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Abstract [Objectives] This study was conducted to investigate the effect of different planting patterns on soil improvement in rocky desertification areas.
[Methods] The one-way ANOVA analysis method was used to statistically analyze the soil physical and chemical properties， enzyme activity， microbial quantity， CEC， ECEC， and aggregate content distribution with different planting patterns.
[Results] The walnut+sesame+mung bean planting pattern showed the highest soil available phosphorus， available potassium， porosity， non-capillary porosity， and contents of free living nitrogen-fixing bacteria， organophosphate-dissolving bacteria， bacteria， fungi and actinomycetes， at 63.2 mg/kg， 178.8 mg/kg， 22.85%， 6.89%， 10.0×106 bacteria/g， 18.0×106 bacteria/g， 21.0×105 CFU/g， 5.7×103 CFU/g and 7.9×105 CFU/g， respectively， and it reduced soil bulk density （the same as treatment F and treatment E） compared with other planting patterns. The walnut+American chicory+sweet potato planting pattern had the highest alkali-hydrolyzale nitrogen， organic matter， CEC， ECEC， water-air ratio and moisture content， which were 227.9 mg/kg， 46.30 g/kg， 36.38 cmol/kg， 24.00 cmol/kg， 8.13， and 32.89%， respectively， and it reduced soil bulk density， increased capillary porosity， acid phosphatase， and contents of bacteria and actinomycetes compared with single cropping of walnut.
[Conclusions] Interplanting crops under walnut forests is an effective measure to improve the ecological environment of rocky desertification farmland.
Key words Stony desertification; Planting pattern; Soil fertility
Received： July 23， 2020 Accepted： September 29， 2020
Supported by Guangxi Innovation-driven Development Project （GK AA17204058-16）; Guangxi Science and Technology Planning Project （GKG 1598016-13）; Basic Scientific and Research Program of Guangxi Academy of Agricultural Sciences （GNK 2021YT041， GNK 2019ZX126）.
Fang QIN （1975-）， female， P. R. China， associate researcher， master， devoted to research about plant nutrition and environmental ecology.
*Corresponding author. E-mail： lirongsu126@126.com.
Rocky desertification is the result of environmental degradation and extreme soil degradation in karst areas under the fragile ecological environment of karst areas， human interference and unreasonable development and utilization[1-2]， mainly manifested as features such as vegetation destruction， soil erosion， bare rock， and land productivity decline and loss[3]. It has influenced the local ecological environment and social and economic development to some extent[4]. Guangxi is located on the edge of the Yunnan-Guizhou Plateau and is one of the provinces with the largest karst area in China， and suffers from very serious rocky desertification. The karst area of the whole region is about 9.87×104 hm2[5]. There are 46 counties with a karst area of more than 20 000 hm2， accounting for 48% of the total number of counties （cities， districts） in the region. The area of karst rocky desertification is 2 379 100 hm2， accounting for 28.56% of the area of karst areas; and the potential karst rocky desertification area is 1 867 100 hm2， accounting for 22.41% of the area of karst areas[6-7]， and karst rocky desertification is still expanding at a rate of 3%-6% per year[5]. Some places in China have explored some better governance patterns and methods， such as the grain-grass-livestock-methane cycle ecological pattern and the forest-grass-livestock three-dimensional ecological cycle pattern formed in the karst peak cluster-depression areas of Pingguo， Guangxi， which have obvious benefits， increase the vegetation coverage rate from less than 10% to more than 50%， and reduce the soil erosion modulus by 30%， forming an ecological industry[8]. The karst mountain eco-industry development patterns formed in the karst rocky mountain area of Nanping Town， Nanchuan City， Chongqing， such as economic or food crops+economic strip hedges+contour plants formed in gentle slope farmland and protective farmland+harmless agricultural production equality pattern formed in rice fields， increases vegetation coverage by 30%[9]. Fu et al.[10] have shown that for the nutrient cycle of multiple cropping farmland ecosystems， the multiple cropping systems are rich in calcium and magnesium nutrients， but insufficient in iron， manganese， nitrogen， phosphorus and potassium nutrients， and the nitrogen nutrient is lost more[10]. The walnut industry in Guangxi has also developed rapidly. Before 2011， it was only 1.0×104 hm2， and it developed to about 2.0×105 hm2 in 2017. There was 1.67×105 hm2 in Hechi City， where the economic benefits had initially appeared. In 2016， the fruit-bearing area reached 4.7×103 hm2， producing 2 000 t， with the economic benefit of 80 million yuan， and the understory interplanting area was more than 1.33×104 hm2， where 1 million chickens were raised. In this study， with single cropping of walnut as a control group， different interplanting treatment groups were set up to analyze and compare the physical and chemical properties and microbial quantity of the soil， so as to explore the planting patterns with a better soil-improving effect. This study provides a theoretical basis for screening the planting patterns capable of improving cultivated land quality and ecological environment in rocky desertification areas. Materials and Methods
General situation of the experimental site
The experimental site was located in Kaqingtun， Ding’an Village， Donglan County， Hechi City， which is located in the subtropical monsoon climate zone with an average annual temperature of around 20.5 ℃. The test field was flat and the soil fertility was basically the same. The physical and chemical properties of soil were as follows： total nitrogen 1.31 g/kg， total phosphorus 2.90 g/kg， total potassium 5.28 g/kg， alkali-hydrolyzale nitrogen 141.3 mg/kg， available phosphorus 20.6 mg/kg， available potassium 90.0 mg/kg， organic matter 30.20 g/kg， pH 6.34， CEC 17.93 cmol/kg， ECEC 15.84 cmol/kg， ECEC/CEC 88.3， bulk density 1.32 g/100 cm3， pore void 21.42%， capillary pore 16.65%， non-capillary pore 4.77%， water-air ratio 3.67， and moisture content 31.75%.
