Effects of different mulching materials on soil and near-surface environment and of apple orchard tree growth

Apple is one of the top four fruits in the world and Chinese apple area and output are all the highest. Longdong, the Loess Plateau lying in East Gansu Province, is the largest apple cultivation region in China, with apple cultivated area and yield accounting respectively for 11.11% and 8.75% of the country (Gansu Rural Year book 2015). Apple is a major industry towards the alleviation of local poverty and enrichment of the local people. While drought is one of the main limiting factors of apple production under rainfed conditions, and mulching is an important method for improving the yield of apple. A field experiment was designed to find a suitable measure of soil moisture conservation in apple orchards in Longdong. In a 19-year-old 'Nagafu No.2' apple orchard, wheat straw mulching (WM), river sand mulching (SM), black plastic film mulching (FM) and clean tillage (CK) were applied to study effects of mulching matrieals on the apple orchard environment and tree development for the period from November 2010 to November 2013. Results showed that from April to June, average soil moisture content in the 0-100 cm soil layers of three mulching treatments was significantly (P < 0.01) higher than that of CK. Also the order of soil moisture content under treatments was WM > SM > FM > CK. In the third year, soil water consumptions in the 0-500 cm soil layer under WM and SM treatments were significantly (P < 0.01) lower than that of CK during the period from flower bud expansion stage to end of leaf canopy development stage, while significantly (P < 0.05) higher than that of CK for the period from end of leaf canopy development stage to maturity stage. However, total soil water consumptions of the two treatments were slightly lower than that of CK. Soil water consumption under FM treatment was significantly (P < 0.01) higher than that under CK before and after leaf canopy development. Treatment WM reduced soil temperature in the 5-25 cm soil layer during the period from March to August, increased soil temperature from September to November, but significantly (P < 0.05) decreased average daily soil temperature amplitude in the 5-25 cm soil layer, compared with CK. The near-surface temperature decreased while relative humidity increased under WM treatment compared with CK. Treatment SM had a slight effect on soil temperature, but obviously increased near-surface temperature and decreased relative humidity. The soil temperature and daily amplitude of FM treatment from March to November were always higher than those of CK, and the near-surface temperature and relative humidity were close to CK. Three mulching treatments had no obvious effect on phenological period of apple. On the whole, the order of the contents of nutrients in the 0-60 cm soil layer was WM > CK > FM > SM. The increase in almost all forms of nutrient was highest under WM treatment with the years. However, organic matter, alkaline hydrolytic N and total N in the 0-60 cm soil layer under SM treatment and in the 0-20 cm soil layer under FM treatment decreased obviously with the years. Correspondingly, total salt content (including in the 0-20 cm soil layer under WM treatment) was significantly (P < 0.01) lower than that of CK. All the mulching treatments enhanced apple tree growth, but had little effect on shoot ratio. These mulching treatments also increased single fruit weight and yield, which were significantly (P < 0.05) higher under WM treatment than under CK. Water use efficiency of WM was significantly (P < 0.01) higher than those of other treatments, and the order was WM > SM > CK > FM. The results indicated that wheat straw mulching was better for apple orchard cultivation under rainfed conditions in Longdong. The implementation of river sand mulching and black plastic film mulching should be combined with applications of organic fertilizer and other nutrients The dominant single film mulching measure in the region needed to be further improved.