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sustainability Article Effects of Different Fertilization Regimes on Crop Yield and Soil Water Use Efficiency of Millet and Soybean Qiang Liu 1,2, Hongwei Xu 1,2 , Xingmin Mu 1,3,*, Guangju Zhao 1,3, Peng Gao 1,3 and Wenyi Sun 1,3 1 State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China; [email protected] (Q.L.); [email protected] (H.X.); [email protected] (G.Z.); [email protected] (P.G.); [email protected] (W.S.) 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, China * Correspondence: [email protected] Received: 19 April 2020; Accepted: 14 May 2020; Published: 18 May 2020 Abstract: Soil water and nutrients are major factors limiting crop productivity. In the present study, soil water use efficiency (WUE) and crop yield of millet and soybean were investigated under nine 1 fertilization regimes (no nitrogen (N) and no phosphorus (P) (CK), 120 kg ha− N and no P (N1P0), 240 1 1 1 1 kg ha− N and no P (N2P0), 45 kg ha− P and no N (N0P1), 90 kg ha− P and no N (N0P2), 120 kg ha− 1 1 1 1 1 N and 45 kg ha− P (N1P1), 240 kg ha− N and 45 kg ha− P (N2P1), 120 kg ha− N and 90 kg ha− P 1 1 (N1P2), 240 kg ha− N and 90 kg ha− P (N2P2)) in the Loess Plateau, China. We conducted fertilization experiments in two cultivation seasons and collected soil nutrient, water use, and crop yield data. Combined N and P fertilization resulted in the greatest increase in crop yield and WUE, followed by the single P fertilizer application, and single N fertilizer application. The control treatment, which consisted of neither P nor N fertilizer application, had the least effect on crop yield. The combined N and P fertilization increased soil organic matter (SOM) and soil total N, while soil water consumption increased in all treatments. SOM and total N content increased significantly when compared to the control conditions, by 27.1–81.3%, and 301.3–669.2%, respectively, only under combined N and P application. The combined N and P application promoted the formation of a favorable soil aggregate structure and improved soil microbial activity, which accelerated fertilizer use, and enhanced the capacity of soil to maintain fertilizer supply. Crop yield increased significantly in all treatments when compared to the control conditions, with soybean and millet yields increasing by 82.5–560.1% and 55–490.8%, respectively. The combined application of N and P fertilizers increased soil water consumption, improved soil WUE, and satisfied crop growth and development requirements. In addition, soil WUE was significantly positively correlated with crop yield. Our results provide a scientific basis for rational crop fertilization in semi-arid areas on the Loess Plateau. Keywords: soil nutrients; fertilization; WUE; crop yield; Loess Plateau 1. Introduction Crop growth and development activities not only require appropriate light, moisture, air, and temperature conditions but also adequate amounts of diverse soil nutrients [1,2]. Considering limited soil nutrient availability, the application of fertilizer to the soil is necessary [3], which would also address current food security challenges by enhancing soil fertility and, in turn, crop yield. The Sustainability 2020, 12, 4125; doi:10.3390/su12104125 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 4125 2 of 13 increasing rates of fertilizer application in agricultural production over recent years, with generally low utilization rates, have led to the considerable waste of fertilizer resources, in addition to severe environmental pollution [4,5]. Controlled-release fertilization is an efficient and pollution-free method of fertilization, which addresses the low utilization rate challenge associated with ordinary quick-acting fertilizers, in addition to environmental pollution, and is a potentially effective strategy to ensure the sustainability of land use and efficient crop production [1,3]. Inadequate soil water and soil nutrient levels are the major factors limiting agricultural production in the Loess Plateau [6–8]. Agricultural production in such environments should aim at achieving efficient water use [9]. Due to the lack of irrigation water resources in the Loess Plateau, rain-fed agriculture accounts for a large proportion of the agricultural production in the region [5,10]. The considerable fertilizer inputs in the region have promoted crop yield greatly, in addition to the associated economic benefits, and the improvement of soil water use efficiency (WUE) of the cropping systems. Consequently, soil fertilization has become one of the key strategies of achieving higher crop yields in the region [11,12]. Rational fertilization practices that take into account soil water availability, improve soil WUE, and improve crop yield have become key strategies for comprehensive agricultural development on the Loess Plateau [13,14]. Strategies for improving crop yield and soil WUE through rational fertilization under poor water availability conditions have been extensively studied. Numerous studies have demonstrated that appropriate water and fertilization conditions facilitate nutrient absorption by crops which, in turn, facilitate coordinated growth in crops [15,16]. Particularly, nitrogen (N) and phosphorus (P) fertilization could decrease transpiration losses significantly and enhance WUE in crop production on the Loess Plateau [7]. Soil WUE reflects the relationship between crop production and water consumption. It is a comprehensive index for evaluating crop growth suitability, and improving WUE could minimize the consumption of limited water resources, especially in semi-arid areas where precipitation is scarce, with important ecological and economic benefits. In arid areas, the consumption of less water resources could promote sustainable water resource exploitation in ecosystems [3]. Drought stress in the Loess Plateau greatly influences fertilization efficiency in the region [17,18]. Under limited soil water conditions, water availability and N and P fertilization can be exploited collectively to enhance yield and improve crop quality [19]. Fertilization can promote crop growth and development, particularly by promoting the development of shallow roots, and enhancing soil nutrient absorption and utilization efficiency [20–22]. Due to crop cultivation and soil erosion, the soil organic matter (SOM) concentrations in dryland farmlands are generally low, and the ability to supply and maintain fertilizer is usually poor. Consequently, the efficient use of water and nutrients is critical for sustainable crop production on the Loess Plateau. However, only a few studies have investigated the effects of fertilization on soil WUE and yield on the Loess Plateau. In the present study, we established split-plot experiments in Ansai County, Shaanxi Province, China. The study region is in a semi-arid/semi-humid climate zone, and major soil and water conservation projects have been implemented in the area [7,8]. The aim of the present study was to investigate the effects of different fertilization regimes on crop yield and WUE on the Loess Plateau, and to determine fertilization strategies that could facilitate sustainable and efficient utilization of water and fertilizer resources in the arid areas of the Loess Plateau. We hypothesized that (1) fertilization significantly increases water use efficiency and crop yield, and (2) the combined application of nitrogen and phosphorus fertilizer has a greater positive effect on soil water use efficiency and crop yield. 2. Materials and Methods 2.1. Basic Geographic Information The field experiments were conducted in 2017 and 2018 in the middle of the Loess Plateau (36◦510 N, 109◦180 E), at 1068 m above sea level, in an area with a mean annual temperature of 8.8 ◦C and a Sustainability 2020, 12, 4125 3 of 13 Sustainabilitymean annual 2020, 12 precipitation, x FOR PEER REVIEW of 500 mm. The study site falls under the loess hilly and gully area.3 The of 14 soil type is Regosol and the soil nutrient and water loss levels in the area are high. Millet was planted in mm,2017 accounting and soybeans for 79.46% were grownof the total in 2018. annual In 2017, rainfall. the In total 2018, rainfall the total in the rainfall growth in periodthe growth was 443.1period mm, wasaccounting 503.1 mm, foraccounting 79.46% of for the 93.8% total of annual the total rainfall. annual In 2018,rainfall the (Figure total rainfall 1). in the growth period was 503.1 mm, accounting for 93.8% of the total annual rainfall (Figure1). 250 2017 2018 200 10-year average precipitation 150 100 50 precipitation (mm) precipitation 0 Month Figure 1. Average monthly precipitation in Ansai County between 2017 and 2018. Figure 1. Average monthly precipitation in Ansai County between 2017 and 2018. Basic soil physical and chemical properties in the study site were measured at a depth of 0–40 cm beforeBasic fertilizationsoil physical (Table and chemical1). Soil bulk properties density in was the measured study site using were ameasured soil bulk samplerat a depth and of soil0–40 pH cmwas before measured fertilization using (Table a pH meter.1). Soil bulk density was measured using a soil bulk sampler and soil pH was measured using a pH meter. Table 1. Basic physical and chemical properties of the soil before fertilization. Table 1. Basic physical and chemical properties of the soil before fertilization. Soil Depth (cm) Bulk Density SOM TN TP pH Soil depth40 cm (cm) 1.387Bulk density 4.053SOM 0.234TN 0.588TP pH 8.42 40 cm 1.387 4.053 0.234 0.588 8.42 1 Notes: SOM = soil organic matter; TN = soil total nitrogen; TP = soil total phosphorus; all in (g kg− ).
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