Effects of Different Planting Years on Physicochemical Properties and Enzyme Activities in Soil of Rice-Cherry Tomato Rotation ()
1. Introduction
Cherry tomato (Lycopersicon esulentum Mill) is one of the most important vegetables or fruit due to increasing international demands, great economic value, and massive nutritional benefits [1] . Since 1997, its planting area has been expanding and it has become the pillar industry of Lingshui County, Hainan Province, China. However, the soil quality is gradually deteriorating due to the long-term continuously cultivation of rice-cherry tomato rotation, which has become a bottleneck restricting the sustainable development of cherry tomato industry [2] . Previous studies have shown that the continuous cropping obstacles have significant adverse effects on plant growth and development, which mainly manifest in weaker photosynthesis, retarded growth, reduced resistance, and decreased yield and harvest quality [3] [4] [5] . There are many reasons for continuous cropping obstacles, such as soil nutrient imbalance, accumulation of toxic allelopathic substances, propagation of soil-borne pathogens, destruction of soil microbial community, etc. [3] [6] . It is also believed that the causes of continuous cropping obstacles vary markedly among different crops, and the underlying mechanism depends on soil characteristics [3] . Appropriate crop rotation can alleviate continuous cropping obstacles via regulating nutrition and water balance, improving soil enzyme activity, preventing and controlling diseases, pest insects, weeds etc. [7] [8] [9] . However, previous studies have shown that different rotation years have different effects on soil properties improvement [10] . To date, there is limited information for the effects of long-term rotation between the same crops on soil properties. It is lacking for the effects of long-term continuously cultivation of rice-cherry tomato rotation on soil physicochemical properties, nutrient contents and enzyme activities. To study the periodicity of single crop rotation pattern and determine the reasonable crop rotation years according to local conditions is a problem that the majority of researchers have been concerned about. Therefore, in this study, soils from rice-cherry tomato rotation for one year (1a), three years (3a), five years (5a), seven years (7a) and ten years (10a) were selected to as study objects, the major objective was to analyze the optimal rotation years of rice-cherry tomato from soil perspective, so as to provide theoretical basis for effectively avoiding continuous cropping obstacles of cherry tomato via studying the response characteristics of soil physicochemical properties, nutrient contents and enzyme activities to planting years of long-term single cultivation pattern of rice-cherry tomato rotation.
2. Materials and Methods
2.1. Description of Study Area
The plots of rice-cherry tomato rotation for 1 year (1a), 3 years (3a), 5 years (5a), 7 years (7a) and 10 years (10a) were respectively selected to as the study area in Guangpo Town, Lingshui County, Hainan Province, China. Three plots were selected to as triple repetitions in each crop rotation year. And the cherry tomato variety, cultivation conditions and management methods of each plot was basically consistent. The soil type of 15 plots is all paddy soil developed from laterite, which texture is all sandy loam. The study area has a typical tropical maritime monsoon climate with sufficient light and heat conditions and the annual precipitation is approximately 1718 mm, and the annual average sunshine duration is approximately 2262 hours. The mean annual air temperature is 25.4˚C, with a minimum temperature 20.6˚C in January and a maximum temperature 28.6˚C in June.
2.2. Soil Sample Collection
Soil samples were taken from 0 - 20-cm tillage layer during the flowering fruitbearing stage of cherry tomato in mid-January 2022. To avoid effects from adjacent plots, samples were taken from the core area of each plot from five points using the equidistant sampling method. The samples were well mixed into one composite sample (about 500 g per plot), sealed in sterile bags, and transported in an ice cube-filled box to the laboratory within two days. A total of 15 soil samples were obtained for subsequent analysis. Each soil sample was divided into two parts. One subsample was air-dried, passed through a 2-mm sieve to remove large particles and plant debris, and then analyzed for soil pH. The subsample was then sieved again (<0.15 mm) and used for analyses of other physicochemical properties. The second subsample was stored at 4˚C and then used for detecting soil enzyme activities.
