainable production and yield increases in cotton [2,3].

Experimental Site
The field experiments were carried out from 2007 to 2018 at the Xiaxian Experimental Station at the Institute of Cotton Research, Shanxi Agricultural University (35 • 11 N, 111 • 05 E). The annual total sunshine duration is approximately 2293.4 h. The annual rainfall is approximately 530 mm, and 75% occurs in the summer and autumn. The site has yellow soil, and the soil properties are presented in Table 1.

Experimental Design
Four treatments were continuously carried out annually in their respective plots: (1) NP (chemical N and P fertilizers; control); (2) NPS (NP plus cotton straw return); (3) NPM (NP plus manure); and (4) NPSM (NP plus cotton straw return and manure). The area of each treatment was 360 m 2 , from which an area of 15 m 2 (7 m long × 2.5 m wide) with uniform cotton growth was selected as the experimental plot. Each of the four treatments was repeated three times to minimize the influence of spatial heterogeneity. The same rate of nitrogen and phosphorus fertilizers was applied in each treatment, i.e., Agronomy 2023, 13, 2133 3 of 11 172.5 kg N ha −1 and 138 kg P 2 O 5 ha −1 , respectively. The cotton straw utilized in this study originated from a uniform field. The annual application amount was 5212 kg ha −1 , and the content of N in the straw on a dry mass basis was 1.69%. The manure was fermented chicken manure with an application amount of 22.5 m 3 ha −1 , which had AN of 1.86%, P 2 O 5 of 3.67%, K 2 O of 1.74%, and pH of 8.4. In total, 60% of the nitrogen fertilizer was applied as basal dressing using rotary tillage before sowing, and the remaining 40% was top-dressed during the flowering stage. The phosphorus fertilizer and fermented chicken manure were applied in one application before sowing. In the straw return treatment, all the cotton straw was mechanically crushed and returned to the soil after the cotton harvest every year. Deep ploughing (to 25 cm) was carried out in each treatment in early November, and rotary tillage was performed in late March of the next year. Then, the cotton seed was sown in plastic film in mid-to late-April. A transgenic insect-resistant cotton cultivar (Keneng 0518) was planted using a wide-narrow row plantation method at a density of approximately 67,500 plants ha −1 . The same field management was carried out for each treatment according to the conventional high-yield technical requirements.

Soil Samples
Soil samples from different depths were collected using a five-point random sampling method in each plot for measurement and analysis in the later stage of cotton boll opening (2 October 2018). Soil bulk density (BD) and aggregates were measured using the cutting ring method and dry sieving method, respectively [17]. The soil pH value was determined using a pH meter (Delta320, Mettler, China). Soil chemical properties were analysed using the methods of Bao [18]. Soil organic carbon (SOC) was obtained using the potassium dichromate oxidation method. Soil carbon storage (SCS) was calculated using Equation (1) [11]. Dissolved organic carbon (DOC) was extracted with deionized water and then analysed using a TOC analyser (Vario TOC, Elementar, Germany) [19]. Microbial biomass carbon (MBC) was obtained according to the classical chloroform fumigationextraction method [20].
where SCS indicates the soil carbon storage (Mg ha −1 ), SOC indicates the soil organic carbon concentration (g kg −1 ), and BD and T are the mean soil dry bulk density at a depth of 20 cm (g cm −3 ) and soil depth (cm), respectively.

Cotton Yield
The cotton yield in each of the twelve years was measured separately. The yield of cotton was measured after manually harvesting each plot and converting the value into the yield per hectare. The coefficient of variation (CV) and sustainable yield index (SYI) were used as measures of temporal stability and sustainability, respectively [21,22]. The CV approached zero as stability increased, and the SYI increased with sustainability; these values were calculated as follows: where σ indicates the temporal standard deviation of the cotton yield (t ha −1 ), Y indicates the temporal mean of the yield (t ha −1 ), and Y max indicates the highest yield achieved during the study period (t ha −1 ).

Data Analysis
SPSS 19.0 software was used for statistical analysis. A comparison of different treatments was performed using a one-way analysis of variance (ANOVA) and Duncan's multiple range test (DMRT) at a significance level of 5%. The cotton yield and various parameters of soil fertility were analysed using Pearson correlations, and graphs were prepared using Origin Pro 2022.

