Effects of environmental factors on the influence of tillage 1 conversion on saturated soil hydraulic conductivity obtained with 2 different methodologies: A global meta-analysis

. The saturated hydraulic conductivity ( K sat ) is a key soil hydraulic property 13 governing agricultural production. However, the influence of conversion from 14 conventional tillage (CT) to conservation tillage (CS) (including no tillage (NT) and 15 reduced tillage (RT)) on K sat of soils is not well understood and still debated. In this study, we applied a global meta-analysis method to synthesize 227 paired 17 observations for soil K sat from 69 published studies, and investigated factors 18 influencing the effects of conversion to CS on K sat . Results showed that soil layer, 19 conservation tillage type, soil texture type and cropping system management did not 20 have significant effects on the influence of conversion to CS on K sat . When the K sat 21 was measured by rainfall simulator, conversion to CS significantly ( p < 0.05) 22 increased the surface and subsurface soil K sat by 41.7% and 36.9%, respectively. In 23 addition, the subsurface K sat also tended to increase under CS practices when the K sat 24 was measured by tension disc infiltrometer. However, when the K sat was measured by 25 hood infiltrometer, ring infiltrometer, constant/falling head and Guelph permeameter, 26 conversion to CS had no significant effects on the K sat . It is observed that when the 27 conversion period was less than 15 yr, the K sat under CS showed a greater increase for 28 a longer conversion period. Climatic and topographic factors including the mean 29 annual temperature (MAT) and the mean annual precipitation (MAP) were statistically 30 related to the responses of K sat to tillage conversion at the global scale. Quadratic 31 polynomials can describe the relationships between them. These findings suggested that quantifying the effects of tillage conversion on soil K sat needed to consider

increased the surface and subsurface soil K sat by 41.7% and 36.9%, respectively. In 23 addition, the subsurface K sat also tended to increase under CS practices when the K sat 24 was measured by tension disc infiltrometer. However, when the K sat was measured by 25 hood infiltrometer, ring infiltrometer, constant/falling head and Guelph permeameter, 26 conversion to CS had no significant effects on the K sat . It is observed that when the 27 conversion period was less than 15 yr, the K sat under CS showed a greater increase for 28 a longer conversion period. Climatic and topographic factors including the mean 29 annual temperature (MAT) and the mean annual precipitation (MAP) were statistically 30 related to the responses of K sat to tillage conversion at the global scale. Quadratic 31 polynomials can describe the relationships between them. These findings suggested 32 that quantifying the effects of tillage conversion on soil K sat needed to consider 33 Guelph permeameter (Reynolds and Elrick, 1985) used in field and constant/falling 48 head permeameter applied on intact (undisturbed) or repacked soil cores (Klute and 49 Dirksen, 1986). In addition, rainfall simulators have been applied to simulate rainfall 50 events for the infiltration runs (Gupta et al., 1994). 51 Tillage is one of the main causes of spatio-temporal variability in K sat . 52 Conventional tillage (CT), mainly refers to as heavy tillage practices down to 25-30 53 cm soil depths, is a widely adopted management practice which could significantly  Tillage conversion may also lead to different degrees of changes in the factors (e.g., 78 soil structure, organic matter content and bulk density) influencing K sat (Cameira et al.,  differ by an order of magnitude, which is mainly due to the following reasons: (1) the 87 geometry of water application to the soil is different; (2) the strategies to prevent   search were related to: "saturated hydraulic conductivity", "steady-state infiltration 129 rate", "conventional tillage", "conservation tillage", and "till". Using these keywords, information are presented in Tables S1 and S2.

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Of the 69 studies, 15 did not provide K sat values, but steady-state infiltration rate 152 values. The K sat refers to flow through a saturated porous medium, and the infiltration 153 rate represents the imbibition of water from free water above the soil to pore water 154 beneath the soil surface. In this case there are interface issues such as surface tension, 155 surface crust and seal effects, the influence of litter, mulch, and other factors. 156 Nevertheless, the steady-state infiltration rate was assumed to be the K sat by 157 convention in this study (Yolcubal et al., 2004;Kirkham, 2014) (Table S2). A total of 158 6 measurement techniques for infiltration rate and K sat were involved in these 65 159 studies, including hood infiltrometer, tension disc infiltrometer, ring infiltrometer,  rainfall simulator, respectively (Fig. 3a). However, the mean effect sizes of subsurface simulator, respectively (Fig. 3b).

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The CS type, soil texture and cropping system management had no significant (p >

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The relationships between the ln(R) of K sat and MAT, MAP, and elevation can be 249 fitted by quadratic polynomials, with the R 2 values ranging between 0.005 and 0.099 250 (Fig. 4).  correlations between the response of K sat and elevation were very weak (Fig. 4c).

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Based on these results, we argue that in the cold and temperate regions, the 311 improvement of K sat by tillage conversion will be greater than that in the tropical  Our global meta-analysis indicated that conversion from CT to CS had no significant 320 effects on surface and subsurface K sat . However, these effects were related to  The error bars indicate effect sizes and 95% bootstrap confidence intervals (CI). The effect of CS was statistically significant if the 95% CI did not bracket zero. The sample size for each variable is shown next to the bar.