2.3. Analyses of Soil Physicochemical Properties and Enzyme Activities
Soil pH, organic matter (OM), alkali-hydrolyzed nitrogen (AN), available P (AP), and available K (AK), were measured using routine methods described by Lu [11] . Soil pH was measured using a pH electrode (Leici, Shanghai, China) at a soil: water ratio of 1:2.5. The OM content was measured using a potassium dichromate volumetric method. The AN, AP, and AK contents were analyzed using a diffusion method, the Olsen method, and ammonium acetate extraction flame photometry, respectively. The contents of exchangeable calcium (Ca) and exchangeable magnesium (Mg) were analyzed by atomic absorption spectrophotometry. The content of available sulfur (AS) was determined by phosphate-acetic acid-barium sulfate turbidimetric method. The contents of available iron (Fe), available zinc (Zn) and available copper (Cu) were determined by DTPA extraction and atomic absorption spectrophotometry. The available manganese (Mn) was determined by ammonium acetate extraction and atomic absorption spectrophotometry. The available boron (B) was determined by boiling water extraction and curcumin colorimetry. The activities of soil urease, catalase and acid phosphatase were determined by kit (Suzhou Keming Biotechnology Co., LTD.).
2.4. Statistical Analyses
Data from replicates are expressed as the mean ± standard deviation (SD). Statistical analyses of the experimental data including averages, standard deviations and other significance were performed using SPSS v.17.0 software package (IBM Corp., Armonk, NY, United States). Statistical significance was kept at P < 0.05 for all analyses, and the error bars in each figure represented the standard deviation.
3. Results and Discussion
3.1. Effects of Different Rotation Years of Rice-Cherry Tomato on Soil Physicochemical Properties
3.1.1. pH and Organic Matter
The soil pH value was increased year by year during 1a to 5a (Figure 1(a)), reached the highest value of 5.32 at 5a. However, it was sharply reduced at 7a, dropped below 5.0. And soil acidity was sharply enhanced during 7a to 10a (P < 0.05). The inflow of cations was limited after decreasing of soil pH, which leading to a significant reduction in the adsorption capacity of K+, Ca2+, Mg2+ and other nutrient ions, and accelerated leaching of soil nutrients [12] . Soil organic carbon can be used as an ideal indicator to assess soil quality and health. The OM content affects the potential for the sequestration of soil nutrients, especially N, by altering cation exchange capacity and mineralisation-immobilisation turn over. In this study, the soil OM content was obviously decreased by 11.2% to 50.4% with the increase of rotation years (Figure 1(b)), during 3a to 10a (P < 0.05). Notably, the content of soil organic matter was began to reduce below 2 g·kg−1 at 5a of rice-cherry tomato rotation (P < 0.05), which would not be suitable for the growth and development of crops.
3.1.2. Macro Nutrients
The influence of the contents of AN, AP and AK on the plant growth and development is very great. It can be seen from Figure 2(a) that the soil AN contents were significantly increased by 24.4% to 60.4% during 3a to 5a (P < 0.05). And there was no significant difference between during 7a to 10a and at 1a. However, they were all at the enrichment level. The soil AP contents were all very high at different rotation years and significantly increased year by year with the extension of rotation years (P < 0.05), especially increased by 110% to 173% during 3a to 10a (Figure 2(b)). The soil AK contents were also very high at different rotation years, and increased year by year with the increase of rotation years during 3a to 7a, with the increase rate of 36.1% at 7a (Figure 2(c)).
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Figure 1. The pH and OM contents in soils from different rotation years of rice-cherry tomato. Different lowercases indicate significant differences at P < 0.05 level.
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Figure 2. The contents of macro nutrients in soils from different rotation years of rice-cherry tomato. Different lowercases indicate significant differences at P < 0.05 level.
3.1.3. Medium Nutrients
The medium nutrients of Ca, Mg and S, as essential nutrient elements of crops, play an extremely important role in the growth and development of crops, especially the macro elements were applied a lot, exacerbates the imbalance among nutrients. The content of soil available Ca was significantly increased by 27.3% to 111% during 3a to 7a (P < 0.05), but the increase rate was gradually decreased with the increase of rotation years (Figure 3(a)). The content of soil available Ca was lower than the critical value (400 mg∙kg−1) required for crop growth at 10a. The contents of soil available Mg were significantly increased by 287% to 354% during 3a to 10a (P < 0.05). However, which was all lower than the critical value (60 mg∙kg−1) required for crop growth (Figure 3(b)). The content of soil available S was significantly increased at 3a. However, that was significantly decreased during 5a to 10a and were all lower than the critical value (16 mg∙kg−1) required for crop growth and development during 1a to 10a (Figure 3(c)).
