A comparative evaluation of soil preferential flow of mulched drip irrigation cotton field in Xinjiang based on dyed image variability versus fractal characteristic parameter

Highlights

• Dyed area ratio had a tendency of keeping decreasing from soil surface.

• Finger flow mainly appeared in the soil at the early cotton growth stage.

• Macropore flow primarily developed with the maturing stages of the cotton.

• Fractal feature parameter was comparatively effective to evaluate the preferential flow.

Abstract

Preferential flow in soil has a potential impact on the maintenance of the agricultural ecosystem. The aim of this study was to quantify and evaluate the characteristics of preferential flow in drip irrigation cotton fields in Xinjiang. We applied the dye tracer method, addressed the variability in stained images, collected soil samples in the excavated soil profile, and calculated the fractal parameters in four key growth stages of cotton in 2021. According to the dyeing mode, the dyed area ratio tended to keep decreasing from the soil surface; a down movement of points which drastically dropped to zero appeared at pace with the growth of the cotton (from −10 cm to −30 cm). Statistics obtained from the staining images showed that the preferential flow degree was highest at the boll opening stage; no significant difference in non-uniformity emerged from different treatments at the same growing stage (p > 0.05). Conversely, the fractal feature parameters in the active region model probed that the preferential flow degree was developed at the flowering & bolling stage in treatments with opaque oxidation-biodegradable film mulching (B,W), and at boll opening stage with plastic film mulching (P). During the same growth period, a significant difference (p < 0.05) existed between different groups. In comparison, the fractal feature parameter was found more adequate and effective in evaluating the characteristics of preferential flow in drip irrigation cotton fields. This research will provide practical information for the local department to adjust some agronomic measures to reduce the risk of low irrigation water utilization efficiency.

Introduction

The soil, functioning crucially as buffering and filtering systems on earth (Clothier et al., 2008), are deemed the breeding and growing base for natural flora and fauna and are also known as fundamental parts of the terrestrial ecosystem (Allaire et al., 2009). Recently, there has been renewed interest in environmental issues; thus, a primary concern of soil eco-environment with higher standards is raised along with the development of modernization and the economy (Alouai et al., 2015). The soil structure has strong spatial heterogeneity at different scales regarding a porous medium, through which the mechanism of water movement and solute migration is very sophisticated (Beven and Germann, 2013). Commonly, the process of infiltration in the soil is nonequilibrium referring to the preferential flow, which is a general term presented for defining a phenomenon when water flow is allowed to move following favored paths while bypassing most parts of the medium (soil matrix) (Nimmo, 2012). The presence of preferential flow reduces the residence duration of the solutes, resulting in transient contact with the soil matrix and giving rise to the potential risk of direct passage of chemicals from the surface to the groundwater. (Guo and Lin, 2019, Kohler and Abbaspour, 2005). Therefore, in agricultural production, the possible existence of preferential transport will affect the hydraulic activities in fields thus negatively having an effect on the water-fertilization utilization efficiency of the crop and without studying preferential flow, it is impossible to gain insight into the causes of reduced crop water use efficiency. (Wang and Zhang, 2006). Typically, the soil preferential flow in agricultural systems is induced by several drivers, one of which is the alteration of soil fabric caused by individual elongated pores such as the crop root path, soil animals’ holes, and surface fissures, leading to an unusual and rapid downward movement of the water flow (Sen and Giroux, 1988); moreover, the preferential flow is associated with the size, geometry, distribution and continuity of soil pores. (Bundt et al., 2001a; Bundt et al., 2001b; Lukas et al., 2021; Kalkhajeh and Huang, 2021). Additionally, even if there’s devoid of preferential fractures, the soil’s water repellency will prevent the soil from being moisture (Gjettermann and Nielsen, 1997) and restrict the influence by capillarity of soil conduit, thus forcing the flow selecting the zone with higher water conductivity unstably (Dekker and Ritsema, 1993). Furthermore, the irrigation infiltration responds quickly to higher irrigation rate and intensity of precipitation, starting up a quick migration of irrigation water and fertilization (Rooij et al., 2000).