Experimental design and materials
Experimental scheme
The walnut trees were planted in 2016， and the variety was Dapao walnut. The walnut trees were interplanted with different crops， forming six treatments， namely， treatment A： single cropping of walnut， B： walnut+corn+sweet potato， C： walnut+corn+mung bean， treatment D： walnut+herbage+sweet potato， treatment E： walnut+herbage+mung bean， and treatment F： walnut+sesame+mung bean （"+" is interplanting）. The experiment adopted randomized block design. Each treatment was set with three replicates， and each plot had an area of 40 m2.
Experimental materials
The test corn and herbage varieties were， respectively Guitiannuo 525 and American chicory. Mung bean， sesame and sweet potato were all local varieties. The contents of nitrogen， phosphorus and potassium in the tested organic fertilizer were 3.63%， 1.03% and 0.47%， respectively; the organic matter was 71.4 g/kg; and the pH was 5.44. The nitrogen content in urea was 46%. The crop fertilization and planting specifications for different intercropping patterns are shown in Table 1. The crops were sown July 12， and the topdressing time was August 21. During the growing period of the crops， artificial weeding was carried out， and other management was the same as local practice. The harvesting was carried out on October 12.
The test fertilizers are all commercial organic fertilizers and chemical fertilizers， and the urea is urea contains 46% of pure nitrogen （Table 1）.
Sample collection and analysis
Sample collection
The basic soil before crop planting was collected on July 12， 2018， and the soil samples of the 0-20 cm tilled layer was collected on the day of harvest on October 12， 2018. During sampling， weeds and humus on the soil surface were removed， and five samples were collected from each plots according to S shape and then mixed to take 0.5 kg as a soil sample by quartering method. Fresh soil samples were tested for moisture， and the soil was naturally air-dried， ground as required， and stored in ziplock bags for soil nutrient determination. Meanwhile， the five-point method was used to collect soil in each plot， that is， the original soil sample of the 0-20 cm soil layer was collected according to S shape， while trying to avoid squeezing during sampling to maintain the original structure of the soil sample. After sampling， the sample was gently broken into soil blocks with a diameter of about 10 mm along the natural cracks of the soil， and the animal and plant residues and small rocks were picked out. The remaining part was brought back to the laboratory and naturally air-dried for the determination of aggregate indicators[11]. Determination methods
① Determination of soil physical properties： Soil bulk density and porosity were determined by cutting-ring method[12].
② Determination of soil chemical properties： Dry soil samples were analyzed for total nitrogen， total potassium， alkali-hydrolyzale nitrogen， available phosphorus， available potassium， organic matter， pH， CEC and ECEC. PH was determined with a pH meter. The organic matter was determined by the potassium dichromate and concentrated sulfuric acid external heating method. The total nitrogen was determined by the semi-automatic Kjeldahl distillation method. The determination of total phosphorus adopted the acid solution-molybdenum-antimony anti-colorimetric method. The total potassium was determined by the NaOH melting-flame photometry. The alkali-hydrolyzale nitrogen was determined by the alkaline hydrolysis and diffusion method. The determination of available phosphorus adopted the ammonium fluoride and hydrochloric acid extraction-molybdenum-antimony anti-colorimetric method. The rapidly available potassium was determined by the NH4AC extraction-flame photometry. The determination of cation exchange capacity （CEC） adopted the 1 mol/L ammonium acetate exchange method. The effective cation exchange capacity （ECEC） was determined by the additive method （the exchangeable acid adopted the by KCl neutralization titration method， the exchangeable potassium and sodium adopted the flame photometry， and the atomic absorption spectrophotometry was used to determine exchangeable calcium and magnesium）.
Statistical analysis
The Excel-2003 software was used for data processing， and the SPSS19.0 statistical software was used for multiple comparison data analysis.