Cotton Yield
The effects of organic amendment addition on cotton yield are shown in Figure 1. The treatment had a marked impact on cotton yield (p < 0.05). The average cotton yields under the NPS, NPM, and NPSM treatments were 3.12, 3.22, and 3.55 t ha −1 , which increased by 10.04%, 12.53%, and 21.28%, respectively, compared with the control treatment (NP). Cotton straw combined with organic manure had the greatest effect on cotton yield, followed by the addition of cotton straw and organic manure separately (Figure 1a,b and Table 2). Moreover, our long-term experiments showed that the addition of organic amendments considerably improved cotton yield over time, which increased by an average of 1.26% (NPS), 1.23% (NPM), and 1.51% (NPSM) per year, but the yield in the control treatment (NP) increased by only 0.92% per year. With the addition of organic amendments, the average annual increase in the cotton yield was 2.57%, 15.07%, and 32.28% higher, respectively, compared with the control (NP). This growth was derived from the yield trend slopes during the period from 2007 to 2018 ( Figure 1c). The CV and SYI of cotton yield for different treatments over the twelve years were as follows: NP > NPS > NPM > NPSM and NP < NPS < NPM < NPSM. The application of organic amendments had greater yield stability and sustainability, especially the cotton straw plus organic manure treatment ( Table 2).

Data Analysis
SPSS 19.0 software was used for statistical analysis. A comparison of different treatments was performed using a one-way analysis of variance (ANOVA) and Duncan's multiple range test (DMRT) at a significance level of 5%. The cotton yield and various parameters of soil fertility were analysed using Pearson correlations, and graphs were prepared using Origin Pro 2022.

Cotton Yield
The effects of organic amendment addition on cotton yield are shown in Figure 1. The treatment had a marked impact on cotton yield (p < 0.05). The average cotton yields under the NPS, NPM, and NPSM treatments were 3.12, 3.22, and 3.55 t ha −1 , which increased by 10.04%, 12.53%, and 21.28%, respectively, compared with the control treatment (NP). Cotton straw combined with organic manure had the greatest effect on cotton yield, followed by the addition of cotton straw and organic manure separately (Figure 1a,b and Table 2). Moreover, our long-term experiments showed that the addition of organic amendments considerably improved cotton yield over time, which increased by an average of 1.26% (NPS), 1.23% (NPM), and 1.51% (NPSM) per year, but the yield in the control treatment (NP) increased by only 0.92% per year. With the addition of organic amendments, the average annual increase in the cotton yield was 2.57%, 15.07%, and 32.28% higher, respectively, compared with the control (NP). This growth was derived from the yield trend slopes during the period from 2007 to 2018 ( Figure 1c). The CV and SYI of cotton yield for different treatments over the twelve years were as follows: NP > NPS > NPM > NPSM and NP < NPS < NPM < NPSM. The application of organic amendments had greater yield stability and sustainability, especially the cotton straw plus organic manure treatment ( Table 2).  Comparison of cotton yield (a) and the average cotton yield (b) and its temporal trends (c) under various treatments. In the box plots, the solid lines and squares inside the boxes represent the respective median and mean values of all data. The upper and lower edges of the boxes signify the third (Q3) and first quartiles (Q1), while the upper and lower lines extending from the box denote the 1.5 times interquartile range (IQR) above Q3 and below Q1, respectively. The bars are the standard deviation (S.D.). Various letters and asterisks represent marked differences between treatments (** p < 0.05). The shaded region is the 95% confidence band for the relationship. Mean ± S.D. (n = 3) is used to indicate the average yield and different letters within a given column signify significant differences (p < 0.05).
The distribution of soil aggregates at the 0-10 cm depth was influenced by various treatments (Figure 2b). The utilization of cotton straw and organic manure markedly decreased the percentage of <0.25 mm aggregates compared to the control (p < 0.05); the values were 41.00% (NPS), 32.60% (NPM), and 45.04% (NPSM) lower, respectively, than that of NP. The addition of cotton straw induced a greater decline in the fraction of <0.25 mm aggregates compared with organic manure addition. The percentage of 0.5-1 mm aggregates increased with the NPS, NPM, and NPSM treatments. There was no significant disparity observed in the distribution of soil aggregates in the 10-30 cm layer (Figure 2c,d). The distribution of soil aggregates at the 0-10 cm depth was influenced by various treatments (Figure 2b). The utilization of cotton straw and organic manure markedly decreased the percentage of <0.25 mm aggregates compared to the control (p < 0.05); the values were 41.00% (NPS), 32.60% (NPM), and 45.04% (NPSM) lower, respectively, than that of NP. The addition of cotton straw induced a greater decline in the fraction of <0.25 mm aggregates compared with organic manure addition. The percentage of 0.5-1 mm aggregates increased with the NPS, NPM, and NPSM treatments. There was no significant disparity observed in the distribution of soil aggregates in the 10-30 cm layer (Figure 2c,d).

