Soil matric potential and salt transport in response to different irrigated lands and soil heterogeneity in the North China Plain
In the lowland area of the North China Plain (NCP), increasing utilization of brackish water could promote the transformation of precipitation into available water resources, and alleviate the conflict between increase food production and freshwater scarcity. However, the processes of soil water movement and salt migration might be altered, because utilization of brackish water results in frequent changes in groundwater depth and thickness of vadose zone. Thus, it was necessary to understand soil water movement and salt migration when using brackish water for irrigation.
Materials and methods
In this study, soil matric potential (SMP) and total dissolved solids (TDS) at multiple depths were measured in situ to investigate the mechanisms of soil water movement and salt migration at one grassland (site 1) and at three typical irrigated croplands (sites 2, 3, and 4) with different soil textures and groundwater depths in a lowland area of the NCP.
Results and discussion
The study showed that deep soil water and groundwater were recharged generally following heavy precipitation during rainy season. SMP values increased quickly at site 4 due to relatively homogeneous soils, followed by site 3 > site 2 > site 1 with an obvious hysteresis response of SMP at multiple depths to precipitation. Soil water mainly moved downward in piston flow, and preferential flow also existed in the soil above 100 cm in the percolation process at four sites. Generally, SMP values followed the order of site 4 > site 1 > site 2 > site 3 and exhibited an inverse trend for TDS, which was mainly due to soil heterogeneity and soil texture in vertical profiles. The differences in SMP among the four sites were mainly due to land use and groundwater depth. There were significantly differences in spatiotemporal distribution of water and salts between homogenous and heterogeneous soils. The processes of infiltration and water redistribution ended quickly in relatively homogeneous soils after heavy rains. However, there was obvious hysteresis in SMP with an increase in soil depth in heterogeneous soils.
Homogenous soils favored water infiltration, salt leaching, and groundwater recharge, and the flow of soil water flow was blocked and salt accumulated significantly in layered soils. The soil water movement and the transformation relationship between water and salt in the vadose zone provided a basis for utilization of brackish water irrigation in lowland region of the NCP.
KeywordsBrackish water irrigation North China Plain Salt migration Soil water movement Soil matric potential
Increasing freshwater scarcity was a global systemic risk (Mekonnen and Hoekstra 2016). Agricultural irrigation was the largest consumptive sector of global freshwater, and water shortage in agriculture will intensify due to increasing demands for food and biofuels (Siebert et al. 2010; Hu et al. 2010; Wada et al. 2012; Ercin and Hoekstra 2014). Brackish water resources were distributed widely in the North China Plain (NCP), and about 35 billion cubic meters of brackish water and saline water resources can be exploited in the Hebei Plain (Qian et al. 2014). Water scarcity in these regions has forced farmers to explore the use of brackish water for agricultural irrigation. Brackish water can be used successfully for crop irrigation, and it was beneficial for crop production and conservation of fresh water (Malash et al. 2012; Singh and Panda 2012; Chen et al. 2016; Liu et al. 2016). However, irrigation using brackish or saline water that contains large quantities of soluble solutes may affect soil salinity, groundwater quality, soil microbial metabolic activity, and food production profoundly (Russo et al. 2014; Wang et al. 2015b; Chen et al. 2016). In lowland areas of the NCP, freshwater shortage and soil salinization were the major factors that inhibit crop yields and sustainable development of agriculture (Yang et al. 2016). How to utilize and manage brackish water and mitigate or prevent land degradation have become important issues (Li 2016; Wu and Sun 2016).
In lowland areas of NCP, soil water and salt dynamics were the major factors that influence crop growth, and research on the transport of soil water and salt can provide scientific evidence for regulating water and salt. Researchers have focused on soil water dynamics and solute transport to estimate groundwater recharge rates and the potential effects on groundwater quality (Dahan et al. 2014; Turkeltaub et al. 2014; Izbicki et al. 2015). Increasing utilization of brackish water could promote the transformation of precipitation into available water resources, and this could alleviate the serious overexploitation of deep groundwater in the NCP (Zhang et al. 2009; Pang et al. 2010). Brackish water utilization also results in frequent changes in groundwater depth in shallow aquifer and in thickness of vadose zone. The processes of soil water movement and salt migration could be changed with alterations in shallow aquifers and vadose zone, so it was necessary to understand soil water movement and soil salt migration under brackish water irrigation.
Characterizing the processes of soil water flow and salt transport in the vadose zone often relies on measurements of soil water content or soil water potential as basic parameters. The soil matric potential (SMP) was probably the most useful way to describe soil water status and movement. The SMP was critical to evaluate soil water flow and solute transport (Hayashi et al. 2009), calculate propagation velocities of the wetting front (Vereecken et al. 2008), and estimate soil water availability to plants (Pan et al. 2013). However, compared with the abundant research on soil water content, less attention has been paid to understanding the dynamics of SMP in the lowland area of NCP. Knowledge of the variability of SMP at different depths can be used to determine the direction of soil water flow and the magnitude of soil water fluxes and to estimate the rate of groundwater recharge (Hubbell et al. 2004; Scanlon et al. 2010). A precise description of SMP could also be used to determine “when” and “how much” to irrigate and to evaluate the impacts of irrigation practices on water movement and salt leaching (Masseroni et al. 2016; Min et al. 2017). Thus, studying the characteristics of SMP dynamics and distribution in the vadose zone was critical for characterizing transport of soil water and salt.