Fang QIN et al. Improving Soil Fertility with Different Planting Patterns in Rocky Desertification Areas
Results and Analysis
Soil chemical properties of different planting patterns
It can be seen from Table 2 that the soil available nitrogen in different planting patterns directly affected the level of soil nitrogen supply. Treatment D had the highest content of alkali-hydrolyzale nitrogen at 227.9 mg/kg， which was 33.04%， 27.82%， 38.01%， 27.61% and 68.69% higher than treatment A， treatment B， treatment C， treatment E， and treatment F， respectively， and the differences were extremely significant. Soil available phosphorus is an important index for soil to provide phosphorus. Among the different treatments， treatment F showed the highest soil available phosphorus content， being 63.2 mg/kg， followed by treatment D （52.9 mg/kg）. The available potassium content in the soil reflects the timely supply of potassium in the soil. Among the treatments， treatment F had the highest soil available potassium， 178.8 mg/kg， which was 5.18%， 49.0%， 28.54%， 81.52%， and 61.96% higher than treatment A， treatment B， treatment C， treatment D， and treatment E， respectively， and the differences were extremely significant. Therefore， the best performance of both the total nutrient amount and available state content was in the walnut interplanting treatment groups， indicating that reasonable interplanting can enhance soil’s fertility retention capacity. Soil pH affects soil physical properties and nutrient bioavailability. The pH values of treatment B， treatment C， treatment D， treatment E， and treatment F were respectively 24.58%， 12.94%， 19.54%， 22.78% and 14.42% lower than that of treatment A， and the differences were extremely significant. It might be due to the low pH of the applied organic fertilizer. Therefore， in the actual production of walnut intercropped with crops， reasonable fertilization should be combined with the pH of organic fertilizers.
Treatment D had the highest soil organic matter content， which was 46.30 g/kg， followed by treatment A， which was 42.90 g/kg， and treatment B showed the smallest value， which was 33.30 g/kg. Soil cation exchange capacity （CEC） is the total amount of various cations that soil colloids can absorb， which is an important basis for affecting soil buffering capacity and evaluating soil fertility retention capacity. It is of great significance for soil improvement and rational fertilization. The CEC and ECEC of treatment D were the highest， at 36.38 and 24.00 cmol/kg， respectively， followed by treatment C， 27.84 cmol/kg and 21.93 cmol/kg， respectively; the CEC and ECEC values of treatment C， treatment D， and treatment F were higher than treatment A by 26.14% and 35.45%， 64.84% and 48.24%， and 11.83% and 21.37%， respectively; and the CEC/ECEC values were in the range of 41.94-79.61. The results showed that the soil of the walnut+American chicory+sweet potato pattern was rich in available nitrogen that can be directly absorbed and utilized by crops， and had the ability to continuously provide various nutrients and retain fertilizer for crops.
Soil physical properties of different planting patterns
The soil bulk densities of different planting patterns were in the range of 1.20-1.29 g/100 cm3， and the differences between the treatments were not significant. It can be seen from Table 3 that treatment E and treatment F had the largest impact on soil bulk density， and their soil bulk densities were both 1.20 g/100 cm3， followed by treatment D （1.22 g/100 cm3）. The bulk densities of treatment E and F were higher than treatment A， treatment B， treatment C and treatment D by 2.44%， 6.25%， 6.98%， and 1.64%， respectively. The total soil porosity was in the range of 20.40%-22.85%， and the differences between the treatments were not significant. Treatment F had the highest value of 22.85%， which was 4.05%， 6.23%， 4.29%， 12.01% and 9.17% higher than treatment A， treatment B， treatment C， treatment D， and treatment E， respectively. The capillary porosity was in the range of 15.61%-18.05%， and the differences between the treatments were not significant. Treatment B was the largest at 18.05%， which was 15.63%， 1.12%， 1.52%， 6.05% and 4.21% higher than treatment A， treatment C， treatment D， treatment E， and treatment F， respectively. The non-capillary porosity was the largest in treatment F， which was not significantly different from treatment A and treatment C， but was significantly different from treatment B， treatment D， and treatment F. Treatment B had the highest moisture content， which was not significantly different from treatment C， treatment D and treatment F， but had extremely significant differences from treatment A and treatment E. Treatment D had the largest water-air ratio， which was extremely different from other treatments. The above results indicated that interplanting American chicory or sesame under walnut forests could reduce soil bulk density， increase soil capillary porosity， and increase soil water storage capacity. Soil enzyme activity and microbial quantity of different planting patterns
It can be seen from Table 4 that treatment B and treatment C had the highest cellulolytic enzyme， both at 17.5 u/g， which was 7.36%， 16.67%， 9.38%， and 40.0% higher than treatment A， treatment D， treatment E， and treatment F， respectively， and the differences were extremely significant. Treatment A had the highest urease at 2.90 mg/kg， which was not significantly different from other treatments. For acid phosphatase， treatment C was the highest at 1.140 g/h， which was extremely different from other treatments.
The contents of nitrogen-fixing bacteria， organophosphorus-dissolving bacteria， bacteria， fungi， and actinomycetes all had the highest values in treatment F， which were 10.0×106 bacteria/g， 18.0×106 bacteria/g， 21.0×105 CFU/g， 5.7×103 CFU/g， and 7.9 ×105 CFU/g， respectively， and the differences from other treatments were extremely significant. The potassium-dissolving bacteria were the highest in treatment A， at 7.4×106 bacteria/g， and the differences from other treatments were extremely significant. The results showed that intercropping sesame under walnut forest significantly increased the number of microorganisms， and intercropping crops could reduce the content of urease and ammonia volatilization.
