تأثیر سیستم‌های مختلف خاک‌ورزی و بقایای گیاهی بر عملکرد گندم(Triticum aestivum L.) و خصوصیات فیزیکی خاک در تناوب آیش- گندم در شرایط دیم

Introduction
Climatic variation particularly for participation is a constraint for the crops to reach their potential productivity under rainfed conditions. Therefore soil moisture preservation by means of tillage practices and improvement of soil physical criteria, particularly through enhancement of organic matter content is crucial. For this reason soil tillage must be managed in a proper way in order to fulfill this objective. Use of the right implements for this purpose under rainfed conditions together with retention of crop residue are practices most suitable for low rainfall environments. Tillage operation has a great impact on soil properties such as soil particle stability, soil aggregate and also soil water holding capacity. There are good evidences regarding yield improvement under rainfed conditions when implements such as chisel plow and sweep plow is used. Reduced tillage has been referred to as a proper practice under such conditions and degrees of no to minimum tillage have been proposed. These types of tillage combined with retention of crop residue which is referred to as conservation agriculture is being received more attention for the sake of soil conservation and moisture preservation. The purpose of the present study was to investigate these type of management for a rainfed wheat-fallow rotation.
Materials and Methods
In order to study the effect of different tillage practices in fallow-wheat rotation and retention of crop residue on soil properties such as bulk density, soil aggregate, porosity, particle stability and moisture holding capacity and also crop criteria including yield, above ground biomass of wheat and weed under rainfed conditions, an experiment was conducted for two years (2013-2014 and 2014-2015 growing seasons) in Shirvan region, Northeast of Iran. The layout of experiment was strip plots based on a randomized complete block design and three replications. Tillage systems and retention of crop residue were the experimental factors with four levels of tillage (chisel, sweep, moldboard and no tillage) and three levels of crop residue retention (7, 3.5 t.ha-1 and zero residue). Tisdal and Oades method used to determine the soil aggregates size distribution in dry and wet sieving conditions. To determine the soil bulk density, undisturbed samples were picked up from a depth of 0-10, and 10-20 cm and soil bulk density were determined after drying of soil in Oven for 48 hours at a temperature of 105°c. In order to evaluate the soil moisture at the end of the fallow year, sampling was done from each treatment in the depth of 0-20cm. During growth and after harvesting, the plant height, ear length, seed number per ear, thousand kernel weight, seed yield, and biological yield were determined.
Results and Discussion
Results showed that in general soil bulk density was higher at the depth of 0 to 10 cm compared with that at the depth of 0 to 10 cm for the experimental soil. Soil porosity followed a reserve trend of that of bulk density. Retention of crop residue caused a slower bulk density and a higher soil moisture content. Tillage with moldboard at different levels of crop residue retention caused lower weed biomass compared with other tillage operation. Soil practices size higher than 0.5 mm were respectively higher and lower with moldboard plow. Seed yield and also above ground biomass were higher with moldboard plow compared with the conservation tillage practices. Despite lots of reports about the prominence of conservation tillage systems in comparison with moldboard tillage, the reason for the prominence of moldboard plow in comparison with conservation tillage in this experiment can be attributed to better seedbed preparation in moldboard tillage. The other reason for seed yield increment in the treatment of moldboard tillage in comparison with other treatments can be the effective control of weed plants, especially Russian Knapweed.
Conclusion
Despite contradictory reports about the effect of moldboard tillage on wheat yield in comparison with conservation tillage, the results of this experiment showed the prominence of moldboard tillage in contrast with the sweep, chisel, and no-tillage for wheat production. Under rainfer conditions retention of crop residue together with minimum level of soil disturbance seems to be a proper practice. This has been evidenced elsewhere and is confirmed somehow in our experiment too. This is mostly related to enhancement of water holding capacity of the soil.