Soil water movement and salt migration were complicated, and they were affected by numerous factors, which include climate conditions; soil properties; such as soil texture, and structure, groundwater depth; and rooting depths of vegetation (Wiesner et al. 2016; Xia et al. 2016; Jia et al. 2017). The main influential factors and their degree of influence on soil water and salt dynamics were quite complex and remain to be analyzed further (Li et al. 2014). Few studies focus on soil water movement and salt migration under brackish water irrigation, where different soil properties and groundwater depths were combined. In this research, SMP and total dissolved solids (TDS) were monitored at four sites with different soil textures, groundwater depths, and different resources of irrigated water. The four field sites included three croplands with different brackish water irrigation regimes and one grassland without irrigation in lowland area of the NCP. The scientific objectives of this study were (1) to illustrate soil water movement and salt transport vertically under different irrigation regimes using brackish water and with different soil textures and groundwater depths, and (2) to reveal the relationship between the factors that influence transport of soil water and salt. The results of the study could be used to provide guidance for water management under agricultural irrigation, such as determining “when” and “how” to irrigate croplands and to avoid soil salinization in lowland areas of the NCP.
2 Material and methods
2.1 Site description
Field experiments were conducted in Nanpi County, which was located in the Low Plain around the Bohai Sea in North China. The climate was semi-humid monsoon, with hot, humid summers, and a rainy season from July to September. During 1990–2016, the average annual precipitation was 562 mm, with about 80% occurring during the rainy season, and the mean annual temperature was 13.2 °C. Water shortage and soil salinization were the major abiotic factors that affect the sustainable development of agriculture in this region. The extensive brackish water resources in Nanpi County in the upper aquifer (shallow groundwater) were stored at a depth of 10–60 m and have TDS of 2–5 g/L (electrical conductivity (EC) of 2.8–8.2 ds/m).
2.2 Experimental design and field observations
Description of the study sites
GD range in 2016
Winter wheat and Soybean
Winter wheat and summer maize
Winter wheat and summer maize
Twice using shallow groundwater (1.90–2.40 g/L) for winter wheat, and no irrigation during soybean growth
Twice using surface water (1.95–2.96 g/L) and once freshwater (about 0.8 g/L) for winter wheat; once freshwater before glowing maize
Three irrigations for winter wheat and once before glowing maize using surface water (1.65–2.85 g/L)
SMP and TDS values were measured at different depths corresponding to soils sampled. SMP was recorded using waterflood tensionmeters during 14 July–30 September 2015 and during 1 April–30 September 2016. No monitoring data were collected from October to the end of March, because the temperature was generally low enough that water froze after injecting the tensionmeter, which could not measure SMP after freezing. During this period, winter wheat grew slowly and soil water and salt movement were minimal. TDS values were calculated by measuring the main ions of soil pore water samples by ion chromatography (ICS-600). Soil pore water samples were collected by soil water sampler and extracted by vacuum pump. Sampling of soil pore water occurred every day after irrigation and after a large precipitation event for 7 days, and every 7 days at other times. Water samples were stored in 30-ml polyethylene bottles, transported to the laboratory, and analyzed within 1 week. All samples were sealed with adhesive tape to reduce evaporation. According to historical weather records, 2015 and 2016 were defined as a wet year and a dry year with precipitation of 755.4 mm and 484.2 mm, respectively.
Precipitation and other meteorological parameters were recorded by a standard weather station at Nanpi Eco-Agricultural Experimental Station, located at 38° 06′ N, 116° 40′ E. Data analysis was conducted using SPSS 19.0, SigmaPlot 12.5, and Excel 2013.
3 Results and discussion
3.1 The dynamics of soil matric potential
Descriptive statistics of soil matric potential in 2016
3.2 Soil water movement after heavy precipitation
The results showed that there was deep leakage at site 1, site 2, and site 4 after the second heavy precipitation, and it was much more significant in pushing the wetting front to the bottom of the vadose zone. This indicated that shallow groundwater could be recharged at the four sites after the second heavy precipitation, except at site 3 with thick and heavy soil texture layer. In relatively homogeneous soils, the response time of changes in SMP from precipitation was short due to silt loam soils with a faster infiltration rate (site 4). However, hysteresis existed in heterogeneous soils (especially at site 1 and site 2), and soil texture with clay loam and silt clay loam blocked water from moving to deeper depths that resulted in more water stored above 70 cm at site 3. The differences in water infiltration and redistribution were mainly attributed to different soil textures. In a layered soil, water dynamics was affected not only by the interlayer properties and the thickness of the layers, but also by their spatial configuration. Soils with textural layering in profile impeded vertical water movement because of the discontinuity in hydraulic properties (Zornberg et al. 2010; Zettl et al. 2011). Soil profiles with vertical heterogeneity in soil texture hindered vertical water movement, reduced percolation, and stored more water than homogeneous soils with a similar vertical texture (Zornberg et al. 2010; Huang et al. 2011).
3.3 Mechanism of soil water movement
Nevertheless, this did not preclude temporal occurrence of preferential flow, especially in shallow layers. Preferential flow was faster than average water movement that moved along a fraction of the pore space, thereby bypassing most of the matrix (Baram et al. 2012). Preferential flow was expected to have been eliminated by the wetting-front propagation process, which wetted the entire domain, even if it did develop on the wetting front (Rimon et al. 2011). There are some differences in variation in total water potential in the upper 100 cm at the four sites, which suggested that other processes resulted in differences in residence time, mixing effect or preferential flow. Total water potential increased rapidly after a heavy rain on 20 July within 30 cm, 70 cm, 50 cm, and 50 cm at site 1, site 2, site 3, and site 4, respectively, which corresponded to the interface of textural layers. The observed pattern of abrupt changes and rapid increases in the total water potential profiles cannot be explained by the slow matrix flow rate (Baram et al. 2012). These patterns were mainly caused by vertical heterogeneous structures, vegetation root channels, worm holes, and other disturbances that likely created macrospores in the soil to allow water to exhibit of preferential flow (Šimůnek et al. 2003; Li et al. 2013).
3.4 The dominant factors of soil salinity dynamics
The depth of salt accumulation was most thick at site 1, and TDS was largest at site 3. The accumulation layers of soil salt were 50–350 cm at site 1, 70–350 cm at site 2, 50–220 cm at site 3, and 30–170 cm at site 4. TDS decreased first and then increased in the accumulation layers, although TDS values below the accumulation layers showed the inverse tendency at the four sites. Soil layering diluted salt concentrations because of the increase of water content in layered soils (Sadegh-Zadeh et al. 2009). Overall, the TDS was lowest at site 4, followed by site 2 and site 1, and it was largest at site 3. Soil salt leaching was largest at site 4, and least at site 3. This was due to the relatively homogeneous soil texture that resulted in a greater infiltration rate of water and a greater leaching rate of salt at site 4. This was quite different from that in homogeneous soils, because of the discontinuity in hydraulic properties, and both the soil texture and the layering of differently textured soils affected salt migration (Li et al. 2013). Consequently, salt accumulation in the root zone was also tightly linked to the configuration of differently textured soil layers, especially at site 3 with thick loam layers that blocked water and salt transport. Thus, soil texture and layered soil were the dominant factors that influenced the concentration and migration of salt at our sites. Homogeneous soils were more suitable for irrigation with brackish water, and irrigating with brackish water under soils with thick layers of clay loam or silt clay loam should be avoided, especially at shallow depths.
This study investigated mechanisms of soil water movement and salt migration by monitoring SMP and TDS under different soil textures, groundwater depths, and land use in a lowland area of the NCP. The results showed that soil water movement generally occurred with heavy precipitation events during the rainy season. Following heavy precipitation during the rainy season, a rapid increase in SMP was recorded, especially at shallow depths. The differences in SMP among the four sites were mainly due to land use and groundwater depth.
The dominant flow mechanism of soil water that moved downward was piston flow during the percolation process at four sites. However, water movement at the four sites was more complex in the upper depths (0–100 cm) than at deeper depths.
Soil texture and its vertical heterogeneity caused the spatiotemporal distribution of water and salts in the layered soil profile to be very different from that in homogenous soils. The processes of infiltration and water redistribution were completed rapidly in relatively homogeneous soils after heavy rains. However, there was obvious hysteresis of SMP with an increase in soil depth in heterogeneous soils. Homogenous soils favored water infiltration, salt leaching, and groundwater recharge. Although the flow of soil water was blocked and salt accumulated significantly in heterogeneous soils, the blocking effects of clay loam on water and salt transport were more significant. The results provide guidance for irrigating with brackish water under different soil textures and soil layers with vertical spatial heterogeneity.
The authors gratefully appreciate the Nanpi Agro-ecology Research Station of Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences.
The authors received funding from the Science and Technology Service Network (STS) Program of Chinese Academy of Sciences (KFJ-STS-ZDTP-053), the National Natural Science Foundation of China (No. 41601216 and No. 41530859), the Key Support Program of Key Laboratory of Agricultural Water Resources, Chinese Academy of Sciences (ZZKT201603), the 100-Talent Project of Chinese Academy of Sciences, and the Natural Science Foundation of Hebei province (D2017503021).
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