Correlation of soil basic physical and chemical properties and nutrient contents
The soil total potassium had a significant negative correlation with soil pH and organic matter content （Table 5）. The soil alkali-hydrolyzale nitrogen had an extremely significant negative correlation with soil available potassium and non-capillary porosity， a significant negative correlation with soil porosity， but was significantly positively correlated with soil organic matter and CEC. The soil available potassium had an extremely significant positive correlation with pH and non-capillary porosity， and a significant negative correlation with non-capillary porosity and moisture. The soil pH had a significant positive correlation with soil organic matter content， a very significant positive correlation with non-capillary porosity， but a significant negative correlation with capillary porosity， and an extremely significant negative correlation with moisture. The soil organic matter had an extremely significant positive correlation with CEC and ECEC. The soil CEC content had an extremely significant positive correlation with ECEC， and a significant positive correlation with moisture. The soil bulk density was significantly negatively correlated with soil porosity. The soil porosity was extremely significantly positively correlated with soil non-capillary porosity. The soil capillary porosity had an extremely significant positive correlation with soil moisture. The soil non-capillary porosity had a significant negative correlation with soil moisture. Discussion and Conclusions
Interplanting different crops under walnut forests can increase plant coverage in rocky desertification areas， which is of great significance for improving the ecological environment of rocky desertification areas[13]. Nitrogen， phosphorus， and potassium are three important elements for plant growth and development， and maintaining the balance of their application and crop absorption is the main indicator of the level of material circulation in the farmland ecosystem[14]. Total nitrogen is an indicator of soil nitrogen stock. Nitrogen is one of the major elements needed for plant growth and development， and it promotes crop production. The results of this study showed that the best performance of the total nutrient amount and available state content was in the walnut interplanting treatment groups， indicating that reasonable interplanting can enhance soil’s fertility retention capacity.
The content of organic matter is an important indicator of soil fertility level. Studies have shown that soil organic matter almost includes various nutrient elements （nitrogen， phosphorus， sulfur， and trace elements） needed by crops. With the gradual mineralization of soil organic matter， it is transformed into simple inorganic matter， which is absorbed and utilized by crops and microorganisms. Soil CEC is an important chemical property of soil， which directly reflects soil’s fertility retention， fertilizer supply performance and buffer capacity. The results showed that the soil of the walnut+American chicory+sweet potato pattern is rich in available nitrogen that can be directly absorbed and utilized by crops， and could continuously provide crops with various nutrients and retain fertilizer.
Compared with other planting patterns， the walnut+sesame+mung bean planting pattern reduced soil bulk density （the same as treatment E）， increased the contents of soil available phosphorus and potassium， and increased soil porosity， non-capillary porosity， and contents of nitrogen-fixing bacteria， organophosphate-dissolving bacteria， bacteria， fungi， and actinomycetes， is thus suitable for the restoration and sustained and stable development of ecosystems in rocky desertification areas， which is consistent with Huang Guoqin’s research results[15]. Sweet potato is a vine-type plant and has a large coverage of the soil. Compared with the single cropping of walnut， the walnut+American chicory+sweet potato planting pattern reduced soil bulk density and improved soil total nitrogen， alkaline nitrogen， organic matter， CEC， ECEC， water-air ratio and moisture content， increased capillary porosity， acid phosphatase， and contents of bacteria and actinomycetes， and enhanced soil biological activity. The deterioration of the ecological environment in rocky desertification areas is related to various factors such as excessive use of arable land in traditional ways， types of fertilization， frequency of fertilization， and climate. The results of this study showed that walnut and crop interplanting can improve soil physical， chemical and biological properties to a certain extent. Among them， such two planting patterns as treatment F （walnut+sesame+mung bean） and treatment D （walnut+American chicory+sweet potato） had outstanding soil improvement effects， and they are worthy of promotion and application in walnut forests in rocky desertification areas. References
[1] REN H. A Review on the studies of desertification process and restoration mechanism of karst rocky ecosystem[J]. Tropical Geography， 2005， 25（3）： 195-200. （in Chinese）
[1] WANG SJ. Concept deduction and its connottation of karst rocky desertification[J]. Carsologica Sinica， 2002， 21（2）： 101-105. （in Chinese）
[2] XIONG KN， LI P， ZHOU ZF. Remote sensing-GIS typical study of karst rocky desertification-a case study of Guizhou province[M]. Beijing： Geology Press， 2002. （in Chinese）
[3] ZHANG DF， WANG SJ， LI RL. Study on the eco-environmental vulnerability in Gui Zhou karst mountains[J]. Geography and Territorial Studies， 2002， 18（1）： 77-79. （in Chinese）
[4] QIN YR， LAN CY， SHU WS， et al. Problems of karst desertification caused destruction of limestone vegetation in Guangxi， China[J]. Tropical Forestry， 2007（35 supplement）： 48-51. （in Chinese）
[5] WANG HW， HE Q， CHEN XL， et al. Studies on agriculture development strategy for karst stony desertification region in Guangxi autonomous region[J]. Chinese Journal of Agricultural Resources and Regional Planning， 2009， 30（5）： 71-75. （in Chinese）
[6] LI S， DONG YX， WANG JH. Rediscussion on the concept and classification of rocky desertification[J]. Chona Karst， 2007， 26（4）： 279-284. （in Chinese）
[7] JIANG ZC， LI XK， ZENG HP， et al. Study of fragile ecosystem reconstruction technology in the karst peak-cluster mountain[J]. Journal of Earth Science， 2009， 30（2）： 155-166. （in Chinese）
[8] XIONG PS， YUAN DX， XIE SY. Progress of research on rocky desertification in south China karst mountain[J]. Carsologica Sinica， 2010， 29（4）： 355-362. （in Chinese）
[10] FU QL， YU JY， WANG ZQ. Nutrient cycling in easily drought farmland ecosystem[J]. Chinese Journal of Applied Ecology， 1993， 4（2）： 146-149.