Characterizing the preferential flow as being highly dynamic in time and space brings about the challenges of determining proper quantifying techniques, among which the dye tracer imaging approach has been prevailingly applied to observe and assess the soil preferential flow. The Brilliant Blue FCF, for its easy visibility and non-toxicity, has been extensively selected as the dye tracer, whose flowing characteristics are monitored and evaluated by obtaining and analyzing the dyed images along excavated soil profiles (Flurry et al., 1995; Forrer and Papritz, 2000; Morris and Mooney, 2004). Investigations utilizing this method have presented some precious research findings quantifying the soil’s preferential mobility. For example, the preferential paths in soil, contributing significantly to the subsurface leaching of dissolved reactive phosphorus, are considered biological 'hot spots' with higher microbial biomass as well as diverse microbial community architectures than the soil matrix part. Whilst the broad application of dye tracing imaging methods, most work has been done to study the occurrence mechanism and controlling factors of preferential flow without accurate verification which could bias the representativeness of the results. The identification of the nonequilibrium processes thus requires various methods for better quantification (Qi et al., 2013; Wu et al., 2015; Filipović et al., 2020; Zhang et al., 2021).

Hence, researchers have introduced fractal theory into the description of the non-uniform flowing process in soil (Liu et al., 2005). The fractal theory is universally adopted in soil science to study soil particle size distribution, soil saturated hydraulic conductivity and soil aggregate structure, Etc. (Mandelbrot, 1982; Bartoli et al., 1991; Rasiah et al., 1995; Tyler et al., 1992). Meanwhile, past evidence suggests that the non-uniform flow process in the soil also has fractal characteristics (Flurry et al., 1995; Sheng et al., 2009a). In this scenario, the active region model (ARM), which combined the fractal theory with the continuity model, was proposed; the relationship between the channel area of the non-uniform flow field and soil water saturation was established to well-rounded represent the heterogeneous characteristics of soil water flow. This model can not only solve practical problems on large scale but also capture details of soil water flow at a small scale (Liu et al., 2005; Sheng and Zhang, 2011, Sheng and Zhang, 2011; Sheng et al., 2014). Li et al. (2008) constructed constitutive relation of active region model, in which the fractal parameters were considered to be vital roles to quantify the preferential flow. Since the elemental work of the research on the ARM is to determine the flowing patterns of non-uniform flow (active region), the dye tracer can exactly and directly provide this flowing pattern without influencing the initial soil properties (Sheng et al., 2009b). Thus, it is feasible to apply the dye tracing imaging method to explore this model.

Although previous studies of the preferential infiltration have been carried out, they have paid little attention to quantifying the preferential flow of mulched drip irrigation cotton fields in arid regions like Xinjiang. With vast territory and various natural resources, Xinjiang has suffered from scarce precipitation and large evaporation because of its deep inland location. Despite this, agriculture is still known as the economic pillar industry of Xinjiang, with cotton the major economic crop. In addition, irrigation plays a critical role in maintaining the Oasis agriculture in this region (Danierhan et al., 2013). In the early 1990 s, drip irrigation was brought in and combined with film by the Corps 8th Division Government; this brand-new integrated technology was soon practiced by local farmers due to its precise irrigation and fertilization (Rao et al., 2018). Beyond that, this technology can function by promoting the growth of the roots, increasing yield, improving the quality of crops, preserving the soil temperature, moisture and resisting the drought (Zhang et al., 2020, Wang et al., 2020; Jia et al., 2021; Geng et al., 2021). By the end of 2020, the planting area of cotton with film mulched drip irrigation has amounted to 2.50 × 10⁶ ha, which accounts for approximately 78.9% of the nation’s cotton planting area (data from Xinjiang Statistical Yearbook 2020). In this context, the dye tracing imaging approach and the active region model fractal parameters were carried out to quantify the characteristics of preferential flow in drip irrigation cotton fields with different films mulched, and to validate and ascertain which method is suitable to evaluate the developing degree of preferential flow in mulched drip irrigation cotton field in Xinjiang.