Soil Chemical Properties
The effects of organic amendment addition on soil chemical properties are presented in Table 3. The application of cotton straw and organic manure induced markedly higher contents of available nitrogen in the 0-20 cm and 20-40 cm soil layers compared with the control (NP). The highest AN content in the 0-20 cm layer was measured in the NPSM treatment, which increased by 207.90% compared with NP, followed by NPM and NPS, which increased by 57.36% and 30.07%, respectively, compared with NP. A similar trend in AN was shown at 20-40 cm, which was 99.47%, 93.87%, and 39.68% higher, respectively, than that in NP. The contents of available phosphorus and potassium at the two soil depths in the NPM and NPSM treatments were higher than those in the NP treatment, which increased by 181.43% and 357.82%, respectively, at 0-20 cm and by 156.70% and 212.37%, respectively, at 20-40 cm. Although the AP and AK contents in the NPS treatment increased at the two soil depths, the difference was not marked.
The content of soil organic matter (SOM) at the 0-20 cm and 20-40 cm soil depths (p < 0.05) increased markedly with the addition of cotton straw and organic manure compared with the control. The respective values were 14.16% (NPS), 20.18% (NPM), and 42.47% (NPSM) and 37.14% (NPS), 67.03% (NPM), and 85.68% (NPSM) higher than those in NP. Compared with the NP treatment, the soil pH showed a decreasing trend at the 0-20 cm soil depth, which decreased from 8.84 to 8.38. The pH value in the NPSM treatment was the lowest among the treatments. The NPS and NPM treatments also decreased the pH value, but this effect was not significant. The addition of organic manure did not markedly affect pH at the 20-40 cm soil depth.

Soil Organic Carbon
The influence of the various treatments on soil organic carbon (SOC) is presented in Figure 3a. The SOC content was in the range of 6.42-9.15 g kg −1 at the 0-20 cm depth and 3.52-6.54 g kg −1 at the 20-40 cm depth, which was in the following order for all treatments at both soil depths: NPSM > NPM > NPS > NP. The addition of both cotton straw and organic manure significantly affected SOC (p < 0.05) compared with NP. Cotton straw plus organic manure resulted in the greatest improvement at both soil depths. The addition of cotton straw and organic manure alone had a similar effect on SOC at the 0-20 cm depth, but the application of organic manure alone had a more significant effect on SOC than cotton straw at the 20-40 cm depth. A similar trend in the SCS was observed at the 0-20 cm depth. The incorporation of cotton straw and organic manure increased the SCS from 18.68 to 19.93 (NPS), 20.75 (NPM), and 23.60 Mg ha −1 (NPSM). A significant increase was found in the SCS under the cotton straw plus organic manure treatment, followed by the addition of only organic manure and only cotton straw. (NPSM) at the 20-40 cm depth. The cotton straw plus organic manure treatment (NPSM resulted in the highest increase. The MBC content had a similar trend as that observed fo DOC in the 0-20 cm layer, which was improved by 84.22% (NPS), 79.14% (NPM), an 166.77% (NPSM) at the 0-20 cm depth. However, there was no significant variation i MBC at the 20-40 cm soil depth among the treatments.

Association between Cotton Yield and Soil Properties
The relationships between cotton yield and the soil quality parameters are shown i Figure 4. Cotton yield exhibited a positive correlation with AN, AP, AK, SOM, SOC, DOC and MBC and a negative relationship with BD and pH at the 0-20 cm soil depth. Th strongest correlation was observed between cotton yield and SOM, SOC, MBC, and pH The SOC had a significant direct correlation with SOM and MBC and an inverse correla tion with pH.

Association between Cotton Yield and Soil Properties
The relationships between cotton yield and the soil quality parameters are shown in Figure 4. Cotton yield exhibited a positive correlation with AN, AP, AK, SOM, SOC, DOC, and MBC and a negative relationship with BD and pH at the 0-20 cm soil depth. The strongest correlation was observed between cotton yield and SOM, SOC, MBC, and pH. The SOC had a significant direct correlation with SOM and MBC and an inverse correlation with pH.