[11] MA R， YANG S， ZHENG ZC， et al. Composition and stability of soil aggregates in yellow soil slope land under different tillage practices in maize season[J]. Joural of Yangtze River Scientific Research Institute， 2020： 1-9. （in Chinese）
[12] LU RS. Analytical methods for agrochemistry of soils[M]. Beijing： China Agricultural Science and Technology Publishing House， 1999. （in Chinese）
[13] ZHAO RY， LIU ZQ， JIANG JJ， et al. Effect of crop on the Karst environment[J]. Guangdong Agricultural Science， 2013， 40（1）： 158-160. （in Chinese）
[14] XU N， HUANG GQ. Characteris-tics of energy-nutrient flow of multiple cropping rotation systems in paddy field[J]. Chinese Journal of Eco-Agriculture， 2014， 22 （12）： 1491-1497 （ in Chinese）.
[15] HUANG GQ， ZHOU LH， YANG BJ， et al. Improving soil fertility with different multiple cropping patterns in upland red soil[J]. Acta Ecologica Sinica， 2014， 34（18）： 5191-5199. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU
[Methods] The one-way ANOVA analysis method was used to statistically analyze the soil physical and chemical properties， enzyme activity， microbial quantity， CEC， ECEC， and aggregate content distribution with different planting patterns.
[Results] The walnut+sesame+mung bean planting pattern showed the highest soil available phosphorus， available potassium， porosity， non-capillary porosity， and contents of free living nitrogen-fixing bacteria， organophosphate-dissolving bacteria， bacteria， fungi and actinomycetes， at 63.2 mg/kg， 178.8 mg/kg， 22.85%， 6.89%， 10.0×106 bacteria/g， 18.0×106 bacteria/g， 21.0×105 CFU/g， 5.7×103 CFU/g and 7.9×105 CFU/g， respectively， and it reduced soil bulk density （the same as treatment F and treatment E） compared with other planting patterns. The walnut+American chicory+sweet potato planting pattern had the highest alkali-hydrolyzale nitrogen， organic matter， CEC， ECEC， water-air ratio and moisture content， which were 227.9 mg/kg， 46.30 g/kg， 36.38 cmol/kg， 24.00 cmol/kg， 8.13， and 32.89%， respectively， and it reduced soil bulk density， increased capillary porosity， acid phosphatase， and contents of bacteria and actinomycetes compared with single cropping of walnut.
[Conclusions] Interplanting crops under walnut forests is an effective measure to improve the ecological environment of rocky desertification farmland.
Key words Stony desertification; Planting pattern; Soil fertility
Received： July 23， 2020 Accepted： September 29， 2020
Supported by Guangxi Innovation-driven Development Project （GK AA17204058-16）; Guangxi Science and Technology Planning Project （GKG 1598016-13）; Basic Scientific and Research Program of Guangxi Academy of Agricultural Sciences （GNK 2021YT041， GNK 2019ZX126）.
Fang QIN （1975-）， female， P. R. China， associate researcher， master， devoted to research about plant nutrition and environmental ecology.
*Corresponding author. E-mail： lirongsu126@126.com.
