4.3. Variations with soil textures and climatic conditions
Soil and climatic conditions of several regions affected soil water storage with conservation tillage methods compared to conventional tillage methods as coarse-textured soils stored the lowest amount of water compared to fine-textured soils while the RRs of fine and coarse-textured soils were quite similar in overall conservation tillage methods (Figure 2 and Figure 4) which is consistent with (McConkey et al., 1996) who stated that lowest soil water volume observed in sandy loam in 1983 and with silt loam and clay soil in 1990, for silt loam, the estimated SWSp with CT and NT were 123 and 125 mm and 31, 128 and 128 mm for RT and NT, respectively. Increased SWSp in fine soils and PSE, crop yield and WUE with medium-textured soils (Figure 2 and Figure 4) indicate that coarse to medium-textured soils stored more water and improved yields in both crops. The improved initial soil moisture storage was reported in clay soil with NT and as a result, higher crop yield was expected (Cox et al., 1986; Tanaka and Aase, 1987), while lower yield were obtained with NT under silt loam (Tessieret al., 1990).
According to Hammel et al. (1981), soil texture affects soil hydraulic properties that may have an impact on how much water is lost when using different tillage techniques. For example, according to Papendick et al. (1973) the average WUE of the conservation tillage techniques for three years of study was significantly higher than CT. Large fluctuations in diurnal temperature occur in the top 15 cm of soil and may affect the vapor movement in the dry layer in the Northwest, where the majority of water loss occurs in fallow land in a dry layer of 10 cm or more in thickness (Papendick et al., 1973). Macro pores and cracks can also affect water balance with RT in bare fallow period (Hartzog and Adams, 1989) because of the fast-downward movement of water with significant contribution to the fallow performance in cracking clay soil texture in southern Queensland (Marley and Littler, 1989). Deep movement of water in NT fallows explained the cracking behavior of the soil at Dooen (Australia) while at nearby locations, saturated hydraulic conductivity significantly increased from 14 mm to over 200 mm h-1after 10 years of NT (Bissett and Oleary, 1996) while NT accumulated 47 mm additional water than CT (O’leary and Connor, 1997).
Conservation tillage methods had significant effects on winter wheat PSE, SWSp, crop yield, ET and WUE in all precipitation patterns except for WUE with >600 mm MAP which remained non-significant (Figure 3). Similarly, MAP of >600 mm had non-significant effect on spring maize PSE, except for that, conservation tillage had significant effect on all variables (Figure 5). Similarly, greater positive RR for crop yield and WUE was achieved with conservation tillage practices when MAP was <400 mm which corresponds to Jin et al. (2007) who found that NT is the best tillage practice in fallow period for water conservation with the effective storage efficiency of rainwater leading to higher winter wheat yield and PSE. Similarly, spring maize crop yield with >600 mm MAP was relatively low as compared to <400 mm (Figure 5) which could be due to the saturated field conditions at the end of the fallow period and the time of sowing of the main crop is delayed (Jin et al., 2007). Similarly, the difference between conservation and conventional tillage was more pronounced with relatively dry rainfall (365 mm and 292 mm) years such as overall 42% of fallow precipitation was stored in the soil with comparatively low runoff, which means 58% of fallow precipitation was vanished due to evaporation (Jin et al., 2007). However, the advantages of conservation tillage for soil differ depending on periodic rainfall and soil texture (Unger, 1979; Gajri et al., 2002). Previous studies found that the degree and depth of soil disturbance under the NT, ST, and CT treatments affected rainfall infiltration, soil water-holding capacity and ET (Huang et al., 2006). In three-year research, regardless of the amount of rainfall during the fallow season, water storage status improved to varied degrees with all tillage techniques. In comparison to CT, the NT and ST treatments increased SWS and PSE in order to more effectively store fallow precipitation into the soil (Hou et al., 2012). This is due to the fact that conservation tillage techniques with less soil disturbance result in enhanced rainfall penetration and decreased soil water evaporation (Li et al., 2007; Pikul and Aase, 2003; Amir and Sinclair, 1996). By removing the plough pan layer, ST considerably reduces soil bulk density and allows more precipitation to be stored in the soil (Pikul Jr. and Aase, 1999; Mohantyet al., 2007). Contrarily, excessive soil tillage (CT) leads to a relatively high rate of soil water evaporation (Debaeke and Aboudrare, 2004; He et al., 2010).