Influence of Organic Amendments on Soil Quality
Good soil structure is essential for soil fertility and plant growth. Bulk density and soil aggregates are important parameters of soil physical structure [23,24]. Twelve years of continuous incorporation of cotton straw and organic manure significantly decreased the soil bulk density and increased the percentage of larger aggregates (Figure 2). This not only resulted from the lower density and larger particle size of cotton straw and organic manure but was also due to the bonding effect of the organic amendments, which increased the cohesion between organic matter and soil clay minerals, thus promoting the development of larger aggregates [13,25]. This resulted in an improvement in the soil porosity and aggregate stability and a reduction in the soil bulk density. A reduction in bulk density and an increase in larger aggregates can increase soil fertility by enhancing water infiltration, nutrient cycling, and the flow of heat and air [26,27]. These effects are beneficial for plant growth and development, such as the germination of seeds, penetration of roots, and absorption of water and nutrients for plants; therefore, lower bulk density and larger soil aggregates produced by the addition of cotton straw and organic manure could potentially serve as a driving mechanism for a long-term increase in cotton yield.
Available nitrogen, phosphorus, and potassium are the most important nutrients for plant growth. In our study, significant levels of available nutrients were measured following twelve years of organic amendment application. When cotton straw and organic manure were applied in combination, there was a marked increase in the availability of N, P, and K in the topsoil compared with the control (NP). The available N, P, and K contents in the cotton straw-only treatment and organic manure-only treatment were also higher than those in the control (NP) ( Table 3). The variation in nutrient contents among treatments was attributed to the quality of the organic amendments. The increase in the available nutrients might be due to the active N, P, and K in the added organic amendments and the lower soil nutrient fixation caused by the changes in the soil environment under organic amendments [11,28]. In addition, a significant correlation was observed between the concentrations of available nutrients and several soil properties, including MBC, SOC, and soil pH (Figure 4), which revealed that soil microorganisms and low pH played important roles in nutrient transformation. The soil organic matter level in our study was significantly improved with the addition of organic amendments compared with the control (Table 3), as reported in other studies. This could be caused by the higher organic carbon content in the organic amendments [7,29]. The application of organic amendments was a crucial factor in the formation of soil aggregates, and aggregation in turn played a crucial role in reducing soil organic matter decomposition (Figure 2). The continuous application of organic amendments can regulate soil pH, which decreased in the topsoil in our research ( Table 3). The influences of organic amendments on soil pH are contingent

Influence of Organic Amendments on Soil Quality
Good soil structure is essential for soil fertility and plant growth. Bulk density and soil aggregates are important parameters of soil physical structure [23,24]. Twelve years of continuous incorporation of cotton straw and organic manure significantly decreased the soil bulk density and increased the percentage of larger aggregates (Figure 2). This not only resulted from the lower density and larger particle size of cotton straw and organic manure but was also due to the bonding effect of the organic amendments, which increased the cohesion between organic matter and soil clay minerals, thus promoting the development of larger aggregates [13,25]. This resulted in an improvement in the soil porosity and aggregate stability and a reduction in the soil bulk density. A reduction in bulk density and an increase in larger aggregates can increase soil fertility by enhancing water infiltration, nutrient cycling, and the flow of heat and air [26,27]. These effects are beneficial for plant growth and development, such as the germination of seeds, penetration of roots, and absorption of water and nutrients for plants; therefore, lower bulk density and larger soil aggregates produced by the addition of cotton straw and organic manure could potentially serve as a driving mechanism for a long-term increase in cotton yield.
Available nitrogen, phosphorus, and potassium are the most important nutrients for plant growth. In our study, significant levels of available nutrients were measured following twelve years of organic amendment application. When cotton straw and organic manure were applied in combination, there was a marked increase in the availability of N, P, and K in the topsoil compared with the control (NP). The available N, P, and K contents in the cotton straw-only treatment and organic manure-only treatment were also higher than those in the control (NP) ( Table 3). The variation in nutrient contents among treatments was attributed to the quality of the organic amendments. The increase in the available nutrients might be due to the active N, P, and K in the added organic amendments and the lower soil nutrient fixation caused by the changes in the soil environment under organic amendments [11,28]. In addition, a significant correlation was observed between the concentrations of available nutrients and several soil properties, including MBC, SOC, and soil pH (Figure 4), which revealed that soil microorganisms and low pH played important roles in nutrient transformation. The soil organic matter level in our study was significantly improved with the addition of organic amendments compared with the control (Table 3), as reported in other studies. This could be caused by the higher organic carbon content in the organic amendments [7,29]. The application of organic amendments was a crucial factor in the formation of soil aggregates, and aggregation in turn played a crucial role in reducing soil organic matter decomposition (Figure 2). The continuous application of organic amendments can regulate soil pH, which decreased in the topsoil in our research ( Table 3). The influences of organic amendments on soil pH are contingent upon the original soil pH levels and the characteristics of the organic substances used. Abujabhah et al. found that the addition of organic amendments resulted in a similar decreasing trend in soil pH [30,31]. Long-term organic amendment incorporation increased soil fertility, which contributed to providing stable nutrients for the plants and enhancing the continuous yield of cotton.