Rocky desertification is the result of environmental degradation and extreme soil degradation in karst areas under the fragile ecological environment of karst areas， human interference and unreasonable development and utilization[1-2]， mainly manifested as features such as vegetation destruction， soil erosion， bare rock， and land productivity decline and loss[3]. It has influenced the local ecological environment and social and economic development to some extent[4]. Guangxi is located on the edge of the Yunnan-Guizhou Plateau and is one of the provinces with the largest karst area in China， and suffers from very serious rocky desertification. The karst area of the whole region is about 9.87×104 hm2[5]. There are 46 counties with a karst area of more than 20 000 hm2， accounting for 48% of the total number of counties （cities， districts） in the region. The area of karst rocky desertification is 2 379 100 hm2， accounting for 28.56% of the area of karst areas; and the potential karst rocky desertification area is 1 867 100 hm2， accounting for 22.41% of the area of karst areas[6-7]， and karst rocky desertification is still expanding at a rate of 3%-6% per year[5]. Some places in China have explored some better governance patterns and methods， such as the grain-grass-livestock-methane cycle ecological pattern and the forest-grass-livestock three-dimensional ecological cycle pattern formed in the karst peak cluster-depression areas of Pingguo， Guangxi， which have obvious benefits， increase the vegetation coverage rate from less than 10% to more than 50%， and reduce the soil erosion modulus by 30%， forming an ecological industry[8]. The karst mountain eco-industry development patterns formed in the karst rocky mountain area of Nanping Town， Nanchuan City， Chongqing， such as economic or food crops+economic strip hedges+contour plants formed in gentle slope farmland and protective farmland+harmless agricultural production equality pattern formed in rice fields， increases vegetation coverage by 30%[9]. Fu et al.[10] have shown that for the nutrient cycle of multiple cropping farmland ecosystems， the multiple cropping systems are rich in calcium and magnesium nutrients， but insufficient in iron， manganese， nitrogen， phosphorus and potassium nutrients， and the nitrogen nutrient is lost more[10]. The walnut industry in Guangxi has also developed rapidly. Before 2011， it was only 1.0×104 hm2， and it developed to about 2.0×105 hm2 in 2017. There was 1.67×105 hm2 in Hechi City， where the economic benefits had initially appeared. In 2016， the fruit-bearing area reached 4.7×103 hm2， producing 2 000 t， with the economic benefit of 80 million yuan， and the understory interplanting area was more than 1.33×104 hm2， where 1 million chickens were raised. In this study， with single cropping of walnut as a control group， different interplanting treatment groups were set up to analyze and compare the physical and chemical properties and microbial quantity of the soil， so as to explore the planting patterns with a better soil-improving effect. This study provides a theoretical basis for screening the planting patterns capable of improving cultivated land quality and ecological environment in rocky desertification areas. Materials and Methods
General situation of the experimental site
The experimental site was located in Kaqingtun， Ding’an Village， Donglan County， Hechi City， which is located in the subtropical monsoon climate zone with an average annual temperature of around 20.5 ℃. The test field was flat and the soil fertility was basically the same. The physical and chemical properties of soil were as follows： total nitrogen 1.31 g/kg， total phosphorus 2.90 g/kg， total potassium 5.28 g/kg， alkali-hydrolyzale nitrogen 141.3 mg/kg， available phosphorus 20.6 mg/kg， available potassium 90.0 mg/kg， organic matter 30.20 g/kg， pH 6.34， CEC 17.93 cmol/kg， ECEC 15.84 cmol/kg， ECEC/CEC 88.3， bulk density 1.32 g/100 cm3， pore void 21.42%， capillary pore 16.65%， non-capillary pore 4.77%， water-air ratio 3.67， and moisture content 31.75%.
Experimental design and materials
Experimental scheme
The walnut trees were planted in 2016， and the variety was Dapao walnut. The walnut trees were interplanted with different crops， forming six treatments， namely， treatment A： single cropping of walnut， B： walnut+corn+sweet potato， C： walnut+corn+mung bean， treatment D： walnut+herbage+sweet potato， treatment E： walnut+herbage+mung bean， and treatment F： walnut+sesame+mung bean （"+" is interplanting）. The experiment adopted randomized block design. Each treatment was set with three replicates， and each plot had an area of 40 m2.
Experimental materials
The test corn and herbage varieties were， respectively Guitiannuo 525 and American chicory. Mung bean， sesame and sweet potato were all local varieties. The contents of nitrogen， phosphorus and potassium in the tested organic fertilizer were 3.63%， 1.03% and 0.47%， respectively; the organic matter was 71.4 g/kg; and the pH was 5.44. The nitrogen content in urea was 46%. The crop fertilization and planting specifications for different intercropping patterns are shown in Table 1. The crops were sown July 12， and the topdressing time was August 21. During the growing period of the crops， artificial weeding was carried out， and other management was the same as local practice. The harvesting was carried out on October 12.
The test fertilizers are all commercial organic fertilizers and chemical fertilizers， and the urea is urea contains 46% of pure nitrogen （Table 1）.
Sample collection and analysis
Sample collection
The basic soil before crop planting was collected on July 12， 2018， and the soil samples of the 0-20 cm tilled layer was collected on the day of harvest on October 12， 2018. During sampling， weeds and humus on the soil surface were removed， and five samples were collected from each plots according to S shape and then mixed to take 0.5 kg as a soil sample by quartering method. Fresh soil samples were tested for moisture， and the soil was naturally air-dried， ground as required， and stored in ziplock bags for soil nutrient determination. Meanwhile， the five-point method was used to collect soil in each plot， that is， the original soil sample of the 0-20 cm soil layer was collected according to S shape， while trying to avoid squeezing during sampling to maintain the original structure of the soil sample. After sampling， the sample was gently broken into soil blocks with a diameter of about 10 mm along the natural cracks of the soil， and the animal and plant residues and small rocks were picked out. The remaining part was brought back to the laboratory and naturally air-dried for the determination of aggregate indicators[11]. Determination methods
① Determination of soil physical properties： Soil bulk density and porosity were determined by cutting-ring method[12].
