Temperature effects of straw mulching on the agronomic and physiological characteristics of winter wheat

Straw mulching is a popular water-saving technology that is widely used in dry farming areas. However, the negative effects of straw mulching on soil temperature should be avoided when it is used on crops grown in cool seasons. Field experiments were conducted to investigate the temperature effects of straw mulching on the physiological and agronomic properties of winter wheat and to find solutions to reduce the negative effects of straw mulching on winter wheat production. An experiment consisting of four treatments was set up at the Luancheng Agro-Ecosystem Experimental Station, Chinese Academy of Sciences in 2019–2020, which were no mulching (CK), low-rate straw mulching (2450 kg∙hm⁻², LM), medium-rate straw mulching (3675 kg∙hm⁻², MM), and high-rate straw mulching (7350 kg∙hm⁻², HM). The physiological characteristics (endogenous hormones, concentration of stem bleeding ions) and agronomic traits (NDVI, RVI ratio vegetation index, dry matter accumulation, root length density, and winter wheat yield) were monitored. The results showed that straw mulching increased the soil temperature during winter dormancy and decreased the soil temperature in spring when winter wheat entered the recovery stage. The warming effects of mulching did not affect winter wheat growth during the winter dormancy period because the crop was inactive. When winter wheat entered the recovery stage after winter dormancy, the effects of straw mulching on soil temperature inhibited the growth and development of winter wheat. At the recovery stage, the stem auxin (IAA) contents in the LM, MM, and HM treatments were 1.7, 2.5, and 2.7 times that of CK, respectively. The higher IAA content inhibited the growth of winter wheat. The ratio of IAA to zeatin riboside (ZR) increased in the stems of mulched plants, which inhibited tiller growth. Agronomic characteristics, such as root length, NDVI, RVI, plant density, and aboveground biomass of winter wheat at the recovery stage, were all significantly lower than those of CK. At the jointing stage, when the effects of straw mulching on soil temperature were weaker, the IAA content in wheat stems in mulching treatments significantly decreased and was even lower than that of CK treatment. The IAA/ZR ratio also rapidly decreased, and its difference from that of the CK also decreased. The ion concentrations in the stem sap in mulching treatments were higher than those in CK, indicating that the physiological properties of winter wheat under straw mulching were strengthened; however, the increased growth could not compensate for growth loss during the long recovery stage. The values for the agronomic characteristics under straw mulching were lower than those under CK at the jointing stage. The growth of winter wheat under straw mulching was delayed by 2 days at heading and anthesis. The yield of winter wheat under the straw mulching treatments was lower than that under CK. The results showed that the decrease in soil temperature at the recovery stage caused by straw mulching delayed the growth of winter wheat and was the main reason for the decrease in winter wheat production. The effects of straw mulching on soil temperature and the physiological and agronomic properties increased with the increased straw amount. Therefore, breaking the straw layer to reduce the negative effects of straw mulch on soil temperature during the recovery stage would be an effective measure to promote the positive effects of straw mulching on winter wheat.