تأثیر جهت شخم بر نگهداشت آب و عملکرد دانه گندم در کشتزار دیم در منطقه نیمه‌خشک در غرب زنجان

Abstract
Background and objectives: The availability of water is the most important factor controlling the soil productivity in arid and semi-arid regions. In this area, crop yield is strongly affected by precipitation and water retention in the soils. The application of soil management practices can be suitably control surface runoff and enhance water retention in soil. Tillage as the major tool of soil management practices can affect the runoff production and soil erosion and so can influence the availability of water and crop yield in the rainfed lands. Yet, in most semi-arid areas, rainfed lands are ploughed up and down slope, a practice that decline water retention by declining the opportunity of water infiltration in to soil. Although various studies have been done to determine the effect of tillage methods on water conservation and crop yield worldwide, little information is available on the role of plough direction on the water retention and crop yield in wheat rainfed lands in semi-aria areas, particularly in Iran. Therefore this study was conducted to determine the effects of two plough directions (up and down slope and contour line) on soil water content during the growth period and grain yield of wheat in a semi-arid rainfed land in Iran.
Materials and Methods: The study was carried out in a rainfed lands with slope steepness of 10 % and clay loam soil in west of Zanjan, north western Iran in 2015. Winter wheat was cultivated in two directions consisted of 1) up and down slope (as conventional tillage) and 2) contour line (as conservation tillage) early October 2014. A total of twenty four plots, with 1.75 m  8 m dimensions were designed on the land after cultivation. Soil water content was determined in three core samples taken from the rhizosphere in each plot in 5-day interval during the growth period from October 2014 to Jun 2015. Grain yield and yield components of wheat were determined by collecting all crops of plot surface at the harvesting time (Jun). Analysis of variance and the Duncan’s test were conducted for data of soil water content and grain yield of wheat. Statistical analysis was conducted using SPSS 21 software.
Results: Mean soil water content during the growth period in the contour plots (9.1%) was 62% more than the up and down slope plots (5.6%). This difference was statistically significant (p< 0.001). Wheat grain yield was differed significantly between two plough directions (p< 0.001). It was 67% in the contour plots more than up and down slop cultivated plots. In addition, significant differences were also found between the two plough directions in wheat yield components i.e. crop height (p< 0.01), number of grains per spike (p< 0.05), 1000 grain weight (p< 0.001), and biological yield (p < 0.01). Wheat grain yield was significantly related to soil water content (R2= 0.43, p< 0.01). With an increase in soil water content, wheat grain yield considerably increased in the plots.
Conclusion: This study showed that the plough direction is the major factor controlling soil water content in rainfed wheat farms. In the plots ploughed on the contour line, the opportunity of rainwater to infiltrate into the soil is very high. This factor is the most important factor controlling wheat yield under rainfed condition. Increased wheat grain yield under the contour lines cultivation could be attributed to increase in yield components which in turn were influenced by increase soil water content. With regarding these results, application of contour line ploughing method in the rainfed lands of sloped area could be substantial strategy to decline water loss and enhance crop yield in semi-arid regions.