② Determination of soil chemical properties： Dry soil samples were analyzed for total nitrogen， total potassium， alkali-hydrolyzale nitrogen， available phosphorus， available potassium， organic matter， pH， CEC and ECEC. PH was determined with a pH meter. The organic matter was determined by the potassium dichromate and concentrated sulfuric acid external heating method. The total nitrogen was determined by the semi-automatic Kjeldahl distillation method. The determination of total phosphorus adopted the acid solution-molybdenum-antimony anti-colorimetric method. The total potassium was determined by the NaOH melting-flame photometry. The alkali-hydrolyzale nitrogen was determined by the alkaline hydrolysis and diffusion method. The determination of available phosphorus adopted the ammonium fluoride and hydrochloric acid extraction-molybdenum-antimony anti-colorimetric method. The rapidly available potassium was determined by the NH4AC extraction-flame photometry. The determination of cation exchange capacity （CEC） adopted the 1 mol/L ammonium acetate exchange method. The effective cation exchange capacity （ECEC） was determined by the additive method （the exchangeable acid adopted the by KCl neutralization titration method， the exchangeable potassium and sodium adopted the flame photometry， and the atomic absorption spectrophotometry was used to determine exchangeable calcium and magnesium）.
Statistical analysis
The Excel-2003 software was used for data processing， and the SPSS19.0 statistical software was used for multiple comparison data analysis.
Fang QIN et al. Improving Soil Fertility with Different Planting Patterns in Rocky Desertification Areas
Results and Analysis
Soil chemical properties of different planting patterns
It can be seen from Table 2 that the soil available nitrogen in different planting patterns directly affected the level of soil nitrogen supply. Treatment D had the highest content of alkali-hydrolyzale nitrogen at 227.9 mg/kg， which was 33.04%， 27.82%， 38.01%， 27.61% and 68.69% higher than treatment A， treatment B， treatment C， treatment E， and treatment F， respectively， and the differences were extremely significant. Soil available phosphorus is an important index for soil to provide phosphorus. Among the different treatments， treatment F showed the highest soil available phosphorus content， being 63.2 mg/kg， followed by treatment D （52.9 mg/kg）. The available potassium content in the soil reflects the timely supply of potassium in the soil. Among the treatments， treatment F had the highest soil available potassium， 178.8 mg/kg， which was 5.18%， 49.0%， 28.54%， 81.52%， and 61.96% higher than treatment A， treatment B， treatment C， treatment D， and treatment E， respectively， and the differences were extremely significant. Therefore， the best performance of both the total nutrient amount and available state content was in the walnut interplanting treatment groups， indicating that reasonable interplanting can enhance soil’s fertility retention capacity. Soil pH affects soil physical properties and nutrient bioavailability. The pH values of treatment B， treatment C， treatment D， treatment E， and treatment F were respectively 24.58%， 12.94%， 19.54%， 22.78% and 14.42% lower than that of treatment A， and the differences were extremely significant. It might be due to the low pH of the applied organic fertilizer. Therefore， in the actual production of walnut intercropped with crops， reasonable fertilization should be combined with the pH of organic fertilizers.
Treatment D had the highest soil organic matter content， which was 46.30 g/kg， followed by treatment A， which was 42.90 g/kg， and treatment B showed the smallest value， which was 33.30 g/kg. Soil cation exchange capacity （CEC） is the total amount of various cations that soil colloids can absorb， which is an important basis for affecting soil buffering capacity and evaluating soil fertility retention capacity. It is of great significance for soil improvement and rational fertilization. The CEC and ECEC of treatment D were the highest， at 36.38 and 24.00 cmol/kg， respectively， followed by treatment C， 27.84 cmol/kg and 21.93 cmol/kg， respectively; the CEC and ECEC values of treatment C， treatment D， and treatment F were higher than treatment A by 26.14% and 35.45%， 64.84% and 48.24%， and 11.83% and 21.37%， respectively; and the CEC/ECEC values were in the range of 41.94-79.61. The results showed that the soil of the walnut+American chicory+sweet potato pattern was rich in available nitrogen that can be directly absorbed and utilized by crops， and had the ability to continuously provide various nutrients and retain fertilizer for crops.
Soil physical properties of different planting patterns
The soil bulk densities of different planting patterns were in the range of 1.20-1.29 g/100 cm3， and the differences between the treatments were not significant. It can be seen from Table 3 that treatment E and treatment F had the largest impact on soil bulk density， and their soil bulk densities were both 1.20 g/100 cm3， followed by treatment D （1.22 g/100 cm3）. The bulk densities of treatment E and F were higher than treatment A， treatment B， treatment C and treatment D by 2.44%， 6.25%， 6.98%， and 1.64%， respectively. The total soil porosity was in the range of 20.40%-22.85%， and the differences between the treatments were not significant. Treatment F had the highest value of 22.85%， which was 4.05%， 6.23%， 4.29%， 12.01% and 9.17% higher than treatment A， treatment B， treatment C， treatment D， and treatment E， respectively. The capillary porosity was in the range of 15.61%-18.05%， and the differences between the treatments were not significant. Treatment B was the largest at 18.05%， which was 15.63%， 1.12%， 1.52%， 6.05% and 4.21% higher than treatment A， treatment C， treatment D， treatment E， and treatment F， respectively. The non-capillary porosity was the largest in treatment F， which was not significantly different from treatment A and treatment C， but was significantly different from treatment B， treatment D， and treatment F. Treatment B had the highest moisture content， which was not significantly different from treatment C， treatment D and treatment F， but had extremely significant differences from treatment A and treatment E. Treatment D had the largest water-air ratio， which was extremely different from other treatments. The above results indicated that interplanting American chicory or sesame under walnut forests could reduce soil bulk density， increase soil capillary porosity， and increase soil water storage capacity. Soil enzyme activity and microbial quantity of different planting patterns
It can be seen from Table 4 that treatment B and treatment C had the highest cellulolytic enzyme， both at 17.5 u/g， which was 7.36%， 16.67%， 9.38%， and 40.0% higher than treatment A， treatment D， treatment E， and treatment F， respectively， and the differences were extremely significant. Treatment A had the highest urease at 2.90 mg/kg， which was not significantly different from other treatments. For acid phosphatase， treatment C was the highest at 1.140 g/h， which was extremely different from other treatments.
