Effects of gravel mulch on soil hydro-thermal process and rain-fed wheat-maize yields

Abstract: Rain-fed farming in semi-arid areas of the Loess Plateau in China is constrained by low temperatures and water limitations at the early crop growth stage, and the gravel mulch technology as an effective strategy has been used for centuries to conserve soil water and increase soil temperature. To reveal the effects of gravel mulch on soil moisture, soil temperature, rain-fed crop growth as well as yield, a field study of consecutive wheat-maize rotation was conducted in Shannxi Province in the year of 2013-2015. The field experiment was arranged under natural rain-fed conditions with 5 gravel mulching levels, including control with no mulch (CK), surface gravel mulch of 25% (GM1), surface gravel mulch of 50% (GM2), surface gravel mulch of 75% (GM3) and surface gravel mulch of 100% (GM4). The results showed that soil moisture was significantly and positively correlated with gravel mulch degree (GMD), and soil moisture increased with the increase of GMD; compared with the other 4 gravel mulch treatments, GM4 treatment obtained the maximum soil water storage. Gravel mulch acted as a layer with water holding capacity, which not only reduced water loss in the drought period, but also intercepted rainfall in the wet period. Along with the increase of GMD, the water maintaining capacity in the dry period and the ability of rainfall interception in the rainy period were both enhanced. Gravel mulch had an effective effect on soil temperature conservation; the daily average soil temperatures in the different depths of the 4 gravel mulch treatments were all higher than the control, and 100% gravel mulch was the maximum. In order to investigate how gravel mulch affected soil temperature in the hot and cold period, we tested the soil temperature in the extreme coldest (from -5 to 0 ℃) and hottest (40-45 ℃) weather conditions during the winter wheat and summer maize growing season. Gravel mulch acted as a regulator of soil temperature, and thus soil temperature could actively respond to extremely cold or hot weather. In the extremely cold period, air temperature was around from -5 to 0 ℃, soil temperature of GM4 was 5 ℃ higher than CK, and soil temperatures of the other 3 gravel mulch treatments were all higher than CK; in the extremely hot period, air temperature was around 40-45 ℃, soil temperature of GM4 was 3.7 ℃ less than CK, and soil temperature of the other 3 gravel mulch treatments was all less than CK. Soil warming ability responding to air temperature and soil moisture was greatly different; the 4 gravel mulch treatments were all higher than the control treatment, and 100% gravel mulch was the maximum. So gravel mulch could effectively stabilize soil temperature. As for the effects of gravel mulch on crop growth and yield, plant height and leaf area index were not significantly different in the winter wheat growing season, while in summer maize growing season, plant height and leaf area index of the 4 gravel mulch treatments were all significantly higher than the control, and the 100% gravel mulch was the maximum. Study also showed that wheat and maize yields increased with the increase of GMD, and the average yield of 2 growing seasons for winter wheat and summer maize under 100% gravel mulch treatment was 58.55% and 22.50% higher than the control, respectively. These results demonstrated that the gravel mulch technology could effectively increase soil moisture and maintain soil temperature and was beneficial to promote crop growth and ultimately affect the crop production. Overall, we conclude that gravel mulch technology provides a new idea to positively cope with water limitation and climate change and promote agricultural production in arid and semi-arid area of Northwest China.