Effect of irrigation frequency on migration regularity of greenhouse soil salt during different growth stages of pepper

Abstract: Horticultural production has been widely used in China, and produces high social and economic benefits as a new model of rural industry. However, mismanagement of irrigation and fertilization in greenhouse vegetables, resulting in the nutrients enrichment in surface soil profile through evaporative water loss is an important reason for greenhouse soil salinization, which makes the greenhouse soil unsuitable for sustainable use. Soil salinization negatively impacts agricultural productivity, environmental health, and economic welfare. In order to find a proper irrigation frequency for each growth stage of pepper cultivated in greenhouse, simulated experiments of soil box in a laboratory were carried out to investigate the migration mechanisms of greenhouse soil salt during seedling, squaring, mature stage of pepper with different irrigation frequencies (irrigation every 5, 10, 15 days) as well as the same amounts of irrigation. The results indicated that the different irrigation frequency treatments played an important role in salt leaching. At the seedling stage of pepper, Soil salt distribution in soil profile was uniform at seedling stage of pepper in each irrigation frequency, and there was a significant surface convergence phenomenon occurred before irrigation, the zone with the highest value of soil salt for each treatment was at a depth of 0-10 cm. Salts were moved downward with water, the soil salt content changed in the entire profile with treatment progress, the desalinization rates of 0-20 cm soil layer decreased by 33.97%-66.20%, 35.96%-67.36%, 37.94%-67.94%, respectively under high, middle and low irrigation frequency, meanwhile, the ratio of desalinization increased with the decreased irrigation frequency, the salt accumulation decreased by 47.57%, 49.85% and 52.41% with different irrigation frequencies after irrigation. The salt accumulation of 20-50 cm soil layer with the middle irrigation frequency was greater than that of high and low frequency. At the squaring stage of pepper, the maximum salt contents with the irrigation frequency of middle and low frequency before irrigation were observed in 10-20 cm soil layer, and that with the high irrigation frequency was noticed in 30-40 cm soil layer. Salt content in 30-40 cm soil layer decreased most in response to the preferential flow with the high irrigation frequency after irrigation. The results indicated that soil salt transportation were influenced by preferential flow, it is obvious that there showed a phenomenon of preferential flow at squaring stage of pepper under high frequency. Besides, at the mature stage of pepper, the salt accumulation in soil surface with the high and low irrigation frequency were more than that with the middle irrigation frequency before irrigation. After irrigation, the desalinization rate in 0-20 cm soil layer with the high irrigation frequency remained large and there was a preponderance of soil salt accumulation in 20-50 cm soil layer. The salt content decreased largely in 20-50 cm soil layer with the middle irrigation frequency. Salt accumulation in 20-50 cm soil layer with the high irrigation frequency was 18.86% greater than that with the middle irrigation frequency. Soil salt transportation were influenced by preferential flow, it is obvious that there showed a phenomenon of preferential flow at squaring stage of pepper under high irrigation frequency treatment, and which occurred at mature stage of pepper under middle irrigation frequency treatment. These results indicated that the better irrigation should be with appropriate amounts and middle irrigation frequency before the mature stage of pepper, while that should be with small amounts and high frequent irrigation during the mature stage of pepper. The large amounts and low frequency irrigation can easily cause serious salt leaching in soil profile and even groundwater pollution.