The contents of nitrogen-fixing bacteria， organophosphorus-dissolving bacteria， bacteria， fungi， and actinomycetes all had the highest values in treatment F， which were 10.0×106 bacteria/g， 18.0×106 bacteria/g， 21.0×105 CFU/g， 5.7×103 CFU/g， and 7.9 ×105 CFU/g， respectively， and the differences from other treatments were extremely significant. The potassium-dissolving bacteria were the highest in treatment A， at 7.4×106 bacteria/g， and the differences from other treatments were extremely significant. The results showed that intercropping sesame under walnut forest significantly increased the number of microorganisms， and intercropping crops could reduce the content of urease and ammonia volatilization.
Correlation of soil basic physical and chemical properties and nutrient contents
The soil total potassium had a significant negative correlation with soil pH and organic matter content （Table 5）. The soil alkali-hydrolyzale nitrogen had an extremely significant negative correlation with soil available potassium and non-capillary porosity， a significant negative correlation with soil porosity， but was significantly positively correlated with soil organic matter and CEC. The soil available potassium had an extremely significant positive correlation with pH and non-capillary porosity， and a significant negative correlation with non-capillary porosity and moisture. The soil pH had a significant positive correlation with soil organic matter content， a very significant positive correlation with non-capillary porosity， but a significant negative correlation with capillary porosity， and an extremely significant negative correlation with moisture. The soil organic matter had an extremely significant positive correlation with CEC and ECEC. The soil CEC content had an extremely significant positive correlation with ECEC， and a significant positive correlation with moisture. The soil bulk density was significantly negatively correlated with soil porosity. The soil porosity was extremely significantly positively correlated with soil non-capillary porosity. The soil capillary porosity had an extremely significant positive correlation with soil moisture. The soil non-capillary porosity had a significant negative correlation with soil moisture. Discussion and Conclusions
Interplanting different crops under walnut forests can increase plant coverage in rocky desertification areas， which is of great significance for improving the ecological environment of rocky desertification areas[13]. Nitrogen， phosphorus， and potassium are three important elements for plant growth and development， and maintaining the balance of their application and crop absorption is the main indicator of the level of material circulation in the farmland ecosystem[14]. Total nitrogen is an indicator of soil nitrogen stock. Nitrogen is one of the major elements needed for plant growth and development， and it promotes crop production. The results of this study showed that the best performance of the total nutrient amount and available state content was in the walnut interplanting treatment groups， indicating that reasonable interplanting can enhance soil’s fertility retention capacity.
The content of organic matter is an important indicator of soil fertility level. Studies have shown that soil organic matter almost includes various nutrient elements （nitrogen， phosphorus， sulfur， and trace elements） needed by crops. With the gradual mineralization of soil organic matter， it is transformed into simple inorganic matter， which is absorbed and utilized by crops and microorganisms. Soil CEC is an important chemical property of soil， which directly reflects soil’s fertility retention， fertilizer supply performance and buffer capacity. The results showed that the soil of the walnut+American chicory+sweet potato pattern is rich in available nitrogen that can be directly absorbed and utilized by crops， and could continuously provide crops with various nutrients and retain fertilizer.
Compared with other planting patterns， the walnut+sesame+mung bean planting pattern reduced soil bulk density （the same as treatment E）， increased the contents of soil available phosphorus and potassium， and increased soil porosity， non-capillary porosity， and contents of nitrogen-fixing bacteria， organophosphate-dissolving bacteria， bacteria， fungi， and actinomycetes， is thus suitable for the restoration and sustained and stable development of ecosystems in rocky desertification areas， which is consistent with Huang Guoqin’s research results[15]. Sweet potato is a vine-type plant and has a large coverage of the soil. Compared with the single cropping of walnut， the walnut+American chicory+sweet potato planting pattern reduced soil bulk density and improved soil total nitrogen， alkaline nitrogen， organic matter， CEC， ECEC， water-air ratio and moisture content， increased capillary porosity， acid phosphatase， and contents of bacteria and actinomycetes， and enhanced soil biological activity. The deterioration of the ecological environment in rocky desertification areas is related to various factors such as excessive use of arable land in traditional ways， types of fertilization， frequency of fertilization， and climate. The results of this study showed that walnut and crop interplanting can improve soil physical， chemical and biological properties to a certain extent. Among them， such two planting patterns as treatment F （walnut+sesame+mung bean） and treatment D （walnut+American chicory+sweet potato） had outstanding soil improvement effects， and they are worthy of promotion and application in walnut forests in rocky desertification areas. References
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