Minimum tillage and residue management increase soil water content, soil organic matter and canola seed yield and seed oil content in the semiarid areas of Northern Iraq
Introduction
The long-term sustainability of dryland farming in northern Iraq depends on the ability to develop appropriate farming practices that arrest or even reverse the detrimental effects of tillage on soil physical and chemical properties. Whilst several countries around the world have developed highly productive, profitable and sustainable farming approaches based on minimizing tillage practices, the Iraqi agricultural systems have remained heavily dependent on tillage. Farmers are accustomed to till their lands twice a year, to control weeds and to prepare fine seedbeds. However, extensive and unnecessary cultivations disrupt soil physical properties and reduce its organic matter (Ahmad et al., 1996, Das et al., 2014, Kuotsu et al., 2014, Roper et al., 2013). As a result, drylands productivity in northern Iraq have dropped by more than 40% in the last two decades (Hussein et al., 2007, Jaradat, 2003). This addresses an urgent need for developing farming practices that are able to improve soil quality and crop productivity.
The interest in reducing tillage operations has grown dramatically, particularly in the rainfed cereals agrosystems where this technique has proven to increase soil water recharge and the amount of the available water for sufficient crop yields (Kahlon et al., 2013, Nielsen, 1997). Agriculture in semiarid areas of northern Iraq often experiences considerable annual variation in crop yield and profitability due to two direct reasons. Rainfall volume and distribution during the season. Therefore, this variable rainfall conditions should be accounted for when evaluating the production potential for any alternative cropping systems.
Minimum tillage (MT) is not a new approach. However, the concept of minimizing soil disturbance clashes with most Iraqi farmers who used to plow the soil to obtain softer seedbeds. For the farmers, minimally tilled soils seems harder and resist root penetration than plowed soils. Indeed, hard soils have proven to reduce or even prevent plant root development (Atwell, 1993, Hill, 1990). Reduced tillage with stubble retention increase soil water recharge by reducing surface runoff and evaporation losses and increasing soil water infiltration (Ji and Unger, 2001, Van Eerd et al., 2014). Canola seed yield and seed oil content are positively correlated with soil moisture availability, while unfavorable soil moisture condition particularly during flowering and bud development results in intensive assimilates, pod abortion and loss in seed weight (Johnson et al., 1995, Nielsen, 1997).
MT is known to have positive influences on several soil chemical and physical properties including soil organic matter (Bescansa et al., 2006, Cambardella and Elliott, 1992, Conant et al., 2011, Kahlon et al., 2013). However, there is a conflicting evidence about the predicted period for MT to alter soil organic matter. For example, Hejazi et al. (2010) estimated 10–12 years for the change in soil organic matter to be observed bellow 20 cm soil depth, after switching from moldboard plow to MT. This has occurred when MT retained more than 80% of the previous crop residue. In contrast, only three years of MT were required for the soil to develop favorable organic matter above the 15 cm soil depth, when MT was implemented with the most recent modification by incorporating previous crop residue into the soil (Kahlon et al., 2013, Van Eerd et al., 2014). Generally, changes in soil organic matter due to utilizing MT depend on several factors including initial soil and climate conditions and differences in carbon input/output balances induced by soil management along with these factors interactions (Conant et al., 2011, Deligios et al., 2013, Kuotsu et al., 2014). In fact, it has been concluded that, for the soil, the interaction between the management and the environment is more important that the management alone (Aziz et al., 2013, Roper et al., 2013).
Successful strategies to integrate MT into the Iraqi farming systems require information on how this shift would influence soil chemical and physical properties. Information is also required to determine whether this shift increases soil water availability and whether this increase occurs at periods correspond to greater seed yield and seed quality. Therefore, this study aimed to determine the effects of tillage and residue management on canola seed yield and seed oil content as affected by changes in soil water content and soil organic matter during germination, flowering and maturity of canola in the semiarid climates of northern Iraq.
Section snippets
Site characteristics and crop management
A two year (2009–2011) field experiment was carried out at The University of Mosul, Nineveh, Northern Iraq (36.340°N, 43.130°E, 260 m above sea level). The average seasonal temperature (February–July) was 11 and 6 °C and precipitation was 187 and 221 mm, for 2010 and 2011, respectively (Al-Dabbas et al., 2012). The growing season precipitation was below average in 2010 and above average in 2011. The study area was characterized by a considerable temperature variation from an average of -3 °C in
Seasonal rainfall patterns
Based on rainfall volume and pattern, seasons in which the experiment was carried out can be classified into two groups. In the first season (2009 ± 2010), total rainfall from February–July was slightly more than 180 mm with 30.1, 9.38, 61.3, 74.2, 12.2 and 0.0 mm, respectively. In the second season (2010 ± 2011), rainfall was above average with 50.92, 2.15, 13.4, 11.2, 75.03, and 68.3 mm, respectively, but drier than usual April and May and wetter than usual June and July. Therefore, the climate
Soil water content (SWC)
In the present field experiment, conservation practices of MT and residue management reflected a significant increase in SWC over the seasons. The increased SWC was particularly pronounced at flowering, the stage most sensitive to drought in canola’s development (Nielsen, 1997). Drought stress during flowering results in source limitation for seed yield, pod abortion and reduction in pod formation. This probably due to the susceptibility of meiosis of the pollen mother cells to water stress
Conclusions
The current study showed that minimum tillage and residue application were associated with greater soil water content compared with conventional tillage without residue. The greater soil water content was well reflected in the dry season and when moisture was most required for the critical pre-flowering and flowering periods. Minimum tillage with 4 t ha−1 residue increased canola seed yield and seed oil content by providing favorable rainfall interception and favorable soil surface
Acknowledgments
The author wishes to acknowledge the research finding provided by The University of Mosul – Natural Science Foundation. The author expresses appreciation for ACIAR-Iraq Conservation Agriculture Project for their guidance and professional expertise as well as material support. The assistance of The Agriculture Research Institution Laboratory in Nineveh, Mosul is also acknowledged. The author wishes to acknowledge the assistance of Dr. Abdulsattar Alrejabo (ICARDA/ACIAR project manager at Mosul
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