Influence of Organic Amendments on Soil Organic Carbon
Soil organic carbon is commonly used as an evaluation index for soil productivity due to its benefits on soil physicochemical and biological properties. Extensive research has shown that the incorporation of organic amendments can enhance soil organic carbon [32,33]. Our research showed a similar increasing trend in the concentration and storage of SOC with the addition of cotton straw and organic manure. The combination of cotton straw and organic manure resulted in the greatest increase, followed by the addition of organic manure and cotton straw alone (Figure 3a,b). This was associated with the quantity and quality of the organic materials, which increased the concentration and storage of SOC with both the direct carbon input and the indirect contribution resulting from increased plant residues [33]. The SOC content increased from 6.42 g kg −1 to 9.15 g kg −1 in our study, but this is still lower than that in the topsoil of the Loess Plateau (14.25 g kg −1 ). The SOC should be further improved to benefit the soil agroecosystem environment. Therefore, the sustainable accumulation of SOC will be the focus of future research.
DOC and MBC represent the dynamic fraction of soil organic carbon characterized by enhanced bioavailability and an accelerated turnover rate. Dissolved organic carbon is commonly considered a sensitive indicator for soil quality change, which can reveal the active nutrients in soil [34]. MBC is also a crucial index that not only reflects small changes in total soil carbon but also directly participates in the process of soil biochemical transformation. MBC is a reservoir of plant-effective nutrients in the soil and promotes the effectiveness of soil nutrients [35]. The long-term addition of cotton straw and organic manure in our study elevated the contents of DOC and MBC (Figure 3c,d), which demonstrated that organic amendments could enhance the pool of soil organic carbon quality, facilitate soil nutrient turnover, and improve biological productivity. In general, the improvement in the quality and storage of SOC could provide guarantees for the increase in cotton yield and the sustainable production of cotton farmland.

Influence of Organic Amendments on Cotton Yield
The yield of a crop is intensely affected by soil fertility, which can be regulated with management practices. Intensive fertilization is a key management practice to increase crop production [28,36]. In our twelve-year study, the average and continuous cotton yields were markedly increased with the addition of organic amendments compared with the control, especially under the cotton straw plus organic manure treatment (Figure 1). The temporal stability and sustainability of cotton yield were also improved with the application of organic amendments ( Table 2). These results were due to the enhanced soil fertility caused by the addition of organic amendments. Cotton yield was closely related to soil fertility (Figure 4). The lower bulk density and larger soil aggregates produced with the addition of organic amendments optimized the soil structure, which created a favourable root environment and promoted the growth and yield of cotton [37]. The increase in soil nutrients, including available nitrogen, phosphorus, potassium, and soil organic matter, resulted in more nutrients for plant growth, thus increasing the biomass accumulation of cotton and directly increasing the cotton yield [38]. Moreover, the application of organic amendments improved the content and quality of the SOC pool, which is a decisive indicator of cotton yield, enhancing the supply and cycling of soil nutrients and promoting a stable and sustainable increase in cotton yield [39].

Conclusions
Twelve years of adding cotton straw and organic manure increased soil fertility and cotton yield by improving the soil structure, soil nutrients, and soil organic carbon pool.
The addition of organic amendments exerted a beneficial impact on the storage and quality of soil organic carbon along with increased cotton yield in a continuous cotton cropping system, which confirmed their effectiveness in enhancing the sustainable production of cotton farmland. In addition, the reuse of cotton straw and organic manure provides additional environmental value and has a great potential application for the cleaner and sustainable production of cotton. Currently, the practice of returning cotton straw to a field after cultivation is widely used by farmers; however, the application of organic manure is not performed annually. In order to effectively preserve and enhance the sustainable productivity of cotton farmlands, we recommend the annual application of organic manure during the cotton straw return process.