Effects of moistube depth and spacing on soil water and salt transports of tomato in solar greenhouse

Abstract: Soil secondary salinization has become the most prominent problem in solar greenhouse cultivation. The utilization method of moistube irrigation, a underground irrigation technology with semi-permeable membrane as the core material to supply slow and continuous water flow to crop root zone in soils should be considered to avoid the secondary salinization. In order to explore the effect of moistube irrigation on secondary salinization soil in solar greenhouse, a field experiment was carried out from October 2015 through April 2016 in a 108-m by 8-m solar greenhouse (E108°02′, N34°17′) in Yangling Agricultural Hi-tech Industries Demonstration Zone, Shaanxi Province, China. A total of 3 soil depths (10, 20, 30 cm) and 3 moistube layouts with different spacing (1 tube with 2 lines, 2 tube with 2 lines, 3 tube with 2 lines, the 2 lines refer to the line of tomatoes) were set up to study the changes of soil water and salt distribution in solar greenhouse. Meanwhile, the mulched drip irrigation was used as control in order to analyze the difference of moistube irrigation from the conventional irrigation method. The open field soil nearby the greenhouse was sampled to investigate the soil salinization degree caused by the greenhouse cultivation. The results showed that the soil salinity in solar greenhouse was significantly higher than that in open field, the average salinity of the cultivated layer (0-20 cm) reached 2.745 g/kg after 5 years of tomato cultivation, which was close to the critical point (2.75 g/kg) of crop growth. The mild soil salinization had been occurred in the tested solar greenhouse soil. Under the CK condition, the soil moisture largely fluctuated in the main root zone. The fluctuation was big but stable at the end of the stage. The soil moisture changed in a similar trend for all the moistube irrigation treatments. The soil moisture increased until 45 days of planting and then decreased slowly. The coefficient of variation in the 0-30 cm depth was larger than 10% for the CK treatment, which was obviously higher than the moistube irrigation. Among all the moistube irrigation treatments, the 3 tubes with 2 lines had the more even soil moisture distribution and the coefficient of variation at the depth of 10 cm was smallest (3.62%). Both the buried depth and moistube layout significantly (P<0.05) affected the soil moistube but they didn't show significant interactive effect (P>0.05). Compared with the CK treatment, moistube irrigation had a high degree of desalination. The average relative desalinization rate of the moistube irrigation was 32.49% at 0-60 cm soil layer and 76.30% in the main root zone (0-30 cm) (P<0.05) higher than that of CK. The buried depth of moistube was an important factor affecting the distribution of soil water and salt, a high water and low salinity zone appeared in the soil layer at the moisture buried depth. It was beneficial to the soil salt leaching in the main root zone (0-30 cm) under shallow buried conditions, and in the minor root zone under deep conditions. The soil salinity was increased at the late growth stage of tomato for the treatment of 30 cm buried depth and 1 tubes with 2 lines, which could exacerbate soil secondary salinization. Considering the characteristics of salt accumulation and tomato root distribution in solar greenhouse, we suggested that 10 cm depth and 3 tube with 2 lines were the best for moistube irrigation with the average relative desalinization rate of 0-60 cm soil layer of 22.27% and the relative desalinization rate of 29.86% in the tomato main root zone. This study can provide valuable information for the application of moistube irrigation in solar greenhouse.