Effects of straw layer and flue gas desulfurization gypsum treatments on soil salinity and sodicity in relation to sunflower yield
Introduction
The Hetao Plain, which is located in both the Inner Mongolia and Ningxia Hui Autonomous Regions of China, is an alluvial plain on the coast of the Yellow River. This plain is also one of the major grain production regions in western China but is difficult to farm without irrigation. Because of its shallow water table, low precipitation, high evaporation and salt content of the parent materials, more than half of the irrigated cropland in this region suffers from soil salinization and alkalization (Lei et al., 2001; Zhao et al., 2016a). Both autumn and spring irrigation using very large amounts (>6000 m3 ha−1) of river water are performed for the reduction of soil salinization (Feng et al., 2005; Yue et al., 2016). However, overirrigation and insufficient drainage have aggravated the soil salinization problem. Severely salt-affected lands might eventually become completely unproductive and abandoned if the salt problem cannot be resolved immediately and effectively (Wu et al., 2008).
In this region, water movement in the soil occurs mainly vertically, and evaporation from the soil surface is a major factor (Yu et al., 2010). Because salts are usually transported as solutes within soil water, capillary barriers to water movement might prevent the transfer of dissolved salts from subsoil water to surface soils by either evaporation or transpiration (Khan et al., 2009). Thus, several studies have suggested the placement of a capillary barrier underneath the soil to reduce the accumulation of salt, and the suggested materials have included mainly sand or gravel, a coarse particulate layer, crop straw and plastic sheeting (Porro et al., 1993; Sharma et al., 1995; Rooney et al., 1998). These studies have shown the potential effects of capillary barriers on soil water and salt management, particularly under conditions in which the groundwater table is shallow. Among the varieties of capillary material, crop straw is acceptable due to its high quantity, low cost and high availability, as well as its effectiveness. Qiao et al. (2006) reported that a buried straw layer can limit the evaporation of deep soil water, improve soil water storage and reduce the content of dissolved salt. Our previous studies have confirmed that a buried straw layer reduces the salt content of topsoil (0–20 cm) by 12.1–56.4% (Zhao et al., 2014) and increases the biomass and seed yields of sunflower (Helianthus annuus L.) by 21.2% and 31.0% (Zhao et al., 2014, Zhao et al., 2016a) compared with non-straw control treatment within the first year. In addition, the optimal straw burial depth is 40 cm (Zhao et al., 2016b), and the top-priority material for sunflower production is maize (Zea mays L.) straw (Zhao et al., 2018). In addition to the aforementioned benefit of acting as a capillary barrier, a buried straw layer also increases the soil organic matter and nutrients (Huo et al., 2017) as well as the microbial diversity (Li et al., 2016).
Many studies have focused on the management of salt accumulation, but the effects of high pH in conjunction with high alkalinity on the physical behaviour of soils have generally been overlooked (Gupta et al., 1984). The Hetao Plain is also subject to soil alkalization (Zhao et al., 2016a), which adversely affects both the physical properties of soil and plant growth and thus indirectly reduces the microbial activity (Rengasamy, 2002). Flue gas desulfurization (FGD) gypsum, a byproduct of industrial power generation, has become an effective amendment for alkaline soil reclamation and does not cause contamination (Chen et al., 2011; Wang et al., 2017). Previous studies have demonstrated that the application of FGD gypsum reduces the soil salinity, exchangeable sodium percentage (ESP) and pH, improves the physical properties of soil, and increases crop yields (Matsumoto, 1998; Sakai et al., 2004; Wang et al., 2008; Watts and Dick, 2014). In addition, FGD gypsum acts as a fertilizer source of Ca, S and Mg and thus improves plant growth (Clark et al., 2001; Dick, 2006). To date, this practice has been applied to approximately 120 km2 of land in China (Yu et al., 2014; Wang et al., 2017) and other countries (Bolan et al., 1991; Stout and Priddy, 1996; Watts and Dick, 2014), and positive results have been observed.
Currently, FGD gypsum is widely applied to increase crop yields on the Hetao Plain. The practice of burying a subsoil straw layer will also become widely adopted in the near future with the help of tillage machines. As mentioned above, numerous studies have demonstrated the positive effects of FGD gypsum on soil salinity and sodicity, but few studies have addressed the effects of straw layers on soil water and salinity dynamics. With the exception of our preliminary study on the soil column, the integration of these two treatments has not yet been documented in saline–alkali soils. Our previous study showed that the combined application of a straw layer with FGD gypsum increased the water content by at least 7.8% but moderately increased the salt content in soil at a depth of 0–30 cm 2 d after water infiltration (Zhao et al., 2017). Nevertheless, the available information on the effects of straw layer burial rates on soil water and salt management is scarce, and such information is necessary for the reclaiming of saline–alkali soils and resources in agricultural lands through the application of crop straw and FGD gypsum. Therefore, a 2-year field experiment was conducted to evaluate the effects of buried straw layers in combination with FGD gypsum on sunflower production. We hypothesized that the soil water, salinity and sodicity dynamics in the soil profile as well as the sunflower yield would be affected by the combined application of these two treatments.
Section snippets
Experimental site
A field experiment was conducted from October 2014 to September 2016 at Jungar Banner, Ordos City (40°15′24″N, 110°50′50″E, 994 m above sea level), Inner Mongolia, China. This area has a typical arid continental climate consisting of very cold winters with little snowfall and very dry summers with little rainfall. The mean annual precipitation in the region is approximately 292 mm, and most of the precipitation occurs during July and August. The mean annual evaporation is approximately 2106 mm,
Soil water
The soil water content was significantly affected by the straw layer and soil depth but not by FGD gypsum (Table 3). In the non-gypsum controls, the straw treatments retained more water in the overlying soil at planting than the non-straw treatments (Fig. 3a and b). The water content increased with increases in the straw rates, but significant differences were found only between the 18 t ha−1 straw and non–straw treatments. Within a depth of 0–30 cm, the 18 t ha−1 straw treatment increased the
Discussion
The soil water and salt dynamics obtained after burying a straw layer in the subsoil were more complex than those obtained in the initial state, and this finding could be explained by the capillary break effect of the straw layer during both water infiltration and evaporation processes. Because the straw is relatively dry prior to its application, the hydraulic conductivity of straw layers is notably lower than that of the neighbouring soil layers (McCartney and Zornberg, 2010). In this case,
Conclusions
The results of this 2-year field experiment conducted in severe saline–alkali soils on the Hetao Plain of China suggest that the combined application of a buried straw layer and FGD gypsum exerted positive effects on the salinity and sodicity distributions in soil and on the sunflower yields. Buried straw layers increased the soil water content but reduced the EC, ESP and pH, particularly in the soil layer directly overlying the straw. These beneficial effects increased with increases in the
Acknowledgements
We thank Mr. Zhiyong Wang of the Soil and Fertilizer Station of Baotou City, Inner Mongolia, and all the staff at Tsinghua Agriculture Co., Ltd., for their assistance throughout the experimental process, and we also thank Mr. Zhentao Sun from the Research Center for Saline–Alkali Soil Rectification and Carbon Fixation of Tsinghua University for the assistance provided with the soil property measurements. This research was supported by the National Key Research and Development Program of China (
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