Studies on the physical-chemical and biological properties of soils cropped continuously with melon under protected cultivation condition

Abstract:【Objective】Melon (Cucumis melo L.) is an important fruit crop in China, with a large cultivation area in the north regions. Protected cultivation of melon has created remarkable economic benefits, especially for the early spring crop. Like vegetable crops, continuous cropping of melon causes soil deterioration which leads to lower yield and quality. This phenomenon is called continuous cropping obstacle, and suitable cultivation techniques must be developed to solve this problem. In order to overcome the obstacle caused by continuous cropping of melon, changes in physical-chemical and biological properties of the soils need to be clarified. This study aims to measure the indices of physical-chemical and biological properties of the soils cropped continuously with melon for different years under protected cultivation condition.【Methods】Bali township, Hua county, Henan province, China has cultivated melons for a long history. This was the site where the field experiment in our study was carried out. The greenhouses, which had been continuously used for plantation of melon for 1, 3, 5, 7, and 10 years, were selected for soil sampling.The soils from the field which had never been used for melon plantation before, were used as control. Dur-ing the experiment, all the selected greenhouses and the control field were planted with 'Cuili' melon un-der consistent management. The melon rhizosphere soils were sampled for the examination of physical-chemical properties, microflora, and enzyme activities. The physical-chemical properties included p H val-ue, bulk density, salt content, organic matter content, and available N, P and K contents. The microbialpopulations comprised of bacteria, actinobacteria, and fungi. The activities of five enzymes, urase, cata-lase, sucrase, phospatase, and protease, were determined. Also, correlation analysis among these indiceswas conducted to identify their correlations.【Results】Continuous melon cropping under protected culti-vation caused marked changes in soil physical- chemical properties, microflora, and enzyme activities.With the increase in cropping years, the soil p H and bulk density declined while the salt content and or-ganic matter content raised continuously. Compared with that in the control soil, the salt content increasedby 46.97% in cropping for 3 years and 90.90% in cropping for 10 years. The available N content in thesoil rose rapidly with the increase of cropping years, while the available P and K contents declined contin-uously. The activities of three enzymes, urase, sucrase, and protease, rose first and then declined with theyears of continuous melon cropping, with the maximum activities occurred in cropping for 3 to 5 years.However, the activities of catalase and phospatase displayed a liner drop. As for the microflora, the num-ber of bacteria and actinomycetes first increased and then decreased. However, fungus number increasedcontinuously to 16.08 times that of the control in 10 years of melon cropping. The change patterns of thethree physiological groups of bacteria resembled that of the total bacteria, while the number changes oftwo pathogenic germs were in line with the total number of fungi. Correlation analysis showed that catalaseactivity was positively correlated with available P and K contents but negatively correlated with organicmatter content. A positive correlation was observed between bacteria number and sucrase activity, and be-tween fungus number and organic matter content, available N content, and protease activity. Negative cor-relations were observed between actinomycete number and phospatase activity and available P content,and between fungus number and available K content.【Conclusion】Under protected cultivation condition,acidification and secondary salinization as well as imbalance of N-P-K proportion occurred in the soil ofcontinuous cropping. With continuous cropping for 3 to 5 years, the activities of some soil enzymes beganto decline and the fungus number increased significantly, and the soil showed obvious continuous crop-ping obstacle. From the results, suitable plantation techniques need to be developed for protected cultiva-tion of melons, particularly fertilizer and water management, so as to improve the enzyme activity and nu-trients in the soil. This can efficiently increase the number of beneficial bacteria and inhibit pathogenspread, and thus overcome the obstacles of continuous cropping. Meanwhile, rational crop rotation withother plants(gramineous crops) is necessary in melon plantation, which should be applied before the development of severe obstacles of continuous cropping(about three years after continuous cropping).: Melon; Cropping soil; Physical-chemical properties; Enzyme activity; Microflora