Effect of straw mulch and tillage direction on temporal variation of soil moisture in wheat rainfed system

Soil moisture is a major factor determining the plant growth in rainfed lands. Lack of soil water content causes drought stress at plant growth stages. Soil water content temporally varies due to variations of precipitation, evaporation and other environmental factors. There are different methods to hold soil water content during the growth period. Knowledge of temporal variations in soil water content during wheat growth period can be effective in identifying the critical times of drought stress. Therefore, this study was carried out to investigate temporal variations of soil water content under the influence of straw mulch and tillage direction in rainfed system in semi-arid region. The study was conducted in a rainfed land with 10% slope steepness in University of Zanjan during winter wheat growth period from December 2015 to June 2016. Experiments were carried out in four levels of wheat straw mulch (0, 200, 400, 600 g/m2 equal to 0, 33, 66 and 100% covering surface) in two tillage directions (up to down slope and on counter line) in a factorial arrangement as completely randomized design at three replications. A total of 24 experimental plots with 2m × 5m in dimensions were designed to investigate the soil water content. Soil water content during the growth period, was determined by a Time- Domain Reflectometry (TDR) at 10-day interval. Monthly variations in soil water content were determined for different treatments during the growth period. The temporal variation of soil water content at different straw mulch levels and tillage direction were determined using the Duncan's tests and t-pair test. Based on the result, soil water content was affected by straw mulch level in all months of wheat growth period (p <0.01), and the highest value was observed in April. In April, the amount soil water content in 100% straw mulch level was significantly increased by 30 and 100% compared to control treatment in along slope tillage and contour tillage, respectively. Mean annual soil water content for along slope tillage at the mulch level of 33, 66 and 100% was 11, 13 and 21% higher than the control treatment (p <0.01), respectively. Mean annual Soil water content for the contour tillage was also at the mulch level of 33, 66 and 100 % was increased 6, 8 and 18% higher than the control treatment, respectively (p <0.01). Study of temporal variations of soil water content in two tillage directions showed that soil water content was significantly increased during growth period except on January, February and May. The reasons for this difference can be expressed by changes in the type of precipitation (snow) and the weakness of the vegetation. The mean soil water content in contour tillage was about 6 % more than along slope tillage (p <0.05). The highest difference in soil water content was observed in March (14%) and the lowest value was in May (2%). No interaction was found between straw mulch level and tillage direction in terms of soil water content in any months during the growth period. This study showed, each of the two management had independent effect on soil water content in rainfed wheat land. The roles of the two management methods in reservation of rain water are obvious in the early stages of growth and in spring when rainfalls frequently occur in the area. Application of at least 33% wheat straw mulch equivalent to 2 tons per hectare and tillage on contour lines are essential for improving soil water content in rainfed system.