Effect of cotton stalk returning to fields on residual film distribution in cotton fields under mulched drip irrigation in typical oasis area in Xinjiang

Abstract: The drip irrigation under mulching film technology was applied in Shihezi reclamation area of Xinjiang in 1996. It is more difficult to recover the mulching after the growing period of cotton, so that the density of residual plastic film in cotton fields exceeds the national standard (75.0 kg/hm2) which causes serious soil contamination in experiment area. Stalk returning to fields is a common method in the film mulching area. This study investigated the effect of cotton stalk returning to fields on residual film distribution. A total of 3 cotton fields were selected in typical oasis area of Xinjiang. The 3 fields were different only in the mulching duration. They were governed by the same owner. The mulched drip irrigation started from the year of 1998, 2002 and 2006. By the year of 2017, the mulching duration was 20, 16 and 12 years, respectively. Soil samples were taken from 0-40 cm layer before cotton stalk returning in 2016 and after 1 year of stalk returning in 2017. The cotton stalk was cut into 4-6 cm pieces and applied to the tillage layer with an application rate of 7 500-8 000 kg/hm2. By comparison, the effect of stalk returning was discussed. In the sampling field, the soil blocks (4 800 cm2 in area and 40 cm in height) were collected for determination of area and weight of residual film and weight of cotton stalk residue. The results showed that: 1) the residual plastic film density in the soil tillage layer of drip irrigation cotton fields increased yearly with increasing mulching years, and the cotton fields of mulched film drip irrigation more than 10 year were seriously polluted. 2) After the cotton stalks were crushed and returned to the fields, the cotton stalks in the soil of 0 to 40 cm in the 3 sampling plots were increased from top to bottom and then decreased and concentrated in the soil depth of 15-25 cm. The cotton stalks were evenly distributed on the surface after being chopped, and because the large-scale machinery was used to carry out the arable land operation, the cotton stalks on the soil surface were ploughed into the soil tillage layer of 15-25 cm depth, which was the main reason for the peaks of cotton stalks residue appeared in the soil depths of 18, 20 and 18 cm, respectively. 3) After the cotton stalks were returned to the fields, the residual plastic film area of the 0-15 cm soil layer of the 3 sampled cotton fields accounted for more than 58%, and the residual film weight accounted for more than 60%. In the soil of 0-15 cm tillage layer, the residual plastic film with the area >5-30 cm2 and the weight >50-100 mg increased significantly, and the average was about 20% and 10% higher than that in 2016, respectively. The mulched plastic film with a thickness of 0.008 mm has been broken at different degrees at the end of the cotton growth period, and it is relatively difficult to recover. In the process of smashing and returning cotton stalks with large-scale mechanical arable land, the plastic film is broken into small film in the area and weight and they accumulated continuously in the deep soil. Therefore, local area should increase the degradable biological film and the use of 0.010 mm thickness of the plastic film, improve the efficiency of residual plastic film recovery and other measures to reduce the plastic film residue in the soil layer of the fields, in order to ensure the sustainable development of agriculture in the typical oasis.