3.1.4. Micro Nutrients
The available state of micro nutrients is the part that can be directly absorbed and utilized by plants, which has a direct effect on improving crop yield and quality. Figure 4(a) showed that the contents of soil available Fe were decreased
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Figure 3. The contents of medium nutrients in soils from different rotation years of rice-cherry tomato. Different lowercases indicate significant differences at P < 0.05 level.
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Figure 4. The contents of micro nutrients in soils from different rotation years of rice-cherry tomato. Different lowercases indicate significant differences at P < 0.05 level.
year by year with the extension of rotation years during 1a to 10a, with the decrease rate of 14.9% to 23.9%. The contents of available Mn were significantly increased by 18.2% to 25.7% during 3a to 5a. And there was no significant difference between during 7a to 10a and at 1a, and both were at medium level (Figure 4(b)). Figure 4(c) and Figure 4(d) showed that the contents of soil available Zn and available Cu were decreased year by year with the extension of rotation years during 1a to 10a, with the decrease rate of 5.00% to 40.0% and 17.2% to 53.2%, respectively. Soil available Fe and available Zn were all not deficient for the crop growth and development during 1a to 10a. However, Available Cu was deficient during 3a to 10a. The contents of soil available B were not changed significantly during 1a to 10a of rice-cherry tomato rotation and always remained at the deficiency level.
3.2. Effects of Different Rotation Years of Rice-Cherry Tomato on Soil Enzyme Activities
Soil enzyme is one of the most active parts of soil components. Soil enzyme activities play a key role in nutrients recycling [13] [14] [15] [16] . Soil urease activity reflects the intensity of nitrogen metabolism and conversion in soil. Soil phosphatase can enzymatically decompose various organophosphorus compounds and provide available phosphorus for plant growth [17] . Soil catalase can quickly transform waste generated by soil metabolism into harmless or less toxic substances, and reduce the toxicity of excessive accumulation of hydrogen peroxide to soil microorganisms and plant roots [18] [19] . Previous studies have shown that reasonable crop rotation can increase soil enzyme activity [20] [21] . In this study, the activities of soil urease, acid phosphatase and catalase were all increased year by year during 1a to 5a, reached the highest at 5a, but sharply decreased after 5a (Figures 5(a)-(c)). The activities of soil urease, acid phosphatase and catalase were significantly increased by 72.2% to 91.7%, 11.5% to 73.4%, and 9.67% to 11.6%, respectively, during 3a to 5a (P < 0.05). However, compared with 5a, those were decreased by 56.0% to 63.6%, 27.6% to 34.2% and 26.5% to 37.4%, respectively, during 7a to 10a (P < 0.05). It’s worth noting that the soil catalase activity was significantly enhanced at 5a, which was beneficial to reduce the autotoxic effect of cherry tomato continuous planting. The three kinds of soil enzyme activities were 5a > 3a > 1a > 7a > 10a, 5a > 7a > 10a > 3a > 1a and 5a > 7a > 3a > 1a > 10a, respectively. The above results indicated that declining on activities of urease, acid phosphatase and catalase will cause the continuous cropping obstacles after 5a of rice-cherry tomato rotation. Because the decrease of soil enzyme activity is one of the important factors causing crop continuous cropping obstacles [22] .
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Figure 5. Enzyme activities in soils from different rotation years of rice-cherry tomato. Different lowercases indicate significant differences at P < 0.05 level.
4. Conclusion
Soil acidity was sharply enhanced during 7a to 10a, the pH value began to decline below 5.0 at 7a of rice-cherry tomato rotation. The contents of soil organic matter, available iron and available zinc decreased year by year, while that of available phosphorus increased year by year with the extension of crop rotation years. The content of soil organic matter began to decline below 2.0 g·kg−1 at 5a of rice-cherry tomato rotation. The activities of soil urease, acid phosphatase and catalase were all the highest at 5a. Therefore, it is concluded that long-term single rotation pattern of rice-cherry tomato would aggravate soil acidification, prompt soil nutrient imbalance and reduce soil enzyme activity, and 5a to 7a would be the potential rotation years to avoid or alleviate the continuous cropping obstacle of cherry tomato.
Acknowledgements
This work was supported by the Hainan Province Science and Technology Special Fund of China [Grant No. ZDYF2021XDNY137 and Grant No. ZDYF2022XDNY212]. We thank anonymous reviewers for their very helpful suggestions. We thank LetPub (https://www.letpub.com/) for linguistic assistance and pre-submission expert review.