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Water and salt exchange flux and mechanism in a dry saline soil amended with buried straw of varying thicknesses
Geoderma 365 (2020) 114213
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Water and salt exchange flux and mechanism in a dry saline soil amended with buried straw of varying thicknesses
T
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Hongyuan Zhang, Huancheng Pang, Yonggan Zhao, Chuang Lu, Na Liu, Xiaoli Zhang, Yuyi Li Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Straw layer Thickness Soil moisture China Salt flux Saline soils
Salt stress severely constrains crop productivity in arid lands of the world. Burying straw at the 40 cm soil depth plus plastic film mulching could mitigate root zone salinity, but little is known about how the thickness of buried straw affects soil water and salt transport. Therefore, a three-year field experiment was conducted from 2010 to 2013 to address this issue, with treatments including: compacted straw thickness of 3 cm (T3), 5 cm (T5), or 7 cm (T7) (corresponding to straw application at rates of 6, 12 and 18 t ha−1, respectively). In addition, a supplementary experiment, which included treatments of no buried straw layer (CK) and 5 cm of compacted straw layer thickness (t5) in the same micro-plot experiment, was carried out from 2014 to 2016 to identify soil pore structure and hydraulic parameters after three years of deep straw burial. Results showed that the initial soil water content increased with increasing thickness but significantly (P < 0.05) decreased in later periods. Soil salinity consistently decreased with increasing straw thickness, and T7 was most effective for improving salt leaching. However, the water and salt regulation under T5 gradually decreased mainly due to the change in saturated water conductivity and porosity among layers. After irrigation, the flux of salt leaching (FL) increased with straw thickness, and the FL under T7 significantly exceeded that under T3 by 105, 89 and 33% and that under T5 by 84, 66 and 33% in 2011, 2012 and 2013, respectively. The salt flux during evaporation under T7 was much higher (P < 0.05) than that under T3 and T5 by 92 and 10% in 2012 and 38, 44% in 2013. At harvest, salt storage within the soil consistently ranked as T3 > T5 > T7. Although T7 had the most pronounced effect on salt mitigation, it was difficult to implement under normal field conditions. Thus, for relatively good water infiltration, salt leaching and inhibition of salt return, straw buried to a thickness of 5 cm is recommended.
1. Introduction Soil salinization is an ecological disaster that causes degradation of soil quality, especially in arid and semi-arid areas. This problem is listed among the major ecological and economic issues in the world (Qadir et al., 2000). At present, the global salinized land area is about 1 billion ha, constituting one of the most pressing challenges to sustainable use of soil resources (Chernousenko et al., 2017). In China, the total area of saline-alkali and secondary saline-alkali land amounts to 36 million ha, accounting for nearly 5% of the global salinized land area (Yang, 2008). Since the saline-alkali land accounts for one fourth of the cultivated land in China, rational use and mitigation of saline soil resources are essential to alleviate the increasing food insecurity (Wang et al., 2013; Yang et al., 2019). To mitigate the effects of salt stress on plants, humans seek to leach soils by flooding or drip irrigation with fresh or brackish water to
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reduce the salt content in the surface soil (Qadir et al., 2000; Kang et al., 2012; Fan et al., 2012a). However, the salts in the deep soil layers and shallow groundwater could move upward and accumulate in the surface soil through a capillary process. To solve this problem, establishment of interlayer in the subsoil is needed as it could act as an effective barrier to capillary movement and improve soil quality in salinealkali areas (Akudago et al., 2009; Zhang et al., 2016). The capillary barrier, consisting of sand, gravel, or coarse layer, is important for infiltration of irrigation water in soil strata (Fredlund et al., 2002; Kampf et al., 2003; Guo et al., 2006, 2007; Jia et al., 2006), preventing salt accumulation in the surface soil, and consequently alleviating salt stress on crops (Sun et al., 2011). When other conditions are constant, the interlayer thickness is one of the determinants for the maximum rise of the capillary water in layered soils (Wang et al., 2014). The thicker the interlayer, the longer the retention time of the capillary water in the interior, and the more effective of the interlayer in inhibiting water
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.geoderma.2020.114213 Received 18 June 2019; Received in revised form 12 January 2020; Accepted 20 January 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved.
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than the annual rainfall. The annual average temperature is 8.1 °C, with monthly averages ranging from 23.8 °C in July to −10.1 °C in January (Wu et al., 2008). Mean monthly precipitation and pan evaporation during the experimental years and over the 10 years prior to the experiments (2001–2010) are presented in Fig. S1. The groundwater table at this site fluctuated between 1.2 and 2.6 m during the sunflower growth period (May to September) and salt concentration in the groundwater varied from 1.5 to 1.8 g L–1. The soil was silty loam with severe salinization. The main soil physico-chemical properties at the start of the field experiments are presented in Table S1.
evaporation (Aubertin et al., 2009). A study indicates that the thickness of the underlying coarse layer also plays a critical role in governing the performance of the capillary barrier (Qian et al., 2010). Although a greater thickness of the capillary barrier layer leads to better blocking of solute, under certain conditions of deep diving, there is an upper limit on the thickness index of the barrier that prevents water and salt movement (Qian et al., 2010). Due to the advantages of easy access and low cost, straw has received more and more attention as a potential barrier material for improving saline soils (Chen et al., 2016). In principle, burying a straw layer below the soil surface can effectively cut capillaries in the soil and prevent salt moving upward (Qiao et al., 2006; Chen et al., 2016). Under steady-state flow conditions, a capillary barrier can form at the interface between soil and the straw layer with contrasting hydrological properties; the barrier can substantially reduce the infiltration of water into the underlying region, and consequently alleviate salt stress effects on crops (Qiao et al., 2006). Soil pore is closely related to water infiltration and plays an important role in soil water and salt transport (Smagin et al., 2008). As a way of returning straw to the soil, burying a straw layer can improve soil porosity and aeration (Tejada et al., 2008). A buried straw layer can improve aggregation and soil structure, thereby increasing water storage in the soil (Cao et al., 2012). However, unlike other interlayer materials, the efficiency of a straw interlayer can be influenced by the decomposition process. With the increase in years of deep straw burial, the degree of straw decomposing is increased, and the effect of the interlayer is weakened (Bastian et al., 2009). Therefore, the thickness of the straw barrier is one of the important factors affecting the regulation of water and salt in subsoils. Despite the beneficial effects of burying a straw layer in saline soils, there is a paucity of information on the relative effects of burying straw layers at different thicknesses on the flux of water and salt exchange. Previous research has investigated buried straw layers at converted thickness ranging from 3 to 5 cm (Qiao et al., 2006; Cao et al., 2012; Fan et al., 2012b; Zhao et al., 2013), but the effects of different straw layer thicknesses on water and salt distribution in the soil need further investigation. We hypothesized that different thicknesses of straw layer affect soil water infiltration and the exchange rate of salts above and below the barrier layer, and that a thicker straw layer is favorable for salt leaching. Also, as the straw is gradually decomposed over time, the difference in regulating water and salt transport among different thicknesses of straw layer will decrease, and therefore, there is an optimum thickness of the straw layer allowing the salt to penetrate the layer into the deeper soil layer, and reducing the upward movement of salt below straw layer. In order to test the hypotheses, through two sets of field micro-plot experiments, we examined the effects of different straw layer thicknesses on soil water and salinity dynamics, salt exchange flux and distribution; also, we studied the internal pore structure and saturated hydraulic conductivity after decomposition of straw layer, using the CT scanning technology. Our objectives were to: (i) clarify the effects of thickness of buried straw on soil water and salt regulation and its process influencing salt exchange between the upper and lower soil layer; (ii) explore the mechanism of water and salt regulation by the straw layer; and (iii) determine the relatively optimum straw layer thickness.
2.2. Experimental design and implementation In the field, a randomized experimental design was used, with treatments being randomly assigned to micro-plots (plot size: 1.8 m × 1.8 m). When constructing the plots, the surrounding areas of each plot was scuttled to 1 m, and insulated by double-plastic sheets buried to a depth of 100 cm from the soil surface to minimize the effects of lateral water and salt movement between plots. The middle gap was filled with soil, and each plot was surrounded by a concrete panel with a dimension of 40 cm wide and 60 cm high, and the exposed part of the panel (approximately 20 cm) was hardened by cement. The procedure of plot construction minimized disturbance of internal soil of the plots. 2.2.1. Experiment 1: Effect of different straw interlayer thickness on water and salt regulation In order to clarify the effect of different straw layer thickness on water and salt regulation, three treatments were studied. Treatment I: burial of a 3-cm-thick layer of maize straw (T3) at the rate equivalent to the averaged amount of straw produced in a maize field (6 t ha−1); Treatment II: burial of a 5-cm-thick layer of maize straw (T5), at a straw rate doubled the field maize straw production amount (12 t ha−1). Treatment III: burial of a 7-cm-thick layer at a straw rate of 18 t ha−1 or thrice the averaged maize straw amount produced per year (T7). In all treatments, straw was cut to segments of 10–15 cm and buried at the soil depth of 40 cm, and the surface soil was mulched with plastic film. A sketch of the plot with straw layer burial and plastic film mulch is shown in Fig. S2. The straw layer was buried at the beginning of the experiment and no additional straw was buried in the 2nd and 3rd years. For burying straws, first, soil layers of 0–20 cm and 20–40 cm in the micro-plots were removed by spade; second, air-dried and chopped maize straw was placed; finally, soils were returned to the plot in the order of soil layers. In the plots under T3, T5 and T7 treatments, 1.94, 3.89 and 5.83 kg of straw was buried each plot at compacted thickness of 3, 5 and 7 cm, respectively. The scale was marked on the cement and two parts of dug soil layer-by-layer to 20 cm and 40 cm layers above the straw interlayer was refilled, and then levelled with a harrow. After 5 d, plots were flood-irrigated with water from the Yellow River, which had a mineralization degree of 0.58 g L–1. The irrigation method in this study was the combination of autumn and spring irrigation, which is the traditional local irrigation system. In October each year, approximately 0.6 m3 water was applied to each plot after harvest (i.e., 1850 m3 ha−1), for which water volume was measured with a water meter. The second irrigation (0.6 m3 plot−1) was applied approximately 10 d before sowing in May each year. Fertilizer was applied at 180 kg ha−1N (urea, 46% N), 120 kg ha−1 P2O5 (diammonium phosphate, 18% N and 46% P2O5) and 75 kg ha−1 K (potassium sulphate, 50% K2O). Plots were mulched with plastic film before sowing and sunflower seeds were sown with a hill-drop planter. No further irrigation and fertilizer was applied to the sunflower. Sunflower (cv LD 5009) was seeded at a row spacing of 60 cm and density of 49,000 plants per hectare. Seeding was done manually on 28 May 2011, 8 June 2012 and 2 June 2013, and the crop was harvested on 23 September 2011, 18 September 2012 and 16 September 2013, respectively. After harvest, the stalks and plastic film residues were
2. Materials and methods 2.1. Site description Field experiments were conducted from October 2010 to September 2013 at the experimental station of the Management Department of Yichang Irrigation Sub-district (41°04′N, 108°00′E, 1022 m ASL), located in Wuyuan county, Inner Mongolia, China (Fig. 1). The region has an arid climate, for which the average annual rainfall is 173.5 mm and more than 70% of the rainfall occurs from July to August. The mean annual evaporation is approximately 2068 mm, being 11 times higher 2
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Fig. 1. Location of the study area and sampling fields. The test area was in Wuyuan county, Inner Mongolia, China (the pentagram in the picture on the left).
laboratory and oven-dried at 105 °C for about 24 h to determine the gravimetric soil water content. After drying, the samples were ground to pass through a 2 mm sieve and saturated with water at a soil/water ratio of 1:5 to determine electrical conductivity. The electrical conductivity of the extract was measured using a DDS-307 conductivity meter (Shanghai TecFront Electronics Co., Ltd., Shanghai, China). The soluble salt content was measured by using the residual drying method. Specifically, we added 100 mL leachate into a 150 mL evaporating dish that had been dried to a constant weight, placed the dish on a hot plate, and heated at 250 °C. After the leachate was completely evaporated, the evaporating dish was transferred to a constant temperature drying oven and dried at 105 °C for 2 h. Then, the dish was transferred to a desiccator to achieve constant weight, and results were recorded and used to calculate the total salt content in leachate by a subtraction method.
removed from the soil surface, and each plot was ploughed to a depth of 20 cm with a spade. 2.2.2. Experiment 2: Mechanisms of straw layer controlling water and salt transport Based on the results of Experiment 1, a supplementary experiment was carried out from 2014 to 2016 in the same micro-plots to determine soil pore structure, hydraulic parameters, and water and salt regulation after three years of deep straw burial. Two treatments were studied in this step: treatment I with no straw layer added (CK), and treatment II with a burial of 5-cm-thick maize straw layer (t5), the same treatment as T5 in experiment 1. Each treatment was repeated three times. The experiment was arranged in the fall of 2013. Micro-plots construction and crop cultivation were consistent with experiment 1. Sunflower (cv LD 5009 in 2014–2015, JK601 in 2016) was seeded at a row spacing of 60 cm and density of 49,000 plants per hectare. Seeding was done manually on 8 June 2014, 5 July 2015 and 18 May 2016, and the crop was harvested on 20 September 2014, 23 September 2015 and 19 September 2016, respectively. Other management practices are the same as those used in experiment 1.
2.4. CT scanning and image processing In order to determine soil pore structure after three years of straw burial, soil profile was excavated after harvesting sunflower in 2016, for which a PVC-made ring cutter was used to collect the soils in the upper part of straw layer (35–41 cm from the soil surface), the straw layer (41–47 cm), and the lower part of the straw layer (47–53 cm) for CT scanning. The PVC ring cutter had a diameter of 50 mm and a height of 60 mm. Sampled soil scanning was performed using the Nano-Industrial Micro-CT (Phoenix NanotomS, GE, USA) in the Nanjing Institute of Soil Science, Chinese Academy of Sciences. The scan parameters were set to a scan voltage of 100 kV, a scan current of 100 μA, and a resolution of 30 μm for each process. The projection data was reconstructed using a rear projection algorithm, and a total of about 2000 32-bit tiff format grayscale images were obtained. Image was processed using Image J software, as described by Udawatta et al. (2006). After measuring the pore structure of soil samples, saturated hydraulic conductivity was measured by using saturated hydraulic conductivity meter under a constant head condition (Sepaskhah and Karizi, 2011).
2.3. Soil sampling and measurements Weather data were obtained from a weather station on the experimental site. Soil samples were taken at 0–20, 20–40, 40–60, 60–80 and 80–100 cm depth between two rows of sunflowers in each plot every 15 days, using a soil auger (36 mm in diameter) (Nanjing Aofeng Mechanical and Electrical Co.). The samples were collected from the center of each plot (under plastic film) between two sunflower rows. After each sampling, soil of the same texture was used to fill the original sampling hole to minimize the effects of soil sampling on crop growth and water and salt measurements. Sampling was delayed 1–2 days when there was rainfall. In the same way, additional soil samples were taken at sowing and 2–3 days after harvesting, respectively. Information on sampling date is provided in Table S2. All the sampled soils were immediately transported to the 3
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Where FL is the salt leaching flux (g m−2 d−1); SSB is the salt storage before spring irrigation (g); SSA salt storage after spring irrigation (g); A is the cross-sectional area of the micro-plot or soil column (m2); and Δt is the interval number of days between pre and post-irrigation (days). To determine the effects of straw layer on salt exchange during phreatic evaporation, the evaporation flux of salt in the 0–40 cm soil layer (FE) was calculated with Eq. (4):
FE = (SSH − SSA) (A × Δt )
(4) −2
−1
d ); SSH is the salt Where FE is the salt evaporation flux (g m storage at harvest (g); SSA is the salt storage after spring irrigation (g); A is the cross-sectional area of the micro-plot or soil column (m2); and Δt is the interval number of the days from the end of the spring irrigation infiltration to the harvest (days). Soil saturated hydraulic conductivity (steady infiltration rate of soil) was measured by the cutting ring method, and it was calculated with Eq. (5):
V = (Qn × 10) (Tn×S)
(5) −1
Where V is the infiltration rate of soil at a given time (mm min ); Qn is the water injection amount in the nth measurement time (ml); Tn is the time interval in the nth measurement time (min); and S is the area of permeable surface (cm2). Soil desalination rate was calculated as with Eq. (6):
P(%) = (SB − SA) SB × 100
(6)
Where P is the salt desalination rate (%); SB is the salt content the previous year (g kg−1); and SA is the salt content the current year (g kg−1). Stratification ratio of salt storage was calculated as with Eq. (7):
SR = SS0 − 40cm SS40 − 100cm
(7)
Where SR is the stratification ratio of salt storage; SS0-40cm is the salt storage in 0–40 cm soil layer (g); and SS40-100cm is the salt storage in 40–100 cm soil layer (g).
Fig. 2. Water content in the 0–40 cm soil layer under different treatments during the sunflower growing seasons of 2011 (a), 2012 (b), 2013 (c). T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
2.6. Data analysis 2.5. Calculations
All data within each individual year were analyzed using the analysis of variance (ANOVA) to test the treatment effects on the measured parameters. Mean comparisons were performed using the Fisher’s LSD test at P < 0.05. Statistical analysis of saturated soil hydraulic conductivity and laboratory simulation data of different treatments were performed using independent sample T-test. The analysis was conducted using the SPSS 13.0 program.
−1
The relationship between soil salinity content (g kg ) and electrical conductivity in soil extracts (ms cm−1) was studied, and the following fitting equation was obtained by regression statistical analysis with Eq. (1):
y= 3.0111x
(1)
where y is the salt contents of the soil (g kg−1), x is the electrical conductivity of soil extracts (ms cm−1). The coefficient of determination (r2) indicated a significant correlation between salt content and electrical conductivity (r2 = 0.98, P < 0.01). Salt storage was calculated with Eq. (2):
SS = A × D × H × y × 10−3
3. Results 3.1. Effects of different straw layer thicknesses on water distribution In 2011 (Fig. 2a), the soil water content changed little among treatments within 49 days after sowing (DAS). Significant differences among these treatments were only observed in the late periods of the season (P < 0.05), except between T5 and T7 at 94 DAS, and between T3 and T5 at 109 DAS. In 2012 (Fig. 2b), during the entire growing season, there was a consistently significant difference between T3 and T7 except at 76 DAS. In addition, a significant difference (P < 0.05) occurred between T5 and T7 at 15, 61, 92 and 106 DAS. In 2013 (Fig. 2c), T7 and T3 only differed significantly at sowing and 45 DAS (P < 0.05). After 62 DAS, the soil water under T7 was much lower (P < 0.05) than that under T3 and T5, by 1.00–2.24% and 0.54–0.86%, respectively.
(2)
Where SS is the salt storage (g); A is the cross-sectional area of the micro-plot or soil column (cm2); D is the soil bulk density (g cm−3); H is the soil layer thickness (cm); and y is the salt content of soil layer (g kg−1). To determine the effects of straw layer on salt exchange during irrigation water infiltration, the leaching flux of salt in the 0–40 cm soil layer (FL) was calculated with Eq. (3):
FL = (SSB − SSA) (A × Δt )
(3) 4
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Fig. 3. Water distribution within the 0–100 cm soil profile at sowing (a, b, c) and after harvest (d, e, f) under different treatments in 2011, 2012, 2013, respectively. T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
3.2. Effects of different straw layer thicknesses on salt distribution
T3 had relatively lower water content than T5 and T7 below the 40 cm soil depth by 0.65% and 0.41% during sowing in 2011 (Fig. 3a), but the trend was reversed in 2012 (Fig. 3b). In 2013 (Fig. 3c), T7 had much more water than T3 and T5 by 0.96% and 0.76%, respectively (P < 0.05). At harvest in 2011 (Fig. 3d), T7 and T5 both retained significantly more water than T3 (P < 0.05). There was no significant difference among treatments in 2012 (Fig. 3e), whereas in 2013 (Fig. 3f), T7 retained much more water than T3 and T5 by 1.25% and 1.78% at the 60–100 cm depth, respectively (P < 0.05).
In 2011, (Fig. 4a), significant differences (P < 0.05) in salt content were observed between T3 and T7 at 33, 49, 80, 94 and 109 DAS, and between T3 and T5 at 33 and 49 DAS. In 2012 (Fig. 4b), a significant difference was consistently observed among T3, T5 and T7 before 76 DAS, for which the salt content in the 0–40 cm soil depth under T7 was 20–37% and 14–30% lower (P < 0.05) than that under T3 and T5. In 2013 (Fig. 4c), significant differences in salt content were observed between T3 and T7 at 62, 74, 91, and 109 DAS (P < 0.05). Below the 40 cm soil depth at sowing (Fig. 5a, 5c), the salt content 5
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3.4. Effects of straw layer on saturated water conductivity and soil pore characteristics after three years of deep burial Table 2 showed that the saturated hydraulic conductivity under the straw layer treatment (t5) was not significantly different from that under the control treatment after straw layer was buried for 3 years. Among all the soil layers, the ratio of the equivalent diameter of 0.1–0.5 mm porosity to the total porosity was as high as 56.17–78.17% (Table 3). Compared with CK, the total porosity and the equivalent diameter > 0.5 mm porosity at the straw layer under t5 increased (P < 0.01) by 2.41% and 2.05%, respectively. Below the straw layer, there was no soil pores with an equivalent diameter of > 1.5 mm in both CK and t5 treatments. However, CK showed a significant increase of 0.13% (P < 0.01) in the 1.0–1.5 mm pore size compared to t5. 4. Discussion 4.1. Soil water movement and distribution In this study, the water content in the 0–40 cm soil depth of all treatments showed a gradual decrease during the sunflower growing seasons except for 2012 (Fig. 2). The overall result can be explained by the fact that the experimental field was located in the arid northwest region of China with a high evaporation-precipitation ratio. The exception of 2012 was due to a large rainfall event (about 120 mm) in July (Fig. S1), which led to a peak in the water content during crop growth (Zhang et al., 2016). In heterogeneous soils, the patterns of water movement and soil water depend not only on the texture of each soil layer, but also on the thickness of the barrier layer (Zhang et al., 2007). The difference in porosity between the soil and straw layer influenced its hydraulic conductivity and the soil water potential mutated at the interface (Bernadiner, 1998), resulting in corresponding changes in water infiltration patterns (Cao et al., 2012; Kumar et al., 2012; Zhang et al., 2019). In the present study, there were significant differences in the soil water (0–40 cm) among treatments with different thicknesses in the first year. In the second and third years, nonetheless, only T7 treatment had significantly higher soil water content than the other two treatments in the whole growth period (Fig. 2). This indicated that the thicker straw barrier layer had greater effect on water content in the 0–40 cm soil layer with the increase of years. Zou et al. (2014) also found that in the third year of straw burial, a higher water retention effect occurred in the treatment with larger amounts of maize straw. However, for treatments with thinner straw layers, the difference in straw layer thickness gradually decreased with the increase of the buried age due to compaction and decomposition. As a result, the water content difference among the treatments gradually decreased (Zou et al., 2014). Furthermore, the difference in saturated water conductivity between treatments also decreased (Table 2), which is related to the proportion of soil pores with an equivalent diameter of 0.1–0.5 mm (Table 3). The water content of the 60–100 cm soil layer under T7 was significantly higher than that under T3 and T5 in 2011 and 2013 (Fig. 3), because the effect of blocking soil capillary continuity increased with increasing straw layer thickness, reducing the upward migration of water from deeper soils (Zhao et al., 2016). Additionally, with the decomposition of straw, the increase in soil organic carbon could improve the condition of soil aggregates, enhance soil water holding capacity and reduce soil water diffusivity, thereby increase the soil water retention effect in the third year (Ouattara et al., 2006; Huo et al., 2017). However, the soil water in the 60–100 cm layer whether in the seedling or harvest stage in 2012 showed different trends from that in 2011 and 2013. This may be related to the extreme rainfall in 2012, in which rainfall from sunflower seedling to maturity was much higher than that for the same period in 2011 and 2013 (Fig. S1). Too much rainfall was transformed into soil water, which was easier to replenish the 60–100 cm soil layer through a thinner straw barrier layer (Gao et al.,
Fig. 4. Salt content in the 0–40 cm soil layer under different treatments during the sunflower growing season in 2011 (a), 2012 (b), 2013 (c). T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Soils in all treatments were mulched with a plastic film. Bars represent LSD at P = 0.05.
under T7 was consistently higher (P < 0.05) than that under T3 and T5 by 28–31% and 6–17% in 2011, and 19–32% and 8–17% in 2013. Notably, the trend was opposite in 2012, when the salt content below the 40 cm soil depth under T7 was lower than that under T3 and T5 by 17–26% and 11–16%, respectively (Fig. 5b). After harvesting sunflower (Fig. 5d-f), compared with T3 and T5, the average salinity within the 100 cm profile under T7 decreased by 24.16% and 13.07% in 2011, 14.92% and 6.91% in 2012, and 18.62% and 13.58% in 2013 (P < 0.05). 3.3. Effects of different straw layer thicknesses on soil salt flux The flux of salt in the 0–40 cm soil after soil leaching (FL) increased with an increasing straw layer thickness, and the differences among the three treatments gradually decreased with time (Table 1). The FL under T7 was higher than that under T3 and T5 by 105.35% and 83.81% in 2011, 89.16% and 66.21% in 2012, and 33.19% and 33.03% in 2013 (P < 0.05). The salt flux in the 0–40 cm soil layer during soil evaporation (FE) decreased with increasing straw layer thickness in 2011, and the FE under T5 and T7 was much lower than that under T3 by 58.73% and 54.41%, respectively. Compared with 2011, the FE in 2012 and 2013 showed the opposite trend. The FE under T7 exceeded that under T3 and T5 by 91.71% and 10.45% in 2012, and 37.86% and 44.03% in 2013 (P < 0.05). 6
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Fig. 5. Salt distribution within the 0–100 cm soil profile at sowing (a, b, c) and after harvest (d, e, f) under different treatments in 2011, 2012 and 2013. T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
et al., 2016). Further, from the annual variation of desalination rate in 0–40 cm soil layer, the effect of straw layer on promoting salt downward leaching was mainly reflected in the first year. However, with the extension of time, the accumulated salt in straw layer or deep soil may gradually move upward with evaporation (Zhang et al., 2019), especially for T7 treatment with the highest FE value (Table 1). Thus, the desalination rate of the 0–40 cm soil layer was low in T7 compared with T3 (23.6% and 10.0% in 2012, and 9.1% and 5.0% in 2013 under T3 and T7, respectively). However, since the value of FL was much higher than that of FE (Table 1), the salinity of 0–40 cm layer under T7 was still the lowest. Since plenty of water was stored in the topsoil under T7 at seedling
2010), so soil water under T3 was higher than that of the treatment with a thicker straw layer. 4.2. Salt movement and distribution With the increase of burial years, the regulation of 0–40 cm soil salinity by straw layer with different thicknesses was weakened (Fig. 4), and this is consistent with the result of water content. In 2012, the regulation effect of straw layer on salinity may also be affected by rainfall (Fig. S1), which promoted the efficiency of salt leaching and changed the distribution of salinity. Therefore, the significant difference between treatments was reflected in the early stage of 2012 (Zhao 7
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Table 1 Salt accumulation and flux in the 0–100 cm soil profile as affected by straw layer treatments from 2011 to 2013. Treatments
T3 T5 T7
2011
2012 −2
−2
2013
SS1 (g)
SS2 (g)
FL (g m d−1)
FE (g m d−1)
SS1 (g)
SS2 (g)
FL (g m d−1)
7035.83 a 5704.96b 5324.91c
12233.10 a 10672.77b 9277.37c
15.70c 17.54b 32.24 a
4.87 a 2.01b 2.22b
5373.49 a 5171.82b 4793.12c
12033.22 a 10997.96b 10238.32c
41.79c 47.56b 79.05 a
−2
−2
FE (g m d−1)
SS1 (g)
SS2 (g)
FL (g m−2 d−1)
FE (g m−2 d−1)
1.93c 3.35b 3.70 a
4884.99 a 4861.69 a 4553.35b
6300.12 a 5970.54 a 4549.05b
41.82b 41.87b 55.70 a
1.40b 1.34b 1.93 a
Note: T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. FL, and FE indicate salt flux in the 0–40 cm soil layer during leaching and evaporation; SS1, SS2 indicate salt storage at harvest in 0–40 cm and 0–100 cm soil layers. Treatment effects were evaluated within an experimental year, and significant differences among treatments are indicated by different lower case letters following the mean values (P < 0.05). Table 2 Saturated water conductivity under different treatments and soil layers in another field experiment after straw burial for three years. Treatments
SCU (mm min−1)
SCS (mm min−1)
SCL (mm min−1)
CK t5 t
0.00080 ± 0.00005 0.00148 ± 0.00051 −1.32
0.00084 ± 0.00040 0.01120 ± 0.00081 −12.83
0.00370 ± 0.00036 0.00236 ± 0.00052 2.59
Note: CK, no straw layer; T5, buried maize straw layer with thickness of 5 cm. SCU, saturated water conductivity in the upper soil layer; SCS, saturated water conductivity in the straw layer; SCL, saturated water conductivity in the lower soil layer. Values are means of three replicates ± standard deviation. t is the t value obtained from an independent sample T-test, * significant at the 0.05 probability level, ** significant at the 0.01 probability level. Table 3 2-D geometric soil pore characteristics in Experiment 2 after straw burial for three years. Soil depth
Soil above straw interlayer
Straw interlayer soil
Soil below straw interlayer
Treatments
CK t5 t CK t5 t CK t5 t
Different equivalent diameter porosity (%)
Total Porosity (%)
0.1–0.5 mm
0.5–1.0 mm
1.0–1.5 mm
1.5–2.0 mm
2.0–2.5 mm
2.5–3.0 mm
3.22 ± 0.48 4.23 ± 0.46 −15.141 3.51 ± 0.13 3.87 ± 0.57 −8.570 4.47 ± 0.45 5.20 ± 0.49 −16.165
2.21 ± 0.40 1.84 ± 0.09 −3.620 0.84 ± 0.04 2.11 ± 0.05 −4.112** 1.36 ± 0.041 0.72 ± 0.64 8.361
0.83 ± 0.04 0.62 ± 0.02 5.310 0.13 ± 0.04 0.66 ± 0.01 −2.592* 0.15 ± 0.37 0.02 ± 0.005 3.492**
0.40 ± 0.09 0.12 ± 0.05 5.139** 0.003 ± 0.0002 0.17 ± 0.01 −5.114* – –
0.10 ± 0.05 0.07 ± 0.06 0.625* – 0.08 ± 0.003
0.03 ± 0.001 0.167 ± 0.03 −4.682* – –
– –
– –
6.79 ± 0.15 7.04 ± 0.25 −8.570 4.48 ± 0.80 6.89 ± 0.52 −4.558** 5.98 ± 0.48 5.94 ± 0.56 0.544
Note: CK, no straw layer; T5, buried maize straw layer with thickness of 5 cm. Values are means of three replicates ± standard deviation. t is the t value obtained from an independent sample T-test, * significant at the 0.05 probability level, ** significant at the 0.01 probability level.
worth mentioning that the SR value of T5 was low in all three consecutive years, showing a relatively stable change.
stage after spring irrigation, it promoted ion exchange and increased salt leaching (Gabriel et al., 2012; Stagnari et al., 2014), thereby the increase of salt content in the 60–100 cm soil layer was significantly higher. This suggested that the salt leaching depth under T7 can be deepened. However, due to the excessive rainfall in May 2012 relative to 2011 and 2013, the salt content in deeper soil under T7 was much lower than that under T3 and T5 in 2012, which indicated that the desalination depth under T7 may exceed 1 m. We also found that the salt content in the 60–100 cm soil layer under T7 was significantly lower than that of other treatments at harvest. Because the straw layer acted as a deep cover, which slowed the evaporation rate of groundwater to some extent (Sun et al., 2011), T7 reduced the salt content below the straw layer. Qiao et al. (2006) also showed that deep straw mulching prevented water loss below the straw layer during evaporation, which produced a positive effect on maintaining water in deep soils. The ratio of 0–40 cm to 40–100 cm salt reserves (SR) can also reflect the desalination status of crop root zone among different treatments (Zhang et al., 2014). The SR value of T3 and T5 was less than 1 in 2012–2013, but that of T7 was greater than 1 in 2013, which implied that the thicker straw layer was not conducive to accelerating salt leaching in the 0–40 cm layer at the later stage of the experiment and there may still be some risk of secondary salinization of the soil. It is
5. Conclusions This study demonstrated significant effects of the thickness of buried straw layer on water and salt distribution in a dry saline soil. Compared with T3 and T5, a thicker straw layer (T7) significantly decreased soil water content in the later stage of sunflower growth. T7 decreased salt content in the 0–40 cm soil layer by 3.07–36.82% throughout the sunflower growth period compared to T3 and T5. Furthermore, the salt leaching flux increased with increases in straw thickness, however, the thicker straw layer increased (the rate of) salt upward movement during evaporation due to salt accumulation in the straw layer, and there may still be some risk of secondary soil salinization. Given the fact that T5 is more practical for field applications and more efficient in salt leaching and inhibiting salt return, it can be a beneficial management practice for comprehensive development and utilization of saline-alkali soils. Declaration of Competing Interest The authors declare that they have no known competing financial 8
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interests or personal relationships that could have appeared to influence the work reported in this paper.
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Relationship between soil structure and runoff/soil loss after 24 years of conservation tillage. Soil Till. Res. 92, 122–128. https://doi.org/10.1016/j.still.2006.01.006. Zhang, W.T., Wu, H.Q., Gu, H.B., Feng, G.L., Wang, Z., Sheng, J.D., 2014. Variability of soil salinity at multiple spatio-temporal scales and the related driving factors in the oasis areas of Xinjiang, China. Pedosphere 24 (6), 753–762. https://doi.org/10.1016/ s1002-0160(14)60062-x. Zhang, W.J., Sun, C., Qiu, Q.W., 2016. Characterizing of a capillary barrier evapotranspirative cover under high precipitation conditions. Environ. Earth Sci. 75 (6), 513. https://doi.org/10.1007/s12665-015-5214-9. Zhang, H.Y., Pang, H.C., Lu, C., Liu, N., Zhang, X.L., Li, Y.Y., 2019. Pore characteristics of straw interlayer based on computed tomography images and its influence on soil water infiltration. Trans. Chinese Soc. Agric. Eng. 35, 114–122 (in Chinese).
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos: 31871584, 31471455); the National Key Research and Development Program of China (Grant No: 2016YFC0501302); and the Special Fund for Agro-scientific Research in the Public Interest (Grant No: 201303130). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2020.114213. References Akudago, J.A., Chegbeleh, L.P., Nishigaki, M., Nanedo, N.A., Ewusi, A., Kankam-Yeboah, K., 2009. Borehole drying: a review of the situation in the voltaian hydrogeological system in Ghana. J. Water Res. Protection 1 (3), 153–163. https://doi.org/10.4236/ jwarp.2009.13020. Aubertin, M., Cifuentes, E., Apithy, S.A., Bussire, B., Molson, J., Chapuis, R.P., 2009. Analyses of water diversion along inclined covers with capillary barrier effects. Can. Geotech. J. 46, 1146–1164. https://doi.org/10.1139/T09-050. Bernadiner, M.G., 1998. A capillary microstructure of the wetting front. Transport Porous Med. 30, 251–265. Bastian, F., Bouziri, L., Nicolardot, B., Ranjard, L., 2009. Impact of wheat straw decomposition on successional patterns of soil microbial community structure. Soil Biol. Biochem. 41, 262–275. https://doi.org/10.1016/j.soilbio.2008.10.024. Cao, J.S., Liu, C.M., Zhang, W.J., Guo, Y.L., 2012. Effect of integrating straw into agricultural soil on soil infiltration and evaporation. Water Sci. Technol. 65, 2213–2228. https://doi.org/10.2166/wst.2012.140. Chen, S., Zhang, Z.Y., Wang, Z.C., Guo, X.P., Liu, M.H., Hamoud, Y.A., Zheng, J.C., Qiu, R.J., 2016. Effects of uneven vertical distribution of soil salinity under a buried straw layer on the growth, fruit yield, and fruit quality of tomato plants. Sci. Hortic. 203, 131–142. https://doi.org/10.1016/j.scienta.2016.03.024. Chernousenko, G.I., Pankova, E.I., Kalinina, N.V., Ubugunova, V.I., Rukhovich, D.I., Ubugunov, V.L., Tsyrempilov, E.G., 2017. Salt-affected soils of the Barguzin depression. Eurasian Soil Sci. 50 (6), 646–663. https://doi.org/10.1134/ s1064229317060035. Fan, X., Pedroli, B., Liu, G., Liu, Q., Liu, H., Shu, L., 2012a. Soil salinity development in the yellow river delta in relation to groundwater dynamics. Land Degrad. Dev. 23, 175–189. https://doi.org/10.1002/ldr.1071. Fan, F., Xu, S.J., Song, G.Y., Zhang, Q.G., Hou, M.H., Song, X.F., 2012b. Studies on improvement of saline and alkali soil with the interlayer of maize straw in West Liaohe region. Chin. J. Soil Sci. 43, 696–701 (in Chinese). Fredlund, M.D., Wilson, G.W., Fredlund, D.G., 2002. Use of the grain-size distribution for estimation of the soil-water characteristic curve. Can. Geotech. J. 39, 1103–1117. https://doi.org/10.1139/t02-049. Guo, G., Araya, K., Jia, H., Zhang, Z., Ohomiya, K., Matsuda, J., 2006. Improvement of salt-affected soils, Part 1: Interception of capillarity. Biosyst. Eng. 94, 139–150. https://doi.org/10.1016/j.biosystemseng.2006.01.012. Guo, G., Zhang, H., Araya, K., Jia, H., Ohomiyaand, K., Matsuda, J., 2007. Improvement of salt-affected soils, Part 3: Specific heat of salt- affected soils. Biosyst. Eng. 96, 413–418. https://doi.org/10.1016/j.biosystemseng.2006.11.007. Gao, F., Jia, Z.K., Lu, W.T., Han, Q.F., Yang, B.P., Hou, X.Q., 2010. Effects of different straw returning treatments on soil water, maize growth and photosynthetic characteristics in the semi-arid area of Southern Ningxia. Acta Ecologiac Sinica. 31, 777–783 (in Chinese). Gabriel, J.L., Almendros, P., Hontoria, C., Quemada, M., 2012. The role of cover crops in irrigated systems: soil salinity and salt leaching. Agr. Ecosyst. Environ. 158, 200–207. https://doi.org/10.1016/j.agee.2012.06.012. Huo, L., Pang, H.C., Zhao, Y.G., Wang, J., Lu, C., Li, Y.Y., 2017. Buried straw layer plus plastic mulching improves soil organic carbon fractions in an arid saline soil from northwest china. Soil Till Res. 165, 286–293. https://doi.org/10.1016/j.still.2016. 09.006. Jia, H., Zhang, H., Araya, K., Guo, G., Zhang, Z., Ohomiya, K., 2006. Improvement of saltaffected soils, Part 2: Interception of capillarity by soil sintering. Biosyst. Eng. 94, 263–273. https://doi.org/10.1016/j.biosystemseng.2006.01.013. Kumar, A., Kumar, P., Yadav, S.K., Hasija, R.C., 2012. Effect of integrated nutrient
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Water and salt exchange flux and mechanism in a dry saline soil amended with buried straw of varying thicknesses
T
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Hongyuan Zhang, Huancheng Pang, Yonggan Zhao, Chuang Lu, Na Liu, Xiaoli Zhang, Yuyi Li Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Straw layer Thickness Soil moisture China Salt flux Saline soils
Salt stress severely constrains crop productivity in arid lands of the world. Burying straw at the 40 cm soil depth plus plastic film mulching could mitigate root zone salinity, but little is known about how the thickness of buried straw affects soil water and salt transport. Therefore, a three-year field experiment was conducted from 2010 to 2013 to address this issue, with treatments including: compacted straw thickness of 3 cm (T3), 5 cm (T5), or 7 cm (T7) (corresponding to straw application at rates of 6, 12 and 18 t ha−1, respectively). In addition, a supplementary experiment, which included treatments of no buried straw layer (CK) and 5 cm of compacted straw layer thickness (t5) in the same micro-plot experiment, was carried out from 2014 to 2016 to identify soil pore structure and hydraulic parameters after three years of deep straw burial. Results showed that the initial soil water content increased with increasing thickness but significantly (P < 0.05) decreased in later periods. Soil salinity consistently decreased with increasing straw thickness, and T7 was most effective for improving salt leaching. However, the water and salt regulation under T5 gradually decreased mainly due to the change in saturated water conductivity and porosity among layers. After irrigation, the flux of salt leaching (FL) increased with straw thickness, and the FL under T7 significantly exceeded that under T3 by 105, 89 and 33% and that under T5 by 84, 66 and 33% in 2011, 2012 and 2013, respectively. The salt flux during evaporation under T7 was much higher (P < 0.05) than that under T3 and T5 by 92 and 10% in 2012 and 38, 44% in 2013. At harvest, salt storage within the soil consistently ranked as T3 > T5 > T7. Although T7 had the most pronounced effect on salt mitigation, it was difficult to implement under normal field conditions. Thus, for relatively good water infiltration, salt leaching and inhibition of salt return, straw buried to a thickness of 5 cm is recommended.
1. Introduction Soil salinization is an ecological disaster that causes degradation of soil quality, especially in arid and semi-arid areas. This problem is listed among the major ecological and economic issues in the world (Qadir et al., 2000). At present, the global salinized land area is about 1 billion ha, constituting one of the most pressing challenges to sustainable use of soil resources (Chernousenko et al., 2017). In China, the total area of saline-alkali and secondary saline-alkali land amounts to 36 million ha, accounting for nearly 5% of the global salinized land area (Yang, 2008). Since the saline-alkali land accounts for one fourth of the cultivated land in China, rational use and mitigation of saline soil resources are essential to alleviate the increasing food insecurity (Wang et al., 2013; Yang et al., 2019). To mitigate the effects of salt stress on plants, humans seek to leach soils by flooding or drip irrigation with fresh or brackish water to
⁎
reduce the salt content in the surface soil (Qadir et al., 2000; Kang et al., 2012; Fan et al., 2012a). However, the salts in the deep soil layers and shallow groundwater could move upward and accumulate in the surface soil through a capillary process. To solve this problem, establishment of interlayer in the subsoil is needed as it could act as an effective barrier to capillary movement and improve soil quality in salinealkali areas (Akudago et al., 2009; Zhang et al., 2016). The capillary barrier, consisting of sand, gravel, or coarse layer, is important for infiltration of irrigation water in soil strata (Fredlund et al., 2002; Kampf et al., 2003; Guo et al., 2006, 2007; Jia et al., 2006), preventing salt accumulation in the surface soil, and consequently alleviating salt stress on crops (Sun et al., 2011). When other conditions are constant, the interlayer thickness is one of the determinants for the maximum rise of the capillary water in layered soils (Wang et al., 2014). The thicker the interlayer, the longer the retention time of the capillary water in the interior, and the more effective of the interlayer in inhibiting water
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.geoderma.2020.114213 Received 18 June 2019; Received in revised form 12 January 2020; Accepted 20 January 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved.
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than the annual rainfall. The annual average temperature is 8.1 °C, with monthly averages ranging from 23.8 °C in July to −10.1 °C in January (Wu et al., 2008). Mean monthly precipitation and pan evaporation during the experimental years and over the 10 years prior to the experiments (2001–2010) are presented in Fig. S1. The groundwater table at this site fluctuated between 1.2 and 2.6 m during the sunflower growth period (May to September) and salt concentration in the groundwater varied from 1.5 to 1.8 g L–1. The soil was silty loam with severe salinization. The main soil physico-chemical properties at the start of the field experiments are presented in Table S1.
evaporation (Aubertin et al., 2009). A study indicates that the thickness of the underlying coarse layer also plays a critical role in governing the performance of the capillary barrier (Qian et al., 2010). Although a greater thickness of the capillary barrier layer leads to better blocking of solute, under certain conditions of deep diving, there is an upper limit on the thickness index of the barrier that prevents water and salt movement (Qian et al., 2010). Due to the advantages of easy access and low cost, straw has received more and more attention as a potential barrier material for improving saline soils (Chen et al., 2016). In principle, burying a straw layer below the soil surface can effectively cut capillaries in the soil and prevent salt moving upward (Qiao et al., 2006; Chen et al., 2016). Under steady-state flow conditions, a capillary barrier can form at the interface between soil and the straw layer with contrasting hydrological properties; the barrier can substantially reduce the infiltration of water into the underlying region, and consequently alleviate salt stress effects on crops (Qiao et al., 2006). Soil pore is closely related to water infiltration and plays an important role in soil water and salt transport (Smagin et al., 2008). As a way of returning straw to the soil, burying a straw layer can improve soil porosity and aeration (Tejada et al., 2008). A buried straw layer can improve aggregation and soil structure, thereby increasing water storage in the soil (Cao et al., 2012). However, unlike other interlayer materials, the efficiency of a straw interlayer can be influenced by the decomposition process. With the increase in years of deep straw burial, the degree of straw decomposing is increased, and the effect of the interlayer is weakened (Bastian et al., 2009). Therefore, the thickness of the straw barrier is one of the important factors affecting the regulation of water and salt in subsoils. Despite the beneficial effects of burying a straw layer in saline soils, there is a paucity of information on the relative effects of burying straw layers at different thicknesses on the flux of water and salt exchange. Previous research has investigated buried straw layers at converted thickness ranging from 3 to 5 cm (Qiao et al., 2006; Cao et al., 2012; Fan et al., 2012b; Zhao et al., 2013), but the effects of different straw layer thicknesses on water and salt distribution in the soil need further investigation. We hypothesized that different thicknesses of straw layer affect soil water infiltration and the exchange rate of salts above and below the barrier layer, and that a thicker straw layer is favorable for salt leaching. Also, as the straw is gradually decomposed over time, the difference in regulating water and salt transport among different thicknesses of straw layer will decrease, and therefore, there is an optimum thickness of the straw layer allowing the salt to penetrate the layer into the deeper soil layer, and reducing the upward movement of salt below straw layer. In order to test the hypotheses, through two sets of field micro-plot experiments, we examined the effects of different straw layer thicknesses on soil water and salinity dynamics, salt exchange flux and distribution; also, we studied the internal pore structure and saturated hydraulic conductivity after decomposition of straw layer, using the CT scanning technology. Our objectives were to: (i) clarify the effects of thickness of buried straw on soil water and salt regulation and its process influencing salt exchange between the upper and lower soil layer; (ii) explore the mechanism of water and salt regulation by the straw layer; and (iii) determine the relatively optimum straw layer thickness.
2.2. Experimental design and implementation In the field, a randomized experimental design was used, with treatments being randomly assigned to micro-plots (plot size: 1.8 m × 1.8 m). When constructing the plots, the surrounding areas of each plot was scuttled to 1 m, and insulated by double-plastic sheets buried to a depth of 100 cm from the soil surface to minimize the effects of lateral water and salt movement between plots. The middle gap was filled with soil, and each plot was surrounded by a concrete panel with a dimension of 40 cm wide and 60 cm high, and the exposed part of the panel (approximately 20 cm) was hardened by cement. The procedure of plot construction minimized disturbance of internal soil of the plots. 2.2.1. Experiment 1: Effect of different straw interlayer thickness on water and salt regulation In order to clarify the effect of different straw layer thickness on water and salt regulation, three treatments were studied. Treatment I: burial of a 3-cm-thick layer of maize straw (T3) at the rate equivalent to the averaged amount of straw produced in a maize field (6 t ha−1); Treatment II: burial of a 5-cm-thick layer of maize straw (T5), at a straw rate doubled the field maize straw production amount (12 t ha−1). Treatment III: burial of a 7-cm-thick layer at a straw rate of 18 t ha−1 or thrice the averaged maize straw amount produced per year (T7). In all treatments, straw was cut to segments of 10–15 cm and buried at the soil depth of 40 cm, and the surface soil was mulched with plastic film. A sketch of the plot with straw layer burial and plastic film mulch is shown in Fig. S2. The straw layer was buried at the beginning of the experiment and no additional straw was buried in the 2nd and 3rd years. For burying straws, first, soil layers of 0–20 cm and 20–40 cm in the micro-plots were removed by spade; second, air-dried and chopped maize straw was placed; finally, soils were returned to the plot in the order of soil layers. In the plots under T3, T5 and T7 treatments, 1.94, 3.89 and 5.83 kg of straw was buried each plot at compacted thickness of 3, 5 and 7 cm, respectively. The scale was marked on the cement and two parts of dug soil layer-by-layer to 20 cm and 40 cm layers above the straw interlayer was refilled, and then levelled with a harrow. After 5 d, plots were flood-irrigated with water from the Yellow River, which had a mineralization degree of 0.58 g L–1. The irrigation method in this study was the combination of autumn and spring irrigation, which is the traditional local irrigation system. In October each year, approximately 0.6 m3 water was applied to each plot after harvest (i.e., 1850 m3 ha−1), for which water volume was measured with a water meter. The second irrigation (0.6 m3 plot−1) was applied approximately 10 d before sowing in May each year. Fertilizer was applied at 180 kg ha−1N (urea, 46% N), 120 kg ha−1 P2O5 (diammonium phosphate, 18% N and 46% P2O5) and 75 kg ha−1 K (potassium sulphate, 50% K2O). Plots were mulched with plastic film before sowing and sunflower seeds were sown with a hill-drop planter. No further irrigation and fertilizer was applied to the sunflower. Sunflower (cv LD 5009) was seeded at a row spacing of 60 cm and density of 49,000 plants per hectare. Seeding was done manually on 28 May 2011, 8 June 2012 and 2 June 2013, and the crop was harvested on 23 September 2011, 18 September 2012 and 16 September 2013, respectively. After harvest, the stalks and plastic film residues were
2. Materials and methods 2.1. Site description Field experiments were conducted from October 2010 to September 2013 at the experimental station of the Management Department of Yichang Irrigation Sub-district (41°04′N, 108°00′E, 1022 m ASL), located in Wuyuan county, Inner Mongolia, China (Fig. 1). The region has an arid climate, for which the average annual rainfall is 173.5 mm and more than 70% of the rainfall occurs from July to August. The mean annual evaporation is approximately 2068 mm, being 11 times higher 2
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Fig. 1. Location of the study area and sampling fields. The test area was in Wuyuan county, Inner Mongolia, China (the pentagram in the picture on the left).
laboratory and oven-dried at 105 °C for about 24 h to determine the gravimetric soil water content. After drying, the samples were ground to pass through a 2 mm sieve and saturated with water at a soil/water ratio of 1:5 to determine electrical conductivity. The electrical conductivity of the extract was measured using a DDS-307 conductivity meter (Shanghai TecFront Electronics Co., Ltd., Shanghai, China). The soluble salt content was measured by using the residual drying method. Specifically, we added 100 mL leachate into a 150 mL evaporating dish that had been dried to a constant weight, placed the dish on a hot plate, and heated at 250 °C. After the leachate was completely evaporated, the evaporating dish was transferred to a constant temperature drying oven and dried at 105 °C for 2 h. Then, the dish was transferred to a desiccator to achieve constant weight, and results were recorded and used to calculate the total salt content in leachate by a subtraction method.
removed from the soil surface, and each plot was ploughed to a depth of 20 cm with a spade. 2.2.2. Experiment 2: Mechanisms of straw layer controlling water and salt transport Based on the results of Experiment 1, a supplementary experiment was carried out from 2014 to 2016 in the same micro-plots to determine soil pore structure, hydraulic parameters, and water and salt regulation after three years of deep straw burial. Two treatments were studied in this step: treatment I with no straw layer added (CK), and treatment II with a burial of 5-cm-thick maize straw layer (t5), the same treatment as T5 in experiment 1. Each treatment was repeated three times. The experiment was arranged in the fall of 2013. Micro-plots construction and crop cultivation were consistent with experiment 1. Sunflower (cv LD 5009 in 2014–2015, JK601 in 2016) was seeded at a row spacing of 60 cm and density of 49,000 plants per hectare. Seeding was done manually on 8 June 2014, 5 July 2015 and 18 May 2016, and the crop was harvested on 20 September 2014, 23 September 2015 and 19 September 2016, respectively. Other management practices are the same as those used in experiment 1.
2.4. CT scanning and image processing In order to determine soil pore structure after three years of straw burial, soil profile was excavated after harvesting sunflower in 2016, for which a PVC-made ring cutter was used to collect the soils in the upper part of straw layer (35–41 cm from the soil surface), the straw layer (41–47 cm), and the lower part of the straw layer (47–53 cm) for CT scanning. The PVC ring cutter had a diameter of 50 mm and a height of 60 mm. Sampled soil scanning was performed using the Nano-Industrial Micro-CT (Phoenix NanotomS, GE, USA) in the Nanjing Institute of Soil Science, Chinese Academy of Sciences. The scan parameters were set to a scan voltage of 100 kV, a scan current of 100 μA, and a resolution of 30 μm for each process. The projection data was reconstructed using a rear projection algorithm, and a total of about 2000 32-bit tiff format grayscale images were obtained. Image was processed using Image J software, as described by Udawatta et al. (2006). After measuring the pore structure of soil samples, saturated hydraulic conductivity was measured by using saturated hydraulic conductivity meter under a constant head condition (Sepaskhah and Karizi, 2011).
2.3. Soil sampling and measurements Weather data were obtained from a weather station on the experimental site. Soil samples were taken at 0–20, 20–40, 40–60, 60–80 and 80–100 cm depth between two rows of sunflowers in each plot every 15 days, using a soil auger (36 mm in diameter) (Nanjing Aofeng Mechanical and Electrical Co.). The samples were collected from the center of each plot (under plastic film) between two sunflower rows. After each sampling, soil of the same texture was used to fill the original sampling hole to minimize the effects of soil sampling on crop growth and water and salt measurements. Sampling was delayed 1–2 days when there was rainfall. In the same way, additional soil samples were taken at sowing and 2–3 days after harvesting, respectively. Information on sampling date is provided in Table S2. All the sampled soils were immediately transported to the 3
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Where FL is the salt leaching flux (g m−2 d−1); SSB is the salt storage before spring irrigation (g); SSA salt storage after spring irrigation (g); A is the cross-sectional area of the micro-plot or soil column (m2); and Δt is the interval number of days between pre and post-irrigation (days). To determine the effects of straw layer on salt exchange during phreatic evaporation, the evaporation flux of salt in the 0–40 cm soil layer (FE) was calculated with Eq. (4):
FE = (SSH − SSA) (A × Δt )
(4) −2
−1
d ); SSH is the salt Where FE is the salt evaporation flux (g m storage at harvest (g); SSA is the salt storage after spring irrigation (g); A is the cross-sectional area of the micro-plot or soil column (m2); and Δt is the interval number of the days from the end of the spring irrigation infiltration to the harvest (days). Soil saturated hydraulic conductivity (steady infiltration rate of soil) was measured by the cutting ring method, and it was calculated with Eq. (5):
V = (Qn × 10) (Tn×S)
(5) −1
Where V is the infiltration rate of soil at a given time (mm min ); Qn is the water injection amount in the nth measurement time (ml); Tn is the time interval in the nth measurement time (min); and S is the area of permeable surface (cm2). Soil desalination rate was calculated as with Eq. (6):
P(%) = (SB − SA) SB × 100
(6)
Where P is the salt desalination rate (%); SB is the salt content the previous year (g kg−1); and SA is the salt content the current year (g kg−1). Stratification ratio of salt storage was calculated as with Eq. (7):
SR = SS0 − 40cm SS40 − 100cm
(7)
Where SR is the stratification ratio of salt storage; SS0-40cm is the salt storage in 0–40 cm soil layer (g); and SS40-100cm is the salt storage in 40–100 cm soil layer (g).
Fig. 2. Water content in the 0–40 cm soil layer under different treatments during the sunflower growing seasons of 2011 (a), 2012 (b), 2013 (c). T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
2.6. Data analysis 2.5. Calculations
All data within each individual year were analyzed using the analysis of variance (ANOVA) to test the treatment effects on the measured parameters. Mean comparisons were performed using the Fisher’s LSD test at P < 0.05. Statistical analysis of saturated soil hydraulic conductivity and laboratory simulation data of different treatments were performed using independent sample T-test. The analysis was conducted using the SPSS 13.0 program.
−1
The relationship between soil salinity content (g kg ) and electrical conductivity in soil extracts (ms cm−1) was studied, and the following fitting equation was obtained by regression statistical analysis with Eq. (1):
y= 3.0111x
(1)
where y is the salt contents of the soil (g kg−1), x is the electrical conductivity of soil extracts (ms cm−1). The coefficient of determination (r2) indicated a significant correlation between salt content and electrical conductivity (r2 = 0.98, P < 0.01). Salt storage was calculated with Eq. (2):
SS = A × D × H × y × 10−3
3. Results 3.1. Effects of different straw layer thicknesses on water distribution In 2011 (Fig. 2a), the soil water content changed little among treatments within 49 days after sowing (DAS). Significant differences among these treatments were only observed in the late periods of the season (P < 0.05), except between T5 and T7 at 94 DAS, and between T3 and T5 at 109 DAS. In 2012 (Fig. 2b), during the entire growing season, there was a consistently significant difference between T3 and T7 except at 76 DAS. In addition, a significant difference (P < 0.05) occurred between T5 and T7 at 15, 61, 92 and 106 DAS. In 2013 (Fig. 2c), T7 and T3 only differed significantly at sowing and 45 DAS (P < 0.05). After 62 DAS, the soil water under T7 was much lower (P < 0.05) than that under T3 and T5, by 1.00–2.24% and 0.54–0.86%, respectively.
(2)
Where SS is the salt storage (g); A is the cross-sectional area of the micro-plot or soil column (cm2); D is the soil bulk density (g cm−3); H is the soil layer thickness (cm); and y is the salt content of soil layer (g kg−1). To determine the effects of straw layer on salt exchange during irrigation water infiltration, the leaching flux of salt in the 0–40 cm soil layer (FL) was calculated with Eq. (3):
FL = (SSB − SSA) (A × Δt )
(3) 4
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Fig. 3. Water distribution within the 0–100 cm soil profile at sowing (a, b, c) and after harvest (d, e, f) under different treatments in 2011, 2012, 2013, respectively. T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
3.2. Effects of different straw layer thicknesses on salt distribution
T3 had relatively lower water content than T5 and T7 below the 40 cm soil depth by 0.65% and 0.41% during sowing in 2011 (Fig. 3a), but the trend was reversed in 2012 (Fig. 3b). In 2013 (Fig. 3c), T7 had much more water than T3 and T5 by 0.96% and 0.76%, respectively (P < 0.05). At harvest in 2011 (Fig. 3d), T7 and T5 both retained significantly more water than T3 (P < 0.05). There was no significant difference among treatments in 2012 (Fig. 3e), whereas in 2013 (Fig. 3f), T7 retained much more water than T3 and T5 by 1.25% and 1.78% at the 60–100 cm depth, respectively (P < 0.05).
In 2011, (Fig. 4a), significant differences (P < 0.05) in salt content were observed between T3 and T7 at 33, 49, 80, 94 and 109 DAS, and between T3 and T5 at 33 and 49 DAS. In 2012 (Fig. 4b), a significant difference was consistently observed among T3, T5 and T7 before 76 DAS, for which the salt content in the 0–40 cm soil depth under T7 was 20–37% and 14–30% lower (P < 0.05) than that under T3 and T5. In 2013 (Fig. 4c), significant differences in salt content were observed between T3 and T7 at 62, 74, 91, and 109 DAS (P < 0.05). Below the 40 cm soil depth at sowing (Fig. 5a, 5c), the salt content 5
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3.4. Effects of straw layer on saturated water conductivity and soil pore characteristics after three years of deep burial Table 2 showed that the saturated hydraulic conductivity under the straw layer treatment (t5) was not significantly different from that under the control treatment after straw layer was buried for 3 years. Among all the soil layers, the ratio of the equivalent diameter of 0.1–0.5 mm porosity to the total porosity was as high as 56.17–78.17% (Table 3). Compared with CK, the total porosity and the equivalent diameter > 0.5 mm porosity at the straw layer under t5 increased (P < 0.01) by 2.41% and 2.05%, respectively. Below the straw layer, there was no soil pores with an equivalent diameter of > 1.5 mm in both CK and t5 treatments. However, CK showed a significant increase of 0.13% (P < 0.01) in the 1.0–1.5 mm pore size compared to t5. 4. Discussion 4.1. Soil water movement and distribution In this study, the water content in the 0–40 cm soil depth of all treatments showed a gradual decrease during the sunflower growing seasons except for 2012 (Fig. 2). The overall result can be explained by the fact that the experimental field was located in the arid northwest region of China with a high evaporation-precipitation ratio. The exception of 2012 was due to a large rainfall event (about 120 mm) in July (Fig. S1), which led to a peak in the water content during crop growth (Zhang et al., 2016). In heterogeneous soils, the patterns of water movement and soil water depend not only on the texture of each soil layer, but also on the thickness of the barrier layer (Zhang et al., 2007). The difference in porosity between the soil and straw layer influenced its hydraulic conductivity and the soil water potential mutated at the interface (Bernadiner, 1998), resulting in corresponding changes in water infiltration patterns (Cao et al., 2012; Kumar et al., 2012; Zhang et al., 2019). In the present study, there were significant differences in the soil water (0–40 cm) among treatments with different thicknesses in the first year. In the second and third years, nonetheless, only T7 treatment had significantly higher soil water content than the other two treatments in the whole growth period (Fig. 2). This indicated that the thicker straw barrier layer had greater effect on water content in the 0–40 cm soil layer with the increase of years. Zou et al. (2014) also found that in the third year of straw burial, a higher water retention effect occurred in the treatment with larger amounts of maize straw. However, for treatments with thinner straw layers, the difference in straw layer thickness gradually decreased with the increase of the buried age due to compaction and decomposition. As a result, the water content difference among the treatments gradually decreased (Zou et al., 2014). Furthermore, the difference in saturated water conductivity between treatments also decreased (Table 2), which is related to the proportion of soil pores with an equivalent diameter of 0.1–0.5 mm (Table 3). The water content of the 60–100 cm soil layer under T7 was significantly higher than that under T3 and T5 in 2011 and 2013 (Fig. 3), because the effect of blocking soil capillary continuity increased with increasing straw layer thickness, reducing the upward migration of water from deeper soils (Zhao et al., 2016). Additionally, with the decomposition of straw, the increase in soil organic carbon could improve the condition of soil aggregates, enhance soil water holding capacity and reduce soil water diffusivity, thereby increase the soil water retention effect in the third year (Ouattara et al., 2006; Huo et al., 2017). However, the soil water in the 60–100 cm layer whether in the seedling or harvest stage in 2012 showed different trends from that in 2011 and 2013. This may be related to the extreme rainfall in 2012, in which rainfall from sunflower seedling to maturity was much higher than that for the same period in 2011 and 2013 (Fig. S1). Too much rainfall was transformed into soil water, which was easier to replenish the 60–100 cm soil layer through a thinner straw barrier layer (Gao et al.,
Fig. 4. Salt content in the 0–40 cm soil layer under different treatments during the sunflower growing season in 2011 (a), 2012 (b), 2013 (c). T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Soils in all treatments were mulched with a plastic film. Bars represent LSD at P = 0.05.
under T7 was consistently higher (P < 0.05) than that under T3 and T5 by 28–31% and 6–17% in 2011, and 19–32% and 8–17% in 2013. Notably, the trend was opposite in 2012, when the salt content below the 40 cm soil depth under T7 was lower than that under T3 and T5 by 17–26% and 11–16%, respectively (Fig. 5b). After harvesting sunflower (Fig. 5d-f), compared with T3 and T5, the average salinity within the 100 cm profile under T7 decreased by 24.16% and 13.07% in 2011, 14.92% and 6.91% in 2012, and 18.62% and 13.58% in 2013 (P < 0.05). 3.3. Effects of different straw layer thicknesses on soil salt flux The flux of salt in the 0–40 cm soil after soil leaching (FL) increased with an increasing straw layer thickness, and the differences among the three treatments gradually decreased with time (Table 1). The FL under T7 was higher than that under T3 and T5 by 105.35% and 83.81% in 2011, 89.16% and 66.21% in 2012, and 33.19% and 33.03% in 2013 (P < 0.05). The salt flux in the 0–40 cm soil layer during soil evaporation (FE) decreased with increasing straw layer thickness in 2011, and the FE under T5 and T7 was much lower than that under T3 by 58.73% and 54.41%, respectively. Compared with 2011, the FE in 2012 and 2013 showed the opposite trend. The FE under T7 exceeded that under T3 and T5 by 91.71% and 10.45% in 2012, and 37.86% and 44.03% in 2013 (P < 0.05). 6
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Fig. 5. Salt distribution within the 0–100 cm soil profile at sowing (a, b, c) and after harvest (d, e, f) under different treatments in 2011, 2012 and 2013. T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. Bars represent LSD at P = 0.05.
et al., 2016). Further, from the annual variation of desalination rate in 0–40 cm soil layer, the effect of straw layer on promoting salt downward leaching was mainly reflected in the first year. However, with the extension of time, the accumulated salt in straw layer or deep soil may gradually move upward with evaporation (Zhang et al., 2019), especially for T7 treatment with the highest FE value (Table 1). Thus, the desalination rate of the 0–40 cm soil layer was low in T7 compared with T3 (23.6% and 10.0% in 2012, and 9.1% and 5.0% in 2013 under T3 and T7, respectively). However, since the value of FL was much higher than that of FE (Table 1), the salinity of 0–40 cm layer under T7 was still the lowest. Since plenty of water was stored in the topsoil under T7 at seedling
2010), so soil water under T3 was higher than that of the treatment with a thicker straw layer. 4.2. Salt movement and distribution With the increase of burial years, the regulation of 0–40 cm soil salinity by straw layer with different thicknesses was weakened (Fig. 4), and this is consistent with the result of water content. In 2012, the regulation effect of straw layer on salinity may also be affected by rainfall (Fig. S1), which promoted the efficiency of salt leaching and changed the distribution of salinity. Therefore, the significant difference between treatments was reflected in the early stage of 2012 (Zhao 7
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Table 1 Salt accumulation and flux in the 0–100 cm soil profile as affected by straw layer treatments from 2011 to 2013. Treatments
T3 T5 T7
2011
2012 −2
−2
2013
SS1 (g)
SS2 (g)
FL (g m d−1)
FE (g m d−1)
SS1 (g)
SS2 (g)
FL (g m d−1)
7035.83 a 5704.96b 5324.91c
12233.10 a 10672.77b 9277.37c
15.70c 17.54b 32.24 a
4.87 a 2.01b 2.22b
5373.49 a 5171.82b 4793.12c
12033.22 a 10997.96b 10238.32c
41.79c 47.56b 79.05 a
−2
−2
FE (g m d−1)
SS1 (g)
SS2 (g)
FL (g m−2 d−1)
FE (g m−2 d−1)
1.93c 3.35b 3.70 a
4884.99 a 4861.69 a 4553.35b
6300.12 a 5970.54 a 4549.05b
41.82b 41.87b 55.70 a
1.40b 1.34b 1.93 a
Note: T3, T5, and T7: maize straw layer treatments with thicknesses of 3, 5, and 7 cm, respectively. FL, and FE indicate salt flux in the 0–40 cm soil layer during leaching and evaporation; SS1, SS2 indicate salt storage at harvest in 0–40 cm and 0–100 cm soil layers. Treatment effects were evaluated within an experimental year, and significant differences among treatments are indicated by different lower case letters following the mean values (P < 0.05). Table 2 Saturated water conductivity under different treatments and soil layers in another field experiment after straw burial for three years. Treatments
SCU (mm min−1)
SCS (mm min−1)
SCL (mm min−1)
CK t5 t
0.00080 ± 0.00005 0.00148 ± 0.00051 −1.32
0.00084 ± 0.00040 0.01120 ± 0.00081 −12.83
0.00370 ± 0.00036 0.00236 ± 0.00052 2.59
Note: CK, no straw layer; T5, buried maize straw layer with thickness of 5 cm. SCU, saturated water conductivity in the upper soil layer; SCS, saturated water conductivity in the straw layer; SCL, saturated water conductivity in the lower soil layer. Values are means of three replicates ± standard deviation. t is the t value obtained from an independent sample T-test, * significant at the 0.05 probability level, ** significant at the 0.01 probability level. Table 3 2-D geometric soil pore characteristics in Experiment 2 after straw burial for three years. Soil depth
Soil above straw interlayer
Straw interlayer soil
Soil below straw interlayer
Treatments
CK t5 t CK t5 t CK t5 t
Different equivalent diameter porosity (%)
Total Porosity (%)
0.1–0.5 mm
0.5–1.0 mm
1.0–1.5 mm
1.5–2.0 mm
2.0–2.5 mm
2.5–3.0 mm
3.22 ± 0.48 4.23 ± 0.46 −15.141 3.51 ± 0.13 3.87 ± 0.57 −8.570 4.47 ± 0.45 5.20 ± 0.49 −16.165
2.21 ± 0.40 1.84 ± 0.09 −3.620 0.84 ± 0.04 2.11 ± 0.05 −4.112** 1.36 ± 0.041 0.72 ± 0.64 8.361
0.83 ± 0.04 0.62 ± 0.02 5.310 0.13 ± 0.04 0.66 ± 0.01 −2.592* 0.15 ± 0.37 0.02 ± 0.005 3.492**
0.40 ± 0.09 0.12 ± 0.05 5.139** 0.003 ± 0.0002 0.17 ± 0.01 −5.114* – –
0.10 ± 0.05 0.07 ± 0.06 0.625* – 0.08 ± 0.003
0.03 ± 0.001 0.167 ± 0.03 −4.682* – –
– –
– –
6.79 ± 0.15 7.04 ± 0.25 −8.570 4.48 ± 0.80 6.89 ± 0.52 −4.558** 5.98 ± 0.48 5.94 ± 0.56 0.544
Note: CK, no straw layer; T5, buried maize straw layer with thickness of 5 cm. Values are means of three replicates ± standard deviation. t is the t value obtained from an independent sample T-test, * significant at the 0.05 probability level, ** significant at the 0.01 probability level.
worth mentioning that the SR value of T5 was low in all three consecutive years, showing a relatively stable change.
stage after spring irrigation, it promoted ion exchange and increased salt leaching (Gabriel et al., 2012; Stagnari et al., 2014), thereby the increase of salt content in the 60–100 cm soil layer was significantly higher. This suggested that the salt leaching depth under T7 can be deepened. However, due to the excessive rainfall in May 2012 relative to 2011 and 2013, the salt content in deeper soil under T7 was much lower than that under T3 and T5 in 2012, which indicated that the desalination depth under T7 may exceed 1 m. We also found that the salt content in the 60–100 cm soil layer under T7 was significantly lower than that of other treatments at harvest. Because the straw layer acted as a deep cover, which slowed the evaporation rate of groundwater to some extent (Sun et al., 2011), T7 reduced the salt content below the straw layer. Qiao et al. (2006) also showed that deep straw mulching prevented water loss below the straw layer during evaporation, which produced a positive effect on maintaining water in deep soils. The ratio of 0–40 cm to 40–100 cm salt reserves (SR) can also reflect the desalination status of crop root zone among different treatments (Zhang et al., 2014). The SR value of T3 and T5 was less than 1 in 2012–2013, but that of T7 was greater than 1 in 2013, which implied that the thicker straw layer was not conducive to accelerating salt leaching in the 0–40 cm layer at the later stage of the experiment and there may still be some risk of secondary salinization of the soil. It is
5. Conclusions This study demonstrated significant effects of the thickness of buried straw layer on water and salt distribution in a dry saline soil. Compared with T3 and T5, a thicker straw layer (T7) significantly decreased soil water content in the later stage of sunflower growth. T7 decreased salt content in the 0–40 cm soil layer by 3.07–36.82% throughout the sunflower growth period compared to T3 and T5. Furthermore, the salt leaching flux increased with increases in straw thickness, however, the thicker straw layer increased (the rate of) salt upward movement during evaporation due to salt accumulation in the straw layer, and there may still be some risk of secondary soil salinization. Given the fact that T5 is more practical for field applications and more efficient in salt leaching and inhibiting salt return, it can be a beneficial management practice for comprehensive development and utilization of saline-alkali soils. Declaration of Competing Interest The authors declare that they have no known competing financial 8
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interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos: 31871584, 31471455); the National Key Research and Development Program of China (Grant No: 2016YFC0501302); and the Special Fund for Agro-scientific Research in the Public Interest (Grant No: 201303130). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2020.114213. References Akudago, J.A., Chegbeleh, L.P., Nishigaki, M., Nanedo, N.A., Ewusi, A., Kankam-Yeboah, K., 2009. Borehole drying: a review of the situation in the voltaian hydrogeological system in Ghana. J. Water Res. Protection 1 (3), 153–163. https://doi.org/10.4236/ jwarp.2009.13020. Aubertin, M., Cifuentes, E., Apithy, S.A., Bussire, B., Molson, J., Chapuis, R.P., 2009. Analyses of water diversion along inclined covers with capillary barrier effects. Can. Geotech. J. 46, 1146–1164. https://doi.org/10.1139/T09-050. Bernadiner, M.G., 1998. A capillary microstructure of the wetting front. Transport Porous Med. 30, 251–265. Bastian, F., Bouziri, L., Nicolardot, B., Ranjard, L., 2009. Impact of wheat straw decomposition on successional patterns of soil microbial community structure. Soil Biol. Biochem. 41, 262–275. https://doi.org/10.1016/j.soilbio.2008.10.024. Cao, J.S., Liu, C.M., Zhang, W.J., Guo, Y.L., 2012. Effect of integrating straw into agricultural soil on soil infiltration and evaporation. Water Sci. Technol. 65, 2213–2228. https://doi.org/10.2166/wst.2012.140. Chen, S., Zhang, Z.Y., Wang, Z.C., Guo, X.P., Liu, M.H., Hamoud, Y.A., Zheng, J.C., Qiu, R.J., 2016. Effects of uneven vertical distribution of soil salinity under a buried straw layer on the growth, fruit yield, and fruit quality of tomato plants. Sci. Hortic. 203, 131–142. https://doi.org/10.1016/j.scienta.2016.03.024. Chernousenko, G.I., Pankova, E.I., Kalinina, N.V., Ubugunova, V.I., Rukhovich, D.I., Ubugunov, V.L., Tsyrempilov, E.G., 2017. Salt-affected soils of the Barguzin depression. Eurasian Soil Sci. 50 (6), 646–663. https://doi.org/10.1134/ s1064229317060035. Fan, X., Pedroli, B., Liu, G., Liu, Q., Liu, H., Shu, L., 2012a. Soil salinity development in the yellow river delta in relation to groundwater dynamics. Land Degrad. Dev. 23, 175–189. https://doi.org/10.1002/ldr.1071. Fan, F., Xu, S.J., Song, G.Y., Zhang, Q.G., Hou, M.H., Song, X.F., 2012b. Studies on improvement of saline and alkali soil with the interlayer of maize straw in West Liaohe region. Chin. J. Soil Sci. 43, 696–701 (in Chinese). Fredlund, M.D., Wilson, G.W., Fredlund, D.G., 2002. Use of the grain-size distribution for estimation of the soil-water characteristic curve. Can. Geotech. J. 39, 1103–1117. https://doi.org/10.1139/t02-049. Guo, G., Araya, K., Jia, H., Zhang, Z., Ohomiya, K., Matsuda, J., 2006. Improvement of salt-affected soils, Part 1: Interception of capillarity. Biosyst. Eng. 94, 139–150. https://doi.org/10.1016/j.biosystemseng.2006.01.012. Guo, G., Zhang, H., Araya, K., Jia, H., Ohomiyaand, K., Matsuda, J., 2007. Improvement of salt-affected soils, Part 3: Specific heat of salt- affected soils. Biosyst. Eng. 96, 413–418. https://doi.org/10.1016/j.biosystemseng.2006.11.007. Gao, F., Jia, Z.K., Lu, W.T., Han, Q.F., Yang, B.P., Hou, X.Q., 2010. Effects of different straw returning treatments on soil water, maize growth and photosynthetic characteristics in the semi-arid area of Southern Ningxia. Acta Ecologiac Sinica. 31, 777–783 (in Chinese). Gabriel, J.L., Almendros, P., Hontoria, C., Quemada, M., 2012. The role of cover crops in irrigated systems: soil salinity and salt leaching. Agr. Ecosyst. Environ. 158, 200–207. https://doi.org/10.1016/j.agee.2012.06.012. Huo, L., Pang, H.C., Zhao, Y.G., Wang, J., Lu, C., Li, Y.Y., 2017. Buried straw layer plus plastic mulching improves soil organic carbon fractions in an arid saline soil from northwest china. Soil Till Res. 165, 286–293. https://doi.org/10.1016/j.still.2016. 09.006. Jia, H., Zhang, H., Araya, K., Guo, G., Zhang, Z., Ohomiya, K., 2006. Improvement of saltaffected soils, Part 2: Interception of capillarity by soil sintering. Biosyst. Eng. 94, 263–273. https://doi.org/10.1016/j.biosystemseng.2006.01.013. Kumar, A., Kumar, P., Yadav, S.K., Hasija, R.C., 2012. Effect of